COMPRESSIBILITY CHARACTERISTICS OF ORGANIC SOILS
IN EGYPT
By
Eng. HOSSAM IBRAHIM ABDEL KADER
B.Sc. In Civil Engineering, 1985
Dipl. In Soil Mechanics and Foundation Engineering, 1990
Dipl. In Construction Engineering and Management, 2002
A Thesis Submitted to the Faculty of Engineering
at Cairo University in Partial Fulfillment of
The Requirement for Degree of
MASTER OF SCIENCE
IN
PUBLIC WORKS
FACULTY OF ENGINEERING, CAIRO UNIVERSITY
GIZA, EGYPT
September 2010
COMPRESSIBILITY CHARACTERISTICS OF ORGANIC SOILS
IN EGYPT
By
Eng. HOSSAM IBRAHIM ABDEL KADER
B.Sc. In Civil Engineering, 1985
Dipl. In Soil Mechanics and Foundation Engineering, 1990
Dipl. In Construction Engineering and Management, 2002
A Thesis Submitted to the Faculty of Engineering
at Cairo University in Partial Fulfillment of
The Requirement for Degree of
MASTER OF SCIENCE
IN
PUBLIC WORKS
Under the Supervision of
DR. MOSTAFA E. MOSAAD
Prof. of Soil Mechanics and Foundation
Engineering, Faculty of Engineering,
Cairo University
DR. MARAWAN M. SHAHIEN
Assoc. Prof. of Soil Mechanics and
Foundation Engineering, Faculty of
Engineering, Tanta University
FACULTY OF ENGINEERING, CAIRO UNIVERSITY
GIZA, EGYPT
September 2010
COMPRESSIBILITY CHARACTERISTICS OF ORGANIC SOILS
IN EGYPT
By
Eng. HOSSAM IBRAHIM ABDEL KADER
B.Sc. In Civil Engineering, 1985
Dipl. In Soil Mechanics and Foundation Engineering, 1990
Dipl. In Construction Engineering and Management, 2002
A Thesis Submitted to the Faculty of Engineering
at Cairo University in Partial Fulfillment of
The Requirement for Degree of
MASTER OF SCIENCE
IN
PUBLIC WORKS
Approved by the
Examining Committee:
Prof. Dr. Mohamed Mamdouh. A. Sabry
Member
Prof. Dr. Fathalla M. El-Nahhas
Member
Prof. Dr. Mostafa E. Mosaad
Thesis Main Advisor
Dr. Marawan M. Shahien
Advisor
FACULTY OF ENGINEERING, CAIRO UNIVERSITY
GIZA, EGYPT
September 2010
TABLE OF CONTENTS
Title
Page
LIST OF TABLES
x
LIST OF FIGURES
xiii
LIST OF SYMBOLS AND ABREVIATIONS
xxiii
ACKNOWLEDGEMENT
xxvii
ABSTRACT
xxviii
CHAPTER 1:
INTODUCTION
1.1
Background
1
1.2
Problem Statement
2
1.3
Research Objectives
4
1.4
Scope of Work
5
1.5
Thesis Structure
6
CHAPTER 2:
GENERAL CHARATERISTICS OF ORGANIC
SOILS; LITRATURE REVIEW
2.1
Introduction
8
2.2
Definitions
11
2.3
Distribution
13
2.3.1 Global Distribution of Peat lands
13
2.3.2 Distribution of Organic Soils in Egypt
15
Morphology and Formation of Peat lands
22
2.4.1 Morphology of Peat lands
22
2.4.2 Humification
30
2.4.3 Accumulation of Organic Deposits
33
2.4.4 Wastage or Oxidation of Organic Deposits
34
Physical Properties of Organic Soils
35
2.5.1 Organic Soil Phases
36
2.4
2.5
iv
TABLE OF CONTENTS-Continued
Title
2.6
2.7
Page
2.5.1.1 Solid Phase
36
2.5.1.2 Pore Water
37
2.5.1.3 Pore Gas
37
2.5.2 Organic Soil Structural Elements
37
2.5.3 Fabric or Structure
38
2.5.3.1 Macrostructure
39
2.5.3.2 Microstructure
41
Classification Systems for Organic Soils and Peat
43
2.6.1 Classification Systems for Organic soils
48
2.6.2 Classification Systems for Peat
55
Site Investigations for Organic Soils
58
2.7.1 Sampling
59
2.7.1.1 Disturbed Sampling
61
2.7.1.2 Undisturbed Sampling
62
2.7.1.2.1 Block Sampling
63
2.7.1.2.2 Deep Sampling
64
2.7.1.2.3 The Swedish Geotechnical
Institute Sampler
2.8
65
2.7.2 In-situ Testing
67
Index and Chemical Properties of Organic Soils
69
2.8.1 Chemical Properties of Organic Soils
73
2.8.1.1 Chemical Composition
73
2.8.1.2 pH Value
73
2.8.1.3 Cation Exchange Capacity
74
2.8.2 Index Properties of Organic Soils
76
2.8.2.1 Organic Content
76
2.8.2.2 Fiber Content
80
2.8.2.3 Void Ratio
80
v
TABLE OF CONTENTS-Continued
Title
CHAPTER 3:
Page
2.8.2.4 Water Content
81
2.8.2.5 Bulk Unit Weight
83
2.8.2.6 Dry Unit Weight
84
2.8.2.7 Specific Gravity
84
2.8.2.8 Atterberg Limits
86
2.8.2.9 Shrinkage
88
ENGINEERING CHARATERISTICS OF ORGANIC
SOILS; LITRATURE REVIEW
3.1
Introduction
89
3.2
Shear Strength Characteristics of Organic Soils
92
3.2.1Factors Affecting Shear Strength of Organic
Soils
92
3.2.1.1 Effect of Fibers
92
3.2.1.2 Other Influences
95
3.2.2 Determination of Shear Strength
96
3.2.3 Frictional Resistance
98
3.2.4 Undrained Shear Strength
99
3.3
Permeability Characteristics of Organic Soils
101
3.4
Distinct Compressibility Behavior of Organic soils
108
3.5
Soil Compressibility
113
3.5.1Primary Consolidation
116
3.5.2 Secondary Compression
125
3.5.3 The Cα/Cc Concept
132
3.6
Factors Affecting Compressibility of Organic soils
134
3.7
Distinct Compressibility Characteristics of Organic
Soils
141
vi
TABLE OF CONTENTS-Continued
Title
Page
3.7.1One-Dimensional Oedometer Testing of Organic
Soils
141
3.7.1.1 Problems related to conventional Test
143
3.7.1.2 Biodegradation of Peat in Laboratory
3.8
CHAPTER 4:
Environment
144
3.7.2 Compression Curves
145
3.7.2.1 Time-Compression Curves - e (εv)-log t
145
3.7.2.2 The e (εv)- σ`v Curves
150
3.7.3 Compression Parameters of Organic soils
152
3.7.3.1 Primary Consolidation
152
3.7.3.2 Secondary Compression
161
Compressibility of Natural Organic soils
166
RESEARCH PROGRAM
4.1
Introduction
168
4.2
Sampling
170
4.3
Preliminary Tests
173
4.3.1 Physical and Index Properties
174
4.3.2 Chemical Properties
175
4.3.3 Scanning Electron Microscope
175
4.3.4 Recommended Classification System and
Organic Soil Classification
4.4
176
Engineering Properties
177
4.4.1 Shear Strength
177
4.4.2 Permeability
178
4.4.3 Compressibility
179
4.4.3.1 Testing Program
179
4.4.3.2 Preparation of Samples
182
vii
TABLE OF CONTENTS-Continued
Title
CHAPTER 5:
Page
4.4.3.3 Consolidation Tests
184
4.4.3.4 Data Analysis
185
TEST RESUTS
5.1
Introduction
187
5.2
Physical and Index Properties
187
5.2.1Organic Soils' Description
190
5.2.2 Organic Content
191
5.2.3 Moisture Content
191
5.2.4 Void Ratio
192
5.2.5 Specific Gravity
192
5.2.6 Unit Weight
192
5.2.7 Atterberg Limits
195
5.2.8 Particle Size Distribution
195
Chemical Properties
197
5.3.1pH Level
197
5.3.2 X-Ray Diffraction
197
5.3.3 Environmental Corrosion Tests
202
5.4
Scanning Electron Microscope (SEM)
203
5.5
Organic Soil Classification
207
5.6
Undrained Shear Strength
208
5.6.1 Unconfined Compression Test
208
5.6.2 Pocket Penetrometer
208
5.7
Permeability
212
5.8
Compressibility
215
5.8.1 Primary Consolidation Behavior
221
5.8.2 Secondary Compression Behavior
228
5.3
viii
TABLE OF CONTENTS-Continued
Title
CHAPTER 6:
Page
ANALYSIS AND DISCUSSION OF RESULTS
6.1
Introduction
232
6.2
General Characteristics of Encountered Organic
Soils
6.3
CHAPTER 7:
234
Engineering Characteristics of Encountered
Organic Soils
248
6.3.1 Undrained Shear Strength
249
6.3.2 Permeability Characteristics
256
6.3.3 Compressibility Characteristics
261
6.3.3.1 Primary Compression
266
6.3.3.2 Secondary Compression
282
SUMMARY, CONCLUSION, AND FUTURE WORK
7.1
Summary
289
7.2
Conclusions
290
7.3
Recommendations for Future Work
295
REFRENCES
297
APPENDIX –A
322
ix
LIST OF TABLES
Table
Title
Page
Table 2.1:
Global distribution of peat lands
15
Table 2.2:
Mire stages, morphology, flora and associated
properties of some British peats
26
Table 2.3:
Qualifying Terms for Peat Soils
32
Table 2.4:
von Post Classification system for organic soils
45
Table 2.5:
Classification of Peat Structure by Radforth
47
Table 2.6:
Organic soils and peat classification
51
Table 2.7:
Description of mixtures in the organic – clay – silt
+ sand triangle
53
Table 2.8:
Grouping of Organic Materials (Tentative ASTM
Standard)
54
Table 2.9:
USDA classification of peat
55
Table 2.10:
Classification of peat (ASTM D 4427)
57
Table 2.11:
Specimen quality in terms of volumetric strain
60
Table 2.12:
Mire stages and associated properties of some
British peats
70
Table 2.13:
Organic soil samples at East Nile Delta and their
properties
71
Table 2.14:
Index properties of organic soil and peat based on
location
72
Table 2.15:
Relative values of various peat properties for
predominant types
80
Table 3.1:
Summary of correlations for Cc
119
Table 3.2:
Values of Cα/Cc for Geotechnical Material
134
x
LIST OF TABLES-Continued
Table
Title
Page
Table 4.1:
Details of incremental loading scenarios of
consolidation tests
180
Table 5.1:
Physical and index properties of organic soil
samples of Robaomaah school-Mahmoudia
188
Table 5.2:
Physical and index properties of organic soil
samples of Ezbet El-Domyati school-Motoubes
189
Table 5.3:
Summary of the organic content and the
constituent minerals abundances of the RU, RL,
and D tested samples
201
Table 5.4:
Injurious chemical compounds in organic soil’s
samples
202
Table 5.5:
Chemical analysis of the groundwater for the two
sites
202
Table 5.6:
Undrained shear strength of RU & RL stratum
determined by unconfined compressive strength
and by pocket penetrometer
209
Table 5.7:
Undrained shear strength of D stratum determined
by unconfined compressive strength and by pocket
penetrometer
210
Table 5.8:
Results of permeability tests for RU, RL, and D
214
Table 5.9:
One-dimensional consolidation test results for RU
& RL stratums
219
Table 5.10:
One-dimensional consolidation test results for D
stratum
220
Table 6.1:
Comparison of some index properties, for different
types of soil, from literature
236
Table 6.2:
Summary of index, and chemical properties of RU, RL,
and D stratums
236
xi
LIST OF TABLES-Continued
Table
Title
Page
Table 6.3:
Summary of Moisture Content of RU, RL, and D
Samples
240
Table 6.4:
Summary of Specific Gravity of RU, RL, and D
Samples
242
Table 6.5:
Summary of Bulk Unit Weights of RU, RL, and D
Samples
244
Table 6.6:
Summary of undrained shear strength data
249
Table 6.7:
Strength parameters of peat soils from literature
250
Table 6.8:
Strength parameters of muck and organic silt and
clay soils from literature.
Summary of the permeability characteristics of the
soils
251
Table 6.10:
Summary of the compressibility characteristics of
RU, RL, and D stratums
263
Table 6.11:
Compressibility characteristics of some peat
deposit
264
Table 6.12:
Compressibility characteristics of some muck
deposits
265
Table 6.13:
Compressibility characteristics of some organic silt
and clay deposits
265
Table 6.9:
xii
256
LIST OF FIGURES
Figure
Title
Page
Figure 2.1:
Extent and Location of Global Peat lands
14
Figure 2.2:
Areas of organic deposits in Nile Delta region
16
Figure 2.3:
Areas of soft clay deposits within Delta region
17
Figure 2.4:
The Nile Delta in the first century BCE, showing
the names and locations of Known Nile
Distributaries
18
Figure 2.5:
Geotechnical zones of Nile Delta region
20
Figure 2.6:
Cross-section of Holocene sediments in the eastern
part of Nile Delta, running from El-Mansoura
south to Damietta north
21
Figure 2.7:
Raised bogs in deep and shallow basins
27
Figure 2.8:
Geological development of a riverine depositional
model
29
Figure 2.9:
Scanning Electron Micrographs of Middleton
fibrous peat
40
Figure 2.10:
Schematic diagram of Fibrous Peat Structure
41
Figure 2.11:
Comparison of Classification Systems Used for
Peat and Organic Soils
49
Figure 2.12:
LPC classification of organic soils
50
Figure 2.13:
Organic matter-clay-silt+sand triangle according to
NEN 5104
52
Figure 2.14:
Peat sampler
62
Figure 2.15:
Large block samples
63
Figure 2.16:
Block sampler: University of Sherbrooke sampler
64
Figure 2.17:
The Swedish Geotechnical Institute Sampler in
use.
66
xiii
LIST OF FIGURES - Continued
Figure
Title
Page
Figure 2.18:
pH value and exchangeable cations in relation to
organic content of lake, fen and bog soils
75
Figure 2.19:
Influence of organic content on classification
properties of Juturnaiba organic clay, Brazil
79
Figure 2.20:
Correlation of water content with loss-on-ignition
82
Figure 2.21:
Correlation of specific gravity with loss on ignition
for Irish peat and from literature
86
Figure 2.22:
Plasticity index versus liquid limit for some UK
peats
87
Figure 3.1:
Effect of compression on peat fabric
93
Figure 3.2:
Shear Strength as Function of Effective Stress
94
Figure 3.3:
Shear failure modes
95
Figure 3.4:
Effective friction angle versus organic content
99
Figure 3.5:
Normalized undrained strength versus organic
content for US peats
100
Figure 3.6:
Strength increment ratio versus organic content
101
Figure 3.7:
Vertical
permeability
during
pauses
consolidation tests on undisturbed peat
in
103
Figure 3.8:
Data on vertical coefficient of permeability of
fibrous peats within frame of reference of
permeability data for sodium clay minerals, soft
clay deposits, including Mexico City clay, and
clean sand
104
Figure 3.9:
Comparison of compressibility and permeability
behavior of muck soil compared to inorganic clay
and fibrous peat
106
Figure 3.10:
Explanation of magnitude of Ck/ e or Ck/ eo in
terms of five materials with different pore-size
distribution
108
xiv
LIST OF FIGURES-Continued
Figure
Title
Page
Figure 3.11:
Typical field settlement curve for an embankment
built over organic deposit
109
Figure 3.12:
Typical time-settlement curve of organic-rich soil
111
Figure 3.13:
Time-settlement curve showing total settlement
components
114
Figure 3.14:
Idealized relationship between void ratio e and
logarithm of the effective stress σ`
115
Figure 3.15:
Settlement of a soil sample or layer of soil of
thickness H in situ
117
Figure 3.16:
Typical laboratory consolidation curve
120
Figure 3.17:
Hypothesis A and B
130
Figure 3.18:
e-log σ`-log t Plot showing relationship between Cα
and Cc during secondary compression
133
Figure 3.19:
Perry and Poskitt (1972) model for Fibrous peat
based on the concept of micro and macro pores.
137
Figure 3.20:
Experimental void ratio-log effective pressure and
void ratio-log permeability for amorphous granular
and fibrous peat
137
Figure 3.21:
Scanning Electron Microphotographs (SEM) of
Middleton peat
139
Figure 3.22:
Values of natural water content and compression
index for peats, clays and silts
140
Figure 3.23:
Types of compression versus logarithm of time
curve derived from consolidation test
145
Figure 3.24:
Vertical strain versus logarithmic of time curve of
fibrous peat for one-dimensional consolidation
147
Figure 3.25:
Examples of secondary compression behavior of
Middleton peat
151
xv
LIST OF FIGURES-Continued
Figure
Title
Page
Figure 3.26:
EOP e versus log σ`v curves of 24 undisturbed
specimens of Middleton peat
151
Figure 3.27:
Relationship between preconsolidation pressure
and in-situ void ratio eo for peat deposit (Kogure
and Ohira, 1977) as well as σ`v versus e
relationship resulting from compression of
Meddleton and James Bay peats
154
Figure 3.28:
Relationship
pressure
consolidation
155
Figure 3.29:
Empirical correlation between compression index,
Cc, and natural water content, wo , for peats,
organic soils as well as soft clay and silt deposits
156
Figure 3.30:
Variation of the coefficient of consolidation with
stress level
158
Figure 3.31:
Effective stress versus coefficient of consolidation
cv
159
Figure 3.32:
Relationship between initial water content and void
ratio
161
Figure 3.33:
Relationship between Cα and σ`v
163
Figure 3.34:
Relationship between Cα and Cc
163
Figure 3.35:
Secondary compression of Meddleton peat
predicted by Cα/Cc concept of compressibility
165
Figure 3.36:
Secondary compression behavior of Meddleton
peat for pressure increment that ends near
preconsolidation pressure resulting from secondary
compression aging
166
Figure 4.1:
Specially- designed thick-walled open-drive 100
mm diameter sampler
171
Figure 4.2:
Labeled samples, wooden boxes and groundwater
sample
173
between
xvi
Cc
and
LIST OF FIGURES-Continued
Figure
Title
Page
Figure 4.3:
Muffle furnace and porcelain crucibles used for
LOI determination
174
Figure 4.4:
Scanning Electron Microscope (SEM) used for
micrograph production
176
Figure 4.5:
Compression device used for unconfined
compressive strength and tested samples.
177
Figure 4.6:
Special consolidation cells used for falling-head
permeability tests
178
Figure 4.7:
MIT procedure for obtaining test specimen from
tube sample
183
Figure 4.8:
Tube sample cutting apparatus and the rotational
trimming table
184
Figure 4.9:
Assembly of all
consolidation tests
standard
185
Figure 5.1:
Variation of organic content, moisture content,
void ratio, specific gravity, bulk unit weight, and
dry unit weight with depth for Robaomaah school
site– Mahmoudia.
193
Figure 5.2:
Variation of organic content, moisture content,
void ratio, specific gravity, bulk unit weight, and
dry unit weight with depth for Ezbet El-Domyati
school site – Motoubes
194
Figure 5.3:
Atterberg limits of organic soil’s samples plotted
on Casagrande plasticity chart
195
Figure 5.4:
particle size distribution of RU, RL, and D samples
196
Figure 5.5:
X-Ray diffraction test results for RU samples
198
Figure 5.6:
X-Ray diffraction test results for RL samples
199
Figure 5.7:
X-Ray diffraction test results for D samples
200
components
xvii
of
LIST OF FIGURES-Continued
Figure
Title
Page
Figure 5.8:
The Scanning Electron Micrographs of RU sample
at initial state
204
Figure 5.9:
The Scanning Electron Micrographs of RU sample
under consolidation pressure of 3200 kPa
204
Figure 5.10:
The Scanning Electron Micrographs of RL sample
at initial state
205
Figure 5.11:
The Scanning Electron Micrographs of RL sample
under consolidation pressure of 3200 kPa
205
Figure 5.12:
The Scanning Electron Micrographs of D sample at
initial state
206
Figure 5.13:
The Scanning Electron Micrographs of D sample
under consolidation pressure of 3200 kPa
206
Figure 5.14:
Soil stratigraphy of the two sites; Robaomaah
school – Mahmoudia, and Ezbet El-Domyati
school – Motoubes
207
Figure 5.15:
Variation of undrained shear strength for RU and
RL samples obtained by unconfined compression
tests and by pocket penetrometer with depth.
211
Figure 5.16:
Variation of undrained shear strength for D
samples obtained by unconfined compression tests
and by pocket penetrometer with depth.
211
Figure 5.17:
Data on vertical permeability kv and horizontal
permeability kh of the three stratums as effective
vertical stress increases from σ`vo to σ`vf and void
ratio decreases from eo to ef.
213
Figure 5.18:
Typical s-logt curves for RU stratum
216
Figure 5.19:
Typical s-logt curves for RL stratum
217
Figure 5.20:
Typical s-logt curves for D stratum
218
xviii
LIST OF FIGURES-Continued
Figure
Title
Page
Figure 5.21:
Typical ε-log σ` EOP compression curves for RU
stratum
222
Figure 5.22:
Typical ε-log σ` EOP compression curves for RL
stratum
223
Figure 5.23:
Typical ε-log σ` EOP compression curves for D
stratum
224
Figure 5.24:
The variation of σ`vo, σ`p, OCR, and εvo with depth
for RU & RL stratums
225
Figure 5.25:
The variation of σ`vo, σ`p, OCR, and εvo with depth
for D stratum
226
Figure 5.26:
The variation of coefficient of consolidation in the
vertical direction cv with vertical effective stress
for RU, RL, and D stratums.
229
Figure 5.27:
The variation of coefficient of consolidation in the
vertical direction ch with vertical effective stress
for RU, RL, and D stratums
230
Figure 5.28:
The variation of variation of time to the EOP (tp)
with effective stress for RU, RL, and D stratums
231
Figure 6.1:
Correlation between loss on ignition and void ratio
239
Figure 6.2:
Correlation between loss on ignition and natural
moisture content compared with the data compiled
by O’Loughlin and Lehane (2003)
241
Figure 6.3:
Correlation between loss on ignition and specific
gravity compared with the data compiled by
O’Loughlin and Lehane (2003)
243
Figure 6.4:
Correlation between bulk unit weight and the loss
on ignition (OC)
245
Figure 6.5:
Correlation between dry unit weight and the loss
on ignition (OC)
245
xix
LIST OF FIGURES-Continued
Figure
Title
Page
Figure 6.6:
Correlation between liquid limit and plasticity
index with the loss on ignition
247
Figure 6.7:
Correlation between liquid limit and plasticity
index with the loss on ignition
247
Figure 6.8:
The relationship between undrained shear strength
determined by unconfined compressive strength
(Sun) relative to that determined by pocket
penetrometer (Sup)
252
Figure 6.9:
The variation of normalized undrained shear
strength (Su/σ`p) with loss on ignition (OC)
254
Figure 6.10:
The variation of undrained shear strength (Su)
determined by both methods and normalized
undrained shear strength (Su/σ`p) with depth
255
Figure 6.11:
Correlation between initial void ratios, eo, and
initial permeability, ko.
258
Figure 6.12:
Correlation between initial permeability ko and loss
on ignition (OC)
258
Figure 6.13:
Data on initial permeability ko of RU, RL, and D
stratums within a frame of reference permeability
data from the literature on fibrous and amorphous
peat deposits, peaty muck, pure clay minerals
montmorillonite, illite, and kaolinite, soft clay and
silt deposits and clean sand
259
Figure 6.14:
Data on vertical permeability kv and horizontal
permeability kh of RU, RL, and D stratums within
the same frame of reference permeability data
259
Figure 6.15:
Relationship between Ck and in-situ void ratio (eo)
260
Figure 6.16:
Relationship between the measured OCR after
aging relative to that computed due to aging.
267
xx
LIST OF FIGURES-Continued
Figure
Title
Page
Figure 6.17:
The variation of constrained modulus (D = 1/ mv)
with vertical effective stress for RU, RL, and D
stratums
269
Figure 6.18:
The variation of Cc` with normalized effective
applied pressures (σ`v/σ`p) for RU, RL, and D
stratums
270
Figure 6.19:
Relationship between Cr` and Cc` for RU, RL, and D
272
Figure 6.20:
The variation of coefficient of consolidation in the
vertical direction cv with vertical effective stress
for RU, RL, and D stratums
274
Figure 6.21:
The variation of coefficient of consolidation in the
vertical direction ch with vertical effective stress
for RU, RL, and D stratums
275
Figure 6.22:
The envelope diagrams drawn for the coefficient of
consolidation data of peaty muck from West
Lafayette (Santagata et al., 2008), and Middleton
fibrous peat (Ajlouni, 2000)
277
Figure 6.23:
The variation of vertical coefficient of
consolidation (cv) with normalized applied pressure
(σ`v ⁄σ`p) for RU, RL, and D stratums
278
Figure 6.24:
Correlation between Cc and Wn for RU, RL, and D
samples
281
Figure 6.25:
Correlation between σ`p and eo for RU, RL, and D
samples
281
Figure 6.26:
Typical logarithm of time-compression curves in
the recompression range.
283
Figure 6.27:
Typical logarithm of time-compression curves, in
the compression ranges
284
stratums
xxi
LIST OF FIGURES-Continued
Figure
Title
Page
Figure 6.28:
A sketch explains obviously the possiblilty of
using Casagrande method to define the time to the
end of primary consolidation, tp, for typical
logarithm of time-compression curve with proper
scale
285
Figure 6.29:
The variation of Cα` with normalized effective
applied pressures (σ'v/σ'p) for RU, RL, and D
stratums
286
Figure 6.30:
The variation of Cα` with normalized effective
applied pressures (σ'v/σ'p) for RU, RL, and D
stratums
288
xxii
LIST OF SYMBOLS AND ABREVIATIONS
Symbol
Description
Ac
Ash content
av
Coefficient of axial compressibility
ASTM
American Society for Testing and Material
BP
Before present
C
Carbon
c`
Effective cohesion
C14
Carbon 14 dating
Cc
Compression index
Ck
Permeability change index = ∆e ⁄∆ log kv
Cr
Recompression index
Cs
Rebound index
CRS
Constant-rate-of-strain consolidation tests
D
Constrained modulus (D=1/mv)
DSS
Direct simple shear test
DST
Direct shear test
ECP
Egyptian code of practice
e
Void ratio
ef
Postconstruction void ratio
eo
In situ void ratio under effective overburden pressure
eop
Void ratio at the end of primary consolidation (EOP).
Gs
Specific gravity of soil solids
GWT
Groundwater table
H
Hydrogen
Hdr
Maximum drainage distance
Hn
Degree of humification
h
Hour
IL
Incremental loading
xxiii
LIST OF SYMBOLS AND ABREVIATIONS-Continued
Symbol
Description
ICD
Isotropically Consolidated drained test
ICU
Isotropically Consolidated undrained test
ISSMFE
kh
International Society of Soil Mechanics and Foundation
Engineering
Coefficient of permeability in horizontal direction
kho
Initial coefficient of permeability in horizontal direction
kv
Coefficient of permeability in vertical direction
kvo
Initial coefficient of permeability in vertical direction
kvf
Coefficient of permeability in vertical direction at ef
km
Kilometer
KoCE
Consolidated extension test under laterally constrained condition
KoCU
LID
Consolidated undrained
condition
Load-increment duration
LIR
Load-increment ratio
m
Meter
Mg
Mega gram
MSL
Laboratory multiple-stage-load (MSL) oedometer tests
mv
Coefficient of volume compressibility
N
Loss on ignition (LOI)
nm
Nano millimeter
O
Oxygen
OC
Organic content
pH
Acidity as{log
test
1
}
(H + )
P.P.
Pocket penetrometer
PWP
Pore water pressure
qun
Unconfined compressive strength
RS
Ring shear test
xxiv
under
laterally
constrained
LIST OF SYMBOLS AND ABREVIATIONS-Continued
Symbol
Description
SEM
Scanning Electron Microscope
SGI
Swedish Geotechnical Institute
SQD
Sample Quality Designation
Su(DSS)
Undrained shear strength measured by direct shear test
Su(FV)
Undrained shear strength measured by field vane test
Su
Undrained shear strength
Sun
Undrained shear strength using unconfined compression test
Sup
Undrained shear strength using pocket penetrometer
tp
Duration of primary consolidation
TC
Triaxial compression test
UC
Unconfined compression test
USCS
Unified Soil Classification System
UU
Unconsolidated undrained test
V
Total volume
wo
Natural moister content
Ws
Weight of solid particle
εv
Axial strain
εvo
Volumetric strain at effective overburden pressure
γd
Dry unit weight
γsat
Saturated unit weight
∆σ
Stress increment.
σ`p
Preconsolidation pressure
σ`v
Effective vertical consolidation pressure
σ`vf
Postconstruction effective vertical pressure
σ`vo
Preconstruction effective vertical pressure
τ
Shear stress
µn
Micron
xxv
LIST OF SYMBOLS AND ABREVIATIONS-Continued
Symbol
Description
Ø
Angle of internal friction
Ø`
Effective angle of internal friction
xxvi
ACKNOWLEDGEMENT
I would like to praise deeply ALLAHH SWT for offering me the opportunity
for better learning and knowledge. I would like to express my sincere gratitude
to those who have contributed directly or indirectly towards the completion of
this work.
This work could not have been completed without my supervisor, Prof. Dr
Mostafa E. Mosaad. I am really indebted to him for his guidance throughout
the research process from topic selection, problem solving directions, advice,
encouragement, constructive criticism to thesis revision. His personal kindness
and patience are highly appreciated. Special gratitude is due to my cosupervisor, Dr Marawan M. Shahien for his constructive ideas, continuous
support, and providing facilities as well as references followed by interesting
discussions throughout the course of this work. His personal kindness and
friendship are sincerely acknowledged.
Special thanks to my uncle Prof. Dr Salah Hamam for continuous
encouragement and providing me with references. I am also indebted to my
friend Dr Khaled Helali for his great efforts in providing me with references
and support.
Finally, I would like to deeply thank my parents for their sincere prayers and
encouragement. My sincere gratitude and appreciation are also due to my wife
and children for their patience, encouragement and keeping up with me during
the period of strain. Their continuous support helped me to culminate this
work.
xxvii
ABSTRACT
In this research, a detailed soil investigation were conducted on high quality
samples obtained from two sites located at West-Delta, known to have the
thickest and most extensive organic deposits found in Egypt, to characterize the
general and engineering properties of the organic soils encountered. The
general characteristics included the determination of physical, index, and
chemical properties of the soils. X-Ray diffraction analysis was performed to
identify the different minerals constituting the inorganic portion of the organic
soils. In addition, the differences in the fiber contents, the pore spaces, and the
perforated plant structure of the organic soil samples, in its initial state and
after compression under high stress, were observed using the Scanning Electron
Microscope (SEM). Organic soil samples were classified based on “Tentative
ASTM Standard”, recommended in this research, to identify and classify
various types of organic soils in Egypt. The study was focused toward
evaluating compressibility characteristics of the organic soils encountered in
terms of primary and secondary compression, based on data obtained from the
results of incremental loading consolidation tests on vertically and horizontally
oriented samples. Permeability measurements were determined through fallinghead flow measurements during the secondary compression stage of IL
oedometer tests on vertically and horizontally oriented samples. Undrained
shear strength was determined through unconfined compression test and using
pocket penetrometer. Based on the above study, the organic soils encountered,
which ranged from organic silty clay to amorphous peat, were considered as a
transitional material between soft inorganic silts and clays and fibrous peat
regarding their mechanical behavior. Their engineering properties, including
compressibility, permeability and undrained shear strength, fall in between
those characteristics of soft inorganic silts and clays and those typical of
fibrous peat. In addition, the study indicated that the organic soils encountered
are as problematic as typical Egyptian soft clay deposits, in term of
compressibility, regarding the usual loading scenarios.
xxviii
CHAPTER 1
INTRODUCTION
1.1
Background
Organic soils are known for their very low shear strength and high
compressibility. The highly compressible nature of organic soils makes it one
of the most undesirable foundation materials, and its low strength adds to its
reputation as a poor foundation material (McVay and Nugyen, 2004)
Organic soil is defined as any soil containing a sufficient amount of organic
matter to influence its engineering properties (Arman, 1970). Organic-rich soils
are generally referred to as Muck and when they contain more than 75 %
organics, as Peat. On the other hand, organic silt and/or clay have 25% or less
organics (Tentative ASTM Standard).
In Egypt, organic deposits are mainly found in Delta region, about 80 km to the
north of Cairo. Those areas which contain organic deposits include Dakahlia,
south Domyat, north Sharqia, north-east of Gharbia, middle and west of KafrElsheikh and north-east of Bohira governorates (see Fig. 2.2). These organic
deposits are mainly of buried nature. West-Delta contains the most extensive
deposits which may reach to 9 m thick, in addition to frequent presence in the
form of multiple layers (Geotechnical Encyclopedia of Egypt, 2002)
Engineers try to avoid organic deposits whenever possible. Generally, local
organic pockets and shallow deposits are excavated and replaced by a more
desirable material, i.e., cohesionless soil. Unfortunately, there are situations
where organic deposits cannot be removed. For instance, if the deposits are of
sufficient depth, extent, and/or at some great depth, it may not be economical to
excavate and replace them. In such a situation, the engineer may have to use
organic material as a foundation. Alternative construction and stabilization
1
methods such as surface reinforcement, preloading, thermal preconsolidation,
chemical stabilization, and the use of sand or stone column, pre-fabricated
vertical drains, and piles are discussed in literatures (Noto, 1991; Hartlen and
Wolsky, 1996; Huat, 2004).
However, the selection of the most appropriate method should be based on the
examination of the morphological, physical, index, chemical, and engineering
characteristics of the soil. The knowledge on the permeability, shear strength
and compression behavior, of various types of organic soils, is essential as it
enables designers to understand the response of encountered type to load and to
suggest proper engineering solutions to overcome the problem.
1.2
Problem Statement
In Egypt, organic soils have been identified as subdivision of the very soft
problematic clayey soils found in Egypt (ECP 202/5-2001). Also, they have
been qualitatively classified based on their fabric as: fibrous organic soils, peat,
and muck without differentiation between them based on their index and
engineering properties, or even their organic content. Therefore, there is a great
demand of developing a classification system for organic soils to distinguish
between various types of organic soils based on their physical and engineering
properties, so that the described behavior can be related to the proper material.
Also, the vastness of organic soil deposits in Egypt and their occurrence close
to or within population centers and existing cropped areas (see Fig. 2.2) means
some form of infrastructure development has to be carried out in these areas.
These would include road crossings, irrigation and drainage, housing
development, water supply, etc. However, few researches had been so far
published in Egypt dealing with the engineering characteristics of the organic
soils encountered. Also, the common practice is to use piling wherever organic
soils encountered which is very expensive. It is, therefore, necessary to expand
the current knowledge of the morphological, physical, index, chemical, and
2
engineering characteristics of these soils in order to better understand their
mechanical behavior, devise reasonable design parameters, and suggest proper
and economical construction techniques for foundations and earth structures
founded on these soils.
Moreover, all over the world, organic soils most commonly occur as extremely
soft, wet, unconsolidated surficial deposits normally as an integral part of
wetland systems. They may also occur as strata beneath other surficial deposits
(Huat, 2004). In Egypt, the most distinctive characteristic of organic soils is
occurring as buried deposits, under alluvial soils, thousands of years ago. It is
the major task in this research; to assess the extent of problematic nature of
organic soils in Egypt, in term of compressibility, compared with those highly
compressible surficial deposits typically encountered all over the world.
The high compressibility of organic-rich soils stands out as the most significant
engineering property. Different from both sands and clays, peats and organic
soils generally undergo rapid and large consolidation settlement and extensive
long-term secondary compression (Fox, 2003). For most organic deposits in the
field, rapid dissipation of water pressure are completed within a few weeks or
months (Mesri et al., 1997). Also, long-term compression has no end within the
time of engineering interest. In addition, typical organic-rich soils contain 510% gas which contributes to the immediate compression and the immediate
and complete rebound if the load is removed immediately after application
(Landva and La Rochelle, 1983). On the other hand, organic clays or silts
present similar engineering challenges as soft silts and clays, including low
hydraulic conductivity, low shear strength and high compressibility, in addition
to, significant creep deformations.
Also, physical, index, and engineering properties of organic soils show a great
variation both spatially and with depth depending on the type and amount of
organic matter. That is, organic soils are well known for their high variability in
3
soil properties, especially in organic contents. Samples from Shelby tube may
have their organic content range from Organic Silt to Peat, i.e. may exhibit
vastly different mechanical behavior (McVay and Nugyen, 2004). This
characteristic is related mainly to variable degree of decomposition within an
organic deposit. Therefore, detailed soil investigation, on high quality samples,
need to be conducted whenever a facility is intended to be constructed in a
particular site.
Several testing methods have been used to study the compressibility of organic
soils. The most popular one is the conventional incremental loading (IL)
oedometer test. However, there may be differences in the magnitudes of
various quantities measured for organic soils but the general shapes of the
consolidation curves appear reasonably similar and the formulation developed
for clay compression can be used to predict the magnitude and rate of
settlement. Alternatively, constant-rate-of-strain (CRS) tests are conducted to
obtain equivalent information as generated by the IL tests, except that
information on the creep behavior cannot be derived from the CRS tests.
1.3
Research Objectives
Since few researches had been so far published in Egypt dealing with the
engineering characteristics of the organic soils, and due to the importance of
their compressibility characteristics in predicting and dealing with settlements
of foundations and earth structures founded on such soils, the following
objectives were set forth for this research:
1. To expand knowledge of the formation and morphology of various types of
organic soils, their distribution in Egypt and around the world, their
classification systems, and their physical, index, chemical, and engineering
properties.
2. To recommend a classification system for organic soil to be used in Egypt.
4
3. To correlate various types of organic soils, based on the recommended
classification system, and their index and engineering properties, reported in
literature, to provide rationale of the research.
4. To identify and classify the various types of organic soils encountered in
Egypt, based on the recommended classification system.
5. To determine physical, index, chemical and engineering properties of the
various types of organic soils encountered in Egypt, and compare the results
with those reported in literature.
6. To focus the study toward evaluating compressibility characteristics of the
organic soils encountered in Egypt in order to devise suitable design
parameters for settlement analysis.
7. To assess the extent of problematic nature of organic soils in Egypt, in term
of compressibility, regarding the usual loading scenarios encountered.
1.4
Scope of Work
The study focuses on the compressibility characteristics of organic soils found
in Egypt. Therefore, the research activities will include:
1. Exploring the most extensive thick deposits of organic soils found in WestDelta lacated at north-west of Kafr-Elsheikh and north-east of Bohira.
2. Obtainning representative undisturbed samples using a specially-designed
thick-walled open-drive 100 mm diameter sampler resembles that of SGI
peat sampler, highly recommended for peat sampling.
3. Identifying the physical properties of the various organic soils encountered
including differences in their fiber contents, pore spaces, and the perforated
plant structure, in its initial state and after compression under high stresses,
using the Scanning Electron Microscope (SEM), to evaluate the effect of
fabric on their compressibility characteritics.
4. Identifying the index properties of the various organic soils encountered
including organic content, water content, unit weight, initial void ratio,
specific gravity, Atterberg limits, and particle size distribution.
5
5. Identifying the chemical properties of the soil including pH level, injurious
chemical compounds of groundwater and organic soils (sulfates and
chlorides content), and the different minerals constituting the inorganic
portion of organic soils encountered through X-Ray diffraction analysis.
6. Classifying organic soils encountered based on their organic content.
7. Evaluating undrained shear strength characteristics of the organic soil using
pocket penetrometer and unconfined compressive tests.
8. Evaluating permeability characteristics based on falling-head permeability
test during the secondary compression stage of IL oedometer tests.
9. Evaluating compressibility characteristics, in terms of primary and
secondary compression, based on the results of the incremental loading
oedometer test under different loading scenarios.
10. Comparing general and engineering characteristics measured for organic
soil in Egypt to those published in literature all over the world.
1.5
Thesis Structure
Chapter 2 presents literature review on the general characteristics of organic
soils including morphology and formation, physical properties, index and
chemical properties of organic soils. The distributions of organic soils in Egypt
and around the world are described, and the distinct depositional features of
organic soils in Egypt are outlined. Different classification systems used for
organic soils and peats are illustrated. Different methods for organic soils'
sampling and in-situ testing are discussed. Also, the available data, in literature,
on physical, index and chemical properties of organic soils are illustrated.
Chapter 3 presents literature review on the engineering characteristics of
organic soils including permeability, shear strength, and compressibility
characteristics. The different factors that affect the engineering behavior of
organic soils are discussed. The distinct compressibility behaviors of organic
soils in primary and secondary compression are outlined. The compressibility
of soils, consolidation theories, data analysis methods, and the settlements
6
computations are discussed. Also, the distinct compressibility characteristics of
organic soils in terms of one-dimensional oedometer testing, compression
curves, and compression parameters both in primary and secondary
compression are illustrated. The available published data for organic soils, in
terms of permeability, shear strength and compressibility are illustrated.
Chapter 4 provides the overall experimental program including laboratory tests
and data analysis. The sampling procedure implemented in this research is
outlined. The preliminary tests carried out to obtain the physical, index, and
chemical properties and to classify the organic soil encountered were described.
Also, the details on the protocol that was developed to determine the
engineering properties and the equipment and procedure used were outlined.
Chapter 5 reports the results of the experimental program carried out on
organic soil’s samples obtained from two sites of pre-planned primary schools
located at West-Delta. The test results of physical, index, chemical, and
engineering properties including permeability, undrained shear strength, and
compressibility in terms of primary and secondary compression for the organic
soils encountered, are illustrated.
Chapter 6 presents discussion on the general and engineering characteristics of
the organic soils encountered. The test results of the current study were
compared with the compiled data, and correlations between various index and
engineering properties were investigated. The compressibility characteristics of
the organic soils encountered, in terms of primary and secondary compression,
under different loading scenarios implemented in this research, were discussed
and compared with published data.
Chapter 7 presents summary of the research program, the conclusions derived
based on the results obtained from the study, and the recommendation for
topics, related to the present study, suggested for further investigation.
7
CHAPTER 2
GENERAL CHARACTERISTICS OF ORGANIC SOILS;
LITERATURE REVIEW
2.1
Introduction
Organic soils commonly occur as extremely soft, wet, unconsolidated surficial
deposits normally as an integral part of wetland systems. They may also occur
as strata beneath other surficial deposits (Huat, 2004). Organic-rich soils are
those whose solid constituents consist predominantly of vegetable matter in
various stages of decomposition or preservation. They are commonly
designated as bog, fen, muskeg, and moor soils with differentiation between
peat and muck soils on one hand, and coastal marshland soils on the other
(Winterkorn and Fang, 1975). Organic-rich soils can easily be identified by
their combustibility, and when containing significant amounts of decomposed
organic matter are usually characterized by a dark grey to black color and an
odor of decomposition.
On the other hand, organic silts or clays are not as bad as peat but worse than
inorganic deposits. These soils most probably appear as inorganic fine-grained
soils, probably black to dark brown in color; have an organic odor and possibly
some visible organic remains. Their plasticity limits should be evaluated as for
other fine-grained inorganic soils and then be classified as silts or clays of low,
medium and high plasticity.
The characteristics of the organic soils are the product of the interaction of
topography, vegetation, high water table and decomposition and preservation
processes i.e. morphology of peat land. According to Winterkorn and Fang
(1975), "Pedologically, organic soils are intrazonal hydromorphous soils, and
may occur within any macroclimatic zone as long as hydrologic and
topographic conditions provide basins of standing water or land areas with a
8
rising water table". Organic soils accumulate in a landscape when the natural
decay processes fail to keep up with the amount of vegetation being produced.
They form during the decomposition of dead organic substances; mainly
remnants of plants. Decomposition takes place in different ways, mainly
through bacterial activity, and is intensified by a hot climate, suitable humidity
and access to oxygen from the air. Therefore, the specific properties of the
organic particles vary greatly depending upon parent material, climate, and
stage of decomposition.
The physical, index, and engineering properties of organic soils show a great
variation depending on the type and amount of organic matter. The organic
matter may occur in many forms from small amount of amorphous or colloidal
substance embedded in the pores of a mineral soil to fibrous peat with a
structure resembling a coarse, loosely woven mat. The effect of the organic
content on the engineering properties in relation to the properties of a pure
mineral soil is in the former case mainly confined to a decreased permeability
and a somewhat increased tendency to creep. In the latter case, the properties
are quite different in most respects.
According to MacFarlane (1969), organic-rich soil – especially peat - has
certain characteristics that set it apart from most mineral soils and requires
special consideration for construction on them. These characteristics include:
1. Organic soils especially those of predominantly fibrous constitution have a
sponge like nature which accounts for their high natural water content (500
to 2000 %, which is 10-100 times greater than in mineral soils).
2. High void ratio (normally 5-15 but may be as high as 25).
3. High shrinkage at drying (up to 50%).
4. The specific gravity of organic deposits ranges from 1.1 to 2.5; values
above 2 indicate marked contribution by mineral matter.
5. The low specific gravity of both organic matter and water leads to low unit
weights for the natural organic soil.
9
6. The permeability of natural organic deposits varies widely depending on the
effective size of the voids and on the portion of the water held physicochemically at the external surface of the particulate constituents; also,
permeability is often much greater in the horizontal than in the vertical
direction.
7. Large compressibility (high settlement reach 50% of the layer thickness and
half of the settlement occurs during the early few days or weeks).
8. Low bearing capacity (shear strength Su = 5 – 20 kPa).
9. Tensile and shear strength of fibrous peat even in their natural wet state is
provided by the felt-like inrtweaving of their fibrous constituents.
10. The aqueous phase of most organic deposits is acidic with pH values
ranging from 4 to 7 but values as low as 2 and as high as 8 have been
encountered.
11. After drying out, peat upon rewetting do not regain their original high water
content.
12. Disturbance of the natural organic soil structure decreases its strength
properties. The sensitivity of it ranges from 1.5 to 10.
13. Potential for further decomposition as a result of changing environmental
conditions.
Many approaches have been developed to address the problems associated with
construction over organic deposits (Lea and Brawner, 1963; Berry, 1983;
Hansbo, 1991). Replacing the organic deposits with good quality soil is still a
common practice even though most probably this effort will lead to
uneconomic design. Alternative construction and stabilization methods were
discussed in literatures (Noto, 1991; Hartlen and Wolsky, 1996; Huat, 2004).
However, the selection of the most appropriate method should be based on the
examination of the morphological, physical, index, chemical, and engineering
characteristics of the soil.
10
In this chapter, the general characteristics of organic soils including
morphology and formation, physical properties, index and chemical properties
of organic soils shall be discussed. The distributions of organic soils in Egypt
and around the world are described, and the distinct depositional features of
organic soils in Egypt are outlined. Different classification systems used for
organic soils and peats are illustrated. Different methods for organic soils'
sampling and in-situ testing are discussed. Also, the available data, in literature,
on physical, index and chemical properties of organic soils are illustrated.
2.2
Definitions
In the following, the definitions of the terms, usually used in the context
dealing with organic soils, are introduced. These definitions are:
• Technically any material that contains carbon is called organic.
• An organic soil is one that contains a significant amount of organic
material recently derived from plant remains in various stages of
decomposition or preservation. This implies that it needs to be fresh and
still in the process of decomposition, and thus retains a distinctive texture,
color and odor. Some soils contain carbon, but are not recently derived from
plants and thus are not considered organic in this context (e.g. sand
containing calcium carbonate – chemical precipitate). Also, engineers and
geologists use a more narrow definition when applying the term to soils as
will be seen subsequently.
• Peat land can be defined as an area of land where organic soil is found on
the surface. Also, peat land is commonly designated as mire in Europe,
muskeg in Canada, or moor in Japan. However, it is generally known as
wetland or peat swamp because of its water table, which is close to, or
above the peat surface throughout the year and fluctuates with the intensity
and frequency of rainfall. Rainfall and surface topography regulate the
overall hydrological characteristics of the peat land.
• In temperate regions such as in Canada, Europe and the USA, Mires
(organic deposits) are termed bogs and fen. Bogs and fen are pits or basins
11
filled with organic material. Bogs are typically covered with live moss.
Swamps are larger than bogs and may contain a wide variety of materials.
Slow streams or lakes typically feed them. A noteworthy example is the
Everglades in Florida, USA. Sometimes, thoroughly decomposed peat is
called muck.
• In Japan several types of moors (high moors, transitional moors and low
moors) are recognized depending on the source of water supply,
topographic characteristics, and types of underlying soil or rock.
• In tropical countries like Malaysia and Indonesia, peat land is generally
termed basin and valley peat. Water plays a fundamental role in the
development and maintenance of tropical peat. A balance of rainfall and
evapotranspiration is critical to their sustainability.
• Mire (Moor) – a peat land where peat is currently forming and
accumulating.
• Fen (Low-moor) – a peat land which receives its water and nutrients from
the soil, rock and groundwater as well as from rain and ⁄ or snow falling on
its surface.
• Bog (High-moor) – a peat land which receives its water solely from rain
and ⁄ or snow falling on its surface.
• The terms peat and organic soils, used for describing soils with an organic
content, were once synonymous, i.e. used interchangeably, but the term
organic soil is presently used for soils that contain organic matter. The
precise definition of peat however varies between soil science and
geotechnical engineering, as well as between countries.
• Soil scientists define peat as soil with organic content greater than 35%.
• To a geotechnical engineer, all soils with organic contents greater than
20% are known as organic soil. The engineering definition is essentially
based on the mechanical properties of the soil. It is generally recognized
that when the organic content of the soils exceeds 20%, their mechanical
behavior will start to depart from that of mineral soils (Huat, 2004).
12
• Under the Unified Soil Classification System (USCS), organic soils are
recognized as a separate soil entity and has a major division called Highly
Organic Soils (pt), which refers to peat, muck and highly organic soils. In
general, muck indicates a higher degree of decomposition of the vegetable
matter or intermixing with mineral soil constituents in contrast to the purely
vegetable peat that has well-preserved plant remains (Winterkorn and Fang,
1975).
• Macroscopically, peaty material has been divided by Radforth (1952)
into three basic groups, namely amorphous granular, coarse fibrous
and fine fibrous peat. The amorphous granular peat has a high colloidal
fraction, holding most of their water in an adsorbed rather than a free state,
the adsorption occurring around the grain structure. In the other two types
the peat is composed of fibers, these usually being woody. In the coarse
variety a mesh of second-order size exists within the interstices of the firstorder network, whilst in fine fibrous peat the interstices are very small and
contain colloidal matter (Bell, 2000).
2.3
Distribution
2.3.1 Global Distribution of Peat lands
Organic soils are encountered at widely varying areas of the world. They tend
to be most common in those parts of the world with a comparatively cold and
wet climate, but they are also found in the tropics and within the non-tropical
world (Africa, South America) as can be seen from Figure (2.1). There is over
4 million km2 of peat land worldwide, as most recent estimation (Joosten and
Clarke, 2002), as illustrated in Table (2.1).
Considering the definition of peat land as an area of land where organic soil is
found on surface: Figure (2.1) deos not show any peat land in Egypt. Thus
emphasizing, the most distinctive characteristic of organic soils, in Egypt, as
being deposits that are buried underground, as will be seen in the following
sections.
13
14
Fig. (2.1): Extent and Location of Global Peat lands (after Lappalainen, 1996)
Table (2.1): Global distribution of peat lands (adapted from Lappalainen, 1996)
Area of peat land
Location
(km2)
% peat
land of
total area
Europe
515,000
5.73
North, Central, and South America
2,050,746
4.83
Africa
58,534
0.18
Asia
1,523,287
1.06
8,009
0.04
Australia, New-Zealand, and the Pacific
2.3.2 Distribution of Organic Soils In Egypt:
According to the pioneering work reported in the Geotechnical Encyclopedia of
Egypt (2002), organic deposits are mainly found in Delta region, about 80 km
to the north of Cairo. They are extending as a belt from east to west, around
and between the two branches of Nile River, as shown in Figure (2-2). This
area which contains the organic deposits includes Dakahlia, south Domyat,
north Sharqia, north-east of Gharbia, middle and west of Kafr-Elsheikh and
north-east of Bohira governrates.
Organic deposits in Egypt are mainly of buried nature. They exist as intrusions
and/or interbeded layers, of varying thicknesses and at varying depths, within
the thick soft clayey deposits of Delta region shown in Figure (2-3). These
organic deposits are thought to have been developed in relation to the changes
in sea level which occurred after the retreat of the last ice sheets (Andres and
Wunderlich, 1986) in areas along and around the old Nile river branches as
shown in Figure (2-4). According to Said (1981), the sedimentary sequence in
which the organic deposits are recorded belonged to Holocene age.
15
16
Fig. (2.2): Areas of organic deposits in Nile Delta region.(Geotechnical Encyclopedia of Egypt, 2002)
17
Fig. (2.3): Areas of soft clay deposits within Delta region. (Geotechnical Encyclopedia of Egypt, 2002).
18
Fig. (2.4): The Nile Delta in the first century BCE, showing the names and locations of Known Nile Distributaries
(Source: A Traveler's Guide to the Geology of Egypt, Sampsell B M, 2003)
According to the Geotechnical Encyclopedia of Egypt (2002), the Delta region
was subdivided to 26 geotechnical zones. The organic deposits, encountered in
Nile-Delta region, are covering the geotechnical sub-regions No. 6-11, 14, and
15 as shown in Figure (2-5).
At the eastern part of Nile Delta, in geotechnical zones No. 6, 9, 10, and 11,
organic deposits exist at depths 11.0, 9.0, 5.5 and 5.0 m respectively, with
thickness ranging between 1-2 m. They exist at shallower depths and of less
thickness to the north. At mid-Delta, in geotechnical zone No.7, they exist at
depth of 6.5 m with thickness ranging between 1-4 m, and of 2.0 m in average.
At the north-western part of Nile Delta, in geotechnical zones No.8, 14, and 15,
they form the thicker and deeper deposits of all organic deposits in Egypt, in
addition to frequent presence in the form of multiple layers. These organic
deposits exist at average depths 11, 11, and 14 ms, and extend to varying
thicknesses 1-8 m, 1-9 m, and 1-3 m respectively with average thickness of 2-3
m.
To the south of the area, the organic deposits are underlain by medium stiff to
stiff silty clay deposits or sandy layers in geotechnical zones No 6-8, while to
the north of the area, they are underlain by soft to medium stiff silty clay
deposits in geotechnical zones No 9-11, 14, and 15.
Hegab and Bahloul (1987) studied the organic deposits of the eastern part of
the Nile Delta. They noted that the encountered deposits are dark brown to
black in color, non-compacted, and granular in texture. The higher carbon
content was reported in the peaty soil of the southern area while the higher ash
content was reported in that of the northern area. Figure (2-6) shows a cross
section of Holocene sediments running from the south to the north of the
studied area.
19
20
Fig. (2.5): Geotechnical zones of Nile Delta region.(Geotechnical Encyclopedia of Egypt, 2002)
21
Fig (2.6): Cross-section of Holocene sediments in the eastern part of Nile Delta, running from El-Mansoura south to
Damietta north (after Hegab and Bahloul, 1987)
Zayed (1989) noted that many scattered borings in Dakhlia governorate
recorded organic deposits of thicknesses ranging from 1.1 to 2.4 meters, and at
depth ranges from 5.6 to 14.4 meters from the ground surface in many parts of
this area and it surroundings.
Andres and Wunderlich (1986) recorded a peaty layer in the Holocene
sediments nearby Rosetta branch (the western part of the Delta) at depth
ranging from 6 to 8 meters below ground surface. They carried out C14 dating
which gave (4.595 + 55) to (5.870 + 70) B.P range of age for this peaty soil
(Zayed, 1989).
2.4
Morphology and Formation of Peat land
2.4.1 Morphology of Peat land
Peat lands are normally classified according to their topographical and
hydrological features, otherwise known as their "morphology". Peat lands may
pass through a number of morphological stages, each with its own particular
plant communities, which characterize the type of peat that develops.
Knowledge of the morphology of the peat land and structure of the peat for
works on organic terrain is no less important than knowledge of geology and
material properties for works on mineral soils (Hobbs, 1986).
According to the pedology science, organic soils are recognized as
hydromorphic soils of marshes, swamps, seep areas, and flats which may occur
within any macroclimatic zone (Winterkorn and Fang, 1975). That is, the
conditions under which peat land will begin to accumulate are determined
primarily by climate and topography; there should be a superfluity of water
throughout the year and the landform should be such that a sufficiency of this
water is retained to support vegetation and to provide a permanent reservoir in
which to preserve its remains, i.e. the peat. Waterlogged depressions and lack
22
shores form ideal habitats but river banks also suitable provided the water flow
is not too energetic.
Geology, the third component of the hydrology of the area, plays an important
part in peat land formation; its chief influence is on the chemistry and
concentration of nutrients in the water entering the basins and valleys by runoff and percolation; this especially applies to the pH value of the water. Base
rich rocks, such as limestone encourage more diverse plant communities
dominated by grasses, sedges and bushes than acidic rocks, such as sandstones,
which lead to less diverse vegetation with bog moss and cotton-grass, or, more
accurately, cotton-sedge. Weathered rock masses, particularly if permeable;
permit easy solution of nutrients by percolating water.
In temperate regions such as in Canada, Europe, north Asia and the US with a
comparatively cold wet climate, mires (moors) will form and organic deposits
accumulate in areas where there is an excess of rainfall and the ground is
poorly drained, irrespective of altitude or latitude. However, upland mires
(bogs) are very different in topography, morphology, chemistry and botany
from the low level fenlands. Upland mires exist in a maritime climate where
for most of the year precipitation exceeds evaporation and low fenlands in a
drier continental-type of climate (Hobbs, 1986).
Mires (fens and bogs) normally arise through a process that is commonly called
the wetland succession as explained by Hobbs (1986). This wetland succession
has 3 stages, each with its peculiar plant communities producing their
characteristic peat with distinctive geotechnical properties. These stages can be
set out as follows:
• The rheotrophic stage (low moor), in which the mire develops in a body
of water such as a lake, pool or flooded basin and gets its nutrients through
the feeding streams, ground water and seepage. Initially the mire process
starts with inorganic sedimentation, such as silts and clays, but this becomes
23
increasingly more organic as the detritus from plant communities builds up
in the basin floor. The eventual product of this build up is a marsh-like
known as fen. Such mires are generally underlain by very soft organic muds
which can cause severe engineering problems.
• Transitional stage (transitional moor) characterized by a steady growth of
the fen upwards and out of the standing water and into a raised bog. During
this stage, the bog is still influenced by local water levels but is beginning to
rely on rainwater for sustenance;
• Ombrotrophic stage (high moor), where the mires has grown fully out of
the standing water and out of the influence of the local water table. At this
stage the bog relies totally on rainwater for its survival and holds its own
survival water reservoir within its mass above the local groundwater table.
This type of bog is termed a raised bog for obvious reasons and is generally
acidic in character.
The above process of peat land development is also called a lake-filled
process. Ombrotrophic mires need not necessarily be preceded by transitional
mires, but under favorable climatic and topographic conditions can form
directly on the land surface. Such mires are known as blanket bogs. Blanket
bogs are associated with upland areas, normally are not underlain by soft muds
as the vegetation grows directly upon the ground beneath. The peat develops
where slopes are not excessive and drainage is impeded. The process is often
one of swamping, and it may start in shallow waterlogged depressions. Bog
extends down slope if poorly drained surface water gives rise to water logging.
In fact, high rainfall in such cool upland areas gives rise to leaching which
results in accumulation of impervious humus colloid and iron pan at a slight
distance below the surface, usually between 0.3 and 1.0m. Such an
impermeable layer gives rise to water logging which represents ideal conditions
for the development of ombrotropic peat.
24
Those broad stages and mire types are set out diagrammatically in Table (2-2)
in a simplified form with a small selection of the more common plants and the
corresponding index properties of the peat from which it will be seen that a
broad correlation exists between the morphological state and the properties of
the associated peat. The various stages in the normal development from
shallow open water to raised bog and the corresponding lake mud and peat
stratigraphy are shown in Figure (2-7).
All present-day surface deposits of peat in northern Europe, Asia and Canada
have accumulated since the last ice age and therefore have formed during the
last 20,000 years. On the other hand, some buried peat may have been
developed during inter-glacial periods. Peat also has accumulated in postglacial lakes and marshes where they are interbeded with silts and muds.
Similarly, they may be associated with salt marshes. Fen deposits are thought
to have developed in relation to the eustatic changes in sea level which
occurred after the retreat of the last ice sheets (Bell, 2000).
As can be seen, the morphological differences between fen and bog organic
deposits arise from the circumstances surrounding their formation and the plant
types constituting organic soil. These differences extend to structure, fabric,
humification and proportion of mineral material, factors which have a
considerable influence on the plasticity, permeability, compressibility and
strength of organic soil and so on engineering behavior.
25
26
Table (2.2): Mire stages, morphology, flora and associated properties of some British peats (after Hobbs, 1986)
27
Fig. (2.7): Raised bogs in deep and shallow basins (after Ivanov, 1981)
In the tropics like Malaysia and Indonesia, peat deposits also occur in both
highlands and lowlands area. They are generally termed valley and basin peat
respectively. However lowland or basin peat is more extensive and occurs in
low-lying poorly drained depressions in the coastal areas. It is usually found in
the inward edge of mangrove swamps along the coast. It is generally classified
as ombrogenous or rain fed peat, and is poor in nutrients. The individual peat
bodies may range from a few to 100,000 hectares and they generally have a
dome-shaped surface. Due to coastal and alluvial geomorphology they are often
elongated and irregular, rather than having the ideal round bog shape. The
depth of the peat is generally shallower near the coast and increases inwards,
locally exceeding more than 20 m. Coastal peat land is generally elevated well
above adjacent river courses. Steep gradients are found at the periphery while
the central peat plain is almost flat (Huat, 2004). According to Mutalib et al.
(1991) basin peat forms domes which are up to 15 m high whilst valley peat is
flat or interlayered with river deposits. Normally, sandy ridges bound basin
peat at their seaward side or they gradually merge into muddy coastal flats.
Lam (1989) postulates the possible event leading to the development of peat
deposits as a result of sea level changes. After the last maximum glaciations
(some 20,000 years BP), the sea level rose rapidly and reached a maximum
level 5,500 years BP. The last global glaciations resulted in rapid denudation
and deep incision of the parent rock formation. This would result in
transportation and deposition of large amount of sediment, which formed deltas
and flood plains (Huat, 2004). Peat swamps were initiated in the depressions
and basins between isolated hills and levees, and in the deltas.
During the initial stage, plants developed in mineral soils. The areas were still
under the influence of rivers with an influx of clastic (mineral) sediments
during flood. The accumulation of clastic sediments and plant remains resulted
in the formation of clayey peat (topogenous peat). As plant remains
accumulated, the ground surface levels were elevated. Then it led to formation
28
of peat, which was free from or low in clastic sediments (ombrogenous peat),
and highly acidic. The peat forming vegetation consists mainly of large trees,
resulting in a high lignin content which according to Anderson (1983) is twice
that of bog peat (Huat, 2004). Figure (2-8) illustrates the development of a
revering depositional model leading to the deposition of basin peat.
Fig. (2.8): Geological development of a riverine depositional model (Chen et
al., 1989)
29
2.4.2 Humification:
Various terms are used to describe the change of state from fresh plant tissue to
peat: decay, decomposition, breakdown and humification. Humification or
decomposition involves the loss of organic matter either in gas or in solution
(including leaching), the disappearance of physical structure and change in
chemical state. Breakdown of the plant remains is brought about by soil
microflora, bacteria and fungi which are responsible for aerobic decay.
Therefore, the end products of humification are carbon dioxide and water, the
process being essentially one of biochemical oxidation.
Immersion in water reduces the oxygen supply enormously which, in turn,
reduces aerobic microbial activity and encourages anaerobic decay which is
much less rapid. This results in the accumulation of partially decayed plant
material as peat. Under normal field conditions, total degradation of the organic
fraction under water is limited due to volatile acid toxicity and nutrient
imbalance. Introduction of nutrients by groundwater seepage may initiate or
sustain decomposition over long periods.
Metabolic activity, apart from the supply of oxygen, is very much influenced
by temperature, acidity and availability of nitrogen. Normally, the higher the
temperature and pH value the faster decomposition occurs; the slower the
accumulation of peat in relation to plant production. The optimum temperature
for the decay of plant debris seems to fall within the range 35 - 40 C.
Scorching, irreversible dehydration, and oxidation of organic substances begins
at about 60° C (Terzaghi et.al, 1996). Turning to pH value, decomposition
generally tends to be most active in neutral to weakly alkaline conditions pH
value (7 - 7.5). The more acid the peat, the better the plant remains are
preserved. The acidity of the peat lands depends upon the rock types in the area
draining into the peat land, the types of plants growing there, the supply of
oxygen and the concentration of humic acids. Normally, decomposition takes
place more rapidly as the amount of available nitrogen increases (Bell, 2000).
30
In temperate regions, blanket and raised bogs are generally acidic with pH
values in the range 3.3 - 4.3. Fen peats on the other hand are generally neutral
or slightly alkaline. Therefore, bog peat is generally more fibrous compared
with fen peat. According to Hobbs (1986), some fen peat in Britain, because
they occur in areas of carbonate rocks such as chalk or limestone, is supplied
with water which is slightly alkaline. As such the plant communities are more
diverse, giving rise to what is called rich fen peat. Rich fen peat develops a
much higher degree of humification than acid peat. Because the strength and
permeability of peat declines significantly as humification increases, rich fen
peat represents more problems to engineers than acid peat.
In the Asia tropics, the peat is generally acidic with pH values in the range of
4 – 4.5 (Mutalib et al., 1991). In tropical Africa the acidity is limiting at
higher values; peat does not accumulate unless the pH is less than 5.5; the
temperature is such (20° to 30° C throughout the year) that decomposition is
complete (Thompson and Hamilton, 1983).
In summary, the fresher the peat, the more fibrous material it contains and the
more light in color. As far as engineering is concerned, the more fibrous peat,
the higher the tensile and shear strength, void ratio, and water content. During
humification, the loss of organic matter and change of chemical state is
accompanied by the breakdown of cellulose within plant tissues so that detritus
gradually becomes increasingly finer until the trace of fibrous structure
disappears. The peat then has an amorphous granular appearance; the material
will be dark in color consisting principally of gelatinous organic acids which
have a sponge-like fabric (Bell, 2000). The process is finally complete when
there is only humus (non degradable residue) and microbial cells are left. In
general, decomposition causes a decrease in solid volume, i.e., compression.
Also, the intensity of the humification process varies throughout peat since
some plants are more resistant than others and certain parts of plants are more
31
resistant than others. The change undergone in peat, as a result of increasing
humification, is not uniform since the fibers are reduced in size and strength in
an irregular manner as the quantity of totally humified peat increases.
As physical, index, and engineering properties of peat and organic soil are
closely related to the average state of decomposition or humification; a method
of quantifying this state is therefore useful. The ecologist L Von Post (1922)
proposed a simple field test for assessing the degree of humification on a scale
of 1 to 10; that is, from fresh plant to amorphous-granular completely
decomposed peat. The degree of humification (Hn) is determined based on the
appearance of soil water that is extruded when a handful of peat is squeezed in
the hand, then observations made with the table (Table 2.7) on degree of
humification. This test forms the basis of the Von Post system of classifying
peat, discussed later.
The Von Post scale however is adapted to pure peat containing little or no
mineral matters. Its use in organic soils with more than 25% mineral matter is
difficult (Huat, 2004). As a result, various coarser scales have been devised to
3-5 degrees of humification. Table (2.3) describes the qualifying terms for peat
soil based on degree of decomposition according to Von Post scale.
Table (2.3): Qualifying Terms for Peat Soils (Huat, 2004)
Organic
Von Post Scale
Qualifying terms
Peat
H1 – H3
Fibric/Fibrous
H4 – H6
Hemic/Moderately Decomposed
H7 – H10
Sapric/Amorphous
32
Fibric peat is mostly undecomposed; typically tan to light reddish brown in
color. Hemic peat is intermediate between fibric and sapric peat in degree of
decomposition or humification, organic content and bulk density, and typically
dark reddish brown in color. Sapric peat on the other hand is generally of
darker color than the above two types of peat, and the most decomposed. It
generally has the highest organic content and bulk density of the three types of
peat mentioned above (Huat, 2004).
2.4.3 Accumulation of Organic Deposits:
Peat forms in a landscape when the natural decay processes fail to keep up with
the amount of vegetation being produced. This usually happens on waterlogged
land starved of oxygen, such as found in mires, fens and bogs, where the lack
of oxygen prevents natural microorganisms from decomposing the dead plant
material. That is, they tend to be most common in those regions where there is
an excess of rainfall and the ground is poorly drained. Where these conditions
occur the dying vegetation does not decay at the end of the growing season as
normal but instead accumulates year on year as a peat layer. Peat forms slowly
in this way, involving an accumulation of organic material in water, and taking
approximately 10 years for 1 cm of peat to form.
Approximately 95% of all deposits of peat have been formed from plants
growing under aerobic conditions. The high water-holding capacity of peat
maintains a surplus of water, which ensures continued plant growth and
consequent peat accumulation. The rate of decomposition of plant detritus is
relatively rapid under aerobic conditions but is slowed down several thousandfold under anaerobic conditions. The most important feature in this simple
scenario is water and in particular the water balance within the peat. For a peat
land to survive the water balance can not be negative; i.e. the water input must
keep up with the water loss (Hobbs, 1986).
33
Peat is not the only soil with organic content. Organic soils can occur in many
ways and in many landscapes. The organic material can be deposited in-situ,
like peat, by dying vegetation and it can also be washed into place by
inundation, flood, rivers, etc. These latter soils that have had their organic
material washed into them inevitably have a higher mineral content due to the
minerals carried by the incoming water flows. These high mineral content soils
are usually considered to be out with the classification of a peat land.
2.4.4 Wastage or Oxidation of Organic Deposits:
Climatic changes affect plant growth in peat lands. A period in which increased
rainfall occurs enhances peat growth and development. By contrast, long drier
periods mean that water levels fall, this give rise to shrinkage and wastage of
peat at the surface whilst the peat below is compacted.
Wastage of peat also occurs as a result of permanent drainage works. Drainage
lowers the water table which normally reduces the growth of vegetation and
means that oxygen begins to invade the anaerobic zone. This, together with the
higher temperature, enhances aerobic decay and associated humification. In
addition, capillary suction and desiccation of peat above the water table lead to
its shrinkage.
These activities may result in significant changes in engineering properties. The
fiber structure is destroyed due to accelerated decomposition and becomes
more amorphous granular (Vonk 1994). Also, drying of organic coarse
particles causes shrinkage of thin-walled tissues and collapse of cell structure
and thereby decreases particle porosity and water-holding capacity. It also
promotes aggregation of organic substances, whereupon organic precipitates
bind mineral particles into stable aggregates.
34
Additionally, drying out, groundwater fluctuations and snow loading bring
about compression in the upper layers of a peat deposit. Under these
mechanisms, the effective pressure is raised causing compression of the peat.
For instance, a fall of 1.0 m of GWT imposes an extra load of 10 kPa which
can lead to approximately 1.5 m of settlement in a layer of peat 10 m in
thickness if the water level is maintained at 1m below the subsided surface for
a year. As there is no loss of material these process of reduction in thickness of
peat are not included within wastage. Indeed, these mechanisms are often more
important as far as near-surface compression is concerned than effective
overburden pressure. This is because the unit weight of peat may be similar to
that of water. As the water table in peat generally is near the surface, the
effective overburden pressure is negligible (Bell, 2000).
Even without drying and at moderate laboratory temperatures, the organic solid
content of soil is susceptible to degradation, decomposition, dissolution and
therefore to loss. Scorching, irreversible dehydration, and oxidation of organic
substances begin at about 60° C (Terzaghi et.al, 1996).
2.5
Physical Properties of Organic Soils:
As can be seen from above, different organic parent materials, various aerobic
and anaerobic conditions of degradation, and different degrees of humification
produce organic substances with a wide range of molecular structure and
particle morphology (Terzaghi et al., 1996). That is, organic substances in soil
range from macroscopic incompletely decomposed plant and animal residues to
microscopic dark-colored humus. Also, organic matter may occur in many
forms from small amount of amorphous or colloidal substance embedded in the
pores of a mineral soil to fibrous material with a structure resembling a coarse,
loosely woven mat. Therefore, organic soil may consist mainly of fibers
(fibrous peat), or may tend to be more granular (amorphous-granular peat) or
intermixing with mineral soil constituents (muck). This variability of organic
35
soil is extreme both horizontally and vertically. This variability results in a
wide range of physical properties such as particle size, texture, structure
composition, and color. In this section, physical properties of organic soils
which are of interest to the engineer will be reviewed.
2.5.1 Organic Soil Phases:
As in conventional soil mechanics, organic soils are considered to be
particulate materials, as opposed to rocks, and can simultaneously contain the
three phases, i.e. solid and liquid and gas phases. The liquid and gas phases are
contained in the voids or pores between the solid particles. From these phases
the weight-volume relationships of soils are derived.
2.5.1.1 Solid Phase
In the case of peat and organic soil, the solid phase consists of two components:
organic matter and inorganic earth materials. The relative proportion of these
components and their specific nature determine the physical and geotechnical
properties of these soils (Edil, 2003). The mineral component is similar to that
of inorganic soils consisting mainly of clay minerals but non-clay minerals are
also encountered. The mineral constituent is generally incombustible and nonash forming.
The organic matter, on the other hand, is generally combustible carbonaceous
matter. Soil organic matter has been defined (Russell, 1952) as "… a whole
series of products which range from undecayed plant and animal tissues
through ephemeral products of decomposition to fairly stable amorphous brown
to black material bearing no trace of the anatomical structure of the material
from which it was derived; and it is this latter material that is normally defined
as the soil humus."
36
Humus includes products of advanced decomposition of organic residues,
products of microbial synthesis, precipitates of dissolved organic compounds,
and organic molecules in solution (Gieseking, 1975a). Therefore, soil organic
matter includes: (1) fresh plant and animal residues (decomposable), (2) humus
(resistant), and (3) inert forms of nearly elemental carbon (charcoal, coal, or
graphite). However, carbon is the chief element of soil organic matter that is
readily measured quantitatively by combustion (Huat, 2004).
2.5.1.2 Pore Water
Usually peat and organic soil has an acidic reaction caused by the presence of
carbon dioxide and humic acid resulting from decay. Peaty waters are
practically free of salts and generally show pH values of 4 to 7 (Lea, 1956).
2.5.1.3 Pore Gas
The submerged organic component of peat is not entirely inert but undergoes
very slow decomposition, accompanied by the production of marsh gas
(methane) with lesser amounts of nitrogen and carbon dioxide (Muskeg
Engineering Handbook, 1969). Hydrogen sulfide is another gas encountered in
deposits containing sulfur.
The gas content of peat is difficult to determine and no widely recognized
method is yet available. A gas content of 5 to 10% of the total volume of the
soil is reported for peat and organic soils (Muskeg Engineering Handbook,
1969). Gas content is of considerable practical importance since it affects all
physical properties measured and field performance that relates to compression
and water flow. Consolidation test results are particularly impacted by gas.
2.5.2 Organic Soil Structural Elements:
The structure of peat and organic soils is an arrangement of primary and
secondary elements that make up the soil. These structural elements mainly are
fibers and granules.
37
In general, fiber can be defined as a fragment or piece of plant tissue that
retains a recognizable cellular structure and is large enough to be retained on a
# 100 sieve (> 0.15 mm) and not more than 20 mm in smallest dimension.
Fibers may be fine (woody or non-woody) or coarse (woody). On the other
hand, organic fine substances that do not have an identifiable fiber shape, finer
in size than 0.15 mm, and having a granular appearance are designated as
amorphous-granular matter.
Coarse fibers from stems and roots are greater than 1 mm diameter, fine fibers
from leaves, stems and roots are smaller than 1mm diameter. Fiber content is
determined from the dry weight of fibers retained on # 100 as a percentage of
the oven-dried mass of the original sample (ASTM, D 1997). The organic fiber
content is also referred to as the fabric of organic soil.
Organic fine substances are usually smaller than 100 µm and it may range
down to 0.1 µm in size. They are also referred to as soil humus. Humus consist
of irregularly shaped organic skeletons such as cell fragments and tissue parts,
as well as of globular organic precipitates smaller than 1µm, and of 3 to 9 nm
organic polymolecules (Terzaghi et al, 1996). The specific properties of the
colloidal particles vary greatly depending upon parent material, climate, and
stage of decomposition (Mitchell, 1993).
2.5.3 Fabric or Structure
Fabric or structure refers to the morphology and spatial arrangement of the
constituent of soil elements i.e. fibers and granules. Macro fabric or
macrostructure refers to those features visible to the naked eye whereas micro
fabric or microstructure involves much smaller features at the particle or fiber
level (Huat, 2004). It is the structure of peat, in its various aspects, that affect
the engineering behavior (Muskeg Engineering Handbook 1969; Dhowian and
Edil, 1980). An appreciation of the constituent matter and its attributes - like
38
orientation - aids in the constitutive modeling of this soil type for basic
understanding of its mechanical behavior (Molenkamp, 1994).
2.5.3.1 Macrostructure
Radforth in the Muskeg Engineering Handbook (1969) provides a classification
of peat structure based on the above mentioned two structural elements: fibers
and granules. In this system, peat structure is classified into 3 predominant
characteristics (in addition to 17 categories further subdividing these
characteristics):
1. Amorphous granular: dominated by highly disintegrated formless botanical
tissues (high colloidal fraction), holding most of their water in an adsorbed
rather than a free state. The adsorption is occurring around the grain
structure.
2. Fine fibrous: dominated by an open framework of highly preserved
fossilized plant remnants with the interstices are very small and contain
colloidal matter.
3. Coarse fibrous: these usually being woody. It has essentially an open
structure with interstices filled with a secondary structural arrangement of
non-woody, fine fibrous material.
Dhowian and Edil (1980) showed that fiber arrangement appears to be a major
compositional factor in determining the way in which peat soils behave.
However, the difference in the fiber content plays an equal important role in the
behavior of fibrous peat. The higher the fiber content, the more the peat will
differ from an inorganic soil in its behavior.
Scanning Electron Microscopy (SEM) images can be used for a visual
appreciation of the soil structure. For fibrous structure, the differences in fiber
contents, pores spaces, and perforated plant structure can be observed in the
39
SEM micrographs. On the other hand, in amorphous-granular structure, the
material will be dark in color consisting principally of gelatinous organic acids
which have a sponge-like fabric. Evidence of plant derived organic matter is
more difficult to identify. This may be attributed to the fact that well humified
organic matter is generally difficult to distinguish (Landva and Pheeny, 1980).
Figure (2-9) shows the Scanning Electron Micrograph of Middleton fibrous
peat (Fox and Edil, 1996). The photograph was taken in vertical and horizontal
planes. Comparison of the two micrographs in Figure indicates a pronounced
structural anisotropy for the fibrous peat with the void spaces in the horizontal
direction larger than those in the vertical direction resulting from the fiber
orientation within the soil.
Fig. (2.9): Scanning Electron Micrographs of Middleton fibrous peat; (a)
Horizontal plane, (b) Vertical plane (Fox and Edil, 1996)
40
2.5.3.2 Microstructure
The fibers of peat consist of cellular structures giving rise to a two-level
structure involving macro and micro pores, i.e., those between fibers and within
fibers, respectively. This two-level structure described by Adams (1965) may
be used to describe the unusual compression behavior of fibrous material.
Kogure et al. (1993) presented the concept of multi-phase system of fibrous
peat, which consists of organic bodies and organic space. The organic bodies
consists of solid organic matter with inner voids, while the organic spaces
between the organic bodies, called outer voids, and are filled with solid
particles and water. The solid organic matter is flexible with the inner voids,
which are filled with water. The cross section of deposition and diagram of the
multi-phase system of fibrous peat are schematically shown in Figure (2-10).
Organic
bodies
Organic
spaces
Organic matters
(Solids)
Water (Inner
voids)
Water (Outer
voids)
Soil particles
(Solids)
(a)
(b)
Fig. (2.10): Schematic diagram of (a) Deposition, and (b) Multi-phase system
of fibrous peat (Kogure et al., 1993)
A highly poriferous and flexible cellular structure is the most important
characteristic of organic coarse particles which are either fibrous or granular.
Soil fabrics characterized by organic coarse particles, as in fibrous peat, hold a
considerable amount of water because they are generally very loose, and also
41
because organic particles are hollow and largely full of water (Terzaghi et al,
1996).
On the other hand, organic fine substances (humus) includes products of
advanced decomposition of organic residues, products of microbial resynthesis,
precipitates of dissolved organic compounds, and organic molecules in solution
(Gieseking, 1975a). Unlike phyllosilicate minerals, humic substances have no
regularly repeating structural units. They are made of a variety of functional
groups, although there is still considerable controversy and disagreement about
their specific structure and morphology (Santagata et al., 2008). They represent
one of the most chemically reactive fractions of the soil due to their high
surface area and surface charge, and thus have a critical influence on the
chemical and physical properties of soils (Oades, 1989). Their role might be
compared to that of the clay fraction in inorganic soils (Santagata et al., 2008).
In organic clays and silts, organic polyanions that pigment the surface of fine
mineral particles such as clay minerals result in stable clay-humus complexes
that promote loose and open fabric. Globular organic polyanions attach
themselves, directly through hydrogen bonding or through adsorbed cations, to
specific sites at the surface of minerals and thus promote flocculation and
aggregation of mineral particles (Terzaghi et al, 1996).
In summary, the physical properties of organic soils show a great variation
depending on the type and amount of organic matter. As can be seen from
above, the organic matter may occur in many forms from small amount of
amorphous or colloidal substance embedded in the pores of a mineral soil to
fibrous peat with a structure resembling a coarse, loosely woven mat. There
has been virtually no research to correlate different structural types and
their index and engineering properties (Huat, 2004).
42
2.6
Classification System for Organic Soils and Peat
Jeffries (1936) stated that "Disagreements as to the behavior of peat, as evident
from a review of the literature, generally can be shown to arise from a lack of
proper definition of the material concerned. The term peat has been incorrectly
used to describe organic silts and clays with mineral contents as high as 90 %".
Jarrett (1983) stated that "One problem with peats and organic soils is their
classification and, indeed, even the definition of what is a peat". Larsson (1996)
concluded that organic soils or soils with an organic content have often been a
concept with various meanings in geotechnical engineering and the rules for
division into different groups have often been rather diffuse (Embankments on
Organic Soils, 1996).
Myślińska (2003) stated that "It is obvious now that in engineering geological
investigations of organic soils the main problem is posed by their classification
generalizing their properties, which in many cases requires complex
determinations. Also, the classifications for peats and organic soils have not
been standardized yet, and despite many attempts, the problem is still
unresolved".
Huat (2004) concluded that the reason for confusion is that the terms peat and
organic soils, used for describing soils with an organic content, were once
synonymous, i.e. used interchangeably, but the term organic soils is presently
used for all soils with organic contents greater than 20%.
Under the Unified Soil Classification System (USCS), subdivisions within the
fine-grained group comprise inorganic silts and clays, and organic silts and
clays, which distinct from each other based on Atterberg limits and simple
index tests. On the other hand, organic soils are recognized as a separate soil
entity and has a major division called Highly Organic Soils (pt), which refers to
peat, muck and other organic soils. Peat is described as a naturally occurring
43
highly organic substance derived primarily from plant materials. Peat is
therefore organic soil with high organic content. Apart from purely organic
form of peat, there are a large number of transitional forms towards the mineral
soils. It is important from the geotechnical point of view to distinguish between
peat and other organic soils, so that the described behavior can be related to the
proper material.
The first known classification system used by engineers was von Post
classification system (1922), which was intended for horticultural, agricultural,
and forestry requirements (Landva et al., 1983a). The von Post classification
attempts to describe peat and its structure in quantitative terms. It is based on a
number of factors such as degree of humification, botanical composition, water
content, fiber content, and woody remnants. Table (2-4) shows the detail of the
von Post classification system. To classify the soil, a sample of soil is squeezed
in the hand. The color and form of fluid that is extruded between the fingers is
observed together with the pressed residue remaining in the hand after
squeezing with reference to the ten point scale of decomposition (H1to H10: the
higher the number, the higher the degree of decomposition), wetness, fiber
content, and woodiness.
Various modified forms of von Post classification system were introduced
(Kivinen, 1948; Korpijaakko and Woolnough, 1977, Landva and Pheeney,
1980; Hobbs, 1986) to specify the terms used. Landva and Pheeney (1980)
followed that used by Korpijaakko and Woolnough with numerical values had
been assigned to water contents and fiber contents, and the degree of
humification had been presented in tabular form. Hobbs (1986) extended that
modified by Landva and Pheeney through the addition of symbols for organic
content, structural anisotropy, smell, plasticity and acidity as required, to
correlate the types of peat with their physical, chemical and structural
properties.
44
Table (2.4): von Post Classification system for organic soils (von Post, 1922)
Degree of
Humification
H1
H2
H3
H4
H5
H6
H7
H8
H9
H10
B1
B2
B3
F0
F1
W0
W1
Description
Completely undecomposed peat which releases almost clear water. Plant
remains easily identifiable. No amorphous material present
Almost completely undecomposed peat, which releases clear or yellowish
water. Plant remains still easily identifiable and no amorphous material
present.
Very slightly decomposed peat which releases muddy brown water, but for
which no peat passes between fingers. Plant remains still identifiable and
no amorphous material are present.
Slightly decomposed peat which releases very muddy dark water. No peat
is passed between the fingers but the plant remains are slightly pasty and
have lost some of the identifiable features.
Moderately decomposed peat which releases very “muddy” water with also a
very small amount of amorphous granular peat escaping between the fingers.
The structure of plant remains is quite indistinct, although it is still possible
to recognize certain features. The residue is strongly pasty.
Moderate strongly decomposed peat with very indistinct plant structure.
When squeezed about one third of the peat escapes between the fingers.
The residue is strongly pasty but shows the plant structure more distinctly
than before squeezing.
Strongly decomposed peat. Contains a lot of amorphous material and very
dry indistinct plant structure. When squeezed about one half the peat escapes
between the fingers. The water, if any is released, is very dark and almost
brown.
Very strongly decomposed peat with a large quantity of amorphous material
and very dry indistinct plant structure. When squeezed about two third of the
peat escapes between the fingers. A small quantity of plant material
remaining in the hand consists of residues such as roots and fibers that resist
decomposition.
Practically fully decomposed peat in which there is hardly any recognizable
plant structure. When squeezed, almost all of the peat escapes between the
fingers as fairly uniform paste.
Completely decomposed peat with no discernible plant structure. When
squeezed, all the wet peat escapes between the fingers.
Wetness
B4
B5
High moisture content
Very high moisture
Nil
Low Content
Fibers
F2
F3
Moderate content
High content
Nil
Low content
Woodiness
W2
W3
Moderate content
High content
Dry
Low moisture content
Moderate moisture content
45
However, Landva et al. (1983a) concluded that the current practice of including
all organic soils in the term peat can be traced back to the von Post's original
work, since this classification system includes all organic soils that can support
plant growth! They concluded also that von Post system is extremely useful for
reconnaissance and survey of peats and organic soils. On the other hand, Huat
(2004) concluded that the von Post classification system however is adapted to
pure peat containing little or no mineral matters. Its use in organic soils with
more than 20-25% mineral matter is difficult.
Radforth (1969) following an extensive study of Canadian peat (muskeg - the
Indian word for mire) proposed a classification system for use by engineers,
based on the structure of peat rather than its botanical origin. The
characteristics of the subsurface material, which is the product of the
interaction of topography, vegetation, high water table and decomposition and
preservation processes are shown in the Table (2-5). According to Radforth,
this approach makes it easier to classify structure and also leads to a better
basis for estimating mechanical properties than a purely botanical classification
system. Seventeen categories of organic soil were recognized, based on the
extent to which the following types of structural components are present: (1)
amorphous and granular, (2) woody or nonwoody fine fibrous (fiber diameter ≤
1mm) and (3) wood particles and coarse woody fibrous fiber diameter > 1mm).
Landva et al. (1983a) concluded that Radforth system is not generally
applicable and appears to be limited to highly organic, that is, peat. Hobbs
(1986) stated that "This scheme is highly specific in relation to the predominant
types present and to the structural arrangement including the presence of any
woody material. However, no mention is made of color, wetness, degree of
humification or organic content and whether clay is present or not. The latter
omission being remarkable in view of the fact that Canadian muskeg is not
exclusively highly organic, even it is useful for large areas in Canada".
46
Table (2.5): Classification of Peat Structure by Radforth, (1969)
characteristic
Predominant
Amorphousgranular
Category
1
Amorphous-granular peat
2
Non-woody, fine fibrous peat
Amorphous-granular peat containing non woody fine
fibers
Amorphous-granular peat containing woody fine fibers
Peat, predominantly amorphous-granular containing non
woody fine fibres, held in a woody, fine-fibrous
framework
Peat, predominantly amorphous-granular containing
woody fine fibres, held in a woody, coarse-fibrous
framework
Alternate layering of non-woody, fine-fibrous peat and
amorphous-granular peat containing non woody fine fibers
Non-woody, fine-fibrous peat containing a mound of
coarse fibers
Woody, fine-fibrous peat held in a woody, coarse fibrous
framework
3
4
5
6
7
Fine-fibrous
8
9
10
11
Coarse-fibrous
Name
12
13
14
15
16
17
Woody particles held in non-woody, fine-fibrous peat
Woody and non-woody particles held in fine-fibrous peat
Woody, coarse fibrous peat
Coarse fibres criss-crossing fine-fibrous peat
Non-woody and woody, fine-fibrous peat held in a coarse
fibrous framework
Woody mesh of fibres and particles enclosing amorphousgranular peat containing fine fibres
Woody, coarse-fibrous peat containing scattered woody
chunks
Mesh of closely applied logs and roots enclosing woody
coarse-fibrous peat with woody chunks
However, a number of classification systems for peats and organic soils are
used in various countries and are based on similar grounds. Generally, these
classification systems are developed based on organic content, the vegetation
forming the organic material, texture, fiber content and degree of
decomposition of fiber. Most of them, however, have not been specially
designed for geotechnical purposes. In the following some classification
systems used or suggested in context with soil mechanics are illustrated.
47
2.6.1 Classification Systems for Organic Soils based on Organic Content
As mentioned above, peat is an organic soil with high organic content.
However, the cut-off value of the percentage of organic matter necessary to
classify an organic deposit or soil as peat varies throughout the world, usually
depending on the purpose of classification. This cut-off value also service to
differentiate peat from soils with lesser amounts of organic content. Figure (211) shows a comparison of some classification systems used or suggested in
context with soil mechanics for peat and organic soils, based on organic
content. In the following, some classification systems shall be illustrated in
detail.
In France, the French practice makes no use of the geotechnical name of soils
for estimating their mechanical properties; these needs to be measured by
means of laboratory or field tests, not on soil classification as concluded by
Magnan (1994). Therefore, organic soil was described as soil having greater
than 10% organic content in the so-called "LPC Soil Classification" (Schon,
1965), which was mainly based on the USCS as shown in Figure (2-12).
According to the LPC organic soils are sub-divided into:
• Highly organic soils with organic content exceeding 30%.
• Medium organic soils with organic content between 10 and 30%.
• Slightly organic soils containing 3-10% of organic and are included
within fine soils and sub-divided according to the Casagrande diagram.
The desire to characterize the degree of humification of organic matter led
Perrin and his colleagues of the Lyon Regional Laboratory to introduce the von
Post index as a classification parameter for medium and highly organic soils
(Perrin, 1974; Magnan, 1980).
48
Ash Content Ac (%)
100
Peaty
organic
soils
Peats
low
organic
organic
mineral
content
low organic
mineral
MO, CO
(gytta,
dy,
peat,
humushighly
rich
organic
topsoil) Organic
soils
medium
Peats
High
organic
Medium
90 Mineral with organic organic
80
70
60
50
40
30
20
10
0
Landva et al
(1983)
Canada
ORGANIC
high ash
medium
ash
low ash
Poland (1984)
mineral
Mineral Mineral
sediments organic
medium
ash
low ash
high ash
medium
ash
low ash
Andrejko et al
(1983) USA
Peats
Carbonaceous
Sedements
Karlsson &
Hansbo
(1981)
Sweden
Muck
Peat
Davis (1946)
organic Mineral
clay or silt
Soil
Clayey/
silty /
Sandy/
Gravelly
Peaty
Peats
Jarrett System
Muck
Konvalov
(1980) USSR
calcareous soils, gytta
Clay
20
30
40
50
60
70
80
90
100
Organic
0
Mucky 10
Clayey
Muck
Muck
Peaty
Muck
Peat
LGS System
Fig. (2.11): Comparison of Classification Systems Used for Peat and Organic Soils (Source: Anderjko et al, 1983; Woliski et al., 1988)
Peaty Soils
peats
peaty
muck
organic silt
&clay
49
Organic Content (%)
Fig. (2.12): LPC classification of organic soils (Magnan, 1980)
Landva et al. (1983a) concluded that the major difference between the von Post
and the Radforth classification systems is that the former includes all organic
soils (!), whereas the latter includes only wholly organic deposits, that is, peat.
They suggested that the simplest approach would be to retain the terms peat
and organic soil and to distinguish between various peats and organic soils by
means of standard and special index properties and tests. In their classification
system, organic soils and peat were divided into four groups (Table 2-6):
1. Peats (Pt).
2. Peaty organic soils (PtO).
3. Organic soils (O).
4. Silts and clays with organic content (MO and CO, respectively).
50
Table (2.6): Organic soils and peat classification (Landva et al., 1983)
Ash
Moisture
Specific
Content
Content
Gravity
(Ac%)
(wo%)
(Gs)
Peats (Pt)
< 20
> 500 %
< 1.7
> 50 %
Peaty organic soils (PtO)
20 – 40
150 - 800 %
1.6 – 1.9
< 50 %
Organic soils (O)
40 – 95
100 – 500 %
> 1.7
Insignificant
Silts and clays with
organic content (MO,
CO respectively)
95 – 99
< 100 %
> 2.4
None
Material
Fiber
Content
Hobbs (1987) compares between the American and Russian definitions of peat,
and suggests a cut-off value to classify peat as follows:
1. Russian geotechnical engineers assume that peat soil containing more than
50% of particle weight of vegetable origin while peaty soil contains from
10% to 50% of particles weight of vegetable origin.
2. The (ASTM-79) prefer that a soil should not be called a peat unless its
organic content exceeds 75%.
3. Hobbs suggests that peat is a soil having organic content more than 27.5%
based on volumetric proportions of organic matter and mineral material
encountered in organic soil. He concluded that due to von Post, there would
be little advantage to distinguish a peat from a peat soil in terms of organic
matter content; more important to recognize its morphological stage, i.e.
fen, transition or bog.
In the Netherlands, where peats and organic soils constitute over 7% of the
country's area (Hobbs, 1986), organic soils are subdivided according to the
percentage content of three components: organic matter, clay and the sum of
sand and silt fraction (Venmans and den Haan, 1990), placed on a triplot
diagram as shown in Figure (2-13). According to this subdivision, prepared for
51
the Committee for Embankments (TAW), two groups of soils are distinguished
as detailed in Table (2-7):
1. Peats: 15-100% of organic matter (OM); 0-70% clay and other fractions.
Peats are subdivided into:
• Slightly clayey (30-55% fi – clay fraction),
• Strongly clayey (55-70% fi),
• Slightly sandy (22.5-35% OM; 30-55% fi; 0-8% fp – sand fraction)
• Strongly sandy (15-22.5% OM; over 8% fp).
2. Other organic soils (humus): 0-15% organic matter; 70-100% clay and other
fractions. Humic soils are subdivided into:
• Slightly organic (0-2.2% OM)
• Moderately organic (2.2-8.5% OM)
• Strongly organic (8.5-15% OM).
Fig. (2.13): Organic matter-clay-silt+sand triangle (according to NEN 5104,
1989)
52
Table (2.7): Description of mixtures in the organic – clay – silt + sand triangle
Area in
triangle
Type of soil
Vm
Peat
Description of mixture
Addition
demonating
triangle
Low mineral content
Vk1
Peat
Slightly clayey
(1)
Vk3
Peat
Strongly Clayey
(1)
Vz1
Peat
Slightly sandy
(1)
Vz3
Peat
Strongly sandy
(1)
h1
(2)
h2
(2)
Addition non
demonating
triangle
(1)
Slightly organic
Moderately
organic
Strongly Organic
h3
(2)
(1) Addition from the clay+silt-sand-gravel triangle.
(2) Denomination obtained from clay-silt-sand-gravel triangle.
According to Myślińska (2003), a classification and description of organic soils
was presented recently for discussion by the European Normalization
Committee (CEN) based on two parts of the norms ISO (2001a, 2001b).
According to these proposals, organic soils are macroscopically distinguished
on the basis of their dark color and characteristic smell, and are generally
subdivided based on their organic matter content (determined in relation to the
mass of the dry soil depleted in grains exceeding 2mm in diameter). In this
classification the following soils are distinguished:
• Low-organic soils with an organic matter content of 2-6%.
• Medium-organic soils with an organic matter content of 6-20%.
• High-organic soils with an organic matter content exceeding 20%.
The classification should be regarded thus as descriptive. The norms contain,
however, also a further classification of organic soils based on their origin and
some properties. The following groups are distinguished:
53
• Fibrous peat: fibrous peat is characterized by a fibrous structure, with
easily recognizable plant remains, and retains some strength.
• Pseudo-fibrous peat: pseudo-fibrous peat has an easily recognizable
plant structure but a diminution in strength.
• Amorphous peat: the plant structure is not visible in amorphous peat,
which additionally has a mushy consistency.
• Gyttja (Muck): gyttja comprises decomposed plant and animal remains
and may contain inorganic constituents.
• Humic soils: humic soils contain plant remains, living organisms and
their excreta, as well as a large content of inorganic constituents; they
form the topsoil.
According to McVay and Nugyen (2004), the American Society for Testing
and Materials (ASTM) is currently working on a standard classification system
that would apply to all interested disciplines. The activity is within committee
D18 (Soils and Rocks), and specifically Subcommittee D18.18 (Peats and
Organic Soils). The ASTM Subcommittee D18.18, in an attempt to distinguish
peat from organic soils has proposed the following organic soil classification to
be used as a standard definition (Table 2-8):
Table (2.8): Grouping of Organic Materials (Tentative ASTM Standard)
Material Description
Peat
Muck
Organic Silt or Clay
Organic Content
(%)
> 75
25 to 75
< 25
Subheading
95 - 100
Low ash peat
85 - 95
Medium ash peat
75 - 85
High ash peat
50 - 75
Peaty muck
25 - 50
Silty or clayey muck
10 - 25
Highly organic silt or clay
1- 10
Slightly organic silt or clay
54
2.6.2 Classification Systems for Peat
Generally, these classification systems are developed based on the vegetation
forming the organic material, texture, fiber content and degree of
decomposition of fiber. The classification based on the vegetation forming the
organic material is not usually adopted in engineering practice even though
researches have indicated that the type of plant forming the peat soil, fiber
content, and degree of decomposition significantly affects the behavior of peat.
Based on the botanical composition, peat is classified as Moss peat, Sedge peat,
and Wood peat. In terms of texture, the peat is classified as woody, fibrous,
sedimentary, and granular peat (Radforth, 1969; Davis, 1997).
The classification based on the degree of decomposition was proposed by Von
Post (1922). Table (2-4) shows the detail of the von Post scale for assessing
peat degree of humification. For geotechnical purposes, these 10 degrees of
humification is often reduced to 3 classes: fibric or fibrous (least decomposed),
hemic or semi-fibrous (intermediate) and sapric or amorphous (most
decomposed), respectively (Magnan 1980; ASTM Standard D 5715). The U. S.
Department of Agriculture (USDA) three-point scale classification based on
fiber content resulting from decomposition is described in Table (2-9).
Table (2.9): USDA classification of peat
Type of Peat
Fiber content
Von Post Scale
Fibric peat
Over 66%
H4 or less
Hemic peat
33 – 66%
H5 or H6
Sapric peat
Less than 33%
H7 – H10
Another 3-part division of peat based on the above-mentioned von Post scale
was described as follows by Karlsson and Hansbo (1981):
• Fibrous peat is low-humified and has a distinct plant structure. It is brown
to brownish-yellow in color. If a sample is squeezed in the hand, it gives
55
brown to colorless, cloudy to clear water, but without any peat matter. The
material remaining in the hand has a fibrous structure. Degree of
decomposition on the von Post scale: H1-H4.
• Pseudo-fibrous peat is moderately humified and has an indistinct to
relatively distinct plant structure. It is usually brown. If a sample is
squeezed in the hand, less than half of the peat mass passes between the
fingers. The material remaining in the hand has a more or less mushy
consistency, but with a distinct plant structure, (H5-H7).
• Amorphous peat is highly humified and the plant structure is very indistinct
or invisible. It is brown to brown-black in color. If a sample is squeezed in
the hand, more than half of the peat mass passes between the fingers
without any free water running out. When squeezing, only a few more solid
components, such as root fibers, wood remnants, etc. can be felt. These
constitute any material remaining in the hand, (H8-H10).
According to Yulindasari (2006) the peat should be further classified based on
fiber content since the presence of fiber alters the consolidation process of
fibrous peat from that of organic silt or clay and amorphous peat. He concluded
that fibrous peat is the one having fiber content more than 20 %, while
amorphous peat, on the other hand, is the peat soil with fiber content less than
20 %. It contains mostly particles of colloidal size and the pore water is
absorbed around the particle surface (ASTM D4427).
According to McVay and Nugyen (2004), the proposed classification for peat
would be based on fiber content, ash content, acidity, absorbency and botanical
composition with the subheadings used as descriptors. On the other hand, peat
is distinguished from phytogenic material of higher rank (i.e., lignite coal) by
its lower BTU value on an "as-received", water-saturated basis (ASTM D388).
Table (2-10) shows the ASTM (D4427-1997) proposed classification system of
peat samples by laboratory testing.
56
Table (2.10): Classification of peat (adapted from ASTM D 4427):
Term
Fiber
Content
(ASTM
D1997)
Subheading
Definition
Fibric
Peat with greater than 67 % fibers (H1 – H3)
Hemic
Peat with between 33 % and 67 % fibers (H4 –
H6)
Sapric
Peat with less than 33 % fibers (H7 – H10)
Low Ash
Peat with less than 5 % ash
Ash Content
(ASTM
Medium Ash
D2974)
High Ash
Acidity
(ASTM
D2976)
Absorbency
(ASTM
D2980)
Botanical
composition
Peat with between 5% and 15 % ash
Peat with more than 15 % ash
Highly Acidic
Peat with a pH less than 4.5
Moderately
Acidic
Peat with a pH between 4.5 and 5.5
Slighly Acidic
Peat with a pH greater than 5.5 and less than 7
Basic
Peat with a pH equal or greater than 7
Extremely
Absorebent
Highly
absorebent
Moderately
Absorebent
Slightly
Absorebent
Floristic
Designation
Peat with WHC (water holding capacity) greater
than 1500%
Peat with WHC between 800 and 1500%
Peat with WHC greater than 300 and less than
800%
Peat with WHC less than or equal to 300%
Name dominant plants in the fibers
From the above, it could be concluded that European practice prefers the nongenetic classifications (descriptive) by grouping all highly organic soils in one
group (e.g. Karlsson and Hansbo, 1981; Hobbs, 1986, 1987; Venmans and den
Haan, 1990; Magnan, 1994; and European Norms' Draft). On the other hand,
North American prefers the genetic-descriptive classifications which
distinguish between peat, muck, and organic silt and clay based mainly on
physical, index, and chemical properties (e.g. Davis, 1946; Landva et al.,
57
1983a; Jarret, 1983; Andrejko et al., 1983; and ASTM Tentative standard). The
latter is more realistic since it differentiates between various types of organic
soils, based on simple index test (organic content), so that the described
behavior can be related to the proper material. Also, it could be integrated with
the USCS to bridge the gap between peat as purely vegetable matter, and
purely inorganic silts and clays.
2.7
Site Investigations for Organic Soils
Before locating a structure or embankment, it is important to have an overview
of the distribution of soils and groundwater conditions, in particular, the
location of organic soils. Use of archival materials and maps is important.
Determination of the depth and thickness of soft organic substrata is an
essential task. According to Bergdahl (1996), experience of penetration testing
is good for estimation of the soil profile, but the possibilities for evaluation of
the soil properties are very limited (Embankments on Organic Soils, 1996). Soil
sampling is necessary in organic soils, both for detailed soil identification and
classification and also for determination of physical, index and engineering
properties in laboratory. Local records of previous engineering works can also
give very useful insights into likely performance characteristics and these
should not be ignored.
On the other hand, organic soils are well known for their high variability in soil
properties, especially in organic contents. Samples from Shelby tube may have
their organic content range from Organic Silt to Peat, so in the field it is hard to
describe soil layering system based on organic content (McVay and Nugyen,
2004). Also, engineering properties of organic soils can vary significantly both
spatially and with depth, such that samples obtained within a few feet of each
other may exhibit vastly different behaviors during loading. Therefore,
subsurface investigations that encounter organic soils should involve more
sampling and testing as compared to inorganic soils to adequately characterize
58
the materials (Sabatini, et al, 2002). Also, the properties of organic soils must
be determined in-situ, on location, and in the laboratory in as close a state of
disturbance or undisturbance as the respective engineering use may require
(Arman, 1970). In this section, short summaries of the most common ground
investigation methods suited for organic soils are set out on the following
pages.
2.7.1 Sampling
There is a reasonably well-established understanding of the causes of
disturbance during sampling, transporting, and handling of inorganic soft clays
and the corresponding accepted practices for sampling of these soils. Like most
soft clays, it can be difficult to obtain undisturbed samples of organic soils for
laboratory performance testing; due to their high moisture content, large void
ratio, and the possibility of fibrous structure. The act of obtaining a sample can
affect the sample being recovered particularly when sampling in organic soils.
Both physical intrusions of the sampler and the removal of in-situ stresses can
cause disturbance. Also, for sampling of organic soil and peat, additional
factors such as compression while forcing the sampler into the ground, tensile
resistance of fibers near the sampler edge during extraction of the sampler, and
drainage as well as internal redistribution of water content must be considered
(Yulindasari, 2006).
In 1981, the ISSMFE Subcommittee on Soil Sampling presented an
international manual on soil sampling of soft cohesive soils (ISSMFE
Subcommittee on Soil Sampling 1981). According to the manual, the degree of
disturbance of the soil samples can be defined as:
• Undisturbed samples = the soil retains the same fabric, type and proportion
of constituents and physical and mechanical properties as in the field.
• Disturbed samples = the soil retains the type and proportion of constituents
and water content, but the fabric may have changed. The physical and
mechanical properties have changed.
59
• Remolded samples = the soil structure and its physical and mechanical
properties have changed from the in-situ conditions. The type and
proportion of constituents and the water content remain unchanged.
In 1979, Andresen and Kolstad suggest a measure of sample quality referred to
as Sample Quality Designation (SQD), and is mainly applicable to cohesive
soils with values of OCR less than about 3 to 5 (Terzaghi, et al, 1996). This
characterization of sample quality is based on the magnitude of volumetric
strain caused by reconsolidation to the in-situ vertical stress σ`vo in an
oedometer test, or in triaxial compression test to the effective vertical and
lateral stresses under which it existed in the field. Table (2-11) shows the
specimen quality designation (SQD) in terms of volumetric strain measured in
laboratory.
Table (2.11): Specimen quality designation (Terzaghi et al., 1996)
Volumetric Strain
Specimen Quality Designation
(%)
Designation
State
<1
A
Excellent
1-2
B
Very good
2-4
C
good
4-8
D
Fair
>8
E
Disturbed
However, organic soil samples may be collected using a variety of methods and
equipments depending on the depth of the desired sample, the type of sample
required, and the type of organic soil. In general, all types of sample have a
value depending on the type of laboratory test planned. For soil identification
only, disturbed or remolded samples can be used. When the deformation and
60
the strength characteristics of the soil are to be investigated in the laboratory, it
is necessary to obtain undisturbed samples.
2.7.1.1 Disturbed Sampling
Organic soils and peats are evidenced during subsurface exploration based on
the presence of decaying vegetative matter and a strong odor. Typically, the
materials are light brown to dark brown, greenish, dark gray, or black in color,
and can have very fibrous structures with wood fragments and plant remains.
Disturbed sampling techniques are used to provide a visual confirmation of the
organic material, and to determine the classification and stratigraphy of the
organic deposits. Near-surface soils may be easily sampled, e.g. using a hand
auger while sampling at greater depths may be performed using continuous
flight auger (screw augers) or a split-spoon sampler in SPT test. Remolded
samples, on the other hand, can be obtained with the peat sampler.
Peat Sampler:
In very soft organic soil, the peat sampler may be used to extract samples for
soil identification. The peat sampler is an open, side intake sampler which can
be closed with a shuttle as shown in Figure (2-14). The sampler is closed
during insertion in the soil down to the sampling level. The sampler is turned
while the shuttle first opens the sampler and then forces the soil into the
sampler. After one full turn, the sampler is extracted. The peat sampler is often
the most useful method in extremely soft organic soil, such as the bottom of
lakes or pools.
61
Fig. (2.14): Peat sampler (after Noto, 1991)
Continuous flight auger (screw augers):
The screw auger consists of a steel rod on which a screw-shaped flange is
welded. The length (0.25-1.0 m) and diameter (35-100 mm) may vary greatly.
In sampling, the auger is rotated into the soil down to the sampling level and
then pulled out. During extraction of the sampler, soil from the shaft above the
sampling level may stick to the outside of the sample. Thus the sample must be
gut clean when extracted above ground. A long continuous sample is obtained
by this method and it is also possible to recognize thin layers of different
materials in a soil layer sequence. In fibrous peat, difficulties may arise because
the fibers are not cut off.
2.7.1.2 Undisturbed Sampling
Disturbance can be minimized using certain sampling techniques. Considering
the type of organic soil encountered; most samplers do not cut the peat fibers
causing a great distortion and compression of the peat structure. Therefore the
sharpness of cutting edge is very important to ensure the quality of sample for
fibrous soil. Considering the depth of sample; undisturbed samples can be
obtained at shallow depth by block sampling method, while tube sampler may
be used to obtain sample at depth. Also, sample size is important with respect
to both sampling disturbance and representative sample volume. Therefore,
large diameter tube sampler modified by adding sharp cutting edge may be
used to obtain sample at depth.
62
2.7.1.2.1
Block Sampling
For shallow block sampling, typically a pit is excavated and blocks of peat are
removed from the pit wall. Other way is to excavate the surroundings of a
sampling site so that samples can be removed from the perimeter. Hebib and
Farrell, (2003) describes a novel method that was employed to obtain
undisturbed peat block samples of 1 m3 were recovered from a vertical face 2.5
m high in the Ballydermot bog - Irland. The samples were cubic in shape,
1m × 1m × 1m in dimension and were obtained using a steel box consisting of
three faces having sharp edges as shown in Figure (2.15).
Fig. (2.15): Block samples, (a) steel box used for sampling (b) peat block
sample recovered (after Hebib and Farrell, 2003).
Also, large block samples (250 mm-square) can be obtained from below the
ground and groundwater surface, down to a depth of 7 m, using a block
sampler for peat described by Landva et al. (1983b). Moreover, large-size
down hole block samplers such as Sherbrooke sampler (250-mm. in diameter)
that have been developed for sampling clays at depths, were also used for peat
with some modifications (Lefebvre et al, 1984) as shwon in Figure (2.16).
63
(a)
(b)
Fig. (2.16): (a) University of Sherbrooke clay sampler modified for peat
sampling, b) Block sample of peat recovered from a depth 0.0-0.4 m
(Lefebvre, et al, 1984)
2.7.1.2.2
Deep Sampling
Hobbs (1986) concluded that even-though block sampling is ideal for
minimizing peat sample disturbance; it is only feasible for shallow deposits. He
recommended using tube samples with double barrel cutters to reduce
disturbance. However, Sampling of deeper organic deposits is usually carried
out with a thin-walled tubes (Shelby tube) and thin-walled piston samplers or
similar samplers to extract undisturbed samples at depth.
For organic soils and amorphous peat; thin-wall sampling tubes can be used
but thin walled fixed piston samples are most suitable for undisturbed
sampling. A number of piston samplers with different sample diameter are
available, c. f. ISSMFE Subcommittee on soil sampling (1981).
64
For fibrous peat; where piston sampler is attempted, it is critical that the
sampler have a sharp edge. Also suggested that large diameter (more than 100
mm) thin walled fixed piston sampler can be used such as a 100-mm-diameter
piston sampler have been developed at the University of New Brunswick for
sampling of peat (Landva et al., 1983b).
The piston sampler cuts a sample of soil by being pushed in closed mode down
through the deposit to the test level at which point a piston slowly pushes a
sampler tube into the soil to be extracted. The technique aims to minimize edge
effects on the sample but some disturbance such as smear is inevitable as the
sampler is inserted. Once the cut sample has stabilized the complete assembly
is withdrawn and the test sample recovered. Recovery ratio is above 95 %
except for fibrous peat containing tough fibers (Noto, 1991).
2.7.1.2.3
The Swedish Geotechnical Institute Sampler
Also, the importance of obtaining good quality large size undisturbed samples
capable of adequately represent the in-situ nonhomogeneous nature of the
organic deposit is stressed by the Swedish Geotechnical Institute (SGI) who
have developed his own 100 mm diameter peat sampler for recovering such
undisturbed samples at depth with good results.
The "SGI sampler" has a sharp circular wave-toothed cutting edge mounted on
100 mm diameter plastic tube capped with a robust driving head on top as
shown in Figure (2-17). The length of the tube is variable and dictates the
length of sample recovered but normally a 1.0m long sampler is used. The
extent of disturbance in the sample largely depends on the method used to drive
the sampler into the ground and following testing. It has been established that
the best results are usually achieved when the sampler is driven down into the
peat by means of a lightweight percussive machine (Ron Munro, 2004).
65
Fig. (2-17): Photographs of the Swedish Geotechnical Institute Sampler in use
(after Ron Munro, 2004).
Samples are taken from the ground surface or the bottom of pre-bored holes.
After extraction of the sampler, the cutting edge and driving head are removed
and the sample in the plastic tube is sealed. Laboratory tests show that samples
of fibrous soils taken with this peat sampler have a higher quality than samples
taken with a small diameter piston sampler. Also, practical experience has
shown a good correlation between laboratory test data results from this kind of
sample and measured field behavior under embankment on fibrous peat
(Embankment on organic soils, 1996).
Finally, all samples should be sealed in airtight containers and carefully
transported to the laboratory. Storage should be at ground temperature and the
samples should be tested as soon as possible in order to prevent effects of
chemical and biological changes with time.
66
2.7.2 In-situ Testing
A review in the literature on in-situ testing in organic soil and peat indicates
that there are no special tools available for determining in situ properties of
organic soil and peat, and the state of the art and the practice are not as
developed for inorganic soils. However, selected methods from that have been
developed for use in soft clays are used either directly or in a somewhat
modified manner for testing peat and organic soils, with certain methods have
gained prominence over the others. Even though, the methods of interpretation
of the in-situ test results as applied to peat and organic soils are limited in the
literature and direct use of the methods primarily developed on the basis of
mineral soil experience should be conducted with great caution. Edil (2001)
reviewed some of the more common approaches to in situ testing in such
deposits and their use in peat and organic soils which include the following
tests:
• Vane shear test.
• Cone penetration test.
• Pressuremeter test.
• Dilatometer test.
• Plate load test and screw plate load test.
The review indicates that it is important to recognize the differences between
various organic deposits. Organic Silt or clay (with organic content of 25% or
less) can be treated in a similar manner to inorganic clays and many of the
same in-situ testing tools such the vane shear, cone penetrometer,
pressuremeter, dilatometer and regular or screw plate load tests can be applied.
While there may be some questions regarding the interpretation of the test data
in determining mechanical parameters for design, it appears that the standard
approaches can be followed with a greater degree of cross-calibration of the
various tests and care for greater material variability and compressibility.
67
On the other extreme, there are serious questions regarding the applicability of
the conventional in-situ tests to fibrous, high organic content, superficial
peat (i.e., not buried and compressed). The presence of fibers, inherent
anisotropy, tendency for high compressibility and rapid drainage, and low and
highly variable strength of these materials make use of the conventional field
tests and interpretation of mechanical parameters unviable. Use of large size
test tools (vanes, cones, etc.), more sensitive measuring devices, and more
rapid loading rates to minimize compression may improve the prospects.
However, irrelevance of the various modes of failure induced in the field as
well as laboratory tests relative to fiber interaction, anisotropy, and
compressibility result in unusual values for mechanical parameters and
inconsistencies between various tests. For instance, the shearing surface is
vertical in the vane shear, ill defined in cone penetration, inclined in triaxial
compression, and horizontal in direct, simple or ring shear test. This situation
has lead some investigators like Landva to recommend test fills as opposed to
solely relying on laboratory or field tests in designing embankments on such
deposits.
Amorphous peat and muck is somewhat intermediate between organic silts
and clays and fibrous peat. There are reports of successful use of in situ tests.
Such materials must be handled on a case-by-case basis. Interpretation of in
situ test results requires corrections usually calibrated based on local
experience with organic deposits and laboratory strength tests.
However, combined use of extensive sampling for the definition of site
variability, in situ tests, and laboratory mechanical property tests and where
possible use of test fills provide a reasonable approach in dealing with these
difficult organic deposits (Edil, 2001).
68
2.8
Index and Chemical Properties of Organic Soils
Organic soil is a mixture of fragmented organic material derived from
vegetation which has accumulated in wet areas such as swamps, marshes, or
bogs and inorganic earth material mainly of clay mineral, but non-clay mineral
are also encountered. The relative proportion of these components and their
specific nature determine the physical and geotechnical properties of these soils
(Edil, 2003). On the other hand, soil organic matter is complex both chemically
and physically, and a variety of reactions and interactions between the mineral
soil and the organic matter is possible (Oades, 1989). Therefore, organic matter
in soil may be responsible for high plasticity, high shrinkage, high
compressibility, low shear strength, and their wide range of hydraulic
conductivity.
Physically, the organic matter may occur in many forms from small amount of
highly decomposed amorphous or colloidal substance embedded in the pores of
a mineral soil, in many ways resembling clay, to fibrous peat with a visible
plant structure resembling a coarse, loosely woven mat. This variability occurs
throughout the deposit, both horizontally and vertically, especially in peat
deposits, as direct result of the deposit's morphology. Significant variation can
happen within 10 meters horizontally and even less vertically. Therefore, the
physical, index, and engineering properties of organic soils show a great
variation as a consequence of the formation and morphology; i.e. the type and
amount of organic matter.
Table (2.12) illustrates the broad stages and mire types set out in a simplified
form with a small selection of the corresponding index properties of some
British peat and organic soil, from which it will be seen that a broad correlation
exists between the morphological state and the properties of the associated
organic soil and peat (Hobbs, 1986). Table (2.13) illustrates the index
properties of 22 organic soils samples were collected from different locations in
Dakhlia Governorate at East-Delta region in Egypt (Zayed, 1989). Also, Table
69
(2.14) presents the results of previous researches on the index properties of
peaty soils around the world based on location (Huat, 2004). In this section,
some index and chemical properties of organic soils which are of interest to the
engineer will be reviewed.
Table (2.12): Mire stages and associated properties of some British peats
(Hobbs, 1986)
70
71
8
2
17
16
15
7
1
14
Talkha
Meet Taher
Sherbeen
El-Mansoura
Shobra Sendey
Dekerness
Belkas
6.5
8.5
11
13
9
8
8
7
7
3
Meniet El-Nasr
Simpelaween
11
18
Aga
11.5
10
El-Mansoura
7
8
20
Belkas
9
7.5
21
El-Baramon
Aga
7
5
Takha
8
8
22
El-Baramon
4
6.5
11
Sherbeen
Meat El-Kholey
9
12
Dekerness
6.5
8
6
Dekerness
19
11.5
13
El-Mansoura
El-manzala
Depth of
Sample
BH.
No.
Site Location
1.9
1.56
2.4
2.3
1.5
1.4
2.9
1.4
1.5
2
1.5
1.5
1.1
1.9
1.9
1.9
2.1
1.5
1.3
2.1
2
1.5
Layer
Thickness
72.11
68.39
63.16
62.49
61.41
57.42
54.98
51.88
42.22
40.73
37.21
36.33
35.14
31.71
27.83
27.06
25.35
22.55
21.44
20.9
19.64
16.06
Organic
Content %
625.2
583.9
491
478.8
399.8
312.3
421.4
395.7
373.4
286.1
228.3
214.1
214
144.3
159.8
108.7
178.1
82.4
108.9
106
120
82.8
Moisture
Content %
1.48
1.515
1.54
1.53
1.625
1.755
1.575
1.64
1.625
1.81
1.825
1.81
1.9
1.93
1.92
1.96
2.07
2.18
2.015
2.1
2.165
2.24
Specific
Gravity
9.25
9.03
7.71
7.32
6.56
5.48
6.83
6.69
6.07
5.28
4.22
4.12
4.15
2.83
3.23
2.2
3.76
1.8
2.23
2.06
2.65
1.85
Void
Ratio
1.047
1.04
1.05
1.065
1.08
1.13
1.061
1.06
1.091
1.21
1.15
1.15
1.24
1.24
1.3
1.31
1.22
1.42
1.35
1.37
1.35
1.46
Bulk unit
weight
gm/cm3
0.144
0.151
0.176
0.184
0.215
0.271
0.201
0.213
0.23
0.288
0.351
0.354
0.369
0.504
0.454
0.613
0.435
0.778
0.623
0.637
0.592
0.786
Dry unit
weight
gm/cm3
L.L
96
120 113
152 131
112
77
138 127
253 196
6.5
6.75
7.15
6
6.71
6.9
6.8
7
7.2
7.2
611
570
508
510
388
310
402
385
NP
493
NP
450
NP
262
NP
NP
378 318
301 260
7.15 270 217
7
7.25 225 200
7.27 195 144
7.15 195 181
7.5
7.35 189 158
7.9
7.5
7.5
7.05 135
96
P.L
Description
Dark brown H. O. Silty Clay, slightly fine fibrous
Dark brown Silty Clayey Peaty Muck, S. fine fibrous
77 Dark brown Clayey Peaty Muck, slightly fine fibrous
Dark brown Clayey Silty Peaty Muck, S. fine fibrous
60 Dark brown Silty Clayey Peaty Muck, S. fine fibrous
Dark brown Silty Clayey Peaty Muck, S. fine fibrous
48 Dark brown Silty Clayey Peaty Muck, S. fine fibrous
Dark brown Clayey Peaty Muck, slightly fine fibrous
Dark brown Clayey Silty Peaty Muck, S. fine fibrous
60 Dark brown Clayey Muck, slightly fine fibrous
41 Dark brown Silty Clayey Muck, slightly fine fibrous
53 Dark brown Clayey Silty Muck, slightly fine fibrous
57 Dark brown Clayey Muck, slightly fine fibrous
25 Dark brown Silty Clayey Muck, slightly fine fibrous
51 Dark brown Clayey Muck, slightly fine fibrous
14 Dark brown Silty Clayey Muck, slightly fine fibrous
11 Dark brown Silty Clayey Muck, slightly fine fibrous
31 Dark brown Clayey Muck, slightly fine fibrous
7
21 Dark brown H. O. Clayey Silt, slightly fine fibrous
16 Dark brown H. O. Clayey Silt, slightly fine fibrous
15 Dark brown H. O. Clay, slightly fine fibrous
11 Dark brown H. O. Clayey Silt, slightly fine fibrous
P.I
Cons. Limits
7.71 107
PH
Level
Table (2.13): Organic soil samples from East-Delta and their properties (After Zayed, 1989)
Table (2-14): Index properties of organic soil and peat based on location (Huat,
2004)
Natural
water
content
(wo %)
Unit
weight
γb
3
(kΝ/m )
Specific
gravity
Organic
content
(Gs)
(%)
370-450
8.7-10.4
-
-
310-450
10.5-11.1
-
65-85
865-1400
10.2-11.3
-
98-99
450
10.2
-
80
759-946
10.1-10.4
-
60-77
Peat -Austria
200-800
9.8-13.0
-
-
Peat -Japan
334-1320
-
-
20-98
Peat-Italy
200-300
10.2-14.3
-
70-80
Peat-America
178-600
-
-
-
Peat -Canada
223-1040
-
-
17-80
Peat-Hokkaido
115-1150
9.5-11.2
-
20-98
Peat-West Malaysia
200-700
8.3-11.5
1.38-1.70
65-97
Peat-East Malaysia
200-2207
8.0-12.0
-
76-98
Peat-Central Kalimantan
467-1224
8.0-14.0
1.50-1.77
41-99
Soil
deposits
Fibrous peat Quebec
Fibrous peat,
Antoniny Poland
Fibrous peat,
Co. Offaly Ireland
Amorphous peat,
Cork, Ireland
Cranberry bog peat,
Massachusetts
72
2.8.1 Chemical Properties of Organic Soils
2.8.1.1 Chemical Composition
Soil organic matter, when extracted, can be fractionated into components,
primarily those of plant tissues and those based on humus. Humic substances
are a complex series of relatively high molecular weight, yellow to black
colored organic substances that are formed by secondary synthesis reactions in
soils. They represent one of the most chemically reactive fractions of the soil
due to their high surface area and surface charge, and thus have a critical
influence on the chemical and physical properties of soils (Oades, 1989).
Humus fraction consisting basically of humic and fulvic acids and humin and
exists both in solid and liquid phases (Huttunen et al., 1996). The humic
fraction is gel-like in properties and negatively charged (Marshall, 1964).
Soil organic matter may occur in any of five groups: carbohydrates; proteins;
fats, resins, and waxes; hydrocarbons; and carbon. Cellulose (C6 H10 O5) is the
main organic constituent of soil (Mitchell, 1993). Chemically, organic matter
consists of carbon, hydrogen, oxygen, and small amount of nitrogen. Previous
researches (Chynoweth, 1983; Schelkoph et al., 1983; Cameron et al., 1989)
showed that the percentage of carbon, hydrogen, oxygen, and nitrogen are in
the ranges of 40 - 60 %, 20 -40 %, 4 - 6 %, and 0 - 5 % respectively. The
chemical composition is greatly related to the degree of decomposition, the
more the peat is decomposed, the less the percentage of the carbon is produced.
Also, the submerged organic component of organic soil is not entirely inert but
undergoes very slow decomposition, accompanied by the production of
methane and less amount of nitrogen and carbon dioxide and hydrogen sulfide.
2.8.1.2 pH value
A strong relationship exists between the type of peat and organic soil and the
chemistry of the associated water. Nutrient or base rich waters are
characteristically non-acidic and are associated with fens, while base deficient
waters are acidic and are associated with bogs. This is because decomposition
73
generally tends to be most active in neutral to weakly alkaline conditions pH
value (7 - 7.5). The more acid the peat, the better the plant remains are
preserved (Hobbs, 1986).
Also, in a high-pH, alkaline environment, organic fine substances disperse into
globular polyanions as small as 3 to 9 nm. In a low-pH, acidic environment or
in high-electrolyte concentration, the polyanions coagulate to form large flocs
of a more or less globular shape (Terzaghi et al., 1996).
According to Hobbs (1986) there is a good correlation between pH value of soil
and organic content as shown in Figure (2-18a). As a rough general role pH
value of fen peat frequently is in excess of 5, that of bog peat is usually less
than 4.5 and may be less than 3, and transitional peat fall in the range from 6 to
4. Generally, peat soils are very acidic with low pH values, often lies between 4
and 7 (Lea, 1956). According to Zayed (1989) pH value of organic soil
encountered in East-Delta region fall in the range from 6.0 to 7.9 (see Table
2.13)
2.8.1.3 Cation Exchange Capacity
For an organic particle, the rigidity and thickness of the zone of adsorbed water
is governed by the cation exchange capacity of the tissue and the chemistry of
the water; the higher the cation exchange capacity; the stronger the adsorbtion
complex, and the greater the interparticle adherence. On the other hand, the
cation exchange capacity has an inverse relationship with the mineral
concentration in the water supply. As the supply of nutrients declines a change
occurs in the type of plants, the cation exchange capacity increases, as does the
water content, and the adsorption complex strengthens (Bell, 2000).
Also, organic fine substances are negatively charged and display a substantial
cation exchange capacity which increases with degree of humification and
strongly influenced by the hydrogen concentration in the pore water. Cations
74
such as Ca, Mg, K, Na and also Fe and Al, replace hydrogen at the exchange
sites of the organic polymolecules. The cation exchange capacity of very fine
humic substances may be as high as 1.5 - 5.0 meq/g (Terzaghi et al, 1996).
In the less organic soils most of the cation exchange ability is saturated by
metallic cations from mineral matter in the soil. As the organic content raises
the quantity of exchangeable hydrogen ions slowly increase as shown in Figure
(2-18b). In peats most of the ions are strongly adsorbed into the exchange
complex. The cation exchange capacity of peats present in the ombrotrophic
bogs is very similar to that of Na montmorillonite. Fen peats have similar
cation exchange capacities to illite.
Fig (2.18): (a) pH and (b) exchangeable cations in relation to organic content of
lake, fen and bog soils (after Gorham 1966)
75
Because the specific gravity of the cell walls of plants is half that of clay
minerals, the adsorption complex in peats is approximately twice as effective as
that in clay. This explains why peats possess very high liquid limits as
compared with clays of similar cation exchange capacity. Also, the liquid limit
declines as the degree of humification increases, in other words as the
adsorption complex is weakened due to the destruction of plant material.
Consequently, fibrous peats have higher liquid limits than amorphous peats
(Bell, 2000).
2.8.2 Index Properties of Organic Soils
2.8.2.1 Organic Content
The organic content of such deposits varies appreciably, with the mineral
content, from over 95% to as low as 1%, which provides some indication of
how the organic deposit was formed. The organic constituents of such deposits
are generally combustible. Therefore, organic content can be determined by dry
or wet combustion of the organic matter.
A much used dry-combustion method is the "loss on ignition method" where a
moist sample is first dried in the oven at a temperature of 105° C for 24 hours,
and weighed (m1) and then placed in the muffle furnace and ignited at a high
temperature (400-900°). After it has cooled at room temperature, the sample is
reweighed (m2) and the loss in mass at ignition is determined. This loss in mass
is put in relation to the original mass and the relative value called "loss on
ignition" designated as "N" is assumed to correspond to the organic content
(ASTM D 2974).
Organic content (OC) = N = (m1 - m2)/ m1
(2.1)
However, the temperature and the length of firing vary. The Muskeg
Engineering Handbook (1969) recommends 800 to 900°C for 3 h or "until the
soil has obviously been reduced to an ash". In Egypt as in Europe, a
76
temperature of 550°C is used for combustion. Arman (1971) recommends 440°
C and holds it for 5 hours, claiming that this temperature is very critical: a few
degrees below will result in incomplete combustion, and a few degrees above
will result in a transformation of clay minerals and hence a loss of mineral
matter. A temperature of 440°C and holding until completely ashed are the
recommendations of ASTM (D 2974-00), and this was followed in this study.
Skempton and Petly (1970) proposed an equation for calculating organic
content (OC) as follows:
Organic content OC% = 100 - C (100 – N)
(2.2)
Where C is the correction factor. For a temperature of 440°C, C = 1.0 (Arman,
1971), while in Europe, C = 1.04 is then applied as correction. However, the
difference is usually small, hence not significant for practical considerations
(Edil, 2003).
The loss on ignition test method is only approximate because since during the
firing process, more than just the organics are burned off. According to Jackson
(1958) this method can produce an error from 5% to 15%. The errors at loss on
ignition increase with increasing mineral content, especially if the soil contains
carbonates and sulphides. Unless the mineral content is high or the soil contains
carbonates, the errors can usually be ignored. This is the case in peats and other
highly organic soils (Lechowicz et al., in Embankments on organic soils, 1996).
However, the ash method "loss on ignition" is generally preferred for
engineering purposes (McVay and Nugyen, 2004).
In soils with higher mineral content, the colorimetric method can be
recommended for determination of the organic content. In the colorimetric
method, a dry pulverized sample is mixed with potassium dichromate solution
in a retort. Concentrated sulphuric acid is then added, whereby the organic
77
matter is wet-combusted. At the oxidation of the organic carbon with
dichromate, the color of the oxidation fluid changes from orange to green. A
simple but reliable measurement of the organic carbon content is obtained by
measuring the intensity of the green color with a colorimeter supplied with a
filter for wavelengths close to 620 nm. The colorimeter is calibrated for the
given test procedure with known amounts of organic carbon. The main sources
of error in this method are the relatively small amount of sample in each test
and the fact that the conversion factor used to calculate organic content from
organic carbon may vary somewhat. Usually, organic matter is considered to
contain 58% organic carbon (Lechowicz et al., in Embankments on organic
soils, 1996).
Other types of wet-combustion followed by titration or dry-combustion method
with determination of evolved carbon dioxide may be used, but they are more
complicated, more expensive and often require specially trained staff. They all
suffer from the same general shortcoming as the colorimetric method and are in
practice not more accurate. The colorimetric method can therefore be
recommended for determination of organic content in soils with low and
medium organic contents and for highly organic calciferous soils. The method
is described in detail e.g. in SGI Report No. 27E, (Larsson et al 1987).
As far as engineering is concerned, the organic content is important in that it
influences the water-holding capacity of organic soils. Indeed, most of the
differences in the index characteristics of organic soils are attributable to the
amount of moisture present. Also, organic matter in soil may be responsible for
high plasticity, high shrinkage, high compressibility, wide range of hydraulic
conductivity, and low shear strength. The influences of organic matter content
on the classification properties of soft clay from Brazil are shown in the Figure
(2-19).
78
Fig. (2.19): Influence of organic content on classification properties of
Juturnaiba organic clay, Brazil (from Coutinho and Lacerda, 1987)
In highly organic soils, such as peat it is common to use ash content instead of
organic content as a measure of the mineral content in the soil. The ash content
(or inorganic content) of an organic soil is the percentage of dry material that
remains as ash after controlled combustion. According to Ron Munro (2004)
organic soil that has grown in-situ normally has an ash content of somewhere
between 2% and 20% of its in-situ volume and this range of ash contents can be
an indicator of this type of organic soil (peat). Also, in many peats the mineral
content increases with depth. On the other extreme, organic muds may contain
some 10% of organic detritus (Bell, 2000).
Ash content (Ac) = m2/ m1 = 1 – organic content
79
(2.3)
2.8.2.2 Fiber Content
The amount of fiber material presence has an influence on the physical, index,
and mechanical properties of organic soils. The fiber content is determined by a
wet sieving procedure (ASTM D 1997). Fibrous peat has the higher fiber
content in all organic soils. Fibrous peat is the one having fiber content more
than 20 % of the oven-dried mass of the original sample, while amorphous
peat, on the other hand, is the peat soil with fiber content less than 20 %
(ASTM D 4427). Amorphous-granular peat tends to behave similarly to
mineral soils, whereas this behavior deviates more and more as the fiber
content increases. The effect of fibrosity on peat characteristics is shown in the
table (2.15).
Table (2.15): Relative values of various peat properties for predominant types
(MacFarlane, 1969)
Predominant Structural
Characteristics
Amorphous-granular
Fine Fibrous
Coarse Fibrous
(woody)
Higher
Unit weight
Water content
Void ratio
Compressibility
Permeability
Shear strength
Tensile strength
Properties in Which
Intermediate
Void ratio
Compressibility
Permeability
Shear strength
Tensile strength
Water content
Unit weight
Lowest
Water content
Permeability
Shear strength
Tensile strength
Unit weight
Void ratio
Compressibility
2.8.2.3 Void Ratio
Natural void ratio of organic soils is generally higher than that of inorganic
soils; with fibrous peat having the greater void ratios. Extreme ranges in void
ratio for organic soils have been reported from 2 to 25, with 5 to 15 being a
more usual range (Hanrahan, 1954). The void ratio of peat ranges between 9,
for dense amorphous granular peat, up to 25, for fibrous types with high
contents of sphagnum. It usually tends to decrease with depth within a peat
80
deposit. Such high void ratios give rise to phenomenally high water contents.
The latter is the most distinctive characteristic of organic soils (Bell, 2000).
2.8.2.4 Water Content
Generally, the water-holding capacity of organic soils depends primarily on the
morphology and structure of the material present and on the degree of
humification. However, organic soils have a high water holding capacity, and
the amount of water held in the organic material is directly correlated to its OC
(McVay and Nugyen, 2004).
Soil fabrics characterized by organic coarse particles, as in fibrous peat, hold a
considerable amount of water because they are generally very loose, and also
because organic particles are hollow and largely full of water. According to
Wilson (1978) the water content of peat is held in the cells of plant remains, as
well as in the voids. Water is also absorbed by the cell walls of the plant
detritus. Also, most water occurs as free water in the large pores; it also occurs
as capillary water in the small pores; and as adsorbed chemically bound,
colloidal or osmotic water. The free water is under a suction pressure of less
than 10 kPa. On the other hand, capillary forces hold interparticle water at a
suction pressure greater than 10 kPa. The suction pressure does not exceed 20
kPa in the case of adsorbed water (Bell, 2000).
These three types of held water have different drainage characteristics. Water is
forced out of the voids when peat undergoes stress. With continuing stress the
particles are brought into contact and the cell structure begins to be distorted.
Hence the water in the plant cells is pressurized. Some of this water moves
through openings in the cell walls, but with increasing stress this begin to
rupture. Water is expelled thereby, giving rise to increasing pore water pressure
in the voids. Wilson indicated that at this point the peat behaves as a material
which has become rapidly softened. Further straining and rupture of the cell
walls means that shear failure is imminent (Bell, 2000).
81
In organic fine substances, water of hydration and double-layer water are
important. The decay of peat is by no means uniform and the water content is
reduced with increasing humification. The water content also declines with
increasing mineral content. Hence, fen peats have lower and less variable water
contents than bog peats (Bell, 2000). Tables (2.12-2.14) show typical values of
natural water contents for a variety of organic deposits.
In general, the water content of peats varies from a few hundred percent dry
weights, e.g. 500% in some amorphous granular peats, to over 3000% in some
fibrous varieties due to its natural fabric structure. Put another way the water
content may range from 75 to 98% by volume of peat. Moreover, changes in
the amount of water content can occur over very small distances as direct result
of the deposit's morphology. Figure (2.20) shows the plot of water content
versus loss on ignition complied by O′Loughlin & Lehane (2003) in the case of
peat and organic soils. The relationship shown is linear but only up to
OC=80%. A high degree of scatter between OC and wo exists for soils with
higher organic content, which they attributed to be due to degree of
humification of the organic matter.
Fig. (2.20): Correlation of water content with loss-on-ignition (O`Loughlin &
Lehane, 2003)
82
The water content is determined for organic soils in the same manner as for
mineral soils. For peat soils, there is a general fear that standard drying of the
soil at 105° C for 24 hours will lead to charring of the organic component in
peat, thus producing too large a figure for water content. Some therefore
advocate a lower temperature, between 50° C and 95° C. Skempton and Petley
(1970) and Kapai and Farkas (1988) investigated the effect, and concluded that
the loss of organic matter at 105°C is insignificant, while drying at lower
temperatures retains a small amount of free water (Huat, 2004).
2.8.2.5 Bulk Unit Weight
The natural bulk unit weight of organic soil is both low and variable, being
related to the organic content, mineral content, water content and degree of
saturation. Bulk unit weight increases as more mineral soil becomes intermixed
with the organic matter. MacFarlane (1969) reports that natural bulk unit
weight have been observed to range from 0.4 Mg/m³ for a moss peat to 1.2
Mg/m³ for an amorphous granular peat. Huat (2004) concluded that the bulk
unit weight of peat are in the range of 0.8 – 1.2 Mg/m³ compared with the bulk
unit weight of mineral soils which are in the range of 1.8 – 2.0 Mg/m³. This is
due to the lower specific gravity of the solids found and the higher water
holding capacity in peat compared to inorganic soils. Tables (2.13 & 2.14)
show typical values of bulk unit weight of various types of organic soils from
different locations.
Hobbs (1986) reported that above natural water content of 600% both the
specific gravity and water content do not greatly influence bulk density. The
primary influence is the degree of saturation or gas content. Bell (2000)
concluded that peats frequently are not saturated and may be buoyant under
water due to the presence of gas.
Based on extensive tests performed on Dutch organic soils, den Haan and El
Amir (1994) proposed the following empirical relationship:
83
γsat (kN ⁄m³) = 12.266 – 3.156 OC
(2.4)
Where OC is organic content expressed as ratio.
2.8.2.6 Dry Unit Weight
Dry unit weight, γd is weight of solid particle (Ws) over total volume (V). The
dry unit weight is a more important engineering property of organic soil,
influencing its behavior under load. The dry unit weight itself is influenced by
the effective load to which a deposit of organic soil has been subjected.
Hanrahan (1954) recorded dry unit weight of drained peat within the range
0.065-0.120 Mg/m³. MacFarlane (1969) reports dry unit weight range from
0.082 Mg/m³ to 0.32 Mg/m³. The dry unit weight is influenced by the mineral
content and higher values than that quoted can be obtained when organic soil
possess high mineral residues (Bell, 2000).
2.8.2.7 Specific Gravity
The specific gravity of any material is the ratio of its density to that of water. In
the case of soil, it is computed for the solid phase only. For most mineral soils
the specific gravity ranges from 2.60-2.80. Specific gravity of organic soil
depends greatly on its composition, the amount of mineral matter contained and
percentage of the organic content, and cannot be simply set to somewhere near
the mineral soils. Cellulose has a specific gravity of approximately 1.58, while
for lignin it is approximately 1.40. These low values reduce the compounded
specific gravity of organic soils (Huat, 2004).
MacFarlane (1969) reported that fibrous peats in Canada, in which the water
content was greater than 500% and organic contents in excess of 80%, had
specific gravities in the range 1.4 -1.7. The lower specific gravity indicates a
lower degree of decomposition and low mineral content. Bell (2000) noted that
the specific gravity of peat has been found to range from as low as 1.1 up to
about 1.8, being influenced by the content of mineral matter. Moreover, Huat
84
(2004) concluded that the high lignin content of tropical peat gives it a slightly
lower specific gravity compared with that of temperate peat, with Gs in the
range of 1.07 – 1.70, and an average of about 1.40. Tables (2.12 & 2.14) show
typical values of specific gravity of various organic deposits. According to
Zayed (1989) the values of specific gravity of organic soils obtained from EastDelta region fall in the range of 1.48 – 2.24 with an average of 1.83, which is
high due to their higher mineral content (Table 2.13).
However, the average specific gravity of soil solids for organic soils can be
calculated from:
Gs = 2.7 (1 - OC) + 1.5 OC
(2.5)
Where, "OC" is the organic content or loss on ignition, expressed as ratio. This
assumption may lead to an error of as high as 18% (Edil 2003). Skempton and
Petley (1970) propose the following relationship for the above two parameters:
1 1 − 1.04(1 − N ) 1.04(1 − N )
=
+
Gs
1.4
2.7
(2.6)
Den Haan (1997) simplified the above equation as follows:
(1 − N )
1
N
=
+
Gs 1.365 2.695
(2.7)
The values of specific gravity obtained from the den Haan equation (2.7) are
somewhat lower than given by Skempton and Petley equation (2.6). Figure
(2.21) shows a graph of specific gravity versus loss on ignition of Irish peat
compared with data from different locations.
85
Fig. (2.21): Correlation of specific gravity with loss on ignition for Irsh peat
and from literature (O`Loughlin & Lehane 2003)
2.8.2.8 Atterberg Limits
Organic matter in soil may be responsible for high plasticity. In general, the
liquid limit of organic soil depends on the organic content, the type of plant
detritus contained (this determines the initial cation exchange capacity), on the
degree of humification and on the proportion of clay soil present. The liquid
limit of organic soil and peat increases with increase in organic content. For
peat soils, the liquid limit declines as the degree of humification increases, in
other words as the adsorption complex is weakened due to the destruction of
plant material. Consequently, fibrous peats have higher liquid limits than
amorphous peats. On the other hand, plastic limits can only be obtained from
organic soils which contain a given amount of clay; the clay content required
decreasing as the degree of humification increases. Skempton and Petley
(1970) put the boundary at approximately H3 of the von Post scale for the
liquid limit, and H5 for the plastic limit.
86
In the case of temperate peat, the liquid limit of fen peat ranges from 200 –
600%, and bog peat from 800 to 1500%, with transition peats between. The
liquid limit of the peat, in other words, increases with increase in natural water
content. Also, the liquid limit is reduced by increasing the degree of
humification (Bell, 2000). For the tropical hemic peat of Malaysia, the liquid
limit is in the range of 200-500%, in about the same range to that of temperate
(fen) peat (Huat, 2004). Figure (2.22) shows a graph of plasticity index versus
liquid limit for some UK peats. All the data is plotted under the A-line.
Fig (2.22): Plasticity index versuss liquid limit for some UK peats (after Hobbs,
1986)
In any case, According to Hobbs (1986) it is not possible to carry out plastic
limit tests on pure bog peats on the one hand even if they are highly humified,
or peat whose liquid limit is greater than 1000% on the other. He concluded
that there is little point in performing the plastic limit test on peat soils since the
plasticity gives little indication of their character. MacVay and Nugyen (2004)
87
stated that "When dealing with peat; determination of the Atterberg limits in
general is neither beneficial nor recommended".
2.8.2.9 Shrinkage
Since organic soil and peat has a high void ratio and water content they
undergo significant shrinkage on drying out. Bell (2000) concluded that the
amount of shrinkage can ranges between 10 and 75% of the original volume
and it can involve reductions in void ratio from over 12 down to about 2. Also,
the change in peat is permanent in that it can not recover all the water lost when
wet conditions return. Only 33 % to 55 % of the water can be reabsorbed
(Mochtar, 1997). Hobbs (1986) noted that the more highly humified peats, even
though they have lower water contents, tend to shrink more than the less
humified fibrous peat. He quoted values of linear shrinkage on oven drying
between 35 and 45%.
88
CHAPTER 3
ENGINEERING CHARACTERISTICS OF ORGANIC SOILS;
LITRATURE REVIEW
3.1
Introduction
Organic soils commonly occur as extremely soft, wet, unconsolidated surficial
deposits that are integral parts of wetland systems. They may also occur as
strata beneath other surficial deposits. Organic soils are characterized by their
loose structure and high water content. As a result of their loose state, they
have high permeability. Also, because of their typical locations, i.e., near the
surface, most surficial organic deposits, however, have no significant loading
history as they are fairly recent deposits in waterlogged areas. Larsson (1996)
concluded that many of these profiles do not even have a dry crust and most of
the soil layers have not been subjected to any load other than the weight of
overlying soil (Embankment on Organic Soils, 1996). The resulting effective
stresses are relatively low because of the high groundwater levels and the low
densities of the unconsolidated organic soils.
Therefore, organic soils in its natural state are known for their high
compressibility and low shear strength which often results in difficulties when
construction work has to take place on such deposit. The low strength often
causes stability problem and consequently the applied load is limited or has to
be placed in stages. Large deformation may occur during and after construction
period both vertically and horizontally, and the deformation may continue for a
long time due to creep (McVay and Nugyen, 2004).
Undoubtedly, these lead to the tendency to either avoid construction and
buildings on these soils, or when this is not possible, to simply remove, replace
or displace them. In some instance that may lead to possibly uneconomical
design and construction alternatives. It is therefore necessary to be able to
89
obtain suitable design parameters for strength and compressibility as well as to
find suitable construction techniques on these materials.
However, the engineering behaviors of organic soils show a great variation
depending on the type and amount of organic matter. The organic matter may
occur in many forms from small amount of amorphous or colloidal substance
embedded in the pores of a mineral soil to fibrous peat with a structure
resembling a coarse, loosely woven mat. The effect of the organic content on
the engineering properties in relation to the properties of a pure mineral soil is
in the former case mainly confined to a decreased permeability and a somewhat
increased tendency to creep. In the latter case, the properties are quite different
in most respects.
Highly fibrous and undecomposed organic soils have a pronounced structural
anisotropy. The fibers and the plant remains usually have a horizontal
orientation. The fibers constitute a horizontal reinforcement and failure
surfaces in such materials usually occur as vertical fractures or horizontal shear
planes parallel to the fibers. The distribution of stress from loads on the ground
surface with depth is relatively small because of fibers. The permeability of the
soil is relatively high and is often many times higher horizontally than
vertically. Due to the high permeability and the fibers, stability is usually not a
problem in the fibrous peat itself, provided that measures are taken to prevent
punching or cracking under the loaded area and that the loading is not
extremely rapid. Also, when normalized towards the stress history, the shear
strengths of highly organic soils are usually higher than for mineral soils
(Larsson, 1996; Edil and Wang, 2000; Mesri and Ajlouni, 2007). Many fibrous
peats, however, are underlain by other very soft soils and serious stability
problems often occur.
Moreover, the compressibility of fibrous soils is very high. Even for small
external loads, it is common for the settlements to amount to more than half of
90
the original thickness of the peat layer (Berry and Poskitt, 1972; Larsson,
1996). On top of this, there are considerable creep deformations with time,
which for most engineering tasks cannot be accepted and have to be stopped.
On the other hand, the engineering problems in the more humified organic soils
with low permeabilities resemble the problems encountered in soft mineral
clays, but are often more accentuated because of the higher compressibility, the
enhanced creep effects, the very low effective stresses and strengths and the
sometimes very low permeabilities. Also, the effects of structural anisotropy
decrease with decreasing content of fibers and increasing degree of
humification (Larsson, 1996).
Furthermore, organic clays or silts present similar engineering challenges as
soft silts and clays, including low hydraulic conductivity, low undrained shear
strength and high compressibility. In addition, organic silts and clays undergo
significant creep deformations.
That is, the influence of organic content on the engineering properties of
organic soils is primarily a function of its type, amount, and degree of
decomposition. Also, structural anisotropy is important not only in peat but also
in other organic soils, where significant effects may occur even at relatively
low organic contents.
However, low shear strength and high compressibility of organic soils and
peats confine them in a problematic category. Therefore, the knowledge on
permeability, shear strength and compression behavior of different types of
organic soils and peats is essential as it enables designers to understand the
response of every soil type to load and to suggest proper engineering solutions
to overcome the problem. In this section, the information on evaluating
permeability, shear strength and compression properties for different types of
organic soils and peats are discussed.
91
3.2
Shear Strength Characteristics of Organic Soils
The shear strength of soil is an important parameter in its behavior under
various loading conditions. Like all soils, the shear strength of organic-rich
soils is directly related to the effective stress in the ground and stress history of
the deposit. Since surficial deposits of organic-rich soils are relatively
lightweight (i.e., low dry density), saturated, and have no significant stress
history, such deposits exhibits high porosities and hydraulic conductivities, and
develop very low vertical effective stresses for consolidation. Such a material
can be expected to behave "drained" like sand when subjected to shear stresses.
With consolidation the porosity decreases rapidly and hydraulic conductivity
becomes comparable to that of clay. Thus, there is a rapid transition
immediately from a well-drained material to an "undrained" material (Huat,
2004).
However, accuracy in determining the shear strength of organic soils is
associated with several variables namely organic content, origin of soil (fiber
content), water content, and degree of decomposition which need to be
considered (Huat, 2004). Also, before the shear characteristics of organic soils
can be determined or improved, the mechanism of how shear strength is
mobilized in organic soils should be understood (McVay and Nugyen, 2004).
3.2.1 Factors Affecting Shear Strength of Organic Soils
3.2.1.1 Effect of Fibers
Organic soils with negligible fiber content (amorphous-granular) derive its
shear strength in a similar manner to mineral soils. The shear strength is
developed from cohesion between the particles and from the frictional
resistance between the grains.
On the other hand, fibrous soil mobilizes its shear strength in an entirely
different manner. In the natural fabric, the organic fibers tend to be somewhat
oriented and overlapping. In a loose natural state, the fibers are surrounded by
92
water, and the soil matrix has low shear strength. Most of the shear strength in
this condition is from apparent cohesion due to mineral soil (clay)
contamination and /or entanglement of the fibers. During compression the
fibers tend to align themselves at right angles to the direction of the applied
(vertical) stress as shown in Figure (3.1). Water is expelled and the fibers come
in contact. Thus, as the soil is stressed, the organic fibers become more oriented
and move close together. The fibers in this condition act as reinforcement to
triaxial shear. The shear strength, in this compressed state, is a function of the
friction between the fibers, the tensile strength of the fibers, and the apparent
cohesion is relatively small. This results in a large increase in shear strength
from the fiber reinforcing effect (McVay and Nugyen, 2004).
Fig. (3.1): Effect of compression on peat fabric (after McVay and Nugyen,
2004)
This mechanism of internal resistance seems to agree with Amaryan (1972),
who described the behavior of a fibrous peat under a range of loadings.
According to Amaryan, the angle of friction is small for low stress increases
and the shear strength is mainly due to cohesion as shown in Figure (3.2). At
93
higher stress levels, the shear strength develops mainly from friction, and the
effective cohesion becomes negligible.
Fig. (3.2): Shear Strength as Function of Effective Stress (Landva, 1980)
It has been shown by many investigators (Macfarlane, 1969; Helenelund, 1975;
Landva 1980; Landva and La Rochelle, 1983, Yamaguchi et al, 1985a,c,d) that
the shear strength of fibrous soil is less in the horizontal plane than in vertical
plane. This is to be expected because when fibrous soil is sheared parallel to the
fibers (horizontally), the reinforcement effect of the overlapping is lost, and the
soil will fail with the fibers sliding over each other as shown in Fig. (3.3).
Since the mobilization of shear strength is affected significantly by fiber action,
as discussed above, any variability in fiber type and content will result in
variation in shear behavior (McVay and Nugyen, 2004, Mesri and Ajlouni,
2007). For peat with low fiber content, the effect of fiber reinforcement will be
insignificant and a shear failure may be expected to occur in the matrix, more
or less independent of the fibers, in a manner similar to mineral soils.
94
Fig. (3.3): Shear failure modes (after McVay and Nugyen, 2004)
3.2.1.2 Other Influences
McVay and Nugyen (2004) concluded that other factors have been found to
have an influence on the shear strength of peat. According to Wyld (1956),
"Qualitatively, the shear strength of peat varies inversely with its water content
and directly with ash content and degree of deformation in compression".
Helenelund (1975) notes, "The undrained shear strength diminishes with
increasing water content and an increasing degree of humification."
Shear behavior also depends on permeability. As organic soil is sheared, excess
pore water pressure is generated. The permeability of organic soil governs the
rate of pore pressure dissipation. Thus, the shear strength varies directly with
permeability; the lower the permeability the lower the shear strength. In
general, the permeability decreases with increasing degree of humification.
Since the degree of humification often varies both horizontally and vertically,
in a somewhat erratic pattern, the permeability and shear characteristics will
vary accordingly. This makes the determination of representative shear strength
parameters for a particular deposit difficult to ascertain.
95
3.2.2 Determination of Shear Strength
Where good quality undisturbed samples can be obtained, laboratory strength
testing should be performed to obtain shear strength information for design.
Triaxial shear tests and direct shear tests are the most common, direct simple
shear test and ring shear test are used also, but to a lesser extent. Since fibrous
peat has a strong anisotropic behavior, therefore, a large discrepancy in shear
strength is observed when direct shear results are compared with triaxial shear
strengths. This is because the failure plane is forced parallel to the fibers in a
direct shear test and the reinforcing effect is lost. Therefore, with anisotropic
soils, the laboratory test, as well as sample orientation is of significance.
Many investigators have successfully used both drained and undrained triaxial
tests to observe the shear behavior of an organic soil (Adams, 1961; Gautschi,
1965; Hanrahan et al., 1967; Hollingshead and Raymond, 1972; Yamaguchi et
al, 1985a, c, d). In most cases large strains were reached before failure occurred
specially for fibrous peat (McVay and Nugyen, 2004). Sabatini, et al. (2002)
concluded that in drained laboratory triaxial tests, it may require greater than
20% axial strain to mobilize the full strength of the material, and this needs to
be considered in stability analysis so that the mobilized strength is consistent
with the expected level of deformation. Also, Huat (2004) concluded that
drained triaxial tests for peat are seldom performed since they may take several
weeks due to significant change in hydraulic conductivity upon consolidation,
and also, due to gross changes in specimen dimensions and shape during the
test. Therefor, triaxial consolidated undrained test in compression and extention
modes, with and without pore pressure measurements, are usually used.
In general, a triaxial compression test should be used to determine the shear
strength of an organic soil deposit under road embankment or other vertically
loaded structure resembles that in triaxial compression. On the other hand,
direct shear test would be the appropriate measure of the soil's shear strength if
the soil is exposed to a horizontal load (e. g. lowest point of a slip surface).
96
Considering the presence of organic soils is almost always below the
groundwater level, the determination of undrained shear strength is of great
importance. Aaccording to Tsushima and Mitachi (1998) unconfined
compression test have been widely used to perform the stability analysis of
highly organic soils. However, becaue of the unconfined compression strengths
are usually scattered due to inevitable change of effective stress and mechanical
disturbance during the process from sampling to laboratory testing, they
proposed a method for predicting in-situ undrained strength from the
relationship between residual effective stress obtained by suction measurement
and unconfined compression strength (Mitaachi et al., 2001).
Also, the undrained shear strength of organic soils has been measured by field
vane tests (FVT), cone penetration test (CPT), pressure-meter test, dilatometer
test, plate load test, and screw auger pulling tests (Edil, 2001). Termaat (1999)
concluded that both tools CPT and FVT are good instruments to determine the
mean value of the shear strength with a low variation. It is recommended for
CPT to use large cones with high resolution load cells since it is likely that the
tip resistance in organic soils will be extremely low (Sabatini et al., 2002; Edill,
2003). Also, Edil (2003) recommended large vanes of diameter 55 to 110 mm
and height to diameter ratio of 2 are for peat, and the rate of vane rotation has
to be faster than for clay, i.e. greater than 0.1 degree/s. Moreover, Hanzawa et
al (1994) recommended a combination of direct shear test (DST) and CPT's, to
improve the design quality. He prefers the CPT to the FVT because the
correlation between CPT and DST is more consistant (Termaat, 1999).
However, among them, the vane shear test is the most commonly used. On the
other hand, for organic silts and clays, their very low hydraulic conductivities
allow for the use of CPT, CPTu, and VST devices as a mean to correlate
undrained shear strengt
Moreover, according to McVay and Nugyen (2004) the shear behavior of peat
was studied by Hanrahan et al. (1967) whom found that the Coulomb-Terzaghi
97
failure criterion could be applied to peats with satisfactory results. The shear
strength τf of peat may be represented as:
τf
= c' + σ ' tan φ '
(3.1)
Where c` is the effective stress strength intercept, σ` is the effective normal
stress, and ø` is the effective angle of internal friction.
3.2.3 Frictional Resistance
In normally consolidated peats and organic soils, the strength behavior is
almost entirely frictional so exhibits zero or small effective cohesion and
generally high effective friction angles.
Figure (3.4) reported the effective friction angle, compiled by Edil (2003) as a
function of organic content. In this figure, materials with an organic content
less than approximately 25% (organic silt and clay) are called "organic soils".
The average effective angle of friction is 53° for organic-rich and clearly above
the average angle of 41° for organic silt and clay. Therefore, it could be
concluded that the effective internal friction ø` of organic-rich soils is generally
higher than that of inorganic soil.
Also, Edil and Dhowian (1981) reported that ø` is 50° for amorphous granular
peat and in the range of 53°-57° for fibrous peat. Therefore, it could be
concluded that the angle of friction is generally higher for more fibrous (fabric)
peat, and decreases with increasing degrees of humification (less fiber content).
The trixial compression tests tends to yield higher angles of shearing resistance
than the direct shear and simple shear test, while trixial extension tests tends to
yield the lower values. Therefore, it could be concluded, also, that the effective
internal friction ø` of organic-rich soils vary according to the type of test used.
98
Fig. (3.4): Effective friction angle versus organic content (Edil, 2003)
3.2.4 Undrained Shear Strength:
It was observed that low initial undrained shear strength of organic-rich
deposits has frequently led to spreading type failure of embankments (Mesri
and Ajlouni, 2007). However, primary and secondary compression can lead to
appreciable increase in shear strength, especially for fibrous peat (Lea and
Brawner, 1963; Weber, 1969). According to MacFarlane (1969), experience in
placing fills on peat suggests that a significant increase in shear strength of peat
occurs after the pore-water pressures are largely dissipated.
Data on undrained shear strength to consolidation pressure for organic-rich
soils, reported by the previous authors, suggest that Su/σ`p or Su/σ`v are in the
range of 0.36 to 0.68 for different shear testing procedures, except for low data
of unconfined compression test usually suffering inaccuracy due to change of
effective stress and mechanical disturbances. However, these values appear
unusually high compared to the typical values for inorganic clays, which is
0.32 and 0.22 to 0.28 for TC and DSS respectively (Terzaghi et al., 1996).
Figure (3.5) presents the normalized nudrained shear strength data; compiled
by Edil (2001) for different types of organic soils in US. Organic-rich soils
99
show no evident differences between fibrous and amorphous soils, and give an
average normalized undrained strength of 0.59 with most of the data falling
between 0.5 and 0.7. Organic silt and clay (OC<20%) seem to have lower
normalized undrained strength compared to organic-rich soils. Also, it is
revealed that normalized nudrained shear strength data from laboratory tests is
lower and less scattered compared to field vane tests.
(a)
(b)
Fig. (3.5): Normalized undrained strength versus organic content for US peats
from: (a) CIU triaxial tests and (b) field vane tests (Edil, 2001)
Also, Huat (2004) reported that the undrained shear strength of peat soil (Su)
obtained by vane shear test was in range of 3-15 kPa, which is much lower than
that of the mineral soils. Mesri and Ajlouni (2007) suggest that Su(FV)/σ`p=1.0.
Hartlen and Wolsky (1996) suggested a correction factor of 0.5 for field vane
test results on organic soil with a liquid limit of more than 200 %. The same
correction factor was suggested by Mesri and Ajlouni (2007) for peat.
On the other hand, Mesri and Ajlouni (2007) concluded that the fibrous
structure is strongly responsible for high frictional resistance and undrained
shear strength to consolidation pressure ratio of fibrous peats, and that any
biochemical degradation of the fiber structure can be expected to lead to a
reduction in both properties.
100
This could be observed from Figure (3.6), introduced by Termaat (1999), in
which the normalized test results for peaty soils from different locations around
the world, using different shearing test procedures (TC, TE, DST, and DSS),
had been collected, and plotted as function of the organic content.
Unfortunately, the test data for DST and DSS was restricted to three; therefore,
he concluded that the regression line, shown for these tests, is more or less an
assumption. However, it is simulating the undrained strength behavior of
different organic soils structures more reasonably than that of Edil (2001).
Fig. (3.6): Strength increment ratio versus organic content (Termaat, 1999)
3.3
Permeability Characteristics of Organic Soils
The permeability of all soils is determined by void ratio, size of flow channels
perpendicular to the direction of flow, and shape of flow channels parallel to
the direction of flow. Large void ratios, large pores, and straight flow channels
result in large permeability, while small void ratios, small pores, and tortuous
flow channels result in small permeability (Mesri and Ajlouni, 2007).
In organic soils, the physical structure and arrangement of constituent particles,
e.g., fibers and granules, greatly affect the void ratio and the size and continuity
of pores, resulting in a wide range of hydraulic conductivities. In addition to the
101
material structure and material characteristics, permeability of organic soils
varies widely, depending on amount of mineral matter, degree of consolidation,
degree of decomposition, chemical composition, and the presence of gas.
However, some of these factors can change with time and result in a change in
hydraulic conductivity (Huat, 2004).
Falling-head and constant-head permeability tests have been used to determine
the vertical and horizontal coefficient of permeability of peat and organic soil.
Tavenas et al. (1983) reported that falling-head permeability test could be
conducted during the secondary compression stage of incremental loading (IL)
oedometer tests. To determine the horizontal coefficient of permeability, kh,
oedometer specimens can be cut with their axis parallel to the horizontal
direction (Mesri et al, 1997), so that permeability anisotropy could be
investigated. Also, permeability coefficients could be computed using pore
pressure measurements during constant rate of strain (CRS) oedometer test or
compression measurements during the incremental loading oedometer tests.
It was noted that highly colloidal, mostly decomposed, amorphous-granular
soils tend to be less permeable and to display lower permeability anisotropy
than well preserved fibrous soils (Hartlen and Wolski, 1996; Edil and Wang,
2000; Mesri and Ajlouni, 2007). On the other hand, the fibrous organic soil
provides many interconnected flow channels through which water can easily
flow. These channels tend to have a horizontal orientation, causing
permeability in the horizontal direction to be higher than that in the vertical
direction. At a given void ratio, the horizontal permeability, especially for
predominantly fibrous peats, is larger than the vertical permeability by an order
of magnitude or more (Dhowian and Edil, 1980). In addition to orientation of
fibers in the horizontal direction, the fabric of some peat deposits is laminated.
However, according to Mesri and Ajlouni (2007), permeability measurements
for the covered Middleton fibrous peat deposits suggest kho/kvo=10; for surficial
fibrous peat, kho/kvo is likely to be in the range of 3 to 5.
102
Also, most naturally occurring peats have relatively high initial permeability of
10-3 to 10-5 cm/s (Wyld, 1956). Studies on physical and hydraulic properties of
fibrous peat indicated that it is averagely porous, and this certifies the fact that
fibrous peat has a very high initial permeability (MacFarlane, 1969; Hobbs,
1986; Mesri and Ajlouni, 2007). In its natural state, the hydraulic conductivity
of fibrous peat may be as high as sand, i.e., 10-3 to 10-2 cm/s (Colleselli et. al.,
2000). On the other hand, it should be noted that many highly decomposed
organic soils are, in fact, relatively impermeable. This is evident by the fact that
organic soils have been used for the impermeable core of the rock dams in
Norway (Tveiten 1956, Silburn 1972). This extreme range in void ratios makes
it difficult to assess the mechanical behavior of organic soil (McVay and
Nugyen, 2004). Figures (3.7, 3.8) show the relationship between void ratio and
vertical permeability of undisturbed samples of peat compared with other
geomaterials, compiled by Hobbs (1986) and Mesri and Ajlouni (2007).
Fig. (3.7): Vertical permeability during pauses in consolidation tests on
undisturbed peat (after Hobbs, 1986)
103
Fig. (3.8): Data on vertical coefficient of permeability of fibrous peats within
frame of reference of permeability data for sodium clay minerals, soft clay
deposits, including Mexico City clay, and clean sand (after Mesri and Ajlouni,
2007)
Moreover, permeability of organic soils is one of the most important properties
because it controls the rate of consolidation and increase in the shear strength
of the soil (Hobbs, 1986). Hanrahan (1954) determined the permeability of peat
in a falling-head permeameter and in the consolidation cell. He noted that the
permeability is affected considerably by the magnitude and duration of loading.
He applied a load of 8 psi on a sample of partly humified peat with a natural
void ratio of 12 and initial permeability of 4x10-4 cm/s. After two days, the
void ratio was reduced to 6.75 and the permeability to 2x10-6 cm/s. After 7
months under the same load, the void ratio was reduced to 4.5 and the
permeability to 8x10-9 cm/s. As soil is compressed, the decrease in void ratio
results in large decreases in permeability. The final permeability corresponded
to 1/50,000 of the initial permeability value.
104
Furthermore, research on Portage fibrous peat shows that the soil initially has a
relatively high permeability comparable to fine or silty sand; however, as
compression proceeds, void ratio decreases rapidly and permeability is greatly
reduced to a value comparable to that of clay, i.e. about 10-10 to 10-11 cm/s
(Hillis and Browner, 1961; Lea and Browner, 1963; Dhowian and Edil, 1980).
It can be concluded that the change in permeability as a result of compression
is drastic for peat because of their very large compressibility.
According to Mesri and Ajlouni (2007), it is useful to plot side by side the
relationship between void ratio and log effective vertical stress and the relation
between void ratio and log permeability, kv, for interpreting the time rate of
primary consolidation. The advantage of such plot is that one can directly see
the decrease in permeability from kvo to kvf as effective vertical stress increases
from σ`vo to σ`vf and void ratio decreases from eo to ef. The slopes of e versus
log kv, that is, Ck = ∆e ⁄∆ log kv, measures the reduction in e required to produce
a tenfold decrease in kv. However, when Ck is large, then for a given decrease in
void ratio the decrease in permeability is small, whereas a small Ck means a
large decrease in permeability with a decrease in void ratio.
Following the above mentioned technique, Santagata et al. (2008) illustrates a
side by side a comparison of permeability data (e-log kv curves) and
consolidation data (e-log σ`v curves) for three soils; Boston blue soft inorganic
clay, organic soil (muck) from West Lafayette (Indiana), and Middleton fibrous
peat (Wisconsin) as shown in Figure (3.9). According to Santagata et al.
(2008), these soils had been selected because of the availability of high quality
data for these soils, so that they represent reference materials, within which the
behavior of the organic soil, in general, is expected to lie.
105
Fig. (3.9): Comparison of compressibility and permeability behavior of muck
soil compared to inorganic clay and fibrous peat: (a) compression curves; (b)
hydraulic conductivity versus void ratio (after Santagata et al., 2008).
The first, Boston Blue Clay (BBC), is inorganic illitic low plasticity marine
clay, with average LL and PL of 45 and 23, respectively (Force 1998). The
second geometrical, muck soil with 40-60% organic content, natural water
content in the 209–285% range, specific gravity between 1.90 and 2.00, and insitu void ratio in the 4.30–5.50 range, was investigated by Santagata et al.
(2008). The third geometrical, Middleton fibrous peat with 90–95% organic
content, natural water content in the 620–850% range, specific gravity between
1.53 and 1.65, and in-situ void ratio in the 10–14 range, was extensively
investigated by several researchers (Mesri et al., 1997; Fox et al., 1992,1999).
Its behavior is considered representative of that of natural highly fibrous peats.
It was found that the in-situ hydraulic conductivity for muck soil can be
estimated at around 10-6 cm/s by extrapolating the linear curves of kv versus
106
void ratio. This value lies just below the range reported for fibrous peats
[kvo=6x10-6 – 10-3 cm/s, (Mesri et. al., 1997)], and also, fall in between the data
for the fibrous peat and BBC. That is, kv increases with increasing organic
content as shown in Figure (3.9b).
However, the findings showed that the rate of decrease of hydraulic
conductivity with decreasing void ratio in organic soils is usually higher than
that in clays (Edil, 2003). Whereas for soft clay and silt deposits the value of
Ck= 0.5(eo) (Tavenas et al. 1983; Mesri et al. 1994a), the empirical correlation
for peat deposits is close to Ck = 0.25(eo) (Mesri et al. 1997), while that for
organic soil (muck), Ck = 0.18-0.2(eo) (Santagata et al., 2008) which falls at the
very low end of the data for peats. Also, as shown in Figure (3.9b), Middleton
fibrous peat, Indiana muck, and Boston blue clay show a close to linear
relationship between logs kv and void ratios in the NC region. For these soils,
Ck is approximately equal to 2, 0.95, and 0.45, respectively (and consistent with
other data for similar soils).
According to Mesri et al. (1997), both the low value of Ck/eo and high values of
Ck for peats as compared to clays and silts suggest that only part of the pores in
peat – macropores in between particles – are serving as the flow channels. The
large initial permeability of peat results from flow mainly through large
macropores between fairly large peat particles. Therefore, the decrease in
permeability with compression results from the reduction in the size of the
macropores. However, Mesri and Ajlouni (2007) introduce Figure (3.10) to
compare Ck versus void ratio for five geotechnical materials, including the
Kozeny-Carmen "soil", to put the behavior of fibrous peat within a general
framework. They concluded that the pore-size distribution of fibrous peats is
very nonuniform compared with different geomaterials in the figure, which
leads to a dramatic decrease in permeability under compression.
107
Fig. (3.10): Explanation of magnitude of Ck/e or Ck/eo in terms of five materials
with different pore-size distribution (after Mesri and Ajlouni, 2007)
3. 4
Distinct Compressibility Behavior of Organic Soils
The high compressibility of organic-rich soils stands out as the most significant
engineering property (Kogure, 1999). Different from both sands and clays,
peats and organic soils generally undergo rapid and large consolidation
settlement and extensive long-term secondary compression (Fox, 2003). In
addition, typical organic soils contain 5-10% gas which contributes to the
immediate compression and the immediate and complete rebound if the load is
removed immediately after application (Landva and La Rochelle, 1983).
Berry and Poskitt (1972) stated that "It has long been recognized that the
consolidation of this material, peat, is extremely complex. This is because of
the highly compressible nature of natural deposits which under the action of
loads equivalent to only few feet of fill may undergo strains of the order of
50% and macropermeability reductions of hundredfold. In addition, the
consolidation process is complicated by the occurrence of secondary
compression which, for certain peats, shows an essentially linear relationship
108
with log time that appears to extend indefinitely, although it is realized that the
settlement must ultimately cease."
That is, for most organic deposits in the field, rapid dissipation of water
pressure are completed within a few weeks or months (Mesri et al., 1997). On
the other hand, long-term compression has no end within the time of
engineering interest. For instance, settlement of a peat layer in the field after 91
years has been reported by Van de Burght (1936).
Figure (3.11) shows settlement curve for an embankment built over organicrich deposits as illustrated by Landva and LaRochelle (1983). It was observed
that after the excess pore water pressure has dissipated, primary consolidation
has stopped and the secondary creep continues linearly with the logarithm of
time. They concluded that the secondary compression is of such a large
magnitude, that it masks the primary portion. This masking effect makes it
difficult to demarcate the end of primary consolidation, which leads to
difficulties in analyzing the settlements. They concluded also that the shape of
the settlement versus logarithm time relationship is a function of permeability
and the rate of creep (McVay &Nugyen, 2004).
Fig. (3.11): Typical field settlement curve for an embankment built over
organic deposit (after McVay &Nugyen, 2004)
109
Edil and den Haan (1994) stated that "A large group of organic soils exhibit a
one-dimensional compression behavior which is in general conformity with the
behavior of clays typically encountered in practice. There may be differences in
the magnitudes of various quantities measured but the general shapes of the
consolidation curves appear reasonably similar and the formulation developed
for clay compression can be used to predict the magnitude and rate of
settlement. In general, these formulations treat primary (hydrodynamic)
compression and secondary (creep) compression separately and decouple the
stress and time effects. Typically, laboratory multiple-stage-load (MSL)
oedometer tests are performed for a load-increment ratio (LIR) of unity and
load-increment duration (LID) of 24 hours. Alternatively, constant-rate-ofstrain (CRS) tests are conducted to obtain equivalent information as generated
by the MSL tests. There are however a certain class of peats, typically high
organic and fiber content materials with low degree of humification (fibrous
peat), that do not conform to the basic tenets of the conventional clay
compression behavior because of their highly different solid phase properties
and microstructure. The differences become particularly apparent especially at
low vertical stresses, i.e., for surficial peat deposits in early load increments in
the laboratory. The primary consolidation is very rapid and large secondary is
observed. The analysis of compression of such materials presents certain
difficulties when the conventional methods are applied because the curves
obtained from the conventional oedometer tests and the behavior exhibited by
them show little resemblance to the clay behavior."
Kogure (1999) illustrated a typical laboratory time–settlement curve for
organic-rich soil as shown in Figure (3.12). It will be seen that an initial
compression occurs over a very short time interval, followed by a long-term
compression that is essentially straight with the logarithm of time. It may be
inferred, therefore, that the initial stage is analogous to primary consolidation
and the long-term stage to secondary compression. He stated that "The large
110
magnitude and short duration of the initial stage and the continuous long-term
compression are the major departures from mineral soil behavior".
Fig. (3.12): Typical time-settlement curve of organic-rich soil (after Kogure,
1999)
Kogure (1999) also stated that "In applying the conventional theory to the
consolidation of fibrous soils, there are two major deviations from the wellknown assumptions, namely, the compressibility of solids and the dramatic
decrease in permeability under applied load. These two anomalies are believed
to account for the significant differences in consolidation behavior between
them and mineral soils. The high initial porosity is believed to account for the
rapid and large initial compression; the compressibility of the solid organic
constituents is believed to account, at least in part, for the continuous long-term
compression (Adams 1965; Kogure, 1993)".
Finally, Mesri and Ajlouni (2007) concluded that in spite of the significantly
different engineering properties of fibrous peat from those of most inorganic
soils, the same fundamental mechanisms and factors control the behavior
of both inorganic soils and fibrous peats. The same was concluded by
Magnan (1994) based on French practice.
111
That is, predicting and dealing with settlements of organic soils has been a
problem for highway and foundation engineers. The prediction of field
settlement from laboratory test on undisturbed samples is essentially a direct
extrapolation of soil behavior. It is based on the assumption of linear rate of
settlement with the logarithm of time. However, the prediction of settlement
remains difficult owing to the heterogeneity of organic deposits, the effect of
the underlying mineral soil layers, shear strains, and gas content.
Most peat deposits are highly variable (Hanrhan 1954; Landva and La Rochelle
1983). This characteristic, related mainly to variable degree of decomposition
within a peat deposit, has been a serious impediment to accurate interpretation
of peat behavior from laboratory measurements and field observations [e.g.,
Magnan (1994)]. The other obstacle to laboratory testing of peats and
interpretation of laboratory measurements has been potential for biodegradation
of peat in laboratory environment (Mesri et al., 1997). Also, in many organic
deposits, the underlying soil may be more dangerous from the standard point of
stability and settlement than the organic soil. Seldom is the compressible soil
layer only peat. Generally it also contains organic clay, clay and/or marl, which
may not drain as quickly as peat (McVay &Nugyen, 2004). Also, the
submerged organic component of organic soil is not entirely inert but
undergoes very slow decomposition, accompanied by the production of
methane and less amount of nitrogen and carbon dioxide and hydrogen sulfide.
A gas content of 5 to 10% of the total volume of the soil is reported for peat
and organic soils (Muskeg Engineering Handbook, 1969).
Considering these factors and the empirical nature of the relations used, one
must realize that the predictions of field settlements will be extremely
approximate (Kogure, 1999). Nevertheless, Mesri and Ajlouni (2007)
concluded that compression parameters determined from laboratory oedometer
tests and from field settlement observations are comparable [e.g., Moran et al.,
112
1958; Adams 1963, 1965; MacFarlane, 1969; Samson and La Rochelle, 1972;
Jorgensen, 1987].
A review of compressibility behavior of cohesive soils, in terms of primary and
secondary compression, is presented including a focus on compressibility
behavior of organic soils. Different consolidation theories are explored. Those
suitable to describe the settlements of organic soils are examined in detail.
3.5
Soil Compressibility
In order to evaluate the suitability of a foundation or earth structure, it is
necessary to design against both bearing capacity failure and excessive
settlement. For foundations on cohesive soils, the principal design criterion is
to control the expected settlements within the limits considered tolerable for the
structure. As a result, once allowable foundation displacements have been
established, the estimate of total settlement over the service life of the structure
is a major factor in the choice of the foundation design.
During construction, surface loads from foundations or earth structures are
transmitted to the underlying soil profile. As a result, stresses increase within
the soil mass and the structure undergoes a time-dependent vertical settlement.
In general, this time-settlement can be represented conceptually as shown in
Figure (3.13). However, the total settlements are calculated as the sum of the
following three components:
S = Si + Sc + Ss
Where:
Si = immediate or distortion settlement,
Sc = consolidation or primary settlement, and
Ss = secondary compression settlement.
113
(3-2)
Fig. (3.13): Time-settlement curve showing total settlement components (Fox,
2003).
Immediate settlement is time-independent, results from elastic distortion of
solid particles, shear strains and compression of gases contained within the soil
skeleton, that occur at constant volume as the load is applied to the soil.
Although this settlement component is not elastic, it is generally calculated
using elastic theory.
Both consolidation and secondary compression settlement components are
time-dependant and result from a reduction of void ratio and concurrent
expulsion of water from the voids of the soil skeleton. For consolidation
settlement, the rate of void ratio reduction is controlled by the rate at which
water can escape from the soil. Therefore, during consolidation, pore water
pressure exceeds the steady state condition throughout the depth of the layer.
Over time, the rate of consolidation settlement continuously decreases as
effective stresses increase to approach their equilibrium values. Once the
consolidation process is completed at time tp, settlement continues in the form
of secondary compression. During secondary compression, the rate of void
ratio reduction is controlled by the rate of compression of the soil skeleton
itself. As such, it is essentially a creep phenomenon that occurs at constant
vertical effective stress and without sensible excess pressure in the pore water.
114
The time-settlement relationship, above mentioned, is conceptually valid for all
soil types. However, large differences exist in the magnitude of the components
and the rate at which they occur for different soils. For granular soils, such as
sand, the hydraulic conductivity is sufficiently large that consolidation occurs
nearly instantaneously with the applied load. In addition, although granular
soils do exhibit creep effects, secondary compression is generally insignificant.
On the other hand, for cohesive soils, such as clays, hydraulic conductivity is
very small and the consolidation of a thick deposit may require years or even
decades to complete. Also, secondary compression can be substantial for
cohesive soils. Different from both sands and clays, peats and organic soils
generally undergo rapid and large consolidation settlement and extensive longterm secondary compression (Fox, 2003).
As early as 1923, Terzaghi designed the consolidometer, today known as the
consolidation apparatus or oedometer. Since then, most of the methods to
predict compressibility of soil and to evaluate the soil parameters required for
settlement calculation are obtained from the laboratory consolidation tests.
However, the immediate data from a consolidation test are presented in the
form of settlement versus time. The settlement dial readings are converted by
computations to either void ratio (e) or strain (ε), and plots of e(εv)-log t and
e(εv)- log σ`v are made, then the laboratory-measured consolidation parameters
are interpreted as shown in Figure (3.14).
Fig. (3.14): Idealized relationship between void ratio e and logarithm of the
effective stress σ` (after Havel, 2004)
115
These consolidation parameters relate to primary settlement (coefficient of
axial compressibility, av, coefficient of volume compressibility, mv,
compression index, Cc, recompression index, Cr), secondary settlement
(secondary compression index, Cα), time rate of settlement (cv), and stress
history (σ`p). However, there may be differences in the magnitudes of various
quantities measured, for organic soils and peats, but the general shape of the
consolidation curves appear reasonably similar and the formulation developed
for clay compression can be used to predict the magnitude and rate of
settlement (Edil and den Haan, 1994, Edil, 1997; Mesri et al., 1997; Mesri and
Ajlouni, 2007).
3.5.1 Primary Consolidation
Terzaghi's theory (1921, 1923, and 1924) was developed for the prediction of
long-term settlement of structures. It was the first to describe and formulate the
primary consolidation process for cohesive materials (clays). This theory is a
mathematical expression that portrays the deformation of a porous media
accompanied by a flow of water, which fills the pores of the medium, i.e.
consolidation (McVay and Nugyen, 2004). Also, it considers that the
deformation of a layer of saturated soil is determined only by the rate of
seepage of pore water under the action of external load (Havel, 2004).
However, Terzaghi's theory is applicable to one-dimensional consolidation
with the following assumptions:
1. Soil is saturated and homogenous.
2. Solid and water are incompressible.
3. Compression and flow are one dimensional.
4. Linear relationship between stress and strain.
5. Darcy's law of laminar seepage is valid.
6. Coefficients of compressibility and permeability are constant within the
range of applied stress.
7. Total applied load is constant during the consolidation process.
116
8. Small strains.
Actually, the total amount of consolidation settlement and the rate at which this
settlement occurs is a coupled problem in which neither quantity can be
calculated independently from the other. In geotechnical engineering practice,
total consolidation settlement and rate of consolidation are almost always
computed independently for lack of widely accepted procedures to solve the
coupled problem (Fox, 2003).
Total Consolidation Settlement
Total one-dimensional consolidation settlement, Sc, results from a change in
void ratio, ∆e, over the depth of consolidating layer H. Referring to Figure
(3.15) and by proportion we may write
Fig. (3.15): Settlement of a soil sample or layer of soil of thickness H in situ
(after Bowles, 1984).
Sc
∆e
=
H 1 + eo
and
(
∆H
= ∆ε , strain)
H
From which the in-situ consolidation settlement numerically integrated over
depth H is
S c = H (∆ε ) = H
∆e
1 + eo
117
(3.3)
Once the in-situ compressibility curve e-log σ`v has been established for a given
sublayer, the change of void ratio can be calculated knowing ∆ σ`v
∆e = Cc log
σ v′ o + ∆σ v′
σ vo′
(3.4)
Now we substitute ∆e and obtain the settlement of normally consolidated clay
due to change of stress ∆ σ`v as:
Sc =
σ ′ + ∆σ v′
Cc
H log vo
σ vo′
1 + eo
(3.5)
Using the definition for ∆ε we obtain
Sc = Cc′ H log
σ vo′ + ∆σ v′
σ vo′
(3.6)
For an overconsolidated clay, if σ`vo+ σ`v < σ`p
Sc =
σ ′ + ∆σ v′
Cr
H log vo
1 + eo
σ vo′
(3.7)
And if σ`vo+ σ`v > σ`p
Sc =
σ`
Cr
σ ` + ∆σ `v
C
H log p + c H log vo
σ `vo 1 + eo
1 + eo
σ `p
(3.8)
The compression index, Cc, and recompression index, Cr, are index values
required for primary consolidation settlement predictions. These parameters
have been defined in Figure (2.10). Each of the compression indices can be
determined, from e-log σ`v curve, by the change in void ratio per log cycle of
stress (= ∆e/∆ log σ`v) for the respective ranges of recompression (Cr), virgin
118
compression (Cc), and swelling or rebound (Cs). In case of using a plot of
vertical strain, εv-log σ`v curve, the compression indices are reported as the
recompression ratio, C`r = Cr/(1+eo), and compression ratio, C`c = Cc/(1+eo).
Typically, values of Cr (or C`r) are 10 to 20 percent of the value of Cc (or C`c).
On the other hand, evaluation of Cc and Cr compression parameters can be
estimated by correlation. Numerous correlations relating simple soil
classification properties (e.g., LL, wo) to Cc and C`r are available in the
literature for silts and clays, and organic soils. These correlations can be used to
make first-order predictions of settlements, but should not be relied upon for
final design, unless the correlation has been developed using site-specific
laboratory consolidation test data. Several correlations that are based on
relatively large databases are provided in Table (3.1).
Table (3.1): Summary of correlations for Cc (modified from Bowles, 1984):
Equation
Applicable Soils
Cc = 0.009 (LL – 10)
Undisturbed clays of low to medium sensitivity
Cc = 0.007 (LL – 7)
Remolded clay
Cc = 0.0115 wo
Organic silt and clay, organic soils, peats
However, the standard procedure for consolidation test specified the load
increment ratio (LIR) of one and the load increment duration (LID) is 24 hour.
For some soft soils, especially organic soil and peat, the end of primary
consolidation can be reached at time much less than 24 hour. Thus, the
estimation of the compression index (Cc) for organic-rich soil, based on
standard consolidation test procedure, may not be accurate; since some creep or
secondary compression will took place before the application of the subsequent
pressure (Fox, 2003). Therefore, measurement of excess pore water pressure
119
during the consolidation test is very critical in the observation of the end of
primary consolidation (EOP).
The void ratio obtained from each load increment after 24 hour is plotted as
open points in Figure (3.16). The EOP consolidation can be determined from
the time-settlement curve by graphical procedures such as the Cassagrande
logarithmic of time or Taylor square root of time methods. Then difference
between the void ratio at the EOP consolidation (eop) and the void ratio at 24
hour is used as a correction applied to the original e- log σ`v curve. The
modified curve, EOP e-log σ` v, is plotted as solid line in Figure (3.16).
Fig. (3.16): Typical laboratory consolidation curve (Fox, 2003)
Moreover, for the compressibility curve in Figure (3.16), both the
preconsolidation pressure, σ`p and the in-situ initial vertical stress, σ`vo are
indicated. The characteristic e(εv)-log σ`v graphs show a change in slope at the
preconsolidation stress. This maximum preconsolidation stress delineates the
region of semi-elastic behavior (corresponding to overconsolidated states) from
the region of primarily plastic behavior (associated with normal consolidation)
as shown in Figure (3.14). The preconsolidation stress is normally interpreted
120
from the e(εv)-log σ`v relationship using the Casagrande (1936) graphical
technique.
Furthermore, the stress history of a soil layer is generally expressed by its
overconsolidation ratio (OCR), which is the ratio of these two values OCR =
σ`p /σ`vo. The OCR will not be a unique value for most soils since σ`vo = γz
increase with depth. However, the soil is normally consolidated if σ`vo ≈ σ`p,
with sampling disturbance causing the small discrepancies. If σ`p < σ`vo, the soil
can be underconsolidated. If σ`p > σ`vo, the soil may be taken as preconsolidated
or overconsolidated.
Rate of Consolidation
At any time during the process of consolidation, the amount of settlement is
directly related to the proportion of excess pore pressure that has been
dissipated. Terzaghi's theory of consolidation is used to predict the progress of
excess pore pressure dissipation as a function of time, i.e., prediction of
consolidation settlement rate. Based on the above mentioned assumptions and
using an analogy between the theory of consolidation and the theory of heat
transfer, Terzaghi proposed the pore pressure based differential equation for
one-dimensional consolidation is:
∂u
∂ 2u
= cv 2
∂t
∂z
Where:
k (1 + eo )
k
=
= Coefficient of consolidation.
av γ w
mv γ w
cv
=
k
= Hydraulic conductivity.
eo
= Initial void ratio.
av
= ∆e/ ∆σ` = Coefficient of compressibility.
γw
= Unit weight of pore-water.
121
(3.9)
av
= Coefficient of volume compressibility
1 + eo
mv
=
z
= Time-independent space coordinate (initial coordinate).
u
= Excess pore-water pressure.
t
= Time.
As shown in the equation, in order to solve for pore-water pressure u, the
composite parameter (cv) must be obtained. The parameter cv, is called the
vertical coefficient of consolidation because it contains the material properties
that govern the consolidation process (Holtz and Kovacs, 1981). However, the
vertical coefficient of consolidation, cv, is used for evaluating time rate of
settlement for shallow foundations and large aerial fills whereas the coefficient
of horizontal consolidation, ch, is used for estimating pore pressure dissipation
around driven piles and for designing wick drains.
It should be noted that cv is not a constant value for a test on a particular soil.
Values for cv depend on many factors including whether the preconsolidation
stress has been exceeded (Leonards and Girault, 1961). The typical trend for
most clayey soils is that cv values are higher in the overconsolidated range and
exhibit a relatively rapid decrease as the preconsolidation stress is approached.
Some of the inherent variability associated with evaluation of this parameter
can be minimized by concentrating the interpretation on values corresponding
to a reload cycle and to values associated with virgin compression.
Two curve-fitting methods are often used to determine the coefficient of
consolidation directly from laboratory deformation-time data: the logarithm-oftime proposed by Casagrande (1938) and the square-root-of-time proposed by
Taylor (1942).
Casagrande’s method uses the time to complete 50 percent primary
consolidation and evaluates cv according to:
122
cv =
0.197 H 2 dr
t 50
(3.10)
Where Hdr is the drainage height (equal to one-half the average thickness of the
oedometer test specimen for each load increment for a double drained
specimen) and t50 is the time required to achieve 50 percent primary
consolidation.
For the square root of time method, the time for 90 percent primary
consolidation, t90, is used and cv is calculated according to:
0.848H 2 dr
cv =
t 90
(3.11)
However, each of these methods is approximate and will result in different
calculated values for cv even though the same deformation-time data are used
for both methods. On the other hand, disturbed samples will likely result in an
overprediction of the time actually required for primary consolidation to be
completed in the field. In addition, laboratory tests only simulate vertical
drainage whereas most natural soil deposits have interbedded seams or layers
of more permeable material within the low permeability layer. These smaller
layers will permit lateral drainage as well, which will tend to decrease the time
required to complete primary consolidation.
Therefore, the coefficient of consolidation can be obtained most accurately in
the field using measured pore pressures from piezometers installed at several
depths within a clay layer. Alternatively, monitoring the actual time rate of
settlement in the early stages of loading can be used to assess the appropriate
value of cv and this value can be used to refine the predicted time to complete
primary consolidation.
123
After the initial and boundary conditions are specified, a closed-form solution
can be obtained through separation of variables (Holtz and Kovacs, 1981):
∞ ⎡
⎛
2
Z ⎞ − M 2Tv ⎤
⎟⎟e
u = ∆σ ∑ ⎢ sin ⎜⎜ M
⎥
M
H
N =0 ⎣
⎢
dr ⎠
⎝
⎦⎥
(3.12)
Where:
∆σ
= Stress increment.
M
= (2 N + 1)
Tv
=
cvt
2
H dr
π
2
- an integer =1, 2, 3, etc.
- Dimensionless time factor.
Z/Hdr = Dimensionless geometry parameter.
Hdr
= the longest drainage distance.
This solution is in reasonable agreement with laboratory test results for a wide
range of soils. Therefore, it is widely used in engineering practice to forecast
compression rates and pore-water pressure in clays.
According to McVay and Nugyen (2004), considerable variations between the
predicted and observed settlements in the field have been reported for some
clays and organic soils. The reasons for the departure from Terzaghi's theory
are often due to apparent failure to satisfy the assumptions of the theory, such
as: constant permeability, linear void ratio-effective stress relationship and
small strain in the field (e.g. McNabb, 1960; Schiffman and Gibson, 1964;
Mesri and Rokhsar, 1974; McVay et al., 1986). Experimental evidence
indicates that the coefficient of permeability decreases with decreasing void
ratio and that a nonlinear relationship between void ratio and effective stress
exists as represented by a straight line in a semi-log plot. It is obvious that the
error resulting from these oversimplified assumptions will be minimized if the
strain is small.
124
However, many researchers have tried to modify the Terzaghi's formulation
with some more realistic assumptions. Havel (2004) stated that "General theory
of the three-dimensional consolidation was proposed by Biot, in 1941. Biot
supposed that soil is a porous skeleton filled with pore fluid, where the porous
skeleton is assumed to be an isotropic elastic medium and pore fluid is
incompressible. According to the Terzaghi's classical theory, the strain-based
consolidation theory was developed by Janbu in 1960s ".
McVay and Nugyen (2004) stated that "Richart (1957) removed the small
strain assumption. Schiffman and Gibson (1964) took the variable permeability
and compressibility into account. Davis and Raymond (1965) derived an
equation based on the assumptions that the void ratio is linear with the
logarithm of effective stress and the decrease in permeability during the
consolidation process is proportional to the decrease in compressibility, that is,
coefficient of consolidation remains constant. Gibson, England and Hussey
(1967) derived an equation which allows finite strain with variable
permeability and compressibility".
A lot of different investigations were performed by many authors; there is no
enough space, in this thesis, to describe all of them. Briefly one can mention
study of the consolidation of clay and peat by Barden (1968, 1969), theory of
consolidation for amorphous and fibrous peat by Berry and Poskitt (1972),
work by Mesri and Rokhsar (1974) and Mesri and Choi (1985).
3.5.2 Secondary Compression
Soon after Terzaghi's work on hydrodynamic consolidation it was recognized
that the viscosity of the soil skeleton caused deviations from the ideal elastoplastic behavior (Den Haan, 1996). As early as 1936, Buisman describe the
creep phenomenon as a 'secondary' time effect occurs at constant stress, which
could be observed as straight line on the void ratio-logarithm of time curve.
This is the first attempt to capture the creep behavior for soils (McVay and
125
Nugyen, 2004). Also, he himself preferred to distinguish between 'direct'
effects and 'secular' effects, where the latter refers to compression continuing
for a large number of years (Den Haan, 1996). However, a significant amount
of research has been done in order to correctly address and simulate creep
phenomenon for geomaterials, and to incorporate the intrinsic time effects into
the consolidation theory.
Havel (2004) stated that "In 1940, Taylor and Merchant re-examined the
classical theory and solved the problem of one-dimensional consolidation of
saturated clayey soils with the incorporation of the secondary consolidation, i.e.
prolonged deformation of the soil's skeleton. In 1942, Taylor introduced the socalled hypothesis B. This theory assumed that, for the given load step,
secondary consolidation occurs due to plastic resistance and appears only after
the completion of primary consolidation, which is influenced by the continuing
effect of the previous load step. In 1953, Florin published his theory of the
consolidation of porous creeping saturated soils, which assumed a simultaneous
action of seepage and skeleton creep from the beginning of consolidation,
proceeding in accordance with the linear theory of hereditary creep for aging
materials. This theory was later confirmed by several experimental studies by
Gibson and Lo (1961), Lo (1961) etc".
A lot of different investigations were performed by many authors, which
confirmed this phenomenon for clays and peats (MacFarlane, 1965; Barden,
1969; Ladd, 1971; Berry and Poskitt, 1972; Mesri, 1973; Mesri and Rokhsar,
1974; Ladd et al., 1977; Edil and Dhowian, 1979). The mechanism of
consolidation and secondary compression can be described in terms of a
continuous process of change in soil structure (Mesri, 1973). The amount of
secondary compression is relatively small for inorganic soils but is quite
significant for organic soils and may constitute more than 50% of the total
settlement (Edil and Dhowian, 1979). The exact mechanism of secondary
compression is not fully understood (McVay and Nugyen, 2004).
126
Barden (1969) reported that the different mechanisms responsible for
secondary consolidation can be classified under three types:
1. Terzaghi (1941) and Taylor (1942) attributed secondary compression to the
readjustment of grains. The basic assumption for this mechanism is that
when a soil element is loaded, the total stress is shared by pressure in the
free pore-water, the plastic resistance in the highly viscous absorbed water
(film bond) and the solid to solid contacts between soil particles (grain
bonds). During secondary compression, since the excess pore-water
pressure is negligible, the total stress is share by film and grain bonds. The
pressure from the film is gradually transferred to the grain bond, and this
transferring process is associated with very slow viscous flow. When the
equilibrium state is reached, grain bonds support the applied load only.
2. Tan (1958) believed that the secondary compression is due to the jumping
of bonds formed by soil particles. Soil particles form a network, which can
be described as a "card house" with water in the voids, and contacts
between solids are treated as mechanical linkages. When the soil element is
loaded, the links are broken in certain locations, but due to the attractive
forces, they can be formed again in another stronger and more stable
structural arrangement. The process of breaking and reforming of bonds is
called "jumping bonds".
3. Adams (1965) and De Jong (1968) proposed a theory for secondary
compression specifically for peat. They recognized that there were two
levels of structure (macro and micro) in a peat element and that the
consolidation resulted from the expulsion of water from both the macro and
micro pores. The primary consolidation is due to the dissipation of macropore water pressure, but the secondary compression is attributed to the
expelling of water from the micro-pores. Because of the permeability of the
micro-pores is much lower than that of the macro-pores, the process takes
much longer to finish.
127
Also, it is important to know that different definitions have been given to
describe the rate of secondary compression. The most popular are:
and
Cα =
∆e
∆ log(t )
(3.13)
C `α =
∆ε ver
∆ log(t )
(3.14)
The first is called the secondary compression index (Mesri, 1973). The second
is secondary compression ratio (Ladd, 1971). The second is the expression of
the first normalized with respect to void ratio, which converts it to strain.
According to McVay and Nugyen (2004), Buisman (1936) suggested that Cα is
a constant which means the settlement curve is a straight line for secondary
compression on a semi-logarithm time graph. Since then, many tests have been
devoted to study the influence of other factors on Cα. Some of those factors are
specimen
thickness,
load
increment
ratio,
consolidation
pressure,
precompression, load duration, temperature, etc. Unfortunately, because the
mechanism of secondary compression is not fully understood and the study of
Cα often requires long-term testing, disagreements regarding factors affecting
Cα are abundant in literature. Leonards and Ramiah (1959) and Lo (1961)
indicated that Cα is not a constant. Mesri and Goldlewski (1977) concluded that
Cα may increase, decrease or remain constant with time depending on the slope
of the e – log σ`v. Newland and Alley (1960) showed that Cα was not affected
neither by the specimen thickness nor load increment ratio. Similar results were
also observed by Mesri (1973) and by Raymond and Wahls (1976) for clays. In
contrast, work by Leonards and Girault (1961) for soft Mexico City clay and by
Barden (1969) for peats show opposite conclusions. In terms of temperature,
Gray (1936), Buisman (1936) and Lo (1961) reported that variation in
temperature has a predominant influence on both the rate and magnitude of
128
secondary consolidation, while Cα has been reported as independent of testing
temperature by Mesri (1973).
Mesri (1973) after describing and analyzing the mechanism of secondary
compression in terms of soil composition and environment, and studying the
possible influences of a number of factors on the magnitude and rate of
secondary compression, concluded that:
• The mechanism of consolidation and secondary compression can be
described in terms of a continuous process of change in soil structure.
• The coefficient of secondary compression, Cα, appears to be the most useful
parameter for describing the magnitude of secondary compression.
• The natural soils with large primary compressibility also show high
secondary compressibility. Highly sensitive clays also exhibit a high rate of
secondary compression.
• For most clay, in normally consolidation condition, Cα continuously
decreases with consolidation pressure. Cα, first increases with increasing
recompression stress, in the range of stress less than σ`p, and reaches a
maximum at a stress level just beyond the maximum σ`p, thereafter; it
merges with values for normally consolidated soil. Similar is the influence
of sustained loading.
• Remolding will generally decrease the rate of secondary compression.
• Cα is independent of pressure increment and pressure-increment ratio.
• Cα is independent of sample thickness.
• Cα for normally loaded and overconsolidated specimens is independent of
testing temperature.
• The major factor, which results in difference between Cα in the field and
laboratory, is the departure from the Ko-condition in the field.
Raymond and Wahls (1976) based on the work of Ladd (1971) and others
concluded the following assumptions must hold true (Sobhan, 2007):
• Cα is independent of time (at least during the time of interest).
129
• Cα is independent of the thickness of soil layer.
• Cα is independent of LIR, as long as some primary consolidation
occurs.
• The ratio Cα/Cc is approximately constant for normally consolidated
clays over the normal ranges of engineering stresses.
Also, since consolidation is a continuous process, therefore, the separation of
total strain into primary compression and secondary compression brings out an
interesting issue. Ladd et al. (1977) reviewed two hypothesis regarding soil
consolidation as illustrated in Figure (3.17). Hypothesis A assumes that creep
occurs only after End of Primary (EOP) consolidation, which implies that at
EOP every sample would have the same void ratio regardless of its thickness.
On the other hand, hypothesis B states that intrinsic time effect occurs during
the dissipation of pore water pressure. This leads to the difference in void ratio
at the EOP since thicker sample need more time to expel pore water (McVay
and Nugyen, 2004). The later would call for a new name for the secondary
constitutive phenomenon, and Buisman's 'secular effect' seems suitable (Den
Haan, 1996).
Fig. (3.17): Hypothesis A and B (after McVay and Nugyen, 2004)
130
Lee and Brawner (1963) supported hypothesis A with laboratory and field data.
This later was reconfirmed by Mesri and Choi (1985). However, Samson and
LaRochelle (1972) showed that the EOP void ratio in the field was smaller than
that in the laboratory sample which supported hypothesis B. Many researchers
(Leonards and Girault, 1961; Berry and Vickers, 1975; Hobbs, 1986; Kogure et
al., 1986; Olson, 1998; Robinson, 2003) have shown that both primary
consolidation and secondary compressions can take place simultaneously. So
far, no conclusive statement can be made about the validity of either A or
B (McVay and Nugyen, 2004).
Moreover, there are a lot of models that were used to simulate the consolidation
process for soils exhibiting secondary compression, as well as ones that have
been developed based on Rheological model specifically for peat (Gibson and
Lo, 1961; Barden, 1968; Berry and Poskitt, 1972). The most recent research on
modeling peat consolidation has been the work of Litus-Lan, (1992), Fox,
(1992), and den Haan, (1996).
However, in geotechnical design analyses, it is assumed that secondary
settlement occurs after primary consolidation is completed. Fortunately, for
organic-rich soils because the primary consolidation process usually is a quick,
the intrinsic effect, if it takes place during that period, can be ignored (McVay
and Nugyen, 2004). Moreover, secondary compression is time-dependent and
may continue for a very long time – perhaps hundreds of years. Therefore,
secondary settlement calculation will be concerned only with that occurs within
a lesser time span – the design life of structures.
That is, secondary compression index (Cα) can be obtained from the secondary
compression region of settlement versus log time curves. The settlement-time
curve for that sample loading closest to anticipated field loading should be used
for the secondary compression index. The secondary compression settlement in
an in-situ time increment ∆t for a stratum of thickness H is
131
Ss =
Cα
t
H log
tp
1 + eo
(3.15)
Where tp = duration of primary consolidation computed using theory of
consolidation and t = tp + ∆t. If a plot of εv-log t is used, the slope of secondary
compression branch would be C`α and similarly, the secondary compression is
S s = C `α H log
t
tp
(3.16)
3.5.3 The Cα/Cc Concept
While exploring the compressive behavior of soils, the Cα/Cc law of
compressibility must be examined, where Cα is the secondary compression
index and Cc is the compression index as shown in Figure (3.18). Mesri and
Goldlewski (1977) postulated that:
• For any natural soil, a unique relationship exists between secondary
compression index Cα = ∆e/∆ log t and compression index Cc = ∆e/∆log σ`v.
• The relation between Cα and Cc holds true at any time, effective stress, and
void ratio during the secondary compression. By using this result and a
simple empirical procedure the variation of Cα with time, at any effective
stress, can be predicted.
• Depending on the shape of the e - log σ` curve, Cα remains constant,
decreases or increases with time, in the range of σ`v at which Cc remains
constant, decreases or increases with σ`v respectively.
• For a variety of natural soils the value of Cα/Cc are in the range of 0.0250.10.
According to the concept of compressibility, a single value of Cα/Cc together
with the end-of-primary (EOP) e versus log σ`v relationship define secondary
compression behavior at all values of σ`v in recompression and compression
and throughout the secondary compression stage, where at any instant (e, σ`v ,
132
t); Cα = ∂e ⁄ ∂logt; Cc= ∂e/∂ log σ`v; e = void ratio; t = time; and σ`v = effective
vertical stress. The graphical evaluation of Cα and Cc is described in detail by
Mesri and Castro (1987).
Fig. (3.18): e-log σ`-log t Plot showing relationship between Cα and Cc during
secondary compression (after Mesri and Goldlewski, 1977)
In a recent technical note, Mesri and Vardhanabhuti (2005) stated that reliable
laboratory and field observations of secondary compression of soils suggest
that:
• The secondary compression index Cα may remain constant over a long
period of time, such as the design life of most structure, and still result in
reasonable magnitudes of secondary settlement.
• In almost all cases of observed secondary compression behavior ∆e/∆t (and
therefore, ∆S/∆t) gradually decreases with time.
• There is no logical reason to expect Cα or ∆e/∆t to become zero. In other
words, under constant external conditions and constant soil composition,
there is no logical reason to expect a "final" or "ultimate" settlement.
• The values of Cα/Cc for each soil type are in a very narrow range (Table
3.2). The total range of values of Cα/Cc for all soils is in the range of 0.01 to
0.07.
133
Table (3.2): Values of Cα/Cc for Geotechnical Material:
Cα/Cc
Material
3.6
Granular soils including rockfill
0.02 ± 0.01
Shale and mudstone
0.03 ± 0.01
Inorganic clays and silts
0.04 ± 0.01
Organic clays and silts
0.05 ± 0.01
Fibrous and amorphous peats
0.06 ± 0.01
Factors Affecting Compressibility of Organic Soils
The compressibility of any soil plays an important role in the variation of the
soil strength and deformation characteristics with time. In general, the actual
compressibility of any soil will depend on the structural arrangement of the
solid particles, i.e. in situ void ratio, nature and arrangement of soil particles,
and in the case of some soils, interparticle chemical bonding. However, the
solid phase of peat and organic soil consists of two components: organic matter
(fibers and granules) and inorganic earth materials, such that the soil structure
is an arrangement of these structural elements. It is the relative proportion of
these components and their specific nature that determine the compressibility
behavior of these soils (Edil, 1997). On the other hand, natural void ratio of
organic soils is generally higher than that of inorganic soils; with fibrous peat
having the greater void ratios. Such high void ratios give rise to phenomenally
high water contents; the most distinctive characteristic of organic soils.
MacFarlane and Radforth (1965) describe two extreme structural types of
organic-rich soil and state that all gradations exist between them. These are:
amorphous granular in which the soil particles are mainly of colloidal size and
the majority of the pore water is adsorbed around the grain structure, and
fibrous, which has an essentially open structure with interstices filled with a
secondary structural arrangement of non-woody fine fibrous peat. Most of the
water occurs as free water rather than viscous adsorbed water.
134
According to Mesri and Ajlouni (2007), fibrous peat particles have a hollow
cellular structure largely full of water; one-third to two-thirds of water content
of fibrous peats is within the particles (Ohira, 1977; Landva and Pheeny, 1980).
The highly perforated peat particles are very permeable, very compressible, and
very bendable. Also, fibrous peat particles, which consist of fragments of long
stems, thin leaves, rootlets, cell walls, and fibers, often are quite large. Stem
diameters of 20 to 500 µm, leaf thicknesses of 10 to 15 µm, and width and
length of 100 to 1,200 µm are common ( Landva and Pheeney 1980; Landava
and La Rochelle 1983 ).
On the other hand, they concluded that the difference between the biochemical
composition of amorphous and fibrous peat particles is not a significant factor,
since the amorphous peats are the product of biochemical decomposition and
breakdown of fibrous peats and other plant remains, in addition to a significant
amount of inorganic matter. However, the organic grains of amorphous peats
are smaller and more or less equidimensional [e.g., Ng and Eischens (1983)].
As compared to fibrous peat deposits, the amorphous peat fabric is likely to
exist at lower void ratios and to display lower permeability anisotropy and
lower compressibility.
Therefore, the settlement behavior of organic-rich soil is influenced by many
factors. These are as follows:
1. Texture:
The texture (amorphous-granular, fibrous) has a major effect on the
compressibility behavior of organic soil. Amorphous-granular soil compresses
in a different manner than fibrous soil. The compressibility behavior of
amorphous granular soil (amorphous peat and muck) is known to be similar
with clay soil, while the behavior of fibrous peat is different from mineral soil
because of different phase properties and microstructure (Edil, 2003). Berry
and Poskitt (1972) also realized a difference in compressibility between
135
amorphous granular and fibrous peat. In developing a theory of consolidation
for peat, they found it necessary to derive two separate models; one theory for
amorphous granular and a different one for fibrous peat. The two different
mechanism of compressibility behavior will be illustrated in the following
sections.
Taking into account the highly viscous water adsorbed around the soil particles,
amorphous granular peat exhibits a plastic structural resistance to compression,
and thus shows a similar rheological behavior to clay soils. It would seem
reasonable, therefore, to expect the Terzaghi (1941)-Taylor (1942) concept of
secondary compression of clays also to apply to amorphous granular peat. That
is, the nature of secondary compression is considered to be due to the gradual
readjustment of the soil grains into a more stable configuration following the
rupture of the soil structure that occurs during the primary stage. The rate at
which this process takes place is controlled by the highly viscous adsorbed
water surrounding each soil particle.
On the other hand, based on recognizing two levels of structure, namely, a
macro- and micro-pore network, Adams (1963) introduced a reasonable
description for consolidation mechanism of fibrous peat. On the basis of this
concept, fibrous peat may be regarded as a system of coarse channels between
particles (macro-pores), with the organic material itself (fibers) contains a
system of compressible very fine channels network (micro-pores). Primary
consolidation involves the dissipation of pore-water pressure in the macropores (i.e. the dissipation of excess macro-pore water pressure) and secondary
compression stage to be due to the very slow drainage of water from the micropore to the macro-pore system (i.e. the dissipation of the excess pore pressure
in the peat structure-see Figure 3.19), as stress is transferred to the fibers (Berry
and Poskitt, 1972). Both stages initially occur simultaneously with the micropores taking much longer to compress. Dhowian and Edil (1980) suggested this
might be a valid concept for fibrous peat (McVay and Nugyen, 2004).
136
Fig. (3.19): Perry and Poskitt (1972) model for Fibrous peat based on the
concept of micro and macro pores.
It may be inferred, therefore, that fibrous peat has higher void ratio and
permeability, and consolidation proceeds more rapidly than that for humified
amorphous-granular soil (Karesniomi, 1972). Furthermore, the fine-fibrous
peat will show high compressibility than the coarse-fibrous peat (Kogure,
1999). On the other hand, as compared to fibrous peat deposits, the amorphous
peat will display lower compressibility (Mesri and Ajlouni, 2007) as shown in
Figure (3.20).
Fig. (3.20): Experimental void ratio-log effective pressure and void ratio-log
permeability for amorphous granular and fibrous peat (after Berry and Poskitt,
1972)
137
2. Permeability:
In its natural state, organic-rich soils have a high porosity and are therefore,
pervious. For this reason, the initial compression of organic-rich soils occurs
rapidly. The permeability of organic-rich soils is rapidly reduced as
compression proceeds. Even under moderate compressive loading, the
permeability reduction can be several orders of magnitude (see Fig. 3.20).
Thus, permeability is strongly influenced by compressibility, with permeability
dictating the rate at which water can be expelled from the peat (Wilson, 1963).
3. Fiber content and orientation:
Fiber content appears to be a major compositional factor in determining the
way in which peat soils behave (Dhowian and Edil, 1980). The higher the fiber
content, the more the peat will differ from an inorganic soil in its behavior. On
the other hand, fiber orientation is identified as a dominant factor in the
structure of fibrous peat. The application of consolidation pressure may induce
a rearrangement of fiber orientation and drastically reduces the voids, causing a
significant reduction in the vertical permeability. In order to develop a visual
appreciation of the fiber content and orientation, the microstructure of the peat
would be examined under a Scanning Electron Microscope (SEM) as shown in
Figure (3.21).
4. Organic content:
The organic content has a considerable effect on the compressibility. In
general, the greater the organic content the greater the water content, the void
ratio and the compressibility (Noto, 1991).
5. Ash content:
It has been found that mineral soil content is inversely proportional to
compressibility (Anderson and Hemstock, 1959). In addition, Rutledge and
Johnson (1958) found the rate of creep decreased with increasing mineral
content (McVay and Nugyen, 2004).
138
Fig. (3.21): Scanning Electron Microphotographs (SEM) of Middleton peat: (a)
Horizontal section x 40; (b) Horizontal section x 200; (c) Vertical section x
250; (d) Vertical section after compression under 766 KPa x 250; (e)
Transverse section of fiber x 800; (f) Longitudinal section of fiber x 1,700
(after Mesri et al., 1997)
139
6. Water content:
Water content is directly proportional to compressibility of organic soils and
peats as shown in Figure (3.22) introduced by Mesri et al., (1997). However,
the water content varies widely and may account for a range between 75% and
95% of volume (Kogure, 1999).
Fig (3.22): Values of natural water content and compression index for peats,
clays and silts (after Mesri et al., 1997).
7. Gas content:
Gas content affects the compressibility by reducing the area through which
water can flow. Thus, gas content varies inversely with compressibility
(McVay and Nugyen, 2004). In consolidation test, large initial compression and
an indistinct behavior of primary consolidation in time-compression relation
reflect the presence of gas. Most peats have about 5% to 10% gas (Moran et al.
1958, Lea and Brawner, 1963). This gas is a combination of entrapped air and
gas generated by organic decomposition.
140
3.7
Distinct Compressibility Characteristics of Organic Soils
Of interest is the distinct compressibility characteristics of organic soils; both
short term related to the dissipation of excess pore pressure, as well as long
term creep associated with either compression or rearrangement of the organic
particles. In the following, the distinct compressibility characteristics of organic
soils are outlined, in terms of one-dimensional oedometer testing, compression
curves, and compression parameters. Problems related to one-dimensional
consolidation testing of organic soils and analyzing compression curves will be
discussed in details.
3.7.1 One-Dimensional Oedometer Testing of Organic Soils
As with mineral soils, silt and clay, the settlement parameters of organic soils,
also, can be determined from one-dimensional consolidation tests on
undisturbed samples of these soils. High quality undisturbed samples using
block sampling, thin-walled tubes, piston samplers, or other special samplers
are required for laboratory consolidation tests. However, test performed on
disturbed samples will result in computed consolidation parameters that may
result in conservative or unconservative design estimations.
Several test methods have been used to study the compressibility characteristics
of orgaic soils. The most popular one is the conventional incremental loading
(IL) oedometer test. Typically, laboratory multiple-stage-load (MSL)
oedometer tests are performed for a load-increment ratio (LIR) of unity and
load-increment duration (LID) of 24 hours. Among the advantages of
oedometer test is the relatively small size of specimen. The small specimen size
gives a reasonable consolidation time and the test can be extended to observe
the secondary compression. Many researchers reported that the test results
provide a reasonable estimate of the amount of settlement for structures or
embankments founded on organic deposits (Samson and La Rochelle, 1972;
Lefebvre et al., 1984).
141
Alternatively, constant-rate-of-strain (CRS) tests are conducted to obtain
equivalent information as generated by the MSL tests. In the constant rate of
strain (CRS) test, the loading applies, at fixed rate of strain έv, continuously
while measuring stress and pore pressures by transducers, thereby reducing
testing times from 1 to 2 weeks by IL oedometer to say 1 day by the CRS
consolidomter. While minimizing the testing time duration, information on the
creep behavior cannot be derived from the CRS tests. However, the CRS
consolidation test requires equipment that is not normally available in
laboratories. A discussion of the CRS and other consolidation test methods are
described in Head (1986).
In general, one-dimensional loading of an organic soil results in a volume
change caused by expulsion of water from the pores within the soil mass. If gas
is present, it will be expelled and/or compressed. The soil particles will fill the
space created by escaping water and gas, and solid constituents will continue to
adjust their relative positions during compression. In fibrous peat, during
compression, the fibers are reduced in size, rearranged and reoriented, with
faces normal to the applied load (McVay &Nugyen, 2004).
Also, It is usually observed that the expulsion of water and gas occurs
relatively quickly compare to mineral soils; for highly organic materials,
primary consolidation may be completed in less than a minute in the early
increments (Samson and La Rochelle, 1972; Mesri et al., 1997, Santagata et al.,
2008). On the other hand, the final stages of compression may occur over long
period of time. This is because the permeability of organic soil decreases
drastically during compression due to large void ratio change as mentioned
above, and there is creep associated with particle realignment and compression.
However, the analysis of compression behavior of such materials presents
certain difficulties which will be discussed subsequently.
142
3.7.1.1 Problems Related to Conventional Test
Although more sophisticated consolidation tests are now available, oedometer
test is still recognized as the standard test for determining the consolidation
characteristics of soil. Advantages and disadvantages of oedometer test are
outlined by Head (1986). Some of the problems related to the conventional test
will be illustrated in the following:
• The rate of settlement is often underestimated, that is, the total settlement is
reached in a shorter time than that predicted from the test data. This is
largely due to the size of sample, which does not represent soil fabric and its
profound effect on drainage conditions (e.g. Lefebvre et al., 1984).
• The drainage in oedometer test is entirely vertical. As some soils are
strongly anisotropic, their properties, particularly drainage, are very
different in horizontal and vertical direction. Therefore, soil samples should
be tested in the vertical and horizontal directions to properly assess
anisotropy effects (e.g. Mesri et al, 1997).
• Besides the natural condition of the sample, sampling disturbance will have
a more pronounced effect on the results of the test done on small samples.
Therefore, specimen preparation should be executed with minimum
disturbance (e.g. Germaine, 2003).
• Drainage starts as soon as the load is applied. A uniform pore pressure may
not be developed throughout a sample, and the initial undrained
compression cannot be measured directly.
• There is no means of measuring excess pore water pressures, the dissipation
of which controls the consolidation process. Therefore the estimation of
compressibility is based solely on the change of height of the specimen.
• For standard test, the load is applied through a mechanical lever arm
system, thus measurement can be easily affected by sudden shock.
Excessive disturbance affects the e-log σ`v plot, gives low value of preconsolidation pressure and high coefficient of volume compressibility at
low stresses. Excessive disturbance also reduces the effect of secondary
compression which is a very important characteristic of soft soils.
143
• The boundary effect from the ring enhances the friction of the sample.
Friction reduces the compression during loading and reduces swelling
during unloading. Therefore, highly polished rings should be used with
suitable lubricant material.
• Highly organic soils (especially fibrous peats) are more prone to
decomposition during oedometer testing. Gas content and additional gas
generation also may complicate the interpretation of oedometer tests (Edil,
2003).
These shortcomings should be considered during testing of organic soils to
perfectly assure not affecting the test results.
3.7.1.2 Biodegradation of Peat in Laboratory Environment
According to Mesri and Ajlouni (2007), when peat is sampled out of the
ground and it has access to oxygen, comes in contact with water of pH higher
than in the ground, and is subjected to higher temperatures than those in the
ground, the rate of biochemical degradation is greatly enhanced by aerobic
microbial activities and anaerobic fungi and bacteria and peat solids are lost
(Clymo and Hayward 1982; Ingram 1983; Wardwell et al. 1983; Van der
Heijden et al. 1994). The loss of structural integrity of cell walls and cell
inclusions can be expected to increase compression during long secondary
consolidation stage in the laboratory. This has been a serious impediment to
sampling and laboratory testing of peats and interpretation of laboratory
measurements (Magnan 1994; Mesri et al. 1997). However, they recommended
that the problem is minimized when the test setup is de-aired,
nonbiodegradable filter paper is used at the ends of the specimen, and ambient
temperature is maintained constant at relatively low values.
144
3.7.2 Compression Curves
As with mineral soils, silt and clay, the consolidation parameters of organic
soils are interpreted from the traditional compression curves; e(εv)-log t and
EOP e(εv)-log σ`v plots. However, there may be differences in the magnitudes of
various quantities measured but the general shape of the consolidation curves
appears reasonably similar (Edil, 1997).
3.7.2.1 Time-Compression Curve - e (εv)-log t
Leonards and Girault (1961) classified the time-compression curve derived
from laboratory test on different types of cohesive soils into three types: Type
I, Type ІІ, and Type ІІІ as shown in Figure (3.23). Type I curves are
characterized by the Terzaghi’s theory, with a well-defined S-shaped curve in
the log time plot, and are obtained for large LIRs only (≥1). For very small
LIRs, Type ІІІ curves are obtained; while a transition curve of Type ІІ is
obtained for intermediate LIRs. They attributed this dependence of the shape of
the time-compression curves on the load increment ratio to the influence of
secondary compression (Robinson, 2003).
Fig. (3.23): Types of compression versus logarithm of time curve derived from
consolidation test (Leonards and Girault, 1962).
145
While the separation of primary and secondary compression from type I curve
is relatively simple because it follows that the secondary compression occurs at
a slower constant rate after the dissipation of excess pore water pressure, it is
very difficult to identify the end of primary consolidation (EOP) of Type ІІ and
Type ІІІ curves based on conventional curve-fitting procedures. Also, Type ІІ
and Type ІІІ curves are characterized by the rapid primary consolidation, and
have no constant Cα value.
McVay and Nugyen (2004) recently conducted a comprehensive laboratory and
field investigation involving organic soils ranged from organic silt and clay to
fibrous peat from Florida. They reported that settlement-time curves for organic
soils have distinct shapes depending on their organic content (OC) level. The
low OC soil (<25%) has the typical S-shaped curve, resemble that of Type I
curve, while the medium (25<OC<50%) and high (>50%) OCs soils have
settlement-time curves resemble that of Type IІ and show significant creep
effects. Moreover, the low OC soil has a clearly delineated breaking point at
the end of primary (EOP) consolidation, as well as a constant Cα. On the other
hand, the medium and high OCs soils have no well-defined end of primary
consolidation (EOP), or constant Cα value.
Also, based on a series of long-term laboratory consolidation tests on peat,
Dhowian and Edil (1980) reported that the strain-logarithm of time curve
consists of four components as shown in Figure (3.24):
• An instantaneous strain εi, which takes place immediately after the
application of a pressure increment. Instantaneous strain is due to elastic
compression and the result of shear deformation. Organic soils generally
contain 5% to 10% gas, which may contribute to the initial compression or
immediate compression and rebound if the load is totally removed (Landva
and LaRochelle, 1983). However, instantaneous strain usually is a result of
step loading which is not the case under field conditions (McVay and
Nugyen, 2004).
146
• A primary strain component εp, which occurs at relatively high rate and
continues for a short period of time to tp due to expulsion of water; and
generally accounts for 50% of the total settlement.
• Secondary strain component εs, which results from a linear increase of strain
with the logarithm of time for a number of log cycles of time until a time ts,
after which the time rate of compression increases significantly. Secondary
strain occurs under small to negligible excess pore water pressure.
• The term tertiary strain component εt, is introduced to designate the
increasing coefficient of secondary compression with time, which continues
indefinitely until the whole compression process ceases.
Fig. (3.24): Vertical strain versus logarithmic of time curve of fibrous peat for
one-dimensional consolidation (Dhowain and Edil, 1980)
That is, the curves obtained from the oedometer tests and the behavior
exhibited by such materials show some differences compared to the behavior of
silt and clay deposits. Therefore, the analysis of compression behavior of such
materials presents certain difficulties when the conventional methods are
applied. These difficulties are mainly concerned with:
147
• The end of primary (EOP), tp, is hard to detect due to the rapid and smooth
transition between primary and secondary compression (Dhowian and Edil,
1980; McVay and Nugyen, 2004). Typically Type ІІ curve could be
encountered in early load increments in the laboratory (Edil and den Haan,
1994; Mesri et al., 1997). Also, Type ІІI curve is expected for relatively
small pressure increments that follow sustained secondary compression
(Mesri et al., 1997).
• Consolidation tests indicated that the secondary compression rate is not
constant with time (Dhowian and Edil, 1980; Fox et al, 1992; Mesri et al.,
1997; Colleselli and Cortellazzo, 1998; McVay and Nugyen, 2004).
• There is possibility that secondary compression start before the dissipation
of excess PWP is completed (Leonards and Girault, 1961; Sridharan et al.,
1995; Robinson, 2003).
To overcome these problems different point of view were introduced. Mesri
and Godlewski (1977) ascertained that experimental and interpretational
problems arise when very small load increment ratios are used. Kogure (1999)
concluded that if the relationship between the settlement and the logarithm of
time is as shown in Figure (3.12), the primary compression should be computed
according to conventional practice. If a defined S curve is not obtained, the
primary compression should be taken at the point where the settlement first
becomes linear with logarithm of time. He also concluded that several
empirical approaches have been used with peat, which has met with some
degree of success (Shibata, 1983; Noto, 1987; Kogure and Matsuo, 1988).
McVay and Nugyen (2004) suggest that instead of representing consolidation
on logarithm of time, square root of time can be used in conjunct with Taylor's
method to determine EOP (tp). To the opposite, Edil et al. (1991) found large
differences between tp and eop determined with the Taylor method, for
Meddleton peat, and the same parameters evaluated in terms of pore pressure
148
dissipation. The same was concluded by Colleselli and Cortellazzo (1998) for
Italian peaty soil.
Also, the existence of tertiary compression was questioned, did it really exist in
the field or was it a laboratory effect? Samson (1985) reported an increase in
secondary compression rate, for embankment built over peat soils, after 8 years
from the end of construction. According to McVay and Nugyen (2004), the
existence of tertiary compression was reported by Candler and Chartres (1988)
under three trial embankments underlain by peaty soils. To the opposite, many
researchers (Edil and Dhowian, 1979; Hansbo, 1991; Fox and Edil, 1994)
argued that this part of compression can be neglected because it generally
started after the design life of structure (Yulindasari, 2006).
However, Mesri et al. (1997), based on a series of long-term laboratory
consolidation tests on Meddleton peat, concluded that for pressure-increment
ratios large enough to produce an inflection point in the εv versus log t curve;
the graphical construction of Casagrande (Casagrande and Fadum, 1940)
readily defines the end of primary consolidation stage, and that the pore-water
pressure measurements indicate that the Casagrande construction defines an
EOP consolidation almost identical to that indicated by excess pore-water
pressure measurements.
Also, they stated that "The 'tertiary creep' concept should be reconsidered in
light of tests result, which suggests that the laboratory favor termed 'tertiary
creep' by Fox et al. (1992) may actually represent biodegradation of peat in
uncontrolled laboratory environment." They concluded that unless proper
control over ambient conditions is exercised during exploring long-term
behavior of peat, such measurements during the secondary consolidation stage
may result in misleading interpretations of secondary compression behavior.
Later on, Fox et al. (1999) confirmed experimentally this conclusion.
149
Moreover, Mesri et al. (1997) concluded that the secondary compression
behavior of peat is completely explained and predicted by Cα/Cc law of
compressibility, and this was confirmed experimentally and from field
measurements (Lefebvre et al., 1984; Samson, 1985; Mesri, 1986, Lefebvre,
1986, Mesri et al., 1994b). This means that Cα may remains constant, decreases
or increases with time, in the range of σ`v at which Cc remains constant,
decreases or increases with σ`v respectively. However, Fox et al. (1999)
confirmed experimentally this conclusion.
Furthermore, to stress this fact, Mesri and Ajlouni (2007) illustrated two εv-logt
curves for Meddleton fibrous peat as shown in Figure (3.25). One specimens
was loaded to a consolidation pressure near the preconsolidation pressure σ`p;
its primary consolidation is completed in 2 min due to large cv in
recompression range, and Cα increases with time since Cc rapidly increasing
with effective vertical stress. The other specimen was loaded to the
compression range; its primary consolidation is completed in 45min due to
significant decrease in cv. Also, at this stress range Cc is constant with effective
vertical stress, and Cα remains contant with time.
3.7.2.2 The e (εv)-log σ`v Curve
Primary compression is commonly expressed in terms of end of primary (EOP)
e(εv)-log σ`v relationship. Within the normally consolidation region (NC) on the
curve, the slope is the compression index Cc, and within the overconsolidation
region on the curve, the slope is the recompression index Cr and the total
primary settlement can be calculated with Cr and Cc. However, compared with
mineral soils, the EOP e(εv)-log σ`v curve for organic soils is not a straight-line
in the normally consolidation region, but usually S-shape as shown in Figure
(3.26). Due to the S-shape of the compression curves, the compression index of
the soil changes significantly, over the stress range (Yamaguchi et al., 1985b;
Mesri et al., 1997; Kogure, 1999; Santagata et al., 2008).
150
Fig. (3.25):Examples of secondary compression behavior of Middleton peat at
a consolidation pressure corresponding to (a) σ`v/ σ`p=1; (b) σ`v/ σ`p=3.2 (after
Mesri and Ajlouni, 2007)
Fig. (3.26): EOP e versus log σ`v curves of 24 undisturbed specimens of
Middleton peat (after Mesri et al., 1997).
151
3.7.3 Compression Parameters of Organic Soils
As with mineral soils, silts and clays, the settlement parameters of organic
soils, also, can be determined from one-dimensional consolidation tests on
undisturbed samples of these soils. The e(εv)-log t curves are used to obtain the
rate of primary and secondary consolidation; coefficient of consolidation (cv)
and coefficient of secondary compression (Cα). On the other hand, semi-log
plots of EOP e(εv)-log σ`v allow an estimation of preconsolidation pressure
(σ`p), and also to obtain compression indexes; compression index (Cc),
recompression index (Cr), and swelling index (Cs). However, the formulation
developed for clay compression can be used to predict the magnitude and rate
of settlement (Edil, 1997, Mesri and Ajlouni, 2007).
Also, at the planning stage before a detailed subsurface investigation is carried
out, or for small scale construction projects, undisturbed sampling and
oedometer testing may not be available. In these cases the consolidation
parameters, for settlement analyses may be estimated from empirical
correlations (Moran et al. 1958; MacFarlane 1969; Kogure and Ohira 1977;
Kogure et al. 1986; Mesri et al. 1994a). Some empirical correlations, from
literature, will be illustrated.
3.7.3.1 Primary Consolidation
Most surficial organic deposits, however, have no significant loading history as
they are fairly recent deposits in waterlogged areas. Also, they are
characterized by their loose structure and high water content. As a result of
their loose state, they have high permeability. Therefore, organic deposits
generally undergo rapid and large consolidation settlement under small load
changes. The large magnitude and short duration of the primary consolidation
are the major departures from the consolidation behavior of mineral soil.
152
Preconsolidation Pressure σ`p:
Generally, desiccation, mechanical unloading, water table fluctuation, or aging
including secondary compression are all known to cause overconsolidation of
soil.
For surficial organic deposits, because of their typical locations, i.e., near the
surface, organic soils generally have small to medium maximum past pressures.
Landva and La Rochelle (1983) reported that a 2 m thick packed snow could
apply a pressure of 8 kPa. It is reported that the preconsolidation pressure, σ`p
of Quebec, Canada, is 0.03 to 0.08 kg/cm2, considerably smaller (Lefebvre et
al., 1984). Also, Kogure (1999) reported that σ`p obtained from the
consolidation tests, in Japan, is generally from 0.05 to 0.4 kg/cm2. Moreover,
sometimes cannot be detected from the results of consolidation test (Mesri et
al., 1997). On the other hand, buried organic deposits usually display slight to
moderate overconsolidation ratio (e.g. Mesri et al., 1997; Coutinho et al, 1998)
Kogure and Ohira (1977) developed an empirical correlation between σ`p and
in situ void ratio eo for surficial peat deposits. A correlation between σ`p and eo
for peats based on compression data of Middleton peat and James Bay peat
introduced by Mesri and Ajlouni, (2007), together with Kogure and Ohira
(1977) data, is shown in Figure (3.27).
153
Fig. (3.27): Relationship between preconsolidation pressure and in-situ void
ratio eo for peat deposit (Kogure and Ohira, 1977) as well as σ`v versus e
relationship resulting from compression of Meddleton and James Bay peats
(after Mesri and Ajlouni, 2007).
Compression Parameters (Cr, Cc, and Cs):
The compression index, Cc, for organic soils, depends on the consolidation
pressure and not a constant value (Yamaguchi et al., 1985b; Mesri et al., 1997;
Kogure, 1999; Santagata et al., 2008). Due to the S-shape of the compression
curves, the compression index changes significantly, over the stress range. In
the normally consolidation region, Cc first increases sharply, but then, beyond
approximately 2σ`p, it decreases continuously with increasing vertical effective
stress (Mesri et al., 1997). On the other hand, it is known that the compression
index of peat reduced by remolding and not a unique value for peat deposit
itself (Noto, 1991).
Kogure (1999) illustrates the relationship between instantaneous compression
index Cc = ∆e/∆logσ`v and the consolidation pressure σ`v for Aiko peat as
shown in Figure (3.28). It will be seen that the peak value of Cc appears in the
154
range from about 0.5 to 0.7 kg/cm2 of the applied consolidation pressure; in the
range from 2 to 4 times of the preconsolidation pressure, σ`p, measured. Also, it
will be seen that the variation of Cc with applied pressure is small for low
values of initial void ratio and most significant for large initial void ratios.
Therefore, the initial void ratio appears as the most significant factor such that
the compression index increasing regularly with the initial void ratio (Lefebvre
et al., 1984). However, Kogure concluded that if the e-log σ`v is not a straightline; the maximum slope of the curve is regarded Cc value of the organic soil.
Fig. (3.28): Relationship between Cc and consolidation pressure (after Kogure,
1999)
Also, the e-log σ`v curves, for organic-rich soils, show a steep slope indicating a
high value of compression index Cc. Published data on Cc ranges from 2.5 to 15
for peat, 1 to 9 for muck, and 0.1 to 1.6 for organic silt and clay, compared with
that of clay of only 0.2-0.8. Lefebvre et al. (1984) concluded that the variation
reflects not only the nonhomogeneity of the material but also the difficulty of
defining Cc on the e-log σ`v curve.
Moreover, according to Mesri and Ajlouni (2007), a direct relationship between
the compression index, Cc = ∆e⁄∆log σ`v, and natural water content should exist
for saturated soils because both are controlled by the composition and the
155
structure of the soil. Composition and structure control both the in-situ void
ratio at which a soil comes to equilibrium and the compressibility after the soil
structure yields at the preconsolidation pressure (Hanrahan 1954; Landva and
La Rochelle 1983).
Figure (3.29) is a plot of Cc evaluated from EOP e versus log σ`v relationship in
the consolidation pressure range of σ`p to 2σ`p versus natural water content wo.
It includes Cc data for Middleton peat, James Bay peat, and peats from the
literature, including from Canada, Japan and the United States, as well as for
reference, for soft clay and silt deposits (Mesri and Ajlouni, 2007). It appears
that as organic content (OC) increases, void ratio, water content, and Cc
increases. Howevr, it should be noted that when Cc is estimated from an
empirical correlation, rather than from undisturbed sampling and laboratory
testing, the EOP settlement of a peat sub-layer is often computed assuming that
the peat deposit is normally consolidated and young and is setting on the EOP
compression curve.
Fig. (3.29): Empirical correlation between compression index, Cc, and natural
water content, wo , for peats, organic soils as well as soft clay and silt deposits
(after Mesri and Ajlouni, 2007)
156
Furthermore, reported values for Cr/Cc for fibrous peat are in the range of 0.1 to
0.3 (Mesri and Ajlouni, 2007), while for amorphous organic soils are in the
range of 0.1 to 0.2 similar to that of silt and clay deposits. Also, the swelling
index, Cs = ∆e ⁄ ∆log σ`v, increases with overconsolidation ratio [OCR = σ`v
(max) ⁄ σ`v], and increases slightly with the decrease in σ`v (max) from which
unloading takes place (Yamaguchi et al., 1985b; Mesri et al., 1997).
Based on the above mentioned empirical correlations [(Figure (3.27) & Figure
(3.29)], Mesri and Ajlouni (2007) suggest that an appropriate equation for EOP
settlement of organic soil is
Sc =
Cc
C
σ `p
σ `vf
H o [ r log ` + log `
1 + eo
Cc
σ vo
σ p
]
(3.17)
Additionally, den Haan (1997) reported that the relationship between initial
void ratio (eo) and natural water content (wo) for organic soils could be
expressed by
⎧ w + 0.88 ⎫
eo = 30.65⎨ o
⎬
⎩ 1.12 ⎭
0.116
− 30
(3.18)
And wo is expressed as ratio.
Rate of Primary Consolidation (cv):
For cohesive soils, such as clays, hydraulic conductivity is very small and the
primary consolidation of a thick deposit may require years or even decades to
complete. It is well known that the Terzaghi coefficient of consolidation, cv, for
soft clay and silt deposits is more or less constant in the compression range
from σ`p to 5σ`p, and has values in the range of 0.5 to 5 m²⁄year; in the
recompression range it is 5 to 100 times (typically 10 times) larger (Terzahi et
al. 1996).
157
Compared with mineral soils, the duration of primary consolidation (tp) for
organic deposits is relatively short due to the high initial permeability. In most
field situations primary consolidation of peat is completed within a few weeks
or months. However, the coefficient of consolidation cv, for organic-rich soils,
varies as a function of stress level; it decreases with an increase in effective
stress above the apparent preconsolidation pressure (σ`p). Therefore, primary
consolidation of organic soils proceeds slower than is predicted using a
constant initial coefficient of consolidation (Mesri et al., 1997).
However, the log time method proposed by Casagrande is normally used to
evaluate cv for organic soils.
Also, it could be determined form CRS
oedometer tests since pore pressures are measured during the test. Figure (3.30)
illustrates a comparison of the coefficient of consolidation data for the three
previously mentioned soils; Boston blue soft inorganic clay, muck soil from
West Lafayette (Indiana), and Middleton fibrous peat (Wisconsin), by plotting
cv versus σ`v ⁄σ`p to account for different stress histories of the soils. The key
difference between the inorganic clay and the two organic soils is the change in
cv in the NC region, which reflects both changes in compressibility and in
permeability. For the two organic soils, cv markedly decreases with increasing
σ`v, while it is more or less constant for BBC (Santagata et al., 2008).
Fig. (3.30): Variation of the coefficient of consolidation with stress level:
comparison between muck data to data for inorganic clay (BBC) and Middleton
fibrous peat (after Santagata et al., 2008)
158
This can be observed in the tests on the intact natural soil, in which the duration
of primary consolidation tp varied from increment to increment: from less than
a minute in the early increments, to approximately a day at the maximum stress
(Santagata et al., 2008). Lea and Browner (1963) indicated a significant
decrease of coefficient of consolidation (cv) during application of pressure from
10 to 100 kPa. The significant reduction factor of 5-100 is attributed to the
reduction of permeability due to the appreciation of pressure. Also, Mesri and
Ajlouni, (2007) indicated that the values of cv at σ`p for fibrous peats are at 20
to 100 m²/year and decrease by a factor of 10 to 20 as consolidation pressure
increases to 5 σ`p. In the recompression range, the cv of fibrous peats can be as
high as 500 m²/year. However, this decrease is more marked in the soils which
have higher organic content (Farrell et al., 1994) as shown in Figure (3.31).
Fig. (3.31): Effective stress versus coefficient of consolidation cv (after Farrell
et al., 1994)
Moreover, Mesri et al., (1997) stated that "Primary consolidation of peat
proceeds according to a theory of consolidation that takes place into account
the decrease in both compressibility and permeability during a typical
consolidation pressure increment". The ratio Ck/Cc measures the decrease in kv
deriving from the increase in the vertical effective stress. This ratio falls in
range 0.3-0.5 for peat (Mesri et al., 1997), and 0.4-0.6 for muck (Santagata et
159
al., 2008) as compared to Ck/Cc =1.0 for a large number of soft clay and silt
deposits.
Furthermore, Mesri and Ajlouni, (2007) indicated that the dramatic decrease in
cv for organic-rich soil is expected due to the low values of Ck/Cc. This low
values mean that:
• A significant decrease in permeability (Mesri and Rokhsar, 1974) is
associated with any increase in effective stress in organic soil.
• During the consolidation process, the amount of water to be expelled
remains high (high Cc), while the flow channels that discharge the water
become rapidly constricted (low Ck).
According to Huat (2004), Oikawa and Igarashi (1997) propose the following
equations for calculating cv. The only input needed for these equations is the
natural water content, without having to do the consolidation test.
[
e f = 2.47 1 − 1 / exp(2.91 / p )
0.39
]w
[
0.85 1−1 / exp (1.85. p )0.45
]
o
(3.19)
⎡1.51
⎤ ⎡ eo + e f ⎤
1.12
log cv = ⎢
+ log( p − p o ) − 1.06 (3.20)
+ 20⎥ ⎢
⎥−
0.68
⎣ wo
⎦ ⎣ 2 ⎦ (wo − 0.21)
Where ef and eo are final and initial void ratios, wo is the water content, and p is
the consolidation pressure in kg/cm2, cv is in cm2/sec. Figure (3.32) shows the
plot of water content and void ratio from Oikawa and Igarashi.
160
Fig. (3.32): Relationship between initial water content and void ratio (Oikawa
and Igarashi, 1997)
3.7.3.2 Secondary Compression
One of the most striking differences in the compression behavior of organicrich soils as compared to mineral soils is the long-term secondary (creep)
compression, which appear to be an almost continuous process (Kogure, 1999).
Creep deformations are generally found as the more significant part of
consolidation settlement because their magnitude may account for as much as
half of the total settlement and their time rate are much slower than that of
primary consolidation. However, within the time of engineering interest,
secondary compression of organic-rich soils has no end.
Also, Mesri et al. (1997) have identified that creep compression of peat
deposits is more significant than that of other geotechnical material due to the
following three reasons:
• Peat deposits exist at very high natural water contents and void ratios
because plant matters constitute peat particles are light and hold a
considerable amount of water. Peat grains, plates, fibers, or elements are
light because the specific gravity of organic matter is relatively small (1.5 1.6), and because peat particles are porous. Because of high in-situ void
161
ratios, peat deposits display high values of compression index Cc, and Cα is
directly related to Cc then peat deposits display high Cα (see Table 3.2).
• Among all geotechnical materials, peats have the highest value of Cα/Cc.
The most detailed measurements and existing reliable data suggest a range
of Cα/Cc = 0.06 ± 0.01 for peat deposits. It appears that the peat deposits
with highly deformable organic particles display the highest values of
Cα/Cc, compared with granular soils of least deformable silicate particles
which display the lowest values (Cα/Cc = 0.02 ± 0.01).
• Duration for primary consolidation for peats is relatively short due to the
high initial permeability. The initial permeability of peat is typically 1001,000 times the initial permeability of soft clay and silt deposits and the
initial coefficient of consolidation of peat, cv is 10-100 times larger. Hence,
secondary compression is significant in peat because it appears soon after
the completion of construction loading.
Moreover, Kogure (1999) illustrates the relationships between Cα and the
consolidation pressure σ`v, for some Japanese organic-rich soils, in which the
preconsolidation pressure σ`p, is from 0.4 to 0.5 kg/cm2 as shown in Figure
(3.33). It will be seen that the peak value of Cα in organic-rich soils appears at
about 2 times of σ`p, and the rate of secondary compression is influenced by the
applied load. The value of Cα increases with pressure at small consolidation
pressure, but decreases or almost remains unchanged past some pressure. Also,
for the same soils, the relationship between Cα and compression index Cc is
shown in Figure (3.34). It will be seen that Cα is proportion to Cc and the
relation is as follow: Cα = 0.065 Cc, within the range reported for peat soil
(Table 3.2).
162
Fig. (3.33): Relationship between Cα and σ`v (after Kogure, 1999)
Fig. (3.34): Relationship between Cα and Cc (after Kogure, 1999)
However, an important aspect of Cα/Cc law of compressibility is the very
narrow range of values of Cα/Cc for all geotechnical materials. The magnitude
of Cα/Cc appears to depend on the compressibility and deformability of the soil
particles (Mesri and Ajlouni, 2007). On average, this value for organic clays
and silts is Cα/Cc = 0.05±0.01. For peats, the value averages 0.06±0.01. These
163
values may be used to assess actual values from laboratory tests or for
preliminary analyses.
In geotechnical design analyses, it is assumed that secondary settlement occurs
after primary consolidation is completed. According to Mesri and Ajlouni
(2007), when Cc is constant with consolidation pressure near σ`vf and therefore
Cα is constant with time, secondary settlement is computed using
Ss =
Cc Cα
t
H o log
tp
1 + eo Cc
(3.21)
The tp is frequently estimated as the time at 95% primary consolidation, t95. In
most field situations involving fibrous peat σ`vf is in the compression range and
Cc is practically constant in the ∆e range associated with t⁄tp corresponding to
the design life, and Eq (3.21) is applicable for computing secondary settlement
(Buisman 1936; Lea and Brawner 1963; Weber 1969; MacFarlane 1969; Mesri
et al. 1997).
In the general case where Cc is not constant with consolidation pressure and Cα
is not constant with time, a simple graphical construction is used to compute
secondary settlement using the EOP e versus log σ`v together with Cα/Cc. An
example was introduced by Mesri et al. (1997) for Middleton peat in terms of
vertical strain, εv and is shown in Figure (3.35). In Cα/Cc, Cc = ∆e ⁄ ∆logσ`v
denotes the slope of the e versus log σ`v curve in recompression as well as the
compression range. At a number of points on εv versus log σ`v curve
corresponding to t⁄tp = 1 (i.e. EOP εv versus log σ`v), the slope Cc ⁄ (1+eo) is
calculated and is used together with Cα/Cc to compute Cα ⁄ (1+eo), which is the
vertical strain increment for one log time cycle of secondary compression, ∆εv.
At t⁄tp=10, primary plus secondary vertical strains is [εv]t=[εv]tp+∆εv. The
resulting (εv, σ`v) points are connected to obtain εv versus logσ`v at t⁄tp=10,
which is in turn used together with Cα/Cc to construct the εv versus log σ`v at
164
t⁄tp= σ`v 100, and so on (Mesri and Shahien, 1993). It has to be noted that Cc
denotes slopes of the e versus log σ`v relationship throughout both the
recompression and compression ranges.
Fig. (3.35): Secondary compression of Meddleton peat predicted by Cα/Cc
concept of compressibility (after Mesri et al., 1997)
Near the preconsolidation pressure, the slope of εv versus log σ`v, i.e., Cc/(1+
eo), significantly increases with the increase in σ`v. According to the Cα/Cc
concept of compressibility, because Cα/Cc is a constant Cα/(1+ eo) is expected
to significantly increase with time. A similar increase in Cα with time is
expected for relatively small pressure increments that follow sustained
secondary compression. When a soil is subjected to secondary compression, it
develops a preconsolidation pressure, σ`p, in the sense that upon reloading it
displays a recompression to compression response (Leonards and Ramiah
1959; Bjerrum 1967; Mesri and Choi 1979; Mesri 1993) as shown in Figure
(3.36).
165
Fig. (3.36): Secondary compression behavior of Meddleton peat for pressure
increment that ends near preconsolidation pressure resulting from secondary
compression aging (after Mesri et al., 1997)
A significant feature in Figure (3.36) is that εv versus log σ`v recompression to
compression curves, after sustained secondary compression aging (solid
curves), merge with εv versus log σ`v curves of peat sample (dash curves), at
consolidation pressures higher than the critical pressure resulting from
secondary compression. A similar behavior for clays and silts has been
attributed to thixotropic hardening or other aging mechanisms taking place
together with secondary compression (Mesri and Castro 1987; Mesri 1993). An
important implication of this type of behavior is that considerable care is
required in interpreting Cα versus Cc data from oedometer tests in which
relatively small pressure increments are applied following sustained secondary
compression, because the initial part of postaging EOP e-log σ`v curve, and
therefore the value of Cc is not well defined.
3.8
Compressibility of Natural Organic Deposits
If a shallow foundation is to be constructed in and around an area with a nearsurface deposit of organic soil or peat, the layer will likely be excavated and
166
replaced prior to foundation construction. For embankments and foundations
that are constructed over organic soil or peat layer, primary settlement will
occur over a relatively short time (i.e., within a few days or months), and the
majority of the total settlements will result from the long-term secondary
compression of these soils. Therefore, secondary settlement will be the
dominant component of settlement during the design life of the structure and
should be evaluated as mentioned above.
The depositional and physical characteristics of an organic deposit can make
the estimation of settlement very difficult, mainly because of the variability
with organic. This variability can be traced partly to variations in porosity or
water content, and partly as suggested by Gautschi (1965), to the fabric and
structure of organic soil. The properties of organic soils such as natural water
content, acidity, degree of humification, fiber content, shear strength, and
compressibility is affected by the formation of organic deposit. This indicates
that in term of content, organic soils are different from one location to another
and detailed soil investigations need to be conducted for organic soils at a
particular site.
In many organic deposits, the underlying soil may be more dangerous from the
standard point of stability and settlement than the organic soil. Seldom is the
compressible soil layer only organic soil. In general, organic deposits consist of
layers of peat, organic clays and soft normally consolidated materials with
different consolidation characteristics, which may not drain as quickly as
organic soil. These materials may develop high excess pore pressures, resulting
in loss of effective stress and instability. Consequently, correct estimation of
time versus settlement behavior necessitates simultaneous estimates of
consolidation and creep for different materials.
167
CHAPTER 4
RESEARCH PROGRAM
4.1 Introduction
Predicting and dealing with settlements of organic soil has been a problem for
highway and foundation engineers. Therefore, the main objectives set forth for
this research were: (i) To expand knowledge of the various types of organic
soils, their classification systems, and their distribution in Egypt and around the
world; (ii) To recommend a suitable classification system of organic soil to be
used in Egypt; (iii) To determine physical, index, chemical and engineering
properties of the various types of organic soils found in Egypt and compare the
results with those found around the world; (iv) To correlate various types of
organic soils and their index and engineering properties; (v) To focus the study
toward evaluating compressibility characteristics of the organic soils found in
Egypt in order to devise suitable design parameters for settlement analysis; and
(vi) To assess the extent of problematic nature of organic soils in Egypt, in
term of compressibility, compared with those highly compressible surficial
deposits typically encountered all over the world. This chapter describes the
methodology used to achieve these objectives.
A comprehensive literature review was conducted to provide rationale of the
research. The development of peat lands and their morphological differences,
the various types of organic soils and their classification systems, and the
distribution of organic soils in Egypt and around the world were described. A
“Tentative ASTM Standard” proposed by the ASTM Subcommittee D18.18
(McVay and Nugyen, 2004), was recommended, in this research, as a suitable
classification system to be used in Egypt. The available data, in literature, on
physical, index and chemical properties of the various types of organic soils
were compiled and categorized, based on the recommended classification
system, to correlate various types of organic soils and their index and chemical
168
properties. The distinct engineering behaviors of the various types of organic
soils, namely in terms of permeability, shear strength and compressibility
characteristics were demonstrated and the available published data were
compiled and categorized for comparisons.
The background was used to develop the hypothesis adopted for this research:
that is, in spite of physical, index, and engineering properties of the various
types of organic soils are highly variable and significantly different from those
of inorganic soils, however, the same fundamental mechanisms and factors
determine the behavior and properties of both inorganic soils and organic soils
(Samson and La Rochelle, 1972; Lefebvre et al., 1984; Mesri et al, 1997; Mesri
and Ajlouni, 2007; Santagata, et al, 2008).
Based on the database of Geotechnical Encyclopedia of Egypt (2002), WestDelta is known to have the most extensive thick organic deposits found in
Egypt. Two sites of pre-planned primary schools, located at this area, were
chosen for representing organic soil’s behavior. One site located at Robaomaah
village – Mahmoudia - Bohira Governorate (geotechnical region No. 8) known
to have 2 layers of shallow and deep deposits, at depths 4.0-6.5 m and 10.014.0 m respectively. The other located at Ezbet El-Domyati – Motoubes - Kafr
-Elsheikh Governorate (geotechnical region No. 14) known to have a deep
deposit at depth 8.0-12.0 m.
Physical, index, and chemical properties such as natural moisture content,
organic content, initial void ratio, degree of saturation, specific gravity, unit
weight, particle size distribution, Atterberg limits, and acidity were determined
for the organic soil’s samples to establish the general characteristics of the soil.
The organic soil’s samples were classified based on organic content and
particle size distribution. X-Ray diffraction analysis was performed to identify
the different minerals constituting the inorganic portion of organic soils
encountered. Scanning Electron Microphotographs were taken to evaluate the
169
structural arrangement of the organic soils encountered. The test results were
compared with published data and correlations between different indexes
properties were investigated (Chapter 6).
Engineering characteristics evaluated in this research include permeability,
undrained shear strength, and compressibility. The focus of the research is to
evaluate the compressibility characteristics of organic soils encountered based
on data obtained from the results of incremental loading consolidation tests.
The test results were analyzed and compared with published data (Chapter 6).
Section 4.2 explains the sampling procedure implemented in this research.
Section 4.3 describes the preliminary tests carried out to obtain the physical,
index, and chemical properties and to classify the organic soil encountered.
Section 4.4 gives details on the protocol that was developed and followed to
determine the engineering properties and on the equipment and procedure used.
4.2 Sampling
Six boreholes were executed in Robaomaah primary school site and four
boreholes were executed in Ezbet El-Domyati primary school site, in order to
collect a sufficient number of undisturbed organic soil’s samples. All boreholes
were executed to a depth of 15.0 - 20.0 ms to explore the nature of upper and
lower clayey deposits.
Undisturbed samples were obtained using a specially-designed thick-walled
open-drive 100 mm diameter sampler. The idea of which was taken from the
Swedish Geotechnical Institute peat sampler well-known for recovering
undisturbed samples at depth with good results (section 2.7.1.2.3). The
designed sampler consists of a 101 mm inner diameter plastic tube of thickness
about 4.5 mm, threaded at both ends to take an outside tapered cutting shoe and
a sampler head as shown in Figure (4.1). The sampler cutting shoe has a
uniform internal diameter of 99.5 mm, and thus stepping out abruptly at the
170
junction of the shoe and sampler tube. The sampler, therefore, has a 31% area
ratio, outer clearance of 3.6%, and an inside clearance of 1.5%, which is
acceptable for a thick-walled open-drive sampler (ECP 202/5-2001). The
sampler head incorporates vents and a ball valve assembly to allow air or water
to leave the top of the tube as soil enters its base. The ball valve is also
intended to improve the sampler retention by preventing air or water reentering the top of the tube if the sample starts to slide out. The lengths of the
tube were 0.5 m and 0.9 m which dictates the length of sample recovered.
Drill rod
Water port
Sampler head
Pin
Ball valve
Threads
110
mm
Plastic tube
101
mm
Threads
Cutting shoe
99.5
mm
114
mm
Fig. (4.1): Specially-designed thick-walled open-drive 100 mm diameter
sampler.
171
The following is a summary of the sampling procedures implemented at each
borehole test location:
• The soil was excavated using Geomashina B-150 drilling rig and single
core-barrels 3.5-inch diameter, and disturbed soil samples were collected.
• The sampler needs a borehole of about 150 mm diameter. When the
sampling position was reached, the borehole was enlarged using 6-inch
flight auger, then flushed with bentonite mud and cleaned in order to
minimize the amount of disturbed material left in the base of the hole before
sampling.
• The sampler was lowered to the base of the hole and driven into the soil by
repeatedly lifting the SPT sliding hammer 10-15 cm and allowing it to fall.
The distance moved by the sampler head during the drive was recorded.
• The sampler was left in the soil for few minutes to increase the adhesion
between sample and plastic tube, then was rotated about 360° for 2 times in
order to shear the soil at its base, then pulled gently out of the soil.
• After extraction of the sampler, the cutting shoe and driving head were
carefully removed. A small quantity of soil was removed from either end of
the tube, and was kept as representative disturbed samples. The ends of the
sample were waxed, and the plastic tube was labeled as shown in Figure
(4.2a).
The three organic soil layers encountered were named after their specific
locations. The upper organic soil layer of Robaomaah-Mahmoudia was named
as “RU”, the lower one was named as “RL”, while the organic soil layer of
Ezbet El-Domyati-Motobes was named as “D”.
Thirty two undisturbed organic soils’ samples were collected from the two
sites. Nine undisturbed samples from the upper layer of Robaomaah primary
school site (RU), eight undisturbed samples from the lower layer (RL), and
fifteen undisturbed samples from Ezbet El-Domyati primary school site (D).
172
All samples were stored in wooden boxes (Figure 4.2b), separated from each
other by straw, and carefully transported to the laboratory. Storage of samples
was at room temperature, and the humidity was controlled by daily watering
the straw between samples. All tests involved in this study were done within
six months after sampling process in order to minimize the effect of
biodegradation. Also, twenty liters of groundwater were gathered from every
site for chemical analysis and to be used instead of distilled water in
consolidation test to keep up with the same environmental conditions as in
nature.
(a)
(b)
Fig. (4.2): (a) Labeled samples, (b) Wooden boxes and groundwater sample.
4.3 Preliminary Tests
Obtained samples were inspected visually, and preliminary laboratory tests
were performed to determine the physical, index, and chemical properties in
order to identify and classify the organic soil encountered and to compare the
results with compiled data. The Scanning Electron Micrograph (SEM) was
performed to evaluate the structural arrangement of organic soils encountered.
173
4.3.1 Physical and Index Properties
The water content of all soil samples was obtained in accordance with ECP
202/2 (2/2/2). Measurements of the loss on ignition (LOI) were conducted
following ASTM D2974 – 00 on all soil samples using muffle furnace shown
in Figure (4.3). The specific gravity (Gs) of representative soil samples from
every layer was determined following ASTM D854-02. The initial void ratio
(eo) was calculated based on the height of solids method for each of the
specimens used for consolidation tests. The unit weights (γ) also, were
calculated based on measured weights of known volumes for each of the
specimens used for engineering tests. The value of Atterberg limits for the
various soil samples were determined following ECP 202/2 (2/3, 2/4, 2/5). The
particle size distribution, of representative organic soil samples from every
layer, was obtained through a combination of wet sieving and hydrometer test
following ASTM D422.
Fig. (4.3): Muffle furnace and porcelain crucibles used for LOI determination.
174
4.3.2 Chemical Properties
The acidity of almost all organic soil samples was determined by pH meter
following ECP 202-01 (2/12/3A). Chemical analysis was conducted to
determine the injurious chemical compounds of groundwater and organic soils
encountered (pH, sulfates content and chlorides content) following ECP 202-01
(2/12/1, 2/12/2, 2/12/3A & 1/12/8). Three representative organic soil samples
from every stratum and one groundwater sample for each site were chemically
analyzed.
X-Ray diffraction analysis was performed to identify the different minerals
constituting the inorganic portion of organic soils encountered. X-ray
diffraction analysis was performed in the Faculty of Science-Tanta University
by a RIGAKU RAD-I X-ray diffractometer on bulk samples using smear-on
glass slide technique. X-Ray diffraction analysis was performed to characterize
the mineral composition for eight samples: two samples of each of RU and RL
stratum, and three samples of D stratum of different organic content, and for
one fired sample of RU stratum.
4.3.3 Scanning Electron Microscope
The Scanning Electron Microscope (SEM) was used to observe the differences
in fiber contents, pore spaces, and perforated plant structure in order to evaluate
the effect of fabric (microstructure) on the compressibility characteristics of
organic soil encountered. Scanning Electron Microscopy was performed in
Housing and Building National Research Center using FEI-inspect S SEM
(Figure 4.4). This model is characterized by Low vacuum mode which allows
the investigation of wet samples without any treatment of the test samples
(without coating). Scanning Electron Micrographs were taken for three
samples, one of each layer, of different organic content. The micrographs were
taken in vertical and horizontal directions before and after the consolidation
test. Also micrographs were taken at different magnifications ranged from 150
to 12,000.
175
Fig. (4.4): Scanning Electron Microscope (SEM) used for micrograph
production.
4.3.4 Recommended Classification System and Organic Soil Classification
In Egypt, organic soils have been ill-defined as subdivision of the very soft
problematic clayey soils (ECP 202/5-2001) and qualitatively classified based
on their fabric without differentiation between them based on their geotechnical
properties, or even their organic content. Therefre, in this research, the
classification system proposed by Jarret (1983), and followed by ASTM
Subcommittee D18.18 (Peats and Organic Soils) as a “Tentative ASTM
Standard”, was recommended to be used in Egypt as a standard definition
(Tables 2.8 & 2.10). This classification system distinguishes between various
types of organic soils based on their organic content, and differentiates between
various types of peats based on their index and chemical properties. It
correlates well various types of organic soils and their index and engineering
properties so that the described behavior can be related to the proper material.
It could be integrated with the USCS to bridge the gap between peat as purely
vegetable matter, and purely inorganic silts and clays. It was used, in this
research, to classify organic soil samples and as a basis for categorizing index
and engineering properties data of various types of organic soils.
176
4.4
Engineering Properties
Engineering characteristics evaluated in this research include undrained shear
strength, permeability, and compressibility. The test results were analyzed and
also compared with published data.
4.4.1 Shear Strength
Undrianed shear strength was determined using two methods:
i.
Pocket pentrometer (P.P.) was used, following ECP 202-01 (2/22) to
determine the undrained compressive strength. The P.P. was used at top and
bottom of the undisturbed samples just after extraction of the sample from
the ground, and also when sliced before any engineering test. The mean
value of all tests for every sample was recorded and then used to estimate
the undrained shear strength (Sup) of the undisturbed sample.
ii.
Unconfined compressive strength (qun) was determined in compression
device following ECP 202/2 (2/21) as shown in Figure (4.5), and then used
to estimate the undrained shear strength (Sun) of the undisturbed sample.
The undrained shear strength obtained by those two methods were compared
and correlated relative to each other. Also, the normalized undrained shear
strength relative to preconsolidation pressure (σ`p) was determined (Su/σ`p) for
both methods.
(a)
(b)
Fig. (4.5): a) Compression device used for unconfined compressive strength, b)
Tested samples.
177
4.4.2 Permeability
Six falling-head permeability measurements were carried out during the
secondary compression stage of IL oedometer tests (Tavenas et al, 1983; Mesri,
1997) using special consolidation cells as shown in Figure (4.6). The values of
the coefficient of permeability in the vertical direction, kv, were determined for
three samples that were cut with their axes parallel to the vertical direction. The
values of the coefficient of permeability in the horizontal direction, kh, were
determined, for the same three previous samples, for specimens that were cut
with their axes perpendicular to the vertical direction and from the following 10
cm of the previous specimen, in order to investigate an isotropy of
compressibility and permeability characteristics. The great advantage of such
tests lies in the ability of measuring, rapidly, the law of variation of k with the
void ratio (e) under increasing effective vertical stresses (Ck = ∆e/ log k).
Outlet
O-rings
Loading
cap
Cutting
ring
Load
Upper and lower
porous stones
Copper base
with grooves
Copper
cylinder
Flow
Fig. (4.6): Schematic of the special consolidation cell used for consolidation
and permeability tests, through the falling-head permeability test.
178
4.4.3 Compressibility
A comprehensive laboratory testing protocol was developed and followed to
achieve the main objectives of this research which were: (i) To conduct
laboratory consolidation tests on undisturbed soil samples for evaluating the
primary and secondary compression behavior; (ii) To estimate compression
parameters under simulated loading scenarios usually encountered; (iii) To
investigate the effect of different loading scenarios on the long-term behavior
of organic soils; (iv) To establish the unique Cα /Cc relationship for organic
soils in Egypt, so that the short term data can be used to predict the secondary
compression index for the calculation of long-term settlement in an effort to
avoid expensive and time-consuming laboratory tests in the future when
dealing with similar subsurface conditions; (v) To correlate the compression
parameters with easily determined index properties, so that may be used for
settlement analysis at the planning stage before a detailed subsurface
investigation is carried out; (vi) To assess the extent of problematic nature of
organic soils in Egypt, in term of compressibility, regarding the usual loading
scenarios encountered; (vii) To compare the results with published data.
A series of 24 incremental loading (IL) consolidation tests were performed
using a fixed-ring consolidometers and following ECP 202/2 (2/13). The
oedometer rings is about 50 mm in diameter and about 20 mm in height. A
series of 6 tabletop consolidometers with a 10 to 1 lever arm were used to apply
the loads. The laboratory testing program is detailed in the following section.
4.4.3.1 Testing Program
Eight IL tests were conducted on six different soil samples from different
depths within every stratum of the three organic soil stratums encountered; the
upper and lower stratums of Robaomaah-Mahmoudia (RU & RL) and the
stratum of Ezbet El-Domyati-Motobes (D). The eight samples were
incrementally loaded following different loading scenarios as follows (Table
4.1):
179
Table (4.1): Details of incremental loading scenarios of consolidation tests:
loaded to the end of primary
(EOP) consolidation tp
2
3
For the same previous sample,
specimen was cut with its axes
perpendicular to the vertical
direction, and incrementally
loaded to the end of primary
(EOP) consolidation tp, to assess
isotropy
Sample
was
incrementally
loaded to the end of primary
(EOP) consolidation tp, and then
the coefficient of permeability in
the vertical direction, kv, was
determined during the secondary
compression stage
Designation
Loading Scheme
(IL+FS)V
(IL+FS)H
0
10
20
(IL+FS)V
+Perm
V ertical Strain, εv ( % )
Test
Test description
No.
1 Sample was incrementally
30
40
50
60
70
4
5
For the same previous sample,
specimen was cut with its axes
perpendicular to the vertical
direction, and incrementally
loaded to the end of primary
(EOP) consolidation tp, and then
the coefficient of permeability in
the horizontal direction, kh, was
determined during the secondary
compression stage to assess
isotropy.
10
100
1000
10000
Vertical Effective Stress, σ'v ( kPa)
(IL+FS)H
+Perm
Sample
was
incrementally
loaded and allowed to undergo
secondary compression for one
day, for each load
IL+Sec
FS = for some increments, especially in NC region, the duration of secondary
stage was < 10 tp (Few Secondary).
180
Table (4.1): Details of incremental loading scenarios of consolidation tests
(cont'd):
Test
Test description
Designation
Loading Scheme
No.
6 Sample was incrementally
0
10
20
V ertical Strain, εv ( % )
loaded till σ`p then unloaded
incrementally to less than σ`vo
then reloaded incrementally to
the end of scheduled load then
incrementally unloaded again.
This loading scenario was used IL+FS+Loop
to accurately estimate the
recompression
index
and
swelling index
30
40
50
60
70
10
100
1000
10000
Vertical Effective Stress, σ'v ( kPa)
Sample
was
incrementally
loaded and allowed to undergo
secondary compression for one
day, for each load. One load in
recompression range, between
σ`vo and σ`p, was allowed to
undergo secondary compression
for one week to assess aging
effect in recompression
0
10
20
IL+Long S
V ertical Strain, εv ( % )
7
30
40
50
60
70
10
100
1000
10000
Vertical Effective Stress, σ'v ( kPa)
Sample
was
incrementally
loaded and allowed to undergo
secondary compression for one
day, for each load. One load in
compression range, between σ`p
and 2σ`p, was allowed to
undergo secondary compression
for one week to assess aging
effect in compression range
0
10
20
IL+Late LS
V ertical Strain, εv ( % )
8
30
40
50
60
70
10
100
1000
Vertical Effective Stress, σ'v ( kPa)
181
10000
4.4.3.2 Preparation of Samples
To prepare each specimen for consolidation test with minimum disturbance,
MIT procedure for obtaining test specimen from tube sample recommended by
Germaine (2003) was followed (Ladd and DeGroot, 2003), as illustrated in
Figure (4.7).
Sample preparation was carried out through the following steps:
1. A 3-inch middle-slice was cut from the plastic tube that contains the
undisturbed soil sample (Figure 4.8a). To do so, the plastic tube was sawed
perpendicular to their long axis using a fine-toothed hacksaw.
2. Pocket pentrometer was used to determine the unconfined compressive
strength for the two other separated parts of the undisturbed samples, and
then waxed and sealed.
3. To extrude the specimen with minimum disturbance, the cutted tube slice
was gently sawed parallel to their long axis using a fine-toothed hacksaw.
4. The soil was debonded from the plastic tube with a piano wire. Then, the
plastic tube was enlarged by hand and the specimen gently extruded by
hand to the specially-designed rotating trimming table, illustrated in Figure
(4.8b), which was covered by a paper towel and sprayed with water.
5. Prior to placing the specimen in the consolidation ring, the height, weight,
and diameter of the ring were measured and recorded, then lubricated inside
with a thin film of high-vacuum silicon grease.
6. The specimen was carefully trimmed on the rotational table using wire saw.
When a complete perimeter cut was made, the specimen was inserted into
the consolidation ring by the width of the cut, and repeated until the
specimen protrudes from the bottom of the ring.
7. The top and bottom of the specimen were finished flat by sharpened 12-inch
metal scale-straight edge.
8. The consolidation ring with specimen was weighed and recorded.
9. The trimmings were used to determine the natural moisture content and
organic content of the specimen.
182
10. The specimen was separated from the top and bottom porous disks by
Whatman No. 54 filter paper, and then assembled with capping head in the
consolidometer.
Fig. (4.7): MIT procedure for obtaining test specimen from tube sample
(Germaine, 2003)
The specially-designed rotating trimming table was used to facilitate the
trimming process, minimize the time needed for trimming, and minimize the
losses in natural water content which may lead to propagate of tension cracks
i.e. minimize disturbance. The table consists of two wooden plates; a circular
upper one of diameter 25 cm, and a square lower one of length 25 cm. The
wooden plates separated from each other by 12 cm diameter roller bearing such
that the upper circular plate could be rotated easily as trimming proceed.
183
(a)
(b)
Fig. (4.8): a) Tube sample cutting apparatus, b) The rotating trimming table.
4.4.3.3 Consolidation Test
Prior to testing the tabletop load device and consolidometer were adjusted and
calibrated. After assembling, the consolidometer was placed on the tabletop
load device and a seating pressure of 5 kPa was applied. Immediately after
application of the seating load, the initial zero reading was recorded, then the
specimen was inundated and left for minimum of one hour to fully saturate the
specimen. In inundation, filtered groundwater taken from each site was used to
minimize the biodegradation of the specimen by keeping the same acidity
environment as in the field. All tests were performed under double drainage
conditions except for the 6 tests in which falling-head permeability tests were
performed in the secondary compression stage; only single drainage condition
were valid during permeability tests.
Loads were then applied using the loading scheme detailed earlier. The test was
conducted with load increment ratio (LIR) of one. The applied pressures were
10, 25, 50, 100, 200, 400, 800, 1600, and 3200 kPa. Deformations were
recorded at time intervals of approximately 0.1, 0.25, 0.5, 1, 2, 4, 8, 15, and 30
min, and 1, 2, 4, 8 and 24 h. For the 6 tests where one increment was allowed to
undergo secondary compression at constant stress for 1-week, the
incrementally loading sequence re-started with small suitable loads and also
184
with LIR = 1. Figure (4.9) shows the assembly of the six tabletop load devices
and consolidometers in use.
Fig. (4.9): Assembly of all components of standard consolidation tests.
4.4.3.4 Data Analysis
Given the reduction in height with time of the specimen during an increment,
the end of primary consolidation (EOP) was determined relaying on the
methods based on Terzaghi’s consolidation theory. The s-log t (Casagrande
method) curves were used to obtain the time to the end of primary
consolidation (tp). Also, the s- t (Taylor method) curves were used to obtain
the time to the end of primary consolidation (tp) only for comparison. The slogt curves were used to obtain the rate of primary and secondary
185
consolidation; coefficient of consolidation (cv) and coefficient of secondary
compression (Cα) were derived.
On the other hand, semi-log plots of ε-log σ` were used for estimation of
preconsolidation pressure (σ`p), over consolidation ratio (OCR) and also to
obtain compression parameters; compression ratio (Cc`), recompression ratio
(Cr`), and swelling ratio (Cs`). Also, coefficient of volume compressibility (mv)
and constrained modulus (D=1/mv) were computed.
Computation of the initial void ratio was dependent upon initial height, weight
of the solids and specific gravity of specimen using the height of solid method.
Computed initial void ratio (eo) was used also to obtain compression
coefficients; compression index (Cc), recompression index (Cr), and swelling
index (Cs).
Correlations between various index properties and compression parameters
were investigated. Compression parameters obtained for organic soils were
related to some of the easily determined index parameters such as water
content, organic content or liquid limits, to provide a basis for comparison of
the results.
186
CHAPTER 5
TEST RESULTS
5.1 Introduction
This chapter reports the results of the experimental program carried out on
organic soil’s samples obtained from two sites of pre-planned primary schools
located at West-Delta. One site, located at Robaomaah village – Mahmoudia Bohira governorate, has two stratums: shallow one (RU) at depth 4.0-6.5 m and
deep one (RL) at depth 10.0-14.0 m. The other, located at Ezbet El-Domyati –
Motoubes - Kafr -Elsheikh governorate, has a deep stratum (D) at depth 8.012.0 m.
The experimental program included the determination of physical, index, and
chemical properties for the organic soil’s samples to establish the basic
characteristics and to classify the soil. Scanning Electron Microphotographs
were taken to evaluate the structural arrangement of organic soils. The
engineering characteristics evaluated in this experimental program included
permeability, undrained shear strength, and compressibility based on data
obtained from incremental loading consolidation tests.
5.2
Physical and Index Properties
The obtained organic soils’ samples were visually inspected and described. A
preliminary laboratory tests were conducted to determine their physical and
index properties including water content, organic content, unit weight, initial
void ratio, specific gravity, Atterberg limits, and particle size distribution. The
test results are presented in Tables (5.1, 5.2).
187
188
RL-18
12.40 - 12.70
10.00 - 10.70
6
RL-17
11.50 - 11.75
12.50 - 13.25
4
RL-12
RL-11
11.00 - 11.45
RL-10
11.00 - 11.30
12.10 - 12.55
3
10.40 - 10.70
RL-6
RL-5
RL-4
5.30 - 6.10
4.50 - 5.30
6
RU-15
Robaomaah Mahmoudia RU-16
4.90 - 5.70
RU-14
4.00 - 4.80
RU-13
5
5.40 - 6.10
RU-9
4.60 - 5.30
RU-8
4
5.70 - 6.20
4.70 - 5.50
5.10 - 5.80
Depth (m)
RU-3
3
2
RU-1
RU-2
BH
No.
Sample
Site Location
No.
12.55
10.35
12.85
11.60
11.20
12.30
11.15
10.55
5.70
4.90
5.30
4.40
5.75
4.95
5.95
5.10
5.45
(m)
Mean
Depth
68.76
41.66
54.33
65.54
56.23
62.38
66.37
36.60
29.10
15.83
22.46
11.60
11.66
12.52
7.63
11.86
19.54
%
283.76
158.20
213.14
248.27
209.43
272.27
251.68
139.02
137.60
91.72
114.26
87.99
78.86
81.83
56.85
89.00
96.92
%
-
1.9
1.6
-
-
-
-
-
-
2.36
-
-
2.52
2.6
-
2.46
-
Gs
Organic Moisture Specific
Content Content Gravity
4.306
3.334
5.311
4.516
-
5.094
4.484
-
-
2.51
-
2.729
2.302
1.88
1.674
2.513
-
e
Void
Ratio
98.5
100
98.4
100
-
100
95.3
-
-
100
-
100
100
100
100
100
-
S%
Degree of
Saturation
11.14
12.42
11.18
11.37
11.48
11.30
10.85
11.37
12.92
14.73
12.21
14.50
15.32
15.57
16.51
14.70
2.90
4.81
3.57
3.27
3.71
3.04
3.09
4.76
5.44
7.69
5.71
7.71
8.57
8.56
10.53
7.78
6.81
kN/m3
kN/m3
13.40
Dry
Unit
Weight
Bulk
Unit
Weight
7.5
-
7.2
6.9
7.1
7.2
7.6
7.6
7.5
7.4
7.5
7.4
7.4
7.1
7.4
-
7.5
pH
Level
27.50
51.00
48.50
PL
64.50
89.00
130.00
PI
44.20
40.20
67.20
35.20
129.30
118.80
122.80
109.80
86.20
206.80
84.00
381.00 225.00 156.00
227.00 143.00
324.00 150.00 174.00
393.00 214.20 178.80
293.00
389.00 265.50 123.50
342.00 114.20 227.80
257.00 126.30 130.70
173.50
159.00
190.00
145.00
171.50 55.20 116.30
154.50 34.20 120.30
92.00
140.00
178.50
LL
Consistancy Limits
58.21
73.00
55.00
LL
NP
33.00
29.00
PL
-
40.00
26.00
PI
Consistancy Limits (Oven
Dried)
Table (5.1): Physical and index properties of organic soil samples of Robaomaah school-Mahmoudia
189
Ezbet ElDomyati Motoubes
10.45 - 11.05
D-15
9.05 - 9.95
D-13
9.95 - 10.45
8.15 - 9.05
D-12
D-14
10.75 - 11.65
D-11
4
9.85 - 10.75
D-10
8.90 - 9.80
D-9
3
8.05 - 8.90
10.25 - 11.15
D-8
D-7
9.30 - 10.20
8.45 - 9.25
D-5
2
11.00 - 11.90
D-4
D-6
10.30 - 10.90
D-3
9.65 - 10.30
1
D-2
Depth (m)
8.75 - 9.55
BH
No.
D-1
Sample
Site Location
No.
10.75
10.20
9.50
8.60
11.20
10.30
9.35
8.45
10.70
9.75
8.85
11.45
10.60
9.95
9.15
(m)
Mean
Depth
62.84
74.59
70.13
66.81
55.54
65.15
56.29
64.76
77.42
67.10
48.80
28.47
75.48
76.45
51.04
%
329.68
355.97
340.12
302.17
294.70
337.05
288.33
294.26
352.25
309.30
245.99
173.10
340.55
352.85
247.70
%
-
-
1.71
-
-
-
-
1.54
1.69
-
1.86
-
-
1.69
-
Gs
Organic Moisture Specific
Content Content Gravity
-
-
5.631
-
5.837
-
-
6.171
6.238
-
4.555
-
-
5.668
-
e
Void
Ratio
-
-
98
-
100
-
-
100
100
-
100
-
-
100
-
S%
Degree of
Saturation
10.13
10.14
10.79
10.49
11.55
10.40
11.11
11.22
11.00
10.60
11.29
12.42
10.01
10.88
11.50
kN/m3
Bulk
Unit
Weight
2.36
2.22
2.45
2.61
2.93
2.38
2.86
2.85
2.43
2.59
3.26
4.55
2.29
2.40
3.31
kN/m3
Dry
Unit
Weight
6.3
5.8
6.2
5.7
6.6
6.3
6.1
5.4
-
5.6
6.3
5.6
6.4
6.2
5.8
pH
Level
95.60
PL
166.40
PI
80.00
66.50
141.00
157.90
484.00 277.20 206.80
547.00 322.70 224.30
449.00 233.30 215.70
221.00
224.50
492.00 333.30 158.70
455.00 235.70 219.30
352.00 146.00 206.00
400.00 270.00 130.00
418.00 260.80 157.20
413.00 256.00 157.00
416.00 268.40 147.60
469.00 288.80 180.20
510.00 312.00 189.00
262.00
LL
Consistancy Limits
94.13
LL
NP
PL
PI
Consistancy Limits (Oven
Dried)
Table (5.2): Physical and index properties of organic soil samples of Ezbet El-Domyati school-Motoubes
5.2.1 Organic Soil’s Description
Data from the six boreholes drilled in the Robaomaah school site – Mahmoudia
- Bohira governorate indicate two stratums of organic soil; shallow stratum
(RU) extending between 4.0-6.5 m, and deep stratum (RL) extending between
10.0-14.0 m. The RU stratum is overlaid by dark brown, very stiff silty clay,
extending from ground surface to depth 3.0 m, followed by 1.0 m of dark grey,
soft to medium stiff silty clay extending to RU stratum. The RU stratum is
underlain by dark grey, soft to medium stiff silty clay with interlayers of grey
silty sand extending to the RL stratum. The RL stratum is underlain by dark
grey, medium stiff to stiff silty clay with interlayers of grey silty sand and grey
sandy silt extending to depth 19.0 m, followed by grey, coarse to medium sand
to the maximum depth explored (20.0 m). The groundwater table after drilling
the boreholes was found to be at depth 0.8 m.
Data from the four boreholes drilled in the Ezbet El-Domyati school site –
Motoubes - Kafr -Elsheikh governorate indicate one stratum of organic soil (D)
extending between 10.0-14.0 m. The D stratum is overlaid by dark brown to
dark grey, stiff silty clay, extending from ground surface to depth 2.5 m,
followed by 5.5 m of dark grey, soft to medium stiff silty clay, with interlayers
of grey medium to fine sand and grey silty sand extending to the D stratum.
The D stratum is underlain by pale blue, medium stiff silty clay with minute
fragments of white limestone extending to depth 15.5 m, followed by dark
brown, stiff silty clay with intercalations of yellowish brown silty sand to the
maximum depth explored (20.0 m). The groundwater table after drilling of the
boreholes was found to be at depth 1.1 m.
Visual inspection of organic soil’s samples indicated two distinct types of soils.
Soil samples obtained from RU stratum indicate grey to dark grey and dark
brown colors and an odor of decomposition. To the touch, it is highly plastic in
nature and is easily deformed. Upon deformation it exhibits a high degree of
cohesion. It is consist mainly of silty clay with spots of/or mixed with partly
190
decomposed plant material. On the other hand, soil samples obtained from RL
and D stratums indicate highly organic soils with their characteristic smell and
colors ranging from yellowish brown and reddish brown to black, with D
samples more light in color than RL. They are consisting mainly of partly
decomposed plant material of granular appearance with traces of partly
decomposed woody pieces and very fine fibers. They exhibit more compressive
strength than the RU stratum and are not easily deformed. Upon deformation,
the soil tends to crumble showing less cohesion than the RU stratum.
5.2.2 Organic Content
Organic content (OC) of the undisturbed samples were measured using loss on
ignition method, where a sample is first dried at temperature 105° C and
weighed then burnt off (ignited) at temperature 440° C till organic matter is
completely fired . The OC of RU nine samples ranged from 7% to 29% with an
average of 16%. The OC of RL eight samples ranged from 36% to 69% with an
average of 57%. The OC of D fifteen samples ranged from 28% to 77% with an
average of 63%. Seven samples of D stratum (highly organic soils) were
ignited again at temperature 550° C according to ECP 202/2, the increase in
loss on ignition was only 0.5-1.0%
5.2.3 Moisture Content
The natural water content for all soil samples was obtained in accordance with
ECP 202-01 (2/2/2). The natural moisture content for RU nine samples ranged
from 57% to 138% with an average of 93%. The natural water content of RL
eight samples ranged from 139% to 284% with an average of 222%. The
natural water content of D fifteen samples ranged from 173% to 356% with an
average of 304%.
191
5.2.4 Void Ratio
Void ratios were determined for the 24 specimens used in consolidation tests, 8
specimens of every layer. The void ratio for RU samples ranged from 1.674 to
2.951 with an average of 2.356. The void ratio for RL samples ranged from
3.249 to 5.846 with an average of 4.461. The void ratio for D samples ranged
from 3.729 to 6.371 with an average of 5.612.
5.2.5 Specific Gravity
Limited number of tests conducted in this study to confirm the ranges reported
in literature. The specific gravity for RU samples ranged from 2.36 to 2.6 with
an average of 2.49. The specific gravity for RL samples ranged from 1.6 to 1.9
with an average of 1.75. The specific gravity for D samples ranged from 1.54 to
1.86 with an average of 1.7.
5.2.6 Unit Weight
The unit weights (γ) were calculated based on measured weights of known
volumes for each of the specimens used for engineering tests. The bulk unit
weight for RU samples ranged from 12.2 kN/m3 to 16.5 kN/m3 with an average
of 14.4 kN/m3. The bulk unit weight for RL samples ranged from 10.9 kN/m3
to 12.4 kN/m3 with an average of 11.4 kN/m3. The bulk unit weight for D
samples ranged from 10.0 kN/m3 to 12.4 kN/m3 with an average of 10.9 kN/m3.
The dry unit weight for RU samples ranged from 5.4 kN/m3 to 10.5 kN/m3 with
an average of 7.6 kN/m3. The dry unit weight for RL samples ranged from 2.9
kN/m3 to 4.8 kN/m3 with an average of 3.6 kN/m3. The dry unit weight for D
samples ranged from 2.2 kN/m3 to 3.3 kN/m3 with an average of 2.8 kN/m3.
Figures (5.1 & 5.2) show the variation of loss on ignition (OC), moisture
content, void ratio, specific gravity, bulk unit weight, and dry unit weight with
depth.
192
193
8
20
20
19
Sand, Coarse to Med., Trace of Silt & Fine
Gravel, Very Dense, Grey
18
19
17
18
17
16
15
15
Med. Stiff to Stiff Silty Clay, Dark Grey,
interlayers of Grey Silty Sand & Grey Sandy
Silt
14
14
16
13
13
12
11
11
Highly Organic Soil, Dark Reddish Brown to
Black (RL)
10
10
12
9
9
Soft to Med. Stiff Silty Clay, Dark Grey,
interlayers of Grey Silty Sand
7
8
7
5
4
3
2
1
6
Organic Silt & Clay, Grey to Black (RU)
Soft to Med. Stiff silty Clay, Dark Grey
Very Stiff Silty Clay, Dark Brown
GWT
0
6
5
4
3
2
1
G. S.
0
20
RU
40
60
RL
80
100
D e p th , m s.
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
0
100
RU
200
RL
300
Water Content, %
400
D e p th , m s.
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
0
2
RU
4
6
RL
Void Ratio, e o
8
RL
2
3
9
8
7
6
5
4
3
2
1
0
20
19
18
17
16
15
14
13
12
20
19
18
17
16
15
14
13
12
11
RU
1
10
0
11
D ep th , m s .
10
9
8
7
6
5
4
3
2
1
0
Specific Gravity, G s
D e p th , m s .
0
0.5
RU
1
RL
1.5
Bulk Unit Weight, t/m3
2
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
0
0.5
RU
1
RL
1.5
Dry Unit Weight, t/m3
2
Fig. (5.1): Variation of organic content, moisture content, void ratio, specific gravity, bulk unit weight, and dry unit
weight with depth for Robaomaah school site– Mahmoudia.
E le v a t io n , m s
0
D ep th , m s.
Organic Content, %
D ep th , m s.
194
19
20
19
20
18
17
17
Stiff Silty Clay, Dark Brown, intercalations of
Yellowish Brown Silty Sand
16
16
18
15
14
15
Med. Stiff Silty Clay, Pale Blue, Minute
Fragments of White Limestone
13
13
14
12
12
10
11
Highly Organic Soil, Reddish Brown to Dark
Reddish Brown (D)
9
8
7
6
5
4
3
2
1
0
11
10
9
8
7
6
5
4
Soft to Med. Stiff Silty Clay, Dark Grey,
interlayers of Grey Med. To Fine Sand &
Grey Silty Sand
Stiff Silty Clay, Dark Brown to Grey
2
3
GWT
1
G. S.
0
20
40
D
60
D
80
100
D ep th , m s.
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
0
100
200
D
300
Water Content,
%
D
400
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
0
2
4
D
D
Void Ratio
, eo
6
8
D ep th , m s.
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
0
1
D
D
2
Specific Gravity, G s
3
9
8
7
6
5
4
3
2
1
0
20
19
18
17
16
15
14
13
12
11
10
D ep th , m s.
D e p th , m s.
0
0.5
1
D
1.5
Bulk UnitD Weight, t/m3
2
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
0
0.5
D
1
D
1.5
Dry Unit Weight, t/m3
2
Fig. (5.2): Variation of organic content, moisture content, void ratio, specific gravity, bulk unit weight, and dry unit
weight with depth for Ezbet El-Domyati school site – Motoubes.
E le v a tio n , m s
0
D ep th , m s.
Organic Content, %
D e p th , m s.
5.2.7 Atterberg Limits
The values of the Atterberg limits were determined for all samples. Figure (5.3)
shows the Atterberg limits plotted on Casagrande plasticity chart. For the RU
samples, the majority of data is plotted above the A-Line and could be
classified as CH. On the other hand, the majorities of RL and D data are plotted
below the A-Line and could be classified as OH. On two samples of RU, a
second determination of the liquid limit after oven drying the soils, the liquid
limit was decreased to 39-46 % of the liquid limit of the nondried soil. The
same treatment was followed for one sample of both RL and D; the liquid limit
was decreased to 30% and 28% of the liquid limit of the nondried soil
respectively.
Plasticty Chart
450
Plasticity Index,Ip
400
350
300
250
200
150
RU
RL
D
100
50
0
0
50
100 150 200 250 300 350 400 450 500 550 600
Liquid Limit, %
Fig. (5.3): Atterberg limits of organic soil’s samples plotted on Casagrande
plasticity chart.
5.2.8 Particle Size Distribution
The particle size distribution, of representative organic soil samples from every
layer, was obtained through a combination of wet sieving and hydrometer test
following ASTM D422. Figure (5.4) shows the particle size distribution of RU,
RL, and D representative organic soil samples.
195
% Finer by Weight
RU
100
90
80
70
60
50
40
30
20
10
0
RU-2
RU-8
RU-9
RU-13
RU-15
10
1
0.1
0.01
0.001
0.0001
0.001
0.0001
0.001
0.0001
Particle Size (mm)
100
90
RL
% Finer by Weight
80
70
60
50
40
30
RL-4
20
RL-12
10
RL-17
0
10
1
0.1
0.01
Particle Size (mm)
D
% Finer by Weight
100
90
80
70
60
50
40
D-5
30
D-7
20
D-8
10
D-11
0
10
1
0.1
0.01
Particle Siz e (mm)
Fig. (5.4): particle size distribution of RU, RL, and D samples.
196
The particle size distribution for RU samples indicates that >98% of the soil is
finer than 75 µm, with >80% in the clay size fraction (<5 µm). The particle size
distribution for RL samples indicates that >92% of the soil is finer than 75 µm,
with >50% in the silt size fraction (>5 µm), and 20-40% in the clay size
fraction (<5 µm). The particle size distribution for D samples indicates that
>82% of the soil is finer than 75 µm, with >60% in the silt size fraction (>5
µm), and 15-22% in the clay size fraction (<5 µm). This indicates that as the
average OC increases (D>RL>RU) the average size of the soil particles
increases.
5.3 Chemical Properties
5.3.1 pH Level
In this study, pH value was determined for almost all samples. The pH values
for RU samples ranged from 7.1 to 7.5 with mean value of 7.4. The pH values
for RL samples ranged from 6.9 to 7.6 with mean value of 7.3. The pH values
for D samples ranged from 5.4 to 6.6 with mean value of 6.0.
5.3.2 X-Ray Diffraction
X-Ray diffraction analysis was performed to identify the different minerals
constituting the inorganic portion of the organic soils. X-Ray diffraction
analysis was performed for eight samples of different organic content: two
samples of each of RU and RL stratum, and three samples of D stratum and for
one fired sample of RU stratum. Figures (5.5, 5.6, & 5.7) show the X-Ray
diffraction analysis results for RU, RL, and D samples respectively. Peak
locations and intensities were determined using Diffrac/AT software, and
minerals were identified by their characteristic reflections. Table (5.3)
summarizes the constituent minerals abundances of the tested samples. It is
evident that the inorganic portion of the organic soils consists of different
percentage of clay minerals (Montmorillonite, Illite, and Kaolinite) and nonclay minerals (Halite, Calcite, Quartz, Gypsum, Hematite, and Feldspars).
197
RU-8
RU-8-H (Ignited)
RU-15
Fig. (5.5): X-Ray diffraction test results for RU samples
198
RL-6
RL-10
Fig. (5.6): X-Ray diffraction test results for RL samples
199
D-2
D-9
D-11
Fig. (5.7): X-Ray diffraction test results for D samples
200
201
Depth (m)
4.60 - 5.30
4.60 - 5.30
4.50 - 5.30
12.10 - 12.55
11.00 - 11.45
9.65 - 10.30
8.90 - 9.80
10.75 - 11.65
Sample
No
RU-8
RU-8-H
RU-15
RL-6
RL-10
D-2
D-9
D-11
55.9
56.0
77.4
56.2
63.7
15.8
-
12.5
%
Organic
Content
-
-
-
6.2
11.1
18.4
-
16.4
Montmorillonite
-
-
-
-
-
-
-
9.7
Illite
Clay minerals
17.4
-
-
6.2
8.8
15.7
-
11.9
Kaolinite
32.3
79.0
63.8
47.0
18.5
29.7
61.1
41.4
Halite
23.8
11.8
17.9
7.3
10.6
10.2
7.1
8.3
Calcite
26.5
-
-
10.0
17.8
16.8
9.1
12.3
Quartz
-
9.1
18.3
7.2
8.5
-
6.9
-
-
-
-
7.5
8.4
9.2
8.8
-
-
-
-
8.6
16.2
-
7.0
-
Gypsum Hematite Feldspars
Non-clay minerals
Table (5.3): Summary of the organic content and the constituent minerals abundances of the RU, RL, and D tested samples.
5.3.3 Environmental Corrosion Tests
Chemical analysis was conducted to determine the injurious chemical
compounds of soil and groundwater (pH, sulfates content and chlorides
content). Three representative organic soil samples from every stratum and
groundwater sample from each site were chemically analyzed. The results are
given in Tables (5.4 & 5.5).
Table (5.4): Injurious chemical compounds in organic soil samples.
Result In Weight %
Chemical Compound
RU
RL
D
Total mineral soluble salts
(Ionized salts)
7.52-9.12
5.70-6.14
2.76-4.58
Salinity as sodium chloride
(NaCL)
7.02-8.54
4.10-5.85
2.34-3.98
Sulphate as sulpher trioxide
(SO3)
0.30-0.46
0.14-1.64
0.22-0.59
Table (5.5): Chemical analysis of the groundwater.
Result (ppm)
Chemical Compound
Robaomaah school –
Mahmoudia
Ezbet El-Domyati
school – Motoubes
Total mineral soluble salts
(Ionized salts)
74250
28000
Alkalinity as sodium carbonate
(Na H CO3)
583
1060
Salinity as sodium chloride
(NaCL)
67275
20457
Sulphate as sulpher trioxide
(SO3)
5110
5170
7.6
7.6
PH value
{log
1
}
(H + )
202
5.4 SEM
The Scanning Electron Microscope (SEM) was used to observe the differences
in the fiber contents, the pore spaces, and the perforated plant structure of the
organic soil samples of RU, RL, and D stratums, in its initial state and after
compression under high stress (3200 kPa). It was also used in order to evaluate
the effect of fabric (microstructure) on the compressibility characteristics of
organic soil encountered.
Scanning Electron Micrographs were taken for three samples of different
organic content, one of each stratum. The Micrographs were taken for 12
specimens extracted from the three samples. First, the micrographs were taken
for 6 specimens in its initial state, 2 of each sample; one was cut with their axes
parallel to the vertical direction (Figures 5.8a, 5.10a, & 5.12a), the other was
cut with their axes perpendicular to the vertical direction (Figures 5.8b, 5.10b,
& 5.12b). Second, the micrographs were taken for 6 specimens, 2 of each
sample; after compressibility and permeability characteristics were determined
in the consolidation test number [3 & 4 – see Table 4.1] under effective stress
of 3200 kPa (Figures 5.9, 5.11, & 5.13). Several micrographs were taken for
every specimen. The micrographs were taken at different magnifications
ranged from 150 to 12,000.
It is evident, from the micrographs that the organic soil encountered is highly
decomposed such that even at higher magnifications only trace of fibers could
be recognized. It is evident, also, the sponge-like porous nature of the organic
soil encountered such that even at higher magnifications it is not possible to
recognize particles except that in Figure (5.9b) in which sand and silt particles
were recognized. Moreover, it is evident from the micrographs that the organic
soil encountered is highly porous in the vertical sections (horizontal directions)
compared with the porosity in the horizontal sections (vertical directions). In
addition, Figures (5.9, 5.11, & 5.13) show the thick relatively stiff mats formed
after consolidation under 3200 kPa in the horizontal and vertical sections.
203
RU-13
OC = 13.2%
(a)
(b)
Fig. (5.8): The Scanning Electron Micrographs of RU sample at initial state (a)
horizontal section x 800 (b) vertical section x 500.
OC = 15.09%
OC = 7.11%
(a)
(b)
Fig. (5.9): The Scanning Electron Micrographs of RU sample under
consolidation pressure of 3200 kPa (a) horizontal section x 1600 (b) vertical
section x 700.
204
RL-12
OC = 52.52%
(a)
(b)
Fig. (5.10): The Scanning Electron Micrographs of RL sample at initial state
(a) horizontal section x 500 (b) vertical section x 1000.
OC = 56.22%
OC = 68.25%
(a)
(b)
Fig. (5.11): The Scanning Electron Micrographs of RL sample under
consolidation pressure of 3200 kPa (a) horizontal section x 1067 (b) vertical
section x 800.
205
D-5
OC = 45.33%
(a)
(b)
Fig. (5.12): The Scanning Electron Micrographs of D sample at initial state (a)
horizontal section x 800 (b) vertical section x 665.
OC = 64.10%
OC = 26.74%
(a)
(b)
Fig. (5.13): The Scanning Electron Micrographs of D sample under
consolidation pressure of 3200 kPa (a) horizontal section x 800 (b) vertical
section x 700.
206
5.5 Organic Soils' Classification
In this research, the recommended classification system to be used in Egypt;
“Tentative ASTM Standard” proposed by the ASTM Subcommittee D18.18 for
classifying organic soils and peat (see Table 2.8), was used to classify the
organic soils encountered. The average OCs of RU, RL, and D stratums are
16%, 57%, & 63% respectively. There is only a trace of fibers that could be
recognized from SEM. There is also trace to some sand and woody pieces as
indicated from the particle size distribution and visual inspection of the
samples. Therefore, organic soils encountered in this study could be classified
as peaty muck for RL & D, and as highly organic silty clay for RU based on
this classification system. Soil stratigraphy of the two sites is illustrated in
Figure (5.14).
0
1
2
3
4
5
6
0
G. S.
GWT
1
2
Very Stiff Silty Clay, Dark Brown
5
Highly Organic Silty Clay, Grey to
Black (RU)
6
8
Soft to Med. Stiff Silty Clay, Dark
Grey, interlayers of Grey Silty Sand
11
Silty Muck to Peaty Muck, Dark
Reddish Brown to Black (RL)
9
10
11
13
14
14
15
15
17
Med. Stiff to Stiff Silty Clay, Dark
Grey, interlayers of Grey Silty Sand
& Grey Sandy Silt
Med. Stiff Silty Clay, Pale Blue,
Minute Fragments of White Limestone
16
17
18
18
19
20
Peaty Muck, Reddish Brown to Dark
Reddish Brown (D)
12
13
16
Soft to Med. Stiff Silty Clay, Dark
Grey, interlayers of Grey Med. To
Fine Sand & Grey Silty Sand
7
10
12
Stiff Silty Clay, Dark Brown to Grey
4
Elevation, ms
Elevation, ms
9
GWT
3
Soft to Med. Stiff silty Clay, Dark
Grey
7
8
G. S.
Stiff Silty Clay, Dark Brown,
intercalations of Yellowish Brown Silty
Sand
19
Sand, Coarse to Med., Trace of Silt &
Fine Gravel, Very Dense, Grey
20
(a)
(b)
Fig. (5.14): Soil stratigraphy of the two sites; (a) Robaomaah school –
Mahmoudia, (b) Ezbet El-Domyati school – Motoubes.
207
5.6 Undrained Shear Strength
Undrained shear strength was determined via unconfined compression test by
two methods: in compression device and using pocket penetrometer. The test
results obtained through unconfined compression tests and pocket penetrometer
are presented in Tables (5.6 & 5.7). Figures (5.15, 5.16) show the variation of
undrained shear strength (Su) determined by both methods with depth.
5.6.1 Unconfined Compression Tests
Undrained shear strength (Sun) were obtained through unconfined compression
tests for almost all undisturbed samples of RU, RL, and D stratums. The
undrained shear strength (Sun) of RU stratum, classified as highly organic silty
clay, ranged from 4.5 kPa to 24 kPa with mean undrained shear strength of 15.6
kPa. The undrained shear strength of RL stratum, classified as peaty muck,
ranged from 4 kPa to 76.5 kPa with mean undrained shear strength of 33 kPa.
The undrained shear strength of D stratum, classified as peaty muck, ranged
from 11.5 kPa to 92 kPa with mean undrained shear strength of 37 kPa.
5.6.2 Pocket Penetrometer
The pocket penetrometer, Soiltest CL-700, was used at top and bottom of the
undisturbed plastic tube samples just after extraction of the sample from the
ground, and also when sliced before any engineering test, for minimum of three
times with the mean value recorded as (P.P.). The undrained shear strength was
obtained through pocket penetrometer (Sup) for all undisturbed samples of RU,
RL, and D stratums and considered as in-situ undrained shear strength. The insitu undrained shear strength (Sup) of RU stratum, classified as highly organic
silty clay, ranged from 20 kPa to 6o kPa with mean undrained shear strength of
32 kPa. The undrained shear strength of RL stratum, classified as peaty muck,
ranged from 28 kPa to 162 kPa with mean undrained shear strength of 104 kPa.
The undrained shear strength of D stratum, classified as peaty muck, ranged
from 37 kPa to 105 kPa with mean undrained shear strength of 78 kPa.
208
209
RL-18
12.40 - 12.70
10.00 - 10.70
6
RL-17
11.50 - 11.75
12.50 - 13.25
4
RL-12
RL-11
11.00 - 11.45
RL-10
11.00 - 11.30
12.10 - 12.55
3
10.40 - 10.70
RL-6
RL-5
RL-4
5.30 - 6.10
4.50 - 5.30
6
RU-15
Robaomaah Mahmoudia RU-16
4.90 - 5.70
RU-14
4.00 - 4.80
RU-13
5
5.40 - 6.10
RU-9
4.60 - 5.30
RU-8
4
5.70 - 6.20
4.70 - 5.50
5.10 - 5.80
Depth (m)
RU-3
3
2
RU-1
RU-2
BH
No.
Sample
Site Location
No.
12.55
10.35
12.85
11.60
11.20
12.30
11.15
10.55
5.70
4.90
5.30
4.40
5.75
4.95
5.95
5.10
68.76
41.66
54.33
65.54
56.23
62.38
66.37
36.60
29.10
15.83
22.46
11.60
11.66
12.52
7.63
11.86
19.54
%
(m)
5.45
Organic
Content
Mean
Depth
283.76
158.20
213.14
248.27
209.43
272.27
251.68
139.02
137.60
91.72
114.26
87.99
78.86
81.83
56.85
89.00
96.92
%
Moisture
Content
11.14
12.42
11.18
11.37
11.48
11.30
10.85
11.37
12.92
14.73
12.21
14.50
15.32
15.57
16.51
14.70
13.40
kN/m
3
2.90
4.81
3.57
3.27
3.71
3.04
3.09
4.76
5.44
7.69
5.71
7.71
8.57
8.56
10.53
7.78
6.81
kN/m
3
Bulk
Dry Unit
Unit
Weight
Weight
87.5
84.4
88.0
86.2
85.6
87.2
85.5
84.7
55.8
52.0
53.5
49.0
55.8
51.9
57.0
51.9
54.2
kPa
260.0
252.0
192.0
270.0
-
250.0
190.0
-
-
132.0
-
96.0
85.0
132.0
85.0
130.0
-
kPa
σ` vo σ` p
35.0
8.0
17.0
-
-
-
128.0
153.0
18.0
26.0
48.0
33.0
43.0
30.0
35.0
9.0
38.0
kPa
q un
17.5
4.0
8.5
-
-
-
64.0
76.5
9.0
13.0
24.0
16.5
21.5
15.0
17.5
4.5
19.0
kPa
S un
0.07
0.02
0.04
-
-
-
0.34
-
-
0.10
-
0.17
0.25
0.11
0.21
0.03
-
S un /σ` p
220.0
200.0
250.0
325.0
210.0
55.0
225.0
180.0
60.0
55.0
85.0
40.0
45.0
55.0
55.0
60.0
120.0
kPa
P.P.
110.0
100.0
125.0
162.5
105.0
27.5
112.5
90.0
30.0
27.5
42.5
20.0
22.5
27.5
27.5
30.0
60.0
kPa
S up
0.42
0.40
0.65
0.60
-
0.11
0.59
-
-
0.21
-
0.21
0.26
0.21
0.32
0.23
-
S up /σ` p
Table (5.6): Undrained shear strength of RU & RL stratum determined by unconfined compressive strength and by pocket
penetrometer
210
Ezbet ElDomyati Motoubes
10.45 - 11.05
D-15
9.05 - 9.95
D-13
9.95 - 10.45
8.15 - 9.05
D-12
D-14
10.75 - 11.65
D-11
4
9.85 - 10.75
D-10
8.90 - 9.80
D-9
3
8.05 - 8.90
10.25 - 11.15
D-8
D-7
9.30 - 10.20
8.45 - 9.25
D-5
2
11.00 - 11.90
D-4
D-6
10.30 - 10.90
D-3
9.65 - 10.30
1
D-2
Depth (m)
8.75 - 9.55
BH
No.
D-1
Sample
Site Location
No.
10.75
10.20
9.50
8.60
11.20
10.30
9.35
8.45
10.70
9.75
8.85
11.45
10.60
9.95
62.84
74.59
70.13
66.81
55.54
65.15
56.29
64.76
77.42
67.10
48.80
28.47
75.48
76.45
51.04
%
(m)
9.15
Organic
Content
Mean
Depth
329.68
355.97
340.12
302.17
294.70
337.05
288.33
294.26
352.25
309.30
245.99
173.10
340.55
352.85
247.70
%
Moisture
Content
10.13
10.14
10.79
10.49
11.55
10.40
11.11
11.22
11.00
10.60
11.29
12.42
10.01
10.88
11.50
kN/m3
2.36
2.22
2.45
2.61
2.93
2.38
2.86
2.85
2.43
2.59
3.26
4.55
2.29
2.40
3.31
kN/m3
Bulk
Dry Unit
Unit
Weight
Weight
72.7
72.2
71.4
70.6
73.2
72.3
71.3
70.5
72.7
71.7
70.7
73.0
72.6
72.0
71.1
kPa
-
-
160.0
-
127.0
-
-
117.0
162.0
-
140.0
-
-
190.0
-
kPa
σ` vo σ` p
122.0
95.0
40.0
71.0
31.0
53.0
36.0
42.0
23.0
115.0
111.0
45.0
184.0
86.0
61.0
47.5
20.0
35.5
15.5
26.5
18.0
21.0
11.5
57.5
55.5
22.5
92.0
43.0
26.5
kPa
kPa
53.0
S un
q un
-
-
0.13
-
0.12
-
-
0.18
0.07
-
0.40
-
-
0.23
-
S un /σ` p
200.0
210.0
135.0
200.0
100.0
150.0
125.0
135.0
115.0
175.0
140.0
75.0
210.0
200.0
165.0
kPa
P.P.
100.0
105.0
67.5
100.0
50.0
75.0
62.5
67.5
57.5
87.5
70.0
37.5
105.0
100.0
82.5
kPa
S up
-
-
0.42
-
0.39
-
-
0.58
0.35
-
0.50
-
-
0.53
-
S up /σ` p
Table (5.7): Undrained shear strength of D stratum determined by unconfined compressive strength and by pocket
penetrometer
S u , (kPa)
3
4
0
Soft to Med. Stiff silty Clay, Dark Grey
5
Organic Silty Clay, Grey to Black (RU)
Depth, ms.
Elevation, ms
10
RU-p
RL-p
8
9
10
11
12
200
RL-q
7
Soft to Med. Stiff Silty Clay, Dark Grey,
interlayers of Grey Silty Sand
150
RU-q
6
7
9
100
5
6
8
50
4
11
Silty Muck to Peaty Muck, Dark Reddish
Brown to Black (RL)
12
13
13
14
Fig. (5.15): Variation of undrained shear strength for RU and RL samples
obtained by unconfined compression tests and by pocket penetrometer with
depth.
3
S u , (kPa)
4
0
5
7
8
8
9
9
11
Peaty Muck, Reddish Brown to Dark Reddish
Brown (D)
12
200
10
11
12
13
14
150
6
7
10
100
5
Depth, ms.
Elevation, ms
6
Soft to Med. Stiff Silty Clay, Dark Grey,
interlayers of Grey Med. To Fine Sand &
Grey Silty Sand
50
13
Med. Stiff Silty Clay, Pale Blue, Minute
Fragments of White Limestone
15
14
D-q
D-p
15
Fig. (5.16): Variation of undrained shear strength for D samples obtained by
unconfined compression tests and by pocket penetrometer with depth.
211
5.7 Permeability
Permeability measurements of RU, RL, and D stratum were determined
through falling-head flow measurements during the secondary compression
stage of IL oedometer tests (Tavenas et al, 1983; Mesri, 1997) as effective
vertical stress increases from σ`vo to σ`vf. The values of the coefficient of
permeability in the vertical direction, kv, were determined for three samples,
one sample from each stratum, that were cut with their axes parallel to the
vertical direction. The values of the coefficient of permeability in the horizontal
direction, kh, were determined, for the same three previous samples, for
specimens that were cut with their axes perpendicular to the vertical direction
and from the following 10 cm to the previous specimens. Data on vertical
permeability kv and horizontal permeability kh of these three samples as
effective vertical stress increases from σ`vo to σ`vf and void ratio decreases from
eo to ef are shown in Figure (5.17). Table (5.8) presents the summary of the
permeability test results.
The in-situ coefficients of permeability in the vertical direction, kvo, and in the
horizontal direction, kho, were estimated by extrapolating the linear initial
portions of the e-log k curves to initial void ratios eo. The obtained initial
coefficients of permeability in the vertical direction, kvo, are 4x10-8, 1x10-5, and
7x10-7 cm/sec for RU, RL, and D samples respectively. The obtained initial
coefficients of permeability in the horizontal direction, kho, are 5x10-8, 8x10-5,
and 8x10-8 cm/sec for RU, RL, and D samples respectively.
Upon compression, the permeability of the organic soils decreases as they
compress under loads because of their high compressibility. The slope of the
initial portion of the e versus log k, that is, Ck = ∆e ⁄∆ log k, measures the
reduction in e required to produce a tenfold decrease in k. Ck was estimated in
the vertical direction as 0.94, 0.57, and 1.29, and as 0.70, 0.88, and 0.91 in the
horizontal direction, for RU, RL, and D samples respectively.
212
6.0
5.0
RU
Void ratio, e
4.0
3.0
2.0
1.0
0.0
1.E-10
RU-13-(IL+FS+Perm)H
RU-13-(IL+FS+Perm)V
1.E-09
1.E-08
1.E-07
1.E-06
1.E-05
1.E-04
Permeability, k (cm/sec)
6.0
5.0
RL
Void ratio, e
4.0
3.0
2.0
1.0
0.0
1.E-10
RL-12-(IL+FS+Perm)H
RL-12-(IL+FS+Perm)V
1.E-09
1.E-08
1.E-07
1.E-06
1.E-05
1.E-04
Permeability, k (cm/sec)
6.0
D
Void ratio, e
5.0
4.0
3.0
2.0
1.0
D-5-(IL+FS+Perm)H
D-5-(IL+FS+Perm)V
0.0
1.E-10
1.E-09
1.E-08
1.E-07
1.E-06
1.E-05
1.E-04
Permeability, k (cm/sec)
Fig. (5.17): Data on vertical permeability kv and horizontal permeability kh of the
three stratums as effective vertical stress increases from σ`vo to σ`vf and void ratio
decreases from eo to ef.
213
214
Ezbet ElDomyati Motoubes
Robaomaah Mahmoudia
D-5
RL-12
RU-13
2
4
5
Sample BH
Site Location
No.
No.
8.45 - 9.25
12.50 - 13.25
8.85
12.85
4.40
(m)
(m)
4.00 - 4.80
Mean
Depth
Depth
70.7
88.0
49.0
kPa
177.62
245.99
64.10
26.74
217.52
208.76
56.22
68.25
75.09
95.79
15.09
7.11
%
%
Moisture
σ` vo Organic
Content Content
3.729
5.381
5.846
4.776
2.506
7.00E-07
1.20E-05
3.80E-08
(cm/s)
eo
2.951
k vo
Void
Ratio
7.50E-08
8.00E-05
4.50E-08
(cm/s)
k ho
Table (5.8): Results of permeability tests for RU, RL, and D.
0.107
6.667
1.184
0.910
1.290
0.875
0.570
0.695
0.944
k ho /k vo c k
Horizontal
Vertical
Horizontal
Vertical
Horizontal
Vertical
Specimen
Direction
5.8 Compressibility
The testing program, previously mentioned in chapter (4), included 24
incremental loading (IL) one-dimensional consolidation tests. Eight IL
consolidation tests were carried out on six chosen representative samples from
different depths within every stratum of RU, RL, and D, such that two samples
were tested in the vertical and horizontal directions. For these two samples, two
specimens were sliced and tested; one ordinary specimen, the other was cut
with its axes perpendicular to the vertical direction and from the following 10
cm of the previous specimen, in order to investigate an isotropy of
compressibility and permeability characteristics. One of these samples, RU-2
the first sample was tested; their test results in vertical and horizontal directions
were unfortunately excluded due to lack of accuracy which was only revealed
during analysis.
Given the reduction in height with time during an increment, s-log t
(Casagrande method) curves was used to obtain the time to the end of primary
consolidation (tp) based on Terzaghi’s consolidation theory. The s-logt curves
were used also to obtain the rate of primary and secondary consolidation;
coefficient of consolidation (cv) and coefficient of secondary compression (Cα)
were derived. Figures (5.18, 5.19, & 5.20) show the typical s-logt curves for
RU, RL, and D stratums. On the other hand, semi-log plots of ε-log σ` were
used for estimation of preconsolidation pressure (σ`p), over consolidation ratio
(OCR) and also to obtain compression parameters; compression ratio (Cc`),
recompression ratio (Cr`), and swelling ratio (Cs`). Also, coefficient of volume
compressibility (mv) and constrained modulus (D=1/mv) were computed.
In the following sections, the results of the one-dimensional consolidation
testing program are presented. The results are presented under the following
two headings: (i) Primary consolidation behavior: (ii) Secondary consolidation
behavior. The important findings are summarized in Tables (5.9 & 5.10).
215
RU- 8
Depth= 4.6-5.3 m
OC= 12.5%
Highly Organic Silty Clay
σ`v/σ`p = 0.19
Test No. [5]
σ`v/σ`p = 0.76
10.7000
10.9900
10.6500
10.9800
10.6000
10.9700
10.5500
Dia l Rea ding , m m
Dia l Rea ding , m m
10.9600
10.9500
10.9400
10.5000
10.4500
10.4000
10.9300
10.3500
10.9200
10.3000
10.9100
10.2500
10.2000
10.9000
10.1500
10.8900
0.1
0.1
1
10
100
1000
1
10
10000
100
1000
10000
1000
10000
Time (minutes)
Time (minutes)
σ`v/σ`p = 1.52
σ`v/σ`p = 3.0
8.7000
8.6000
8.5000
10.2000
10.1000
10.0000
8.4000
8.3000
8.2000
8.1000
8.0000
9.9000
9.8000
Dia l Rea ding , m m
Dia l Rea ding , m m
9.7000
9.6000
9.5000
9.4000
9.3000
9.2000
7.9000
7.8000
7.7000
7.6000
7.5000
7.4000
7.3000
7.2000
7.1000
7.0000
6.9000
9.1000
9.0000
8.9000
6.8000
6.7000
6.6000
8.8000
8.7000
0.1
1
10
100
1000
10000
0.1
1
Time (minutes)
100
Time (minutes)
σ`v/σ`p = 12.1
σ`v/σ`p = 6.0
7.4000
9.2000
9.1000
9.0000
8.9000
8.8000
8.7000
8.6000
8.5000
8.4000
8.3000
8.2000
8.1000
8.0000
7.9000
7.8000
7.7000
7.6000
7.5000
7.4000
7.3000
7.2000
7.3000
7.2000
7.1000
7.0000
6.9000
Dial Reading, mm
D ia l Rea ding , m m
10
6.8000
6.7000
6.6000
6.5000
6.4000
6.3000
6.2000
6.1000
6.0000
5.9000
5.8000
0.1
1
10
100
Time (minutes)
1000
10000
0.1
1
10
100
Time (minutes)
Fig. (5.18): Typical s-logt curves for RU stratum
216
1000
10000
RL- 5
Depth= 11.0-11.3 m
Peaty Muck
OC=66.6%
σ`v/σ`p= 0.13
Test No. [5]
σ`v/σ`p= 0.53
9.9400
9.5900
9.5700
9.9300
9.5500
9.5300
9.5100
9.9200
9.4900
9.4700
9.4500
Dial Reading, mm
Dial Rea ding, m m
9.9100
9.9000
9.8900
9.4300
9.4100
9.3900
9.3700
9.3500
9.3300
9.3100
9.2900
9.2700
9.8800
9.2500
9.2300
9.8700
9.2100
9.1900
9.1700
9.8600
9.1500
0.1
1
10
100
1000
10000
0.1
1
10
Time (minutes)
σ`v/σ`p= 1.1
Time (minutes)
100
1000
10000
1000
10000
1000
10000
σ`v/σ`p= 2.1
9.1000
8.0000
7.9000
7.8000
9.0000
7.7000
7.6000
8.9000
7.5000
7.4000
7.3000
8.8000
7.2000
Dial Reading, mm
Dial Reading, mm
7.1000
8.7000
8.6000
8.5000
7.0000
6.9000
6.8000
6.7000
6.6000
6.5000
6.4000
6.3000
8.4000
6.2000
6.1000
8.3000
6.0000
5.9000
5.8000
8.2000
5.7000
5.6000
8.1000
5.5000
0.1
1
10
100
Time (minutes)
1000
10000
0.1
1
σ`v/σ`p= 8.4
9.2000
9.0000
8.8000
8.6000
8.4000
8.2000
Dial Reading, m m
Dial Reading, mm
8.0000
7.8000
7.6000
7.4000
7.2000
7.0000
6.8000
6.6000
6.4000
6.2000
6.0000
5.8000
1
10
100
Time (minutes)
100
Time (minutes)
σ`v/σ`p= 4.2
0.1
10
1000
5.8000
5.7000
5.6000
5.5000
5.4000
5.3000
5.2000
5.1000
5.0000
4.9000
4.8000
4.7000
4.6000
4.5000
4.4000
4.3000
4.2000
4.1000
4.0000
3.9000
3.8000
3.7000
3.6000
3.5000
0.1
10000
1
10
100
Time (minutes)
Fig. (5.19): Typical s-logt curves for RL stratum
217
D- 11
Depth= 10.8-11.7 m
OC = 55.9%
Peaty Muck
σ`v/σ`p= 0.19
Test No. [5]
σ`v/σ`p= 0.79
10.1800
9.7000
10.1700
9.6000
10.1600
10.1500
9.5000
10.1400
9.4000
10.1200
Dial Reading, mm
Dia l Rea ding , m m
10.1300
10.1100
10.1000
10.0900
10.0800
9.3000
9.2000
9.1000
9.0000
10.0700
8.9000
10.0600
10.0500
8.8000
10.0400
8.7000
10.0300
10.0200
0.1
1
10
100
1000
8.6000
10000
0.1
1
10
Time (minutes)
100
1000
10000
1000
10000
1000
10000
Time (minutes)
σ`v/σ`p= 1.57
σ`v/σ`p= 3.0
8.700
5.800
8.500
5.600
8.300
5.400
5.200
7.900
5.000
7.700
4.800
7.500
4.600
Dial Reading, mm
Dial Reading, mm
8.100
7.300
7.100
6.900
4.400
4.200
4.000
3.800
6.700
3.600
6.500
3.400
6.300
3.200
6.100
3.000
5.900
2.800
2.600
5.700
0.1
1
10
100
1000
0.1
10000
1
10
100
Time (minutes)
Time (minutes)
σ`v/σ`p= 25.2
σ`v/σ`p= 6.0
9.1000
4.8000
8.9000
4.6000
8.7000
8.5000
4.4000
Dial Reading, mm
Dial Reading, mm
8.3000
8.1000
7.9000
7.7000
7.5000
4.2000
4.0000
3.8000
7.3000
3.6000
7.1000
6.9000
3.4000
6.7000
6.5000
3.2000
0.1
1
10
100
1000
10000
0.1
Time (minutes)
1
10
100
Time (minutes)
Fig. (5.20): Typical s-logt curves for D stratum
218
219
Robaomaah Mahmoudia
10.00 - 10.70
RL-17
RL-18
12.40 - 12.70
10.00 - 10.70
12.50 - 13.25
RL-12
6
12.50 - 13.25
RL-12
RL-17
11.50 - 11.75
RL-11
11.00 - 11.30
12.10 - 12.55
3
RL-6
RL-5
4.50 - 5.30
RU-15
6
4.00 - 4.80
RU-13
4.00 - 4.80
RU-13
5
5.40 - 6.10
RU-9
4.60 - 5.30
4
RU-8
4.70 - 5.50
4.70 - 5.50
Depth (m)
5.70 - 6.20
3
BH
No.
RU-3
RU-2
RU-2
Sample
Site Location
No.
12.55
10.35
10.35
12.85
12.85
11.60
12.30
11.15
4.90
4.40
4.40
5.75
4.95
5.95
5.10
5.10
(m)
Mean
Depth
68.55
41.43
41.88
68.25
56.22
69.01
63.73
66.56
15.83
7.11
15.09
11.66
12.52
7.63
11.01
12.70
%
283.76
164.78
158.20
217.52
208.76
248.27
272.27
251.68
91.72
75.09
95.79
78.86
81.83
56.85
85.56
89.00
%
Organic Moisture
Content Content
4.306
3.419
3.249
5.846
4.776
4.516
5.094
4.484
2.510
2.506
2.951
2.302
1.880
1.674
2.290
2.735
eo
Void
Ratio
2.250
2.900
2.250
7.000
3.300
2.000
11.600
3.200
1.300
5.650
3.100
4.200
2.000
3.000
-
-
%
εvo
85.0
-
-
kPa
σ` p
1.50
-
-
OCR
67.0
96.0
85.0
1.37
1.96
1.52
230.0 2.73
252.0 2.99
186.0 2.11
192.0 2.18
87.5 280.0 3.20
84.4
88.0
86.2 270.0 3.13
87.2 250.0 2.87
85.5 190.0 2.22
52.0 132.0 2.54
49.0
55.8
51.9 132.0 2.54
57.0
51.9
kPa
σ` vo
5.15E-03
3.30E-03
8.00E-03
8.00E-03
5.50E-03
8.25E-03
1.03E-03
3.25E-04
6.00E-04
5.75E-04
4.75E-04
-
-
2
(cm2/s)
1.20E-02
1.75E-02
2.10E-04
-
(cm /s)
ch
cv
Ave. in recompn.
1.15E-03
3.50E-04
2.15E-03
2.12E-03
2.68E-03
8.15E-04
1.69E-04
1.39E-04
3.15E-04
1.69E-04
3.65E-04
-
(cm2/s)
cv
1.15E-03
6.60E-03
1.30E-04
-
2
(cm /s)
ch
Ave. in compn.
0.046
0.061
0.047
0.159
0.087
0.083
0.235
0.094
0.044
0.129
0.096
0.050
0.055
0.091
-
-
c' r
0.244
0.270
0.200
1.089
0.503
0.458
1.432
0.515
0.154
0.452
0.379
0.165
0.158
0.243
-
-
cr
0.656
0.489
0.501
0.550
0.555
0.576
0.425
0.444
0.320
0.250
0.361
0.384
0.323
0.245
-
-
c' c
3.481
2.161
2.129
3.765
3.206
3.177
2.590
2.435
1.123
0.877
1.426
1.268
0.930
0.655
-
-
cc
0.070
0.125
0.094
0.289
0.157
0.144
0.553
0.212
0.138
0.516
0.266
0.130
0.170
0.371
-
-
c r /c c
Ratio
0.023
0.021
0.022
0.021
0.031
0.032
0.026
0.026
0.016
0.011
0.014
0.016
0.012
0.006
-
-
c' α
Table (5.9): One-dimensional consolidation test results for RU & RL stratums
0.120
0.094
0.095
0.142
0.177
0.175
0.156
0.141
0.055
0.037
0.056
0.053
0.034
0.015
-
-
cα
0.044
0.038
0.038
0.038
0.046
0.050
0.058
0.066
0.039
0.043
0.040
0.046
0.037
0.033
-
-
cα/cc
Ratio
IL+Late Long Sec
(IL+FS)H
(IL+FS)V
(IL+FS+Perm)H
(IL+FS+Perm)V
IL+FS+Loop
IL+Long Sec
IL+ Sec
IL+Long Sec
(IL+FS+Perm)H
(IL+FS+Perm)V
IL+FS+Loop
IL+ Sec
IL+Late Long Sec
(IL+FS)H
(IL+FS)V
Consolidation Test
Type
220
Ezbet ElDomyati Motoubes
9.05 - 9.95
D-13
4
10.75 - 11.65
D-11
8.05 - 8.90
D-8
3
10.25 - 11.15
10.25 - 11.15
D-7
D-7
8.45 - 9.25
2
D-5
9.65 - 10.30
8.45 - 9.25
1
Depth (m)
D-5
D-2
Sample BH
Site Location
No.
No.
9.50
11.20
8.45
10.70
10.70
8.85
8.85
9.95
(m)
69.00
55.89
74.85
76.36
81.53
26.74
64.10
77.42
%
340.12
294.70
294.26
319.49
352.25
172.64
245.99
352.85
%
Mean Organic Moisture
Depth Content Content
5.631
5.837
6.171
6.371
6.105
3.729
5.381
5.668
eo
Void
Ratio
3.100
4.000
4.300
2.950
3.400
2.000
2.300
2.700
%
εvo
kPa
σ` p
OCR
168.0 2.31
162.0 2.22
148.0 2.09
142.0 2.00
71.4 160.0 2.24
73.2 127.0 1.73
70.5 117.0 1.66
72.7
70.7
72.0 190.0 2.64
kPa
σ` vo
2
6.00E-03
2.95E-03
1.10E-02
2.35E-03
1.75E-03
1.90E-03
ch
5.50E-03
1.75E-03
(cm /s)
cv
(cm2/s)
Ave. in recompn.
1.74E-03
9.85E-04
5.10E-03
6.80E-04
3.80E-04
2.23E-04
(cm2/s)
cv
1.38E-03
2.95E-04
2
(cm /s)
ch
Ave. in compn.
0.081
0.137
0.080
0.067
0.060
0.073
0.077
0.066
c' r
0.537
0.933
0.574
0.494
0.426
0.345
0.491
0.440
cr
0.533
0.641
0.404
0.600
0.625
0.431
0.568
0.557
c' c
3.531
4.383
2.897
4.423
4.441
2.038
3.621
3.714
cc
0.152
0.213
0.198
0.112
0.096
0.169
0.136
0.118
c r /c c
Ratio
Table (5.10): One-dimensional consolidation test results for D stratum
0.018
0.032
0.030
0.030
0.026
0.019
0.037
0.030
c' α
0.122
0.219
0.217
0.223
0.183
0.089
0.234
0.198
cα
0.045
0.046
0.057
0.045
0.042
0.051
0.066
0.049
cα/cc
Ratio
IL+Late Long Sec
IL+ Sec
IL+FS+Loop
(IL+FS)H
(IL+FS)V
(IL+FS+Perm)H
(IL+FS+Perm)V
IL+Long Sec
Consolidation Test
Type
5.8.1 Primary Consolidation Behavior
Figures (5.21, 5.22, & 5.23) show the typical ε-log σ` EOP compression curves
for RU, RL, and D Stratums. The compression curves are characterized by the
S-shape and clear break at the preconsolidation pressure (σ`p). Preconsolidation
pressure σ`p was obtained using Casagrande’s graphical method. Values of σ`p
are in the range of 67-132 kPa, 190-280 kPa, and 117-190 kPa for RU, RL, and
D stratum respectively. This implies that the in-situ over consolidation ratio
(OCR) are in the ranges 1.4-2.5, 2.2-3.4, and 1.7-2.6 for RU, RL, and D
respectively, given that the mean overburden pressure (σ`vo) for the tested
samples are 49-57 kPa, 84-88 kPa, and 70-73 kPa respectively. The mean
overburden pressure (σ`vo) and the ranges of preconsolidation pressure (σ`p) are
indicated with arrows on the plots. Figures (5.24 & 5.25) show the variation of
σ`vo, σ`p, OCR, and εvo with depth.
For six samples, two of each stratum, one load increment was allowed to
undergo secondary compression for one week to assess aging effect on
preconsolidation pressure (σ`p). This was followed for three samples, one of
each stratum, in recompression range, and three samples, one of each stratum,
in compression range, between σ`p and 2σ`p. The preconsolidation pressure
(σ`p) due to aging in recompression range was 132.0 kPa, 250.0 kPa, and 190.0
kPa, and due to aging in compression range was 200.0 kPa, 670.0 kPa, and
300.0 kPa for RU, RL, and D respectively.
The quality of the samples obtained using the sampling technique described
earlier and the procedures followed for specimen preparation, were evaluated
based on the SQD method suggested by Andresen and Closeted (1979). In this
characterization, the magnitude of volumetric strain (εvo) caused by
reconsolidation to the in-situ vertical stress σ`vo in an oedometer test is
determined and compared to the scale shown in Table (2.11). The specimens’
qualities were designated as very good for 3 specimens, good for 13 specimens,
fair for 4 specimens, and disturbed for 1 specimen according to this scale.
221
σ’vo σ’p
0
RU-15-(IL+Long S)
RU-3-(IL+Late LS)
RU-13-(IL+FS)V+Perm
10
RU-13-(IL+FS)H+Perm
Vertical Strain, εv ( % )
RU-9-(IL+FS+Loop)
RU-8-(IL+Sec)
20
30
40
50
60
70
10
100
1000
10000
Vertical Effective Stress, σ'v ( kPa)
Fig. (5.21): Typical ε-log σ` EOP compression curves for RU stratum
222
σ’vo
σ’p
0
RL-5-(IL+Sec)
RL-17-(IL+FS)V
10
RL-17-(IL+FS)H
RL-6-(IL+Long S)
V ertical Strain, εv ( % )
RL-18-(IL+Late LS)
20
RL-12-(IL+FS)V+Perm
RL-12-(IL+FS)H+Perm
RL-11-(IL+FS+Loop)
30
40
50
60
70
10
100
1000
10000
Vertical Effective Stress, σ'v ( kPa)
Fig. (5.22): Typical ε-log σ` EOP compression curves for RL stratum
223
σ’vo σ’p
0
D-7-(IL+FS)V
D-7-(IL+FS)H
D-2-(IL+Long S)
10
D-13-(IL+Late LS)
Vertica l Stra in, εv ( % )
D-5-(IL+FS)V+Perm
D-5-(IL+FS)H+Perm
20
D-8-(IL+FS+Loop)
D-11-(IL+Sec)
30
40
50
60
70
10
100
1000
10000
Vertical Effective Stress, σ'v ( kPa)
Fig. (5.23): Typical ε-log σ` EOP compression curves for D stratum
224
225
Elevation, ms
20
0
RU
50
RL
100
150
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
0
RU
100
RL
200
σ'p, (kPa)
300
Depth, ms.
Depth, ms.
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
0
1
RU
2
OCR
RL
3
4
9
8
7
6
5
4
3
2
1
0
20
19
18
17
16
15
14
13
12
11
10
Fig. (5. 24): The variation of σ`vo, σ`p, OCR, and εvo with depth for RU & RL stratums
Sand, Coarse to Med., Trace of Silt & Fine
Gravel, Very Dense, Grey
19
20
18
19
17
18
17
16
15
Med. Stiff to Stiff Silty Clay, Dark Grey,
interlayers of Grey Silty Sand & Grey Sandy
Silt
15
16
14
14
12
11
10
9
8
7
6
5
4
3
2
1
0
13
Silty Muck to Peaty Muck, Dark Reddish
Brown to Black (RL)
Soft to Med. Stiff Silty Clay, Dark Grey,
interlayers of Grey Silty Sand
Organic Silty Clay, Grey to Black (RU)
Soft to Med. Stiff silty Clay, Dark Grey
Very Stiff Silty Clay, Dark Brown
GWT
G. S.
13
12
11
10
9
8
7
6
5
4
3
2
1
0
Depth, ms.
σ'v , (kPa)
Depth, ms.
0
2
A B
1
C
RU
3
4
5
D
RL
6
7
Volumetric Strain, εvo %
8
226
Elevation, ms
20
19
18
20
19
18
17
0
50
D
100
150
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
0
100
D
200
300
σ'p , (kPa)
400
Depth, ms.
Depth, ms.
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
0
1
2
OCR
D
3
4
Fig. (5.25 ): The variation of σ`vo, σ`p, OCR, and εvo with depth for D stratum
Stiff Silty Clay, Dark Brown, intercalations of
Yellowish Brown Silty Sand
16
16
17
15
15
14
13
Med. Stiff Silty Clay, Pale Blue, Minute
Fragments of White Limestone
12
13
14
11
10
12
Peaty Muck, Reddish Brown to Dark Reddish
Brown (D)
9
8
7
6
5
4
3
2
1
0
11
10
9
8
7
6
5
4
Soft to Med. Stiff Silty Clay, Dark Grey,
interlayers of Grey Med. To Fine Sand &
Grey Silty Sand
Stiff Silty Clay, Dark Brown to Grey
2
3
GWT
G. S.
1
0
Depth, ms.
σ'v , (kPa)
Depth, ms.
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
0
A B
1
2
C
3
4
D
5
D
6
Volumetric Strain, εvo %
7
8
Due to the S-shape of the compression curve, the compression index (Cc) of the
soil changes significantly over the stress range investigated. It was found that
Cc values for RU stratum vary within the range 0.66-1.43 with mean value of
1.05, for RL stratum Cc vary within the range 2.1-3.77 with mean value of 2.86,
and for D stratum Cc vary within the range 2.0-4.4 with mean value of 3.631.
In this research, one sample of every stratum RU, RL, and D was unloaded near
σ`p incrementally to less than σ`vo then subsequently reloaded incrementally to
the desired effective stress. Values of recompression index Cr for RU, RL, and
D stratums, measured from the developed loops, are 0.165, 0.458, and 0.574
respectively. On the other hand, the slope of the tangent at σ`vo for the other εlog σ` curves were measured, in a trial to estimate an approximate value of
recompression index Cr Tables (5.9 & 5.10). For the above three samples, at
the end of the scheduled load sequence, samples were then incrementally
unloaded again to estimate the swelling index (Cs). The values of swelling
index (Cs) were 0.25, 0.35, and 0.43 for RU, RL, and D respectively.
Figure (5.26) shows the variation of the coefficient of consolidation in the
vertical direction (cv) with vertical effective stress. The average vertical
coefficients of consolidation in the recompression (σ`vo- σ`p) are in the ranges
3x10-4 – 1x10-3 cm2/sec, 3x10-3 – 8x10-3 cm2/sec, and 1.8x10-3 – 1x10-2 cm2/sec
for RU, RL, and D stratums respectively. The average vertical coefficients of
consolidation in the compression (σ`p- 3 σ`p) are in the ranges 1.4x10-4 –
3.7x10-4 cm2/sec, 3.5x10-4 – 2.7x10-3 cm2/sec, and 2.2x10-3 – 5.1x10-3 cm2/sec
for RU, RL, and D stratums respectively. As vertical effective stress increases
permeability decreases and necessarily cv decreases continuously, especially
once the soil is loaded well into NC region in the range from σ`p to 2 σ`p.
Figure (5.27) shows the variation of the coefficient of consolidation in the
horizontal direction (ch) with horizontal effective stress. The average horizontal
coefficient of consolidation in the recompression (σ`vo- σ`p) is 2.1x10-4 cm2/sec,
227
1.5x10-2 cm2/sec, and 3.6x10-3 cm2/sec for RU, RL, and D stratums
respectively. The average horizontal coefficient of consolidation in the
compression (σ`p - 3 σ`p) are 1.3x10-4 cm2/sec, 3.9x10-3 cm2/sec, and 8.4x10-4
cm2/sec for RU, RL, and D stratums respectively. As horizontal effective stress
increases permeability decreases and necessarily ch decreases continuously,
especially once the soil is loaded well into NC region in the range from σ`p to
2 σ`p.
Therefore, the duration of primary consolidation (tp=t100) increased
significantly throughout the tests from increment to increment as effective
stress increase. The duration of primary consolidation (tp=t100) vary from less
than a minute in the early increments to several hours at the maximum stress.
Figure (5.28) shows the variation of time to the EOP (tp) with effective stress.
5.8.2 Secondary Compression Behavior
As can be seen from above, the primary consolidation of highly organic soil
(RL & D stratums) is very rapid and relatively rapid for organic silt and clay
(RU Stratum) in laboratory. This was confirmed in the field (Mesri et al.,
1997), which suggests that significant settlement occurs due to secondary
compression under a constant effective stress. On the other hand, this long-term
compression has no end within the time of engineering interest (Berry and
Poskitt, 1972).
The values of secondary compression index (Cα) varied considerably as a
function of the organic content. It was found that values of Cα for RU stratum
vary within the range 0.0154 – 0.0558 with mean value of 0.0418, for RL
stratum Cα vary within the range 0.0937 – 0.1768 with mean value of 0.1376,
and for D stratum Cα vary within the range 0.0894 – 0.2335 with mean value of
0.1854.
228
RU-15-(IL+Long S)
2
Coeffcient of Consolidation, cv (cm /sec
1.E+00
RU
RU-3-(IL+Late LS)
1.E-01
RU-13-(IL+FS)V+Perm
RU-9-(IL+FS+Loop)
RU-8-(IL+Sec)
1.E-02
1.E-03
1.E-04
1.E-05
10
100
1000
10000
1.E+00
RL-5-(IL+Sec)
2
Coeffcient of Consolidation, cv ( cm /sec
Vertical Effective Stress, σ' v ( kPa)
RL
RL-17-(IL+FS)V
1.E-01
RL-6-(IL+Long S)
RL-18-(IL+Late LS)
RL-12-(IL+FS)V+Perm
1.E-02
RL-11-(IL+FS+Loop)
1.E-03
1.E-04
1.E-05
10
100
1000
10000
1.E+00
D-11-(IL+Sec)
2
Coeffcient of Consolidation, cv ( cm /sec
Effective Vertical Stress, σ'v ( kPa)
D
D-7-(IL+FS)V
1.E-01
D-2-(IL+Long S)
D-18-(IL+Late LS)
D-5-(IL+FS)V+Perm
1.E-02
D-8-(IL+FS+Loop)
1.E-03
1.E-04
1.E-05
10
100
1000
10000
Effective Vertical Stress, σ' v ( kPa)
Fig. (5.26): The variation of coefficient of consolidation in the vertical
direction cv with vertical effective stress for RU, RL, and D stratums.
229
RU-13-(IL+FS)H+Perm
2
Coeffcient of Consolidation, ch ( cm /sec
1.E+00
RU
1.E-01
1.E-02
1.E-03
1.E-04
1.E-05
10
100
1000
10000
Effective Vertical Stress, σ' v (kPa)
RL-17-(IL+FS)H
RL-12-(IL+FS)H+Perm
2
Coeffcient of Consolidation, ch ( cm /sec
1.E+00
RL
1.E-01
1.E-02
1.E-03
1.E-04
1.E-05
10
100
1000
10000
Effective Vertical Stress, σ' v (kPa)
D-7-(IL+FS)H
D-5-(IL+FS)H+Perm
2
Coeffcient of Consolidation, ch ( cm /sec
1.E+00
D
1.E-01
1.E-02
1.E-03
1.E-04
1.E-05
10
100
1000
10000
Effective Vertical Stress, σ'v (kPa)
Fig. (5.27): The variation of coefficient of consolidation in the vertical
direction ch with vertical effective stress for RU, RL, and D stratums.
230
1000
RU
tp (minutes)
100
10
RU-15-(IL+Long S)
RU-3-(IL+Late LS)
1
RU-13-(IL+FS)V+Perm
RU-9-(IL+FS+Loop)
RU-8-(IL+Sec)
RU-13-(IL+FS+Perm )H
0.1
10
100
1000
10000
Vertical Effective Stress, σ'v ( kPa)
1000
RL
tp (minutes)
100
10
RL-5-(IL+Sec)
RL-17-(IL+FS)V
RL-6-(IL+Long S)
RL-18-(IL+Late LS)
1
RL-12-(IL+FS)V+Perm
RL-11-(IL+FS+Loop)
RL-12-(IL+FS+Perm)H
RL-17-(IL+FS)H
0.1
10
100
1000
10000
Effective Vertical Stress, σ'v ( kPa)
1000
D
tp (minutes)
100
10
D-11-(IL+Sec)
D-7-(IL+FS)V
D-2-(IL+Long S)
D-18-(IL+Late LS)
1
D-5-(IL+FS)V+Perm
D-8-(IL+FS+Loop)
D-5-(IL+FS+Perm )H
D-7-(IL+FS)H
0.1
10
100
1000
10000
Effective Vertical Stress, σ'v ( kPa)
Fig. (5.28): The variation of variation of time to the EOP (tp) with effective
stress for RU, RL, and D stratums.
231
CHAPTER 6
ANALYSIS AND DISCUSSION OF RESULTS
6.1
Introduction
An organic soil is an accumulation of partially decomposed and disintegrated
plant remains, in various stages of decomposition or preservation, recently
derived by physico-chemical and biochemical processes, and retains a
distinctive texture, color and odor. Organic soils commonly occur as extremely
soft, wet, unconsolidated surficial deposits normally as an integral part of
wetland systems. They may also occur as strata beneath other surficial deposits.
As in conventional soil mechanics, organic soil is considered to be particulate
material and can simultaneously contain the three phases, i.e. solid and liquid
and gas phases. The solid phase of organic soil consists of two components:
organic matter and inorganic earth material. The relative proportion of these
components and their specific nature determine the physical and geotechnical
properties of the soil (Edil, 2003). On the other hand, soil organic matter is
complex both chemically and physically, and a variety of reactions and
interactions between the mineral soil and the organic matter is possible (Oades,
1989). Therefore, organic matter in soil may be responsible for high plasticity,
high shrinkage, high compressibility, low shear strength, and their wide range
of hydraulic conductivity.
Also, organic soils are well known for their high variability in soil properties,
especially in organic contents. Therefore, physical and geotechnical properties
of organic soils show a great variation both spatially and with depth, such that
samples obtained within a few feet of each other may exhibit vastly different
mechanical behavior, depending on the type and amount of organic matter.
232
However, in spite of physical, index, and engineering properties of the various
types of organic soils are highly variable and significantly different from those
of inorganic soils, it has been established that the same fundamental
mechanisms and factors determine the behavior and properties of both
inorganic soils and organic soils (Samson and La Rochelle, 1972; Lefebvre et
al., 1984; Mesri et al, 1997; Mesri and Ajlouni, 2007; Santagata, et al, 2008).
The main objective of this study is to characterize the general and engineering
properties of the organic soils found in Egypt. The study is focused toward
evaluating compressibility characteristics in order to devise suitable design
parameters for settlement analysis and to assess the extent of problematic
nature of buried organic deposits found in Egypt, in term of compressibility,
regarding the usual loading scenarios and, also, compared with those highly
compressible surficial deposits typically encountered all over the world.
Based on the database previously mentioned, two sites located at West-Delta
known to have the thickest and most extensive organic deposits, were explored
and undisturbed organic soil samples were obtained. One site, located at
Robaomaah village- Mahmoudia - Bohira governorate, has two stratums:
shallow one (RU) at depth 4.0-6.5 m classified as highly organic silty clay, and
deep one (RL) at depth 10.0-14.0 m classified as peaty muck. The other,
located at Ezbet El-Domyati - Motoubes - Kafr -Elsheikh governorate, has a
deep stratum (D) at depth 8.0-12.0 m classified as peaty muck.
A comprehensive experimental program was developed and carried out on the
organic soil samples obtained from the two sites. Obtained undisturbed samples
were inspected visually, and preliminary laboratory tests were performed to
characterize the organic soils encountered. Engineering properties evaluated in
this
research
include
undrained
shear
strength,
permeability,
and
compressibility. The experimental program test results were presented in
chapter 5.
233
Edil (1994) emphasizes the importance of characterizing peat and organic soils
by certain index parameters, to provide a basis for comparison of results of
mechanical tests. Huat (2004) concluded that there has been virtually no
research to correlate different structural types of peat and organic soils and
their index and engineering properties. Hobbs (1986) suggests that it is
convenient to relate the basic geotechnical properties of organic soils to some
of the easily determined index parameters such as water content and organic
content. In this chapter, the available published data on index, chemical and
engineering properties of various organic soils were compiled and categorized,
based on the recommended classification system, to correlate various types of
organic soils and their index, chemical and engineering properties, and to
provide a basis for comparison of the results. The test results, of the current
study, were discussed, compared with the compiled data and correlations
between various index and engineering properties were investigated.
6.2 General Characteristics of Encountered Organic Soil
The general characteristics of the encountered organic soils include the
determination of physical, index, and chemical properties and the soil
classification. The physical and index properties include the determination of
organic content, water content, unit weight, initial void ratio, specific gravity,
Atterberg limits, and particle size distribution. Also, the differences in the fiber
contents, the pore spaces, and the perforated plant structure of the organic soil
samples, in its initial state and after compression under high stress (3200 kPa),
were observed using the Scanning Electron Microscope (SEM). The chemical
properties include the determination of pH level, injurious chemical
compounds of groundwater and organic soils (sulfates and chlorides content),
and the different minerals constituting the inorganic portion of organic soils
encountered (X-Ray diffraction analysis). Table (6.1) shows a comprehensive
comparison of some index and chemical properties of different types of organic
soils (based on compiled data presented in Tables 1-5 in appendix A) and
234
mineral soils. Summary of index, and chemical properties of the organic soils
encountered (RU, RL, and D stratums) are presented in Table (6.2). In the
following sections, the general characteristics of the organic soils encountered
will be discussed and compared to the compiled data.
The most distinctive characteristic of organic soils in Egypt is occurring as
deposits buried under alluvial soils thousands of years ago. The buried nature
means that; plant remains stop to accumulate thousands of years ago, there is
an enormous reduction of oxygen supply which reduces the aerobic microbial
activity, and an encouragement of anaerobic decay which is much less rapid.
This leads to that organic soils encountered are under moderate degree of
decomposition and preservation processes long time ago.
The three stratums encountered in this study RU, RL, and D is buried at depths
4.0 m, 10.0 m, an 8.0 m respectively. C14 dating for Holocene organic
sediments nearby Rosetta branch – West-Delta at depth ranging from 6 to 8
meters below ground surface, gave (4.595 + 55) to (5.870 + 70) B.P range of
age for this organic soil (Zayed, 1989). This is interpreting the disappearance of
fibrous structure as observed from SEM images (Figures 5.8 – 5.13) and that
detritus gradually becomes finer as indicated from particle size distribution
(Fig. 5.4). The material encountered consisting mainly of partly decomposed
plant material has an amorphous granular appearance, a sponge-like fabric, and
dark in color as evident from visual inspection.
The mean OCs for RU, RL, and D stratums are 16%, 57%, & 63% respectively.
The organic soils encountered were classified as peaty muck for RL & D, and
as highly organic silty clay for RU based on “Tentative ASTM Standard
classification system” previously described. On the other hand, specimens
obtained from the same tube within a 10 cm of each other exhibited vastly
different organic contents (Tables 5.7, 5.8, & 5.9) which confirm the high
variability of organic soil within each deposit.
235
Table (6.1): Comparison of some index and chemical properties, for different
types of soil, from literature.
Organic
content
Natural
water
content
Bulk
Density
Specific
Gravity
Acidity
%
(wo %)
kN/m3
Gs
pH
Fibrous peat
>75
200-1590
8.4-12.2
1.1-1.9
3.3-8.8
Amorphous peat
>75
200-875
9.2-12.3
1.59-1.73
4.1-7.3
Peaty muck
50-75
125-971
10.0-12.3
1.48-1.89
3.8-7.15
Silty or clayey muck
25-50
105-525
10.6-13.1
1.43-2.27
2.8-7.35
Highly organic silt/clay
10-25
29-234
11.6-19.4
1.9-2.67
7.0-8.6
Slightly organic silt/clay
1-10
22-155
14.8-18.4
1.88-2.69
8.0-8.5
11.2-19.9
2.68-2.72
>7
11.7-23.4
2.65-2.67
>7
Soil type
Cohesive soil
3-70
Cohesionless soil
-
Table (6.2): Summary of index and chemical properties of RU, RL, and D
stratums
Property
RU
RL
D
Organic content (%)
7-29
37-69
29-77
Natural moisture content (%)
57-138
139-284
173-356
Specific gravity of solids
2.36-2.60
1.60-1.90
1.54-1.86
In situ void ratio
1.67-2.95
3.25-5.85
3.73-6.37
Degree of saturation (%)
100
95-100
98-100
3
12.21-16.51
10.85-12.42
10.13-12.42
Dry Unit Weight (kN/m )
5.44-10.53
2.90-4.81
2.22-4.55
Liquid Limit (%)
92-179
196-393
221-510
After Drying (% of initial LL)
32-46
30
28
Plastic Limit (%)
28-67
86-266
67-333
Plasticity Index (%)
65-130
84-227
141-224
pH Level
7.1-7.5
6.9-7.6
5.4-6.6
Bulk Unit Weight (kN/m )
3
236
According to Hobbs (1986), there is a good correlation between pH value and
organic content (Fig. 2.18). The more acid the organic soil, the better the plant
remains are preserved. This is because decomposition generally tends to be
most active in neutral to weakly alkaline conditions pH value (7 - 7.5). In this
study, pH value was determined for almost all samples. The lowest values were
recorded for D stratum, ranging between 5.4 and 6.6 with an average of 6.0,
which reported the highest average organic content of 63%. To the opposite
side, RU and RL average pH values were 7.4 and 7.3, yielding organic content
of 16% and 57% respectively. Moreover, D samples were lighter in color than
RU and RL samples, which means that they are less decomposed.
The particle size distribution for RU samples indicates that >98% of the soil is
finer than 75 µm, with >70% in the clay size fraction (<2 µm). The particle size
distribution for RL samples indicates that >92% of the soil is finer than 75 µm,
with >60% in the silt size fraction (>2 µm), and 10-30% in the clay size
fraction (<2 µm). The particle size distribution for D samples indicates that
>82% of the soil is finer than 75 µm, with >65% in the silt size fraction (>2
µm), and 10-15% in the clay size fraction (<2 µm). It is evident that the average
particles size of D samples is greater than that of RL samples which confirms
that are less decomposed.
The X-Ray diffraction analysis indicates that the inorganic portion of the
organic soils consists of clay and non-clay minerals (Table 5.3). The clay
minerals include montmorillonite [(Al167 Mg33) Si4 O10 (OH)2], illite [Ky (Al
Fe4 Mg6) Si8 Aly O20 (OH)4], and kaolinite [Al4 Si4 (OH)]. The non-clay
minerals include halite [NaCl], calcite [CaCo3], quartz [SiO2], gypsum
[CaSO4], hematite [Fe2O3], and feldspars [K or Na or Ca Al Si3 O8]. The
analysis indicates that RU samples are richer in clay minerals than RL samples
while often missed in D samples. It also indicates the existence of different
percentage of non-clay minerals with predominant existence of halite and
calcite minerals in all samples with D samples have the highest concentration.
237
Organic contents of the undisturbed samples were measured using loss on
ignition method. Specimens were ignited at temperature 440° C for 12-18 h till
organic matter was completely fired (ASTM D2974 – 00). Five hours was not
enough for complete firing as concluded by Arman (1971). Moreover, the
increase in loss on ignition was only 0.5-1.0% when seven samples of highly
organic soils (D samples) were re-fired at temperature 550° C according to
ECP 202-01, hence not significant for practical considerations. Therefore,
igniting at temperature 440° C could be specified in ECP instead of 550° C.
One sample (RU-8) classified as highly organic silty clay was analyzed using
X-ray diffraction analysis in its initial state and after it has been ignited. The
clay minerals encountered in its initial state were missed after it has been
ignited (Table 5.3), which indicates that clay minerals were destructed through
ignition process. This confirms the fact that errors in loss on ignition method
increase with increasing mineral content (Hartlėn and Wolski, 1996). Also,
specified time required for complete firing needs to be more investigated.
The type and amount of organic matter (OC) has a considerable effect on the
physical and index properties of organic soil. Soil organic matter may be
responsible for high water holding capacity, high plasticity, and high shrinkage.
In general the greater the OC the greater the water content, void ratio, liquid
limit, plasticity index, and the lower the specific gravity.
The void ratio of organic soils is generally higher than that of mineral soils.
The average void ratio for RU, RL, and D stratums are 2.356, 4.461, & 5.612
respectively, which is generally higher than that of mineral soils. It is evident
that as OC increases and degree of decomposition decreases void ratio
increases. Figure (6.1) illustrates the correlation between OC and initial void
ratio.
238
8
7
Void Ratio, eo
6
5
4
3
RU
2
RL
1
D
0
0
20
40
60
80
100
Loss on Ignition, %
Fig. (6-1): Correlation between loss on ignition and void ratio
The average natural moisture content for RU, RL, and D stratums are 93%,
222%, & 304% respectively. It is evident that as OC increases natural moisture
content increases, which indicates the high water-holding capacity of soil
organic matter. Table (6.3) illustrates the data of moisture content determined
compared with categorized compiled data which indicates that the data is
comparable.
Figure (2.20) shows the plot of water content versus loss on ignition complied
by O′Loughlin & Lehane (2003) for peat and organic soils from all around the
world. The relationship shown is linear up to OC = 80%. Figure (6.2) illustrates
the correlation between OC and natural moisture content of current study
within an envelope diagram drawn for the data compiled by O’Loughlin and
Lehane (2003). The relationship is linear, except that the majority of the data is
lying to the left of curve drawn by O’Loughlin and Lehane (2003), i.e.
somewhat lower than expected, indicating that some drying occurred during
storage.
239
240
>75
Amorphous Peat
25-50
Silty or Clayey Muck
50-75
10-25
Highly Organic silt or Clay
Peaty Muck
1-10
Slightly Organic silt or Clay
Category
RU
RL
D
Compiled Data
-
-
1
7
1
-
-
138
79-114
57
-
6
2
-
-
-
209-294
139-158
-
-
3
10
2
-
-
341-353
248-356
193-246
-
-
200-875
125-971
105-525
29-234
22-155
Organic
Moisture
Moisture
Moisture
No. of
No. of
No. of
Range of Moisture
Content
Content
Content
Content
samples
samples
samples
Content
%
%
%
%
%
Organic Soil Classification
Table (6.3): Summary of Moisture Content of RU, RL, and D Samples
100
Loss on Ignition (%)
90
80
70
60
50
RU
RL
40
D
30
O'Loughlin & Lehane, 2003
20
MacFarlane & Rutka, 1961
Miyakawa, 1960
10
Env. of data,2003
0
0
200
400
600
800
1000
1200
1400
1600
Natural Water Content (% )
Fig. (6.2): Correlation between loss on ignition and natural moisture content
compared with the data compiled by O’Loughlin and Lehane (2003)
The specific gravity of organic soil is highly variable compared with that of
mineral soils. Limited number of tests conducted in this study to confirm the
ranges reported in literature. The average specific gravity for RU, RL, and D
stratums are 2.49, 1.75, & 1.7 respectively, which is less than that of mineral
soils (2.6-2.8) and higher than that of peat (1.5). It is evident that as OC
increases specific gravity decreases. Table (6.4) illustrates the data of specific
gravity determined compared with categorized compiled data which indicates
that the data is comparable. Figure (6.3) illustrates the correlation between OC
and the specific gravity of current study compared with the data compiled by
O’Loughlin and Lehane (2003) and den Hann (1997). It is evident that the
specific gravity values of current study are within the envelope diagram drawn
for the compiled data.
241
242
>75
Amorphous Peat
25-50
Silty or Clayey Muck
50-75
10-25
Highly Organic silt or Clay
Peaty Muck
1-10
Slightly Organic silt or Clay
Category
RU
-
-
-
5
-
-
-
-
2.36-2.6
-
Organic
No. of Specific
Content
samples Gravity
%
Organic Soil Classification
-
1
1
-
-
-
1.6
1.9
-
-
No. of Specific
samples Gravity
RL
2
2
1
-
-
1.69
1.54-1.71
1.86
-
-
No. of Specific
samples Gravity
D
Table (6.4): Summary of Specific Gravity of RU, RL, and D Samples
1.59-1.73
1.48-1.89
1.43-2.27
1.90-2.67
1.88-2.69
Range of Specific
Gravity
Compiled Data
3.5
RU
RL
3.0
D
Specific Gravity Gs
O'Loughlin & Lehane Env.
O'Loughlin & Lehane Line,2003
2.5
den Hann Line, 1997
2.0
1.5
1.0
0
10
20
30
40
50
60
70
80
90
100
Loss on Ignition (% )
Fig. (6.3): Correlation between loss on ignition and specific gravity compared
with the data compiled by O’Loughlin and Lehane (2003)
The unit weight (γ) of organic soil samples were calculated based on measured
weights of known volumes for each of the specimens used for engineering
tests. The average bulk unit weights for RU, RL, and D stratums are 14.4
kN/m3, 11.4 kN/m3 & 10.9 kN/m3 respectively. That is, as OC and natural
moisture content increases and specific gravity decreases bulk unit weight
decreases. Table (6.5) illustrates the data of bulk unit weight determined
compared with compiled data which indicates that the data is comparable.
Figure (6.4) shows the correlation between bulk unit weight and the loss on
ignition (OC) which confirms this fact.
The average dry unit weights for RU, RL, and D stratums are 7.6 kN/m3, 3.6
kN/m3 & 2.8 kN/m3 respectively. That is, as OC and natural moisture content
increases and specific gravity decreases dry unit weight decreases. Figure (6.5)
shows that, the dry unit weight decreases as the loss on ignition (OC) increases
which confirms this fact.
243
244
Organic
Content %
1-10
10-25
25-50
50-75
>75
Categori
Slightly Organic silt or Clay
Highly Organic silt or Clay
Silty or Clayey Muck
Peaty Muck
Amorphous Peat
Organic Soil Classification
-
-
1
7
1
No. of
samples
-
-
12.9
12.2-15.3
16.5
kN/m
3
Bulk Unit Weight
RU
-
6
2
-
-
-
10.9-11.5
11.4-12.4
-
-
No. of Bulk Unit Weight
3
samples
kN/m
RL
3
10
2
-
-
No. of
samples
10.0-11.0
10.1-11.6
11.3-12.4
-
-
kN/m
3
Bulk Unit Weight
D
Table (6.5): Summary of Bulk Unit Weight of RU, RL, and D Samples
09.2-12.3
10.0-12.3
10.6-13.1
11.6-19.4
14.8-18.4
3
kN/m
Range of Bulk Unit Weight
Compiled Data
Bulk Unit Weight, t/m
3
1.8
RU
RL
D
1.6
1.4
1.2
1
0.8
0
20
40
60
80
10 0
Loss on Ignition, (% )
Fig. (6.4): Correlation between bulk unit weight and the loss on ignition (OC)
Dry Unit Weighty, t/m3
1.2
RU
RL
D
1.0
0.8
0.6
0.4
0.2
0.0
0
20
40
60
80
1 00
Loss on Ignition, (%)
Fig. (6.5): Correlation between dry unit weight and the loss on ignition (OC)
245
For an organic particle, the rigidity and thickness of the zone of adsorbed water
is governed by the cation exchange capacity of the tissue and the chemistry of
the water; the higher the cation exchange capacity, the stronger the adsorbtion
complex, and the greater the interparticle adherence. Because the specific
gravity of the cell walls of plants is half that of clay minerals, the adsorption
complex in organic-rich soils is approximately twice as effective as that in clay.
This explains why organic-rich soils possess very high liquid limits as
compared with clays of similar cation exchange capacity. Also, the liquid limit
declines as the degree of humification increases, in other words as the
adsorption complex is weakened due to the destruction of plant material.
Organic fine substances are negatively charged and display a substantial cation
exchange capacity which increases with degree of humification and strongly
influenced by the hydrogen concentration in the pore water. Cations such as
Ca, Mg, K, Na and also Fe and Al, replace hydrogen at the exchange sites of
the organic polymolecules. The cation exchange capacity of very fine humic
substances may be as high as 1.5 - 5.0 meq/g (Terzaghi et al, 1996). In the less
organic soil most of the cation exchange ability is saturated by metallic cations
from mineral matter in the soil. As the organic content raises the quantity of
exchangeable hydrogen ions slowly increase as shown in Figure (2.18b).
The Atterberg limits, similar to natural water content, showed significant
variation between samples as a result of differences in organic content. The
average liquid limit was 156%, 326%, and 408% for RU, RL, and D stratums
respectively, while the average plastic limit was 45%, 166%, and 230%
respectively. This implies that average plasticity index was 111%, 160%, and
178% for RU, RL, and D stratums respectively. Figures (6-6 & 6-7) show the
correlations between liquid limit and plasticity index and OC which indicate
that as OC increases the Atterberg limits increase. Also, it is evident the
consistency of the data for RU samples of low OC and the highly scatter of the
data for RL and D samples of high OC.
246
600
Liquid Limit, (%)
500
400
300
200
RU
RL
D
100
0
0
20
40
60
80
10 0
Loss on Ignition, (%)
Fig. (6.6): Correlation between liquid limit with the loss on ignition
Plasticity Index,Ip
250
200
150
100
RU
RL
D
50
0
20
40
60
80
loss on Ignition, (% )
Fig. (6.7): Correlation between plasticity index with the loss on ignition
247
10 0
On four samples, a second determination of the liquid limit after oven drying
the soils, the liquid limit was decreased to 39-46 %, 30%, and 28% of the liquid
limit of the nondried soil for RU, RL, and D samples respectively, which is
lower than the criteria (75 %) recommended by USCS to identify organic soil.
Also, RU samples were classified as CH, while RL and D samples were
classified as OH when plotted on Casagrande plasticity chart (Fig. 5.3).
Chemical analysis was conducted to determine the injurious chemical
compounds of soil and groundwater (pH, sulfates content and chlorides
content). Three representative organic soil samples from every stratum and
groundwater sample for each site were chemically analyzed. The results are
given in Tables (5.3 & 5.4). Avery high concentration of sodium chloride and
sulpher trioxide exists in both organic soil and groundwater, such that they are
highly aggressive with regarding to concrete and steel reinforcement.
6.3 Engineering Characteristics of Encountered Organic Soil
Peat and organic soils commonly occur as extremely soft, wet, unconsolidated
surficial deposits that are integral parts of wetland systems. They may also
occur as strata beneath other surficial deposits. Therefore, because of their
typical locations, i.e., near the surface, organic-rich soils are characterized by
their loose structure, high water content, and generally have small to medium
past pressures. As a result of their loose state, they have high permeability, high
compressibility, and low shear strength.
On the other hand, the most distinctive characteristic of organic soils in Egypt
is occurring as buried deposits thousands of years ago. The main objective of
this study is to characterize the engineering properties of the organic soils
found in Egypt in terms of undrained shear strength, permeability, and
compressibility with focusing the study toward evaluating compressibility
characteristics. Also, to assess the difference in mechanical behavior between
248
these buried deposits and those of severely problematic surficial deposits all
around the world.
The experimental program test results were presented in chapter 5. Engineering
properties of organic soils show a great variation such that samples obtained
within a few centimeters of each other exhibited different mechanical behavior
depending on the type and amount of organic matter (Tables, 5.6 – 5.10). The
distinct engineering properties shall be discussed in the following sections.
6.3.1 Undrained Shear Strength
Undrained shear strength was determined for almost all undisturbed samples,
by two methods: in compression device and using pocket penetrometer. The
undrained shear strength values obtained through unconfined compression tests
were referred to as Sun, while that obtained through pocket penetrometer tests
were referred to as Sup, and recorded as the in-situ undrained shear strength.
Also, the normalized undrained shear strength was obtained as (Su/σ`p). The
undrained shear strength data are summarized in Table (6.6), and compared
with compiled data in Tables (6.7 & 6.8).
Table (6.6): Summary of undrained shear strength data
Property
RU
RL
D
Organic content (%)
7-29
37-69
29-77
Overburden pressure (kPa)
49-57
84-88
70-73
Preconsolidation pressure (kPa)
67-132
190-280
117-190
OCR
1.4-2.5
2.1-3.2
1.7-2.6
S un (kPa)
4-24
4-77
12-92
S up (kPa)
20-60
28-162
38-105
S un /σ` p
0.03-0.25
0.01-0.34
0.07-0.40
S up /σ` p
0.21-0.32
0.11-0.65
0.35-0.58
249
Table (6.7): Strength parameters of peat soils from literature*
LOI
Peat
Tests
ø
wo
%
(%)
or
ø`
Su/σ`p
Su/σ`v
Authors
(°)
Antioch,
Algiers
TC
>75
230-1,000
-
-
0.48-0.6
Moran et al
(1958)
Burnaby
TC
>75
400-1,200
-
-
0.47-0.58
Lea &
Brawner
(1959)
Muskeg
ICU
77-88
375-430
50-60
-
0.38
Adams
(1961)
Moose
River
ICU
-
330-600
48
0.68
0.37
Adams
(1965)
MacFarlane
& Forrest
(1969)
Ozden &
Wilson
(1970)
Yasuhara
&
Takenaka
(1977)
Edil &
Dhowain
(1981)
Landva &
LaRochelle
(1983)
Ottawa
TC
>75
900-1,200
-
-
-
Muskeg
ICU
96
800
46
-
0.36
Omono
TC
-
-
-
0.54
-
Middleton
KoCU
20 (1)
31 (1)
64 (1)
500-600
50
54
57
-
0.38
0.41
0.42
Escminac
TC
RS
>75
1,240-1,380
40-50
32-40
-
-
San
Joaquin
TC
>75
200-500
44
-
-
Marachi et
al (1983)
Ohmia
Urawa
TC
>75
>75
960-1,190
980-1,260
51-55
53
-
0.55
0.52
Yamaguchi
et al
(1985a,c,d)
Haastrecht
KoCU (2)
KoCU (3)
79
669
-
0.42
0.42
Termmat et
al (1994)
Raheenmore Bog
ICU
ICD
ICE
DST
DSS
RS +
79-80
800-900
55
18-39
38
31
38
-
0.5-0.6
Farrel &
Hebib
(1998)
Middleton
TC
90-95
510-850
60
-
0.53a
Ajlouni
(2000)
* See next page
(1) Fiber content
(2)Biaxial test compression
250
(3) Biaxial test extention
Soil Type
Table (6.8): Strength parameters from present study and from literature* for
muck and organic silt and clay soils
ø
Authors
Tests
LOI
wo
Or
%
(%)
ø`
Su/σ`p
Su/σ`v
0.54
0.52
0.40
0.43
Muck
(°)
Tsushima et al (1977)
ICU
KoCU
ICD
KoCD
57-58
-
52
60
52
52
Oikawa & Miyakawa
(1980)
ICU
56-67
-
78
0.63
0.50
Tsushima & Oikawa
(1982)
ICU
ICD
56
-
51
50
-
0.40
0.38
Yamaguchi et al
(1985c)
ICU
ICU
70
40
-
-
-
0.43
0.40
Woliski et al (1989)
KoCU
DSS
50
-
-
-
0.47
0.38
Kanmuri et al (1998)
KoCU
KoCE
58
507
67
28
-
0.54
Tsushima & Mitachi
(1998)
KoCU (1)
60-70
560-680
-
-
0.54
PP (2)
UC (3)
37-69
139-284
-
0.45
0.12
-
PP
UC
29-77
173-356
-
0.46
0.19
-
Yamaguchi et al
(1985c)
ICU
10
-
-
-
0.33
Nishimura & Tanaka
(1998)
ICU
22
-
-
-
0.42
PP
UC
7-29
57-138
0.25
0.15
-
Mahmoudia peaty muck
Present study (2010)
Motoubes peaty muck
Organic Silt & Clay
Present study (2010)
Mahmoudia organic clay
Present study (2010)
(1) UU tests on Ko consolidated samples
(2) Pocket penetrometer
(3) Unconfined compression test
* As a starting point, the data obtained from publications by Yamaguchi et al. (1985c),
Termaat (1999), and Mesri and Ajlouni (2007) have been used.
251
The mean undrained shear strength (Sun) for RU, RL, and D stratums is 16, 33,
and 37 kPa respectively. The mean undrained shear strength (Sup) for RU, RL,
and D stratums is 32, 104, and 78 kPa respectively. It is evident that the mean
undrained shear strength values obtained through pocket penetrometer tests
(Sup) are twice or more the mean undrained shear strength values obtained
through unconfined compression tests (Sun). However, like all soils, the shear
strength of organic soils is directly related to the effective stress in the ground
and stress history of the deposit. It can be concluded that Sup values are more
represtative than Sun values, since they increase with depth (mean depth is 5.25,
12.0, and 10.0 m for RU, RL, and D respectively) and OCR ratio (mean OCR
ratio is 1.91, 2.68, and 2.19 for RU, RL, and D respectively). Also this
confirms that undrained shear strength values obtained through unconfined
compressive tests are usually scattered due to inevitable change of effective
stress and mechanical disturbances during the process from sampling to
laboratory testing as concluded by Tsushima and Mitachi (1998). Figure (6-8)
shows the relationship between Sun relative to Sup.
120
RU
Sun (Unconfined compressive strength), kPa
110
RL
100
D
90
80
70
60
50
40
30
20
10
0
0
10
20
30
40
50
60
70
80
90
100
110
120
S up (Pocket Penetrometer), kPa
Fig. (6.8): The relationship between undrained shear strength determined by
unconfined compressive strength (Sun) relative to that determined by pocket
penetrometer (Sup)
252
Data on undrained shear strength to consolidation pressure for organic-rich
soils from Tables (6.7 & 6.8) suggest that Su/σ`p or Su/σ`v are in the range of
0.36 to 0.68 for different shear testing procedures, which are high compared to
the Su(TC)/σ`p = 0.32 and Su(DSS)/σ`p = 0.22 to 0.28 for inorganic soft clay and
silt deposits (Terzaghi et al., 1996). Also, Magnan (1994) suggests value for
highly organic soils close to 0.5, while Farrel (1997) suggests value of 0.45.
Moreover, according to Mesri and Ajlouni (2007) the normalized undrained
shear strength Su/σ`p in the compression mode of shear for fibrous peat is
almost twice that of inorganic soft clay and silt deposits. They also concluded
that the fibrous structure is strongly responsible for high ratio of Su/σ`p for
fibrous peats, and that any biochemical degradation of the fiber structure can be
expected to lead to a reduction in property.
Figure (6.9) shows the variation of normalized undrained shear strength (Su/σ`p)
with OC. The mean normalized undrained shear strength (Sun/σ`p) for RU, RL,
and D stratums is 0.15, 0.12, and 0.19 respectively. The mean normalized
undrained shear strength (Sup/σ`p) for RU, RL, and D stratums is 0.25, 0.45, and
0.46 respectively. Again, the mean normalized undrained shear strength
(Sup/σ`p) values is more reasonable and comparable to that reported in literature,
since (Sup/σ`p) value for RU is comparable to Su(DSS)/σ`p of inorganic soft clay
and silt deposits, and (Sup/σ`p) values for RL and D is comparable to that of
highly organic soils taking into account the disappearing of fibrous structure of
encountered soils (Mesri and Ajlouni, 2007). Also the variation of Sup/σ`p data
with OC is comparable to that suggested by Termaat (1999) in Figure (3.6).
Figure (6.10) shows the variation of undrained shear strength (Su) determined
by both methods and normalized undrained shear strength (Su/σ`p) with depth.
It could be concluded that Sup and Sup/σ`p increases with depth (effective stress)
for RU and RL samples, while this criteria is not valid for Sun and Sun/σ`p. On
the other hand, for D samples, Su and Su/σ`p are more or less constant with
depth for both methods.
253
Normalized undrained shear strength,Sun/σ`p
1.0
RU
RL
0.8
D
0.6
0.4
0.2
0.0
0
10
20
30
40
50
60
70
80
90
100
Loss on Ignition (%)
(a)
Normalized undrained shear strength,Sup /σ`p
1.0
RU
RL
0.8
D
0.6
0.4
0.2
0.0
0
10
20
30
40
50
60
70
80
90
100
Loss on Ignition (% )
(b)
Fig. (6.9): The variation of normalized undrained shear strength (Su/σ`p) with
loss on ignition (OC); (a) Sun/σ`p, and (b) Sup/σ`p
254
σ` p , (kPa)
S u , (kPa)
0
GWT
3
Soft to Med. Stiff silty Clay, Dark Grey
4
5
6
7
7
Soft to Med. Stiff Silty Clay, Dark Grey,
interlayers of Grey Silty Sand
10
8
Depth, ms.
9
10
11
11
Silty Muck to Peaty Muck, Dark Reddish
Brown to Black (RL)
200
300
400
0
0
0
1
1
2
2
3
3
4
4
5
5
6
6
7
7
8
8
9
10
11
12
13
13
13
14
14
14
14
15
15
15
15
16
16
16
16
17
17
17
18
18
18
18
19
20
RU-un
RL-un
RU-p
RL-p
19
Sand, Coarse to Med., Trace of Silt & Fine
Gravel, Very Dense, Grey
19
20
19
RL
RU
S u , (kPa)
50
100
D
200
0
100
200
300
400
0
0
0.00
GWT
1
1
1.00
Stiff Silty Clay, Dark Brown to Grey
2
2
2.00
3
3
3
3.00
4
4
4
4.00
5
5
5.00
6
6
6.00
7
7
7
7.00
8
8
8
8.00
9
9
9
9.00
2
5
6
10
11
Soft to Med. Stiff Silty Clay, Dark Grey,
interlayers of Grey Med. To Fine Sand &
Grey Silty Sand
Peaty Muck, Reddish Brown to Dark Reddish
Brown (D)
Depth, ms.
1
G. S.
Depth, ms.
0
0
10
11
10
11.00
12
12
12.00
13
13
13
13.00
14
14
14.00
15
15
15.00
16
16
16.00
17
17
17.00
18
18
18.00
19
19
Med. Stiff Silty Clay, Pale Blue, Minute
Fragments of White Limestone
15
16
17
18
19
20
Stiff Silty Clay, Dark Brown, intercalations of
Yellowish Brown Silty Sand
D-un
D-p
20
20
RL-un
RL-p
D
0.2
0.4
0.6
D-un
D-p
0.8
10.00
11
12
14
RU-un
RU-p
S u /σ` p
σ` p , (kPa)
150
0.8
20
20
0
0.6
11
13
Med. Stiff to Stiff Silty Clay, Dark Grey,
interlayers of Grey Silty Sand & Grey Sandy
Silt
0.4
9
12
17
0.2
10
12
12
Elevation, ms
100
S u /σ` p
Depth, ms.
Elevation, ms
Organic Silty Clay, Grey to Black (RU)
6
9
0
2
5
8
200
1
Very
3
4
150
Depth, ms.
2
100
0
G. S.
1
50
Depth, ms.
0
19.00
20.00
Fig. (6.10): The variation of undrained shear strength (Su) determined by both
methods and normalized undrained shear strength (Su/σ`p) with depth.
255
6.3.2 Permeability Characteristics
Permeability of organic soils is one of the most important properties because it
controls the rate of both; consolidation and increase in the shear strength of the
soil (Hobbs, 1986). The physical structure and arrangement of constituent, i.e.,
fibers and granules, of organic soil greatly affect the void ratio and the size and
continuity of pores, resulting in a wide range of hydraulic conductivities. In
addition to the material structure, permeability of organic soils varies widely,
depending on amount of mineral matter, degree of consolidation, degree of
decomposition, chemical composition, and the presence of gas.
Permeability measurements of RU, RL, and D stratum were determined from
falling-head flow measurements during the secondary compression stage of IL
oedometer tests (Tavenas et al, 1983; Mesri, 1997) as effective vertical stress
increases from σ`vo to σ`vf. The values of the coefficient of permeability in the
vertical and horizontal direction (kv & kh) were determined for three samples,
one sample from each stratum (Figure 5.17). The in-situ coefficients of
permeability in the vertical and horizontal direction (kvo & kho) were estimated
by extrapolating the linear initial portions of the e-log k curves. Table (6.9)
presents the summary of the permeability characteristics of the soils.
Table (6.9): Summary of the permeability characteristics of the soils
Property
RU
RL
D
Organic content (%)
15-7*
56-68*
64-27*
Natural moisture content (%)
96-75*
208-217*
246-178*
2.951-2.506*
4.776-5.846*
5.381-3.729*
OCR
2.0-1.4*
2.2-2.1*
2.0-2.1*
k vo (cm/sec)
3.80E-08
1.20E-05
7.00E-07
k ho (cm/sec)
4.50E-08
8.00E-05
7.50E-08
C k -V
0.94
0.57
1.29
C k -H
0.7
0.88
0.91
C k /e o -V
0.32
0.12
0.24
C k /e o -H
0.28
0.15
0.24
In situ void ratio
* Soil properties for samples tested in the horizontal direction
256
The work performed demonstrates the highly variable nature of organic soils
especially in organic content. The OCs were varied over a fairly wide range for
samples obtained in close proximity to each other, accompanied by a varied
initial void ratios (porosity) and initial coefficients of permeability, especially
for highly organic soils (RL & D samples). Therefore, it was not possible to
evaluate the permeability anisotropy due to varied OCs. Even though, it can be
observed from the SEM images that the organic soil encountered is highly
porous in both vertical and horizontal directions.
Figure (6.11) shows the correlation between initial void ratios, eo, and initial
permeability, ko. Figure (6.12) shows the correlation between initial
permeability ko and loss on ignition (OC). It is evident that initial void ratios,
eo, and coefficients of permeability, k, increase as organic content (OC)
increase.
Figure (6.13) shows the data on initial vertical permeability kvo and horizontal
permeability kho of these three samples together with permeability data from the
literature on fibrous peat (Mesri and Ajlouni, 2007) and amorphous peat
deposits and peaty muck (Hobbs, 1986; Santagata et. al., 2008) within a frame
of reference permeability data on pure clay minerals montmorillonite, illite, and
kaolinite, as well as data on a large number of soft clay and silt deposits and
clean sand (Mesri and Ajlouni, 2007). Figure (6.14) shows the data on vertical
permeability kv and horizontal permeability kh of these three samples within the
same frame of reference permeability data.
It was revealed that RL & D stratums classified as peaty muck (highly organic
soil) possess medium initial permeability (ko); just below the range reported for
peats and around 100 to 1000 times the initial permeability of soft clay and silt
deposits. This is because highly colloidal (Figure 5.4), mostly decomposed
(Figures 5.10, 5.12), amorphous-granular soils tend to be less permeable than
well preserved fibrous soils. On the other hand, initial permeability (ko) for
257
highly organic silty clay soil (RU stratum) exists just at the upper limit of soft
clays and silts deposits, which indicates that organic matter encountered in soil
matrix promotes loose and open fabric (Figure 5.8).
8
Void ratio, eo
6
4
RU-13-V
RU-13-H
2
RL-12-V
RL-12-H
D-5-V
D-5-H
0
1.E-10
1.E-09
1.E-08
1.E-07
1.E-06
1.E-05
1.E-04
1.E-03
1.E-02
Permeability, k (cm/sec)
Fig. (6.11): Correlation between initial void ratios, eo, and initial permeability,
ko.
1.E-03
Permeability,k (cm/sec)
1.E-04
1.E-05
1.E-06
RU-V
1.E-07
RU-H
RL-V
RL-H
1.E-08
D-V
D-H
1.E-09
0
10
20
30
40
50
60
70
80
Loss on Ignition, (%)
Fig. (6.12): Correlation between initial permeability ko and loss on ignition
(OC)
258
32
PeatEnv.
Montmorillonite
Iilite
Soft clay
Kaolinite
Sand
RU-13-V
RU-13-H
RL-12-V
RL-12-H
D-5-V
D-5-H
Santagata,2008
Lieszkowsky, 1977
Lee & Brawner, 1963
28
24
Void ratio, e
20
16
Montmorillonite
Peat Envelope
Iilite
12
8
Soft Clays
4
Sand
Kaolinite
0
1.E-12
1.E-11
1.E-10
1.E-09
1.E-08
1.E-07
1.E-06
1.E-05
1.E-04
1.E-03
1.E-02
1.E-01
1.E+00
Permeability, k (cm/sec)
Fig. (6.13): Data on initial permeability ko of RU, RL, and D stratums within a
frame of reference permeability data from the literature on fibrous and
amorphous peat deposits, peaty muck, pure clay minerals montmorillonite,
illite, and kaolinite, soft clay and silt deposits and clean sand.
D-H
28
D-V
Peat Env.
Montmorillonite
24
Montmorillonite
Iilite
Soft clay
Kaolinite
Void ratio, e
20
Peat
Envelope
Sand
RU-V
RU-H
RL-V
16
RL-H
Santagata, 2008
Lee & Brawner (1963)
12
Iilite
Lieszkowsky (1977)
8
4
Soft Clays
Sand
Kaolinite
0
1.E-12
1.E-11
1.E-10
1.E-09
1.E-08
1.E-07
1.E-06
1.E-05
1.E-04
1.E-03
1.E-02
1.E-01
1.E+00
Permeability, k (cm/sec)
Fig. (6.14): Data on vertical permeability kv and horizontal permeability kh of
RU, RL, and D stratums within the same frame of reference permeability data.
259
Upon compression, the permeability of the organic soils decreases dramatically
as they compress under loads because of their high compressibility. The slope
of the initial portion of the e versus log k, that is, Ck = ∆e ⁄∆ log k, which
measures the reduction in e required to produce a tenfold decrease in k, is
commonly used in empirical correlations as Ck /eo. It was estimated as 0.30,
0.14, and 0.24 for RU, RL, and D stratums respectively compared to 0.25 for
fibrous peat and 0.5 for clay and silt deposits. Figure (6.15a) shows the
relationship between Ck and in-situ void ratio (eo) for the three stratum
compared with peaty muck soil from Indiana – USA (Santagata et. al., 2008).
Figure (6.15b) shows the same relationship for fibrous peats and soft clay and
silt deposits (Mesri and Ajlouni, 2007).
3
RU
RL
D
Santagata, 2008
2
Ck
Ck = 0.24 eo
Ck = 0.3 eo
Ck = 0.18 eo
1
Ck = 0.14 eo
0
0
1
2
3
4
5
6
7
8
In situe Void Ratio, e o
(a)
(b)
Fig. (6.15): (a) Relationship between Ck and in-situ void ratio (eo) for the three
stratums compared with muck soil (Santagata et. al., 2008) (b) The same
relationship for fibrous peats and soft clay and silt deposits (Mesri and Ajlouni,
2007.
It is evident now that highly organic soils (RL, & D stratums) and highly
organic silty clay (RU stratum), encountered in this study, can be considered as
transitional material between soft inorganic silts and clays deposits and fibrous
peats with regarding to permeability characteristics.
260
6.3.3 Compressibility Characteristics
Compression of soils under a laterally constrained condition may be
conventionally divided into primary compression observed during the increase
in effective vertical stress, and secondary compression that follows at constant
effective vertical stress. Both primary and secondary compression is timedependant and result from a reduction of void ratio and concurrent expulsion of
water from the voids of the soil skeleton.
Compressibility of soils is determined by their in-situ void ratio, nature and
arrangement of soil particles, and in the case of some soils, interparticle
chemical bonding (Mesri and Ajlouni, 2007). For primary compression, the rate
of void ratio reduction is controlled by the rate at which water can escape from
the soil. Therefore, during primary compression, pore water pressure exceeds
the steady state condition throughout the depth of the layer. Over time, the rate
of primary compression continuously decreases as effective stresses increase to
approach their equilibrium values. Once the primary compression is completed
at time tp, compression continues in the form of secondary compression. During
secondary compression, the rate of void ratio reduction is controlled by the rate
of compression of the soil skeleton itself, at constant vertical effective stress
and without sensible excess pressure in the pore water.
The time-compression relationship, above mentioned, is conceptually valid for
all soil types. However, large differences exist in the magnitude of the
components and the rate at which they occur for different soils. For granular
soils, such as sand, the hydraulic conductivity is sufficiently large that
consolidation occurs nearly instantaneously with the applied load. In addition,
although granular soils do exhibit creep effects, secondary compression is
generally insignificant. On the other hand, for cohesive soils, such as clays,
hydraulic conductivity is very small and the consolidation of a thick deposit
may require years or even decades to complete. Also, secondary compression
can be substantial for cohesive soils.
261
Different from both sands and clays, organic-rich soils generally undergo rapid
and large consolidation settlement and extensive long-term secondary
compression (Fox, 2003). The high compressibility of organic-rich soils may
be attributed to their loose structure and high water content. As a result of their
loose state, they have high permeability. Consequently when load is applied,
water quickly flows out causing large volumetric deformations in the near term,
as well as large creep deformations in the long-term (McVay and Nugyen,
2004). On the other hand, organic silts and/or clays present similar engineering
challenges as soft silts and clays, including low hydraulic conductivity, high
compressibility and significant creep deformations.
This study was focused toward evaluating and outlining the distinct
compressibility behaviors of the organic soils found in Egypt. The organic soils
encountered in this study were classified as peaty muck for RL & D stratums,
and as highly organic silty clay for RU stratum. The RL & D stratums are
consisting mainly of partly decomposed plant material, have an amorphous
granular appearance with traces of partly decomposed woody pieces and very
fine fibers, and have a sponge-like fabric. Upon deformation, the soil tends to
crumble showing less cohesion than the RU stratum. On the other hand, RU
stratum is consisting mainly of silty clay with spots of/or mixed with partly
decomposed plant material. It is highly plastic in nature and is easily deformed.
Upon deformation it exhibits a high degree of cohesion.
A comprehensive laboratory testing program was carried out to outline the
distinct primary and secondary compression behavior of encountered organic
soils based on data obtained from incremental loading consolidation tests. The
results of the one-dimensional consolidation testing program were presented in
chapter 5. The obtained compression parameters and the important findings are
summarized in Table (6.10), and compared with compiled data in Tables (6.116.13). The distinct primary and secondary compression behavior of organic
soils encountered will be discussed in the following sections.
262
263
0.7
0.4-1.2
0.4
0.6-1.4
Ave. Hor. coeff. of cons. in recomp, ch (m2/year)
Ave. Ver. coeff. of cons. in comp., cv (m2/year)
Ave. Hor. coeff. of cons. in comp, ch (m2/year)
Compression index C c
0.25
0.041
0.130
0.66
0.80
C a /C c
C r /C c
C k /C c -V
C k /C c -H
0.015-0.056
Swelling index C s
Coefficient of secondary compression C α
0.17
2.1-3.8
1.0-3.3
Ave. Ver. coeff. of cons. in recomp., cv (m2/year)
Recompression index C r
3.6-20.8
1.4-2.5
Overconsolidation ratio, OCR
0.23
0.18
0.144
0.049
0.35
0.094-0.177
0.46
1.1-8.5
37.9-55.2
10.4-26.0
2.1-3.2
186-280
84-88
3.25-5.85
67-132
1.67-2.95
In situ void ratio
158-338
Preconsolidation pressure (kPa)
70-139
Natural moisture content (%)
41-69
49-57
7-16
Organic content (%)
RL
Overburden pressure (kPa)
RU
Property
178-353
27-82
D
0.45
0.36
0.198
0.050
0.43
0.089-0.234
0.57
2.0-4.4
0.9-4.4
0.7-16.1
5.5-17.4
5.5-34.7
1.7-2.6
117-190
70-73
3.73-6.37
Table (6.10): Summary of the compressibility characteristics of RU, RL, and D stratums
Table (6.11): Compressibility characteristics of some peat deposit from
laterature (based on Ajlouni, 2000)
wo
Peat
(%)
kvo
cvo
or
(m/s)
(m /year)
2
Cc
Ratio
Cα/Cc
Reference
eo
Fibrous peat
850
4x10
-
10
0.06-0.1
Hanrahan 1954
Peat
520
-
-
-
0.061-0.078
Lewis 1956
Amorphous
and fibrous
peat
500-1500
10 -10
14-17
2.5-5
0.035-0.083
Lea and Browner
1963
Canadian
muskeg
200-600
10
-
-
0.09-0.1
Adams
1965
-6
-7
-6
-5
55.6
4.7-10.3
0.073-0.091
Keene and
Zawodniak
1968
-
-
0.075-0.085
Weber 1969
-7
64
2.6
0.05
-7
16.1
4.4
0.05
Berry and
Poskitt
1972
-
-
0.052-0.072
Samson and
La Rochelle 1972
10 -10
9.1
-
0.06-0.085
Berry and Vickers
1975
-6
-
-
0.042-0.083
Dhowian and Edil
1980
-
6.4
0.055-0.064
Berry 1983
3x10 -5x10
-
4.5-15
0.06
Lefebvre et al. 1984
200-875
-
27.2
-
-
Olson
1970
Amorphous
125-375
-
3.79
-
-
Peat
419
3x10
>6.4
-
-
Jones et al. 1986
Fibrous peat
700-800
10
3-6
-
0.042-0.083
Hansbo 1991
Fibrous peat
370
1.4x10
-
-
0.06
den Haan 1994
Fibrous peat
610-850
6.8x10 - x10
-
-
0.052
Mesri et al. 1997
Fibrous peat
-
-
-
-
0.065
Kogure 1999
Fibrous peat
(Middleton)
510-850
3x10 -10
20-150
6-9
0.053
Fibrous peat
James Bay
1000-1340
-7
4x10 - 7x10
30-300
10-12
Fibrous peat
608
1.2x10-4
4.9
3.1
Fibrous peat
370-650
-
56-82
4.9-6.6
Amorphous to
fibrous peat
705
-
Peat
400-750
10
Amorphous
granular peat
eo=7
4x10
Fibrous peat
e =11
8x10
Fibrous peat
605-1290
10
Fibrous peat
613-886
Fibrous peat
600
10
Coarse
fibrous
202-1159
1.1x10
Fibrous peat
660-1590
Fibrous peat
o
-5
-6
-6
-5
-6
-7
-5
-8
-6
-12
-8
-8
-7
-6
Ajlouni 2000
-6
264
0.059
Yulindasari 2006
0.026-0.038
Sobhan 2007
Table (6.12): Compressibility characteristics of some muck deposits from
present study and from laterature
Muck
wo
cvo
kvo
2
Cc
Ratio
Cα/Cc
Reference
(%)
(m/s)
Fibrous peaty
muck
660-1060
10 -5x10
-
4.5-9.1
0.06
Lefebvre et al. 1984
Italian peaty
muck
250-400
10 -10
-9
-
2.7-3.5
0.05-0.07
Colleselli and
Cortellazzo 1998
Kanagawa
muck
350-550
-
7.3-29.2
1.5-8
-
Matsuda et al. 1998
Iwamizawa
peaty muck
-
-
-
-
0.065
Kogure 1999
Florida muck
105-183
-
-
0.73-2.53
-
McVay and Nugyen
2004
Florida sity
muck
162-328
-
53-126
1.0-2.6
0.51
Sobhan 2007
Indiana muck
210-285
10
-
2.62
0.95
Santagata et al., 2008
Mahmoudia
peaty muck
158-338
1.2x10
-7
10.4-26.0
2.1-3.8
0.49
Present study 2010
Motoubes
peaty muck
178-353
7x10
-9
5.5-34.7
2-4.4
0.5
Present study 2010
-6
(m /year)
-5
-8
-8
Table (6.13): Compressibility characteristics of some organic silt and clay
deposits from present study and from laterature
Organic silt
& Clay
wo
cvo
kvo
2
Cc
Ratio
Cα/Cc
Reference
(%)
(m/s)
(m /year)
Norfolk
organic silt
-
-
-
-
0.05
Barber 1961
Calcareous
organic silt
-
-
-
-
0.035-0.06
Wahls 1962
Post-glacial
organic clay
-
-
-
-
0.05-0.07
Chang 1969
Organic clays
and silts
-
-
-
-
0.04-0.06
Ladd 1971
New Haven
organic
clayey silt
60-117
-
-
0.1-1.6
0.04-0.075
Mesri and Godlewski
1977
Lagos organic
clay
50-90
-
0.3-10
-
-
Ajayi, 1980
Bagerhat
organic clay
silt
30-165
-
4.9-27
0.16-0.63
-
Hoque et al., 2004
Florida
organic silt
25-92
-
-
o.46-0.77
-
McVay and Nugyen
2004
Mahmoudia
organic clay
70-139
3.8x10
1.0-3.3
0.6-1.4
0.041
Present study 2010
-10
265
6.3.3.1 Primary Compression
Natural void ratio of organic soils is generally higher than that of inorganic
soils; with fibrous peat having the greater void ratios. Extreme ranges in void
ratio for organic soils have been reported from 2 to 25 (Hanrahan, 1954), with
void ratio of peat ranges between 9, for dense amorphous granular peat, up to
25, for fibrous types with high contents of sphagnum. It usually tends to
decrease with depth within a peat deposit. Such high void ratios give rise to
phenomenally high water contents. The latter is the most distinctive
characteristic of organic soils (Bell, 2000). The average void ratio for RU, RL,
and D samples is 2.356, 4.461, and 5.612 respectively which is comparable
taking into account that the encountered soil is ranging between organic silty
clay and peaty muck, and of buried nature.
Surficial organic deposits generally have small to medium past pressures
because of their typical locations, i.e., near the surface (Kogure and Ohira,
1977; Hobbs, 1986; Landva and La Rochelle, 1983, McVay and Nugyen, 2004;
Mesri and Ajlouni, 2007). The σ`p of surficial organic deposits may result from
desiccation, mechanical unloading, water table fluctuations, and aging.
Compared with surficial organic deposits, the three stratums encountered in this
study RU, RL, and D are buried at depths 4.0 m, 10.0 m, an 8.0 m respectively.
They have range of age about 5000 year B.P. A profile of σ`vo, σ`p, OCR, and
εvo with depth was developed for every stratum as shown in Figures (5.24,
5.25). It was noted that preconsolidation pressure σ`p was easily determined
from EOP εv -log σ`v compression curves which are characterized by the Sshape and clear break at the preconsolidation pressure. Also, the mean in-situ
overconsolidation ratio (OCR) was computed as 1.91, 2.68, and 2.19 for RU,
RL, and D respectively. Furthermore, the majority of the specimens’ qualities
were considered as good according to Sample Quality Designation scale (εvo =
2-4 %). From the data verification, it could be concluded that the higher values
of σ`p and OCR are corresponding to the lower values of εvo and vise versa.
266
For six samples, two of each stratum, one load increment was allowed to
undergo secondary compression for one week to assess aging effect on
preconsolidation pressure (σ`p). This was followed for three samples, one of
each stratum, in recompression range, and three samples, one of each stratum,
in compression range, between σ`p and 2 σ`p. The measured OCR for these
samples after aging in recompression range was 2.5, 2.9, and 2.6 and after
aging in compression range was 3.5, 7.7, and 4.2 for RU, RL, and D
respectively. The computed OCR = (t/tp)(Cα/Cc / (1-Cr/Cc)) due to aging in
recompression range is 1.43, 1.58, and 1.54, and due to aging in compression
range is 1.25, 1.30, and 1.51 for RU, RL, and D respectively. It could be
concluded that the measured OCR after aging is higher than that computed due
to aging which indicates that aging is not the only reason for overconsolidation.
Figure (6.16) shows the relationship between the measured OCR after aging
relative to that computed due to aging.
8
7
Measured OCR after aging
6
5
4
RU-Recomp
3
RL-Recomp
D-Recomp
2
RU-Comp
1
D-Comp
RL-Comp
0
0
1
2
3
4
5
6
7
8
Computed OCR due to aging
Fig. (6.16): Relationship between the measured OCR after aging relative to that
computed due to aging.
267
According to Mesri and Ajlouni (2007), amorphous peat fabric is likely to exist
at lower void ratios and to display lower permeability anisotropy and lower
compressibility [e.g., Edil and Wang, 2000] as compared to fibrous peat
deposits. Also, Matsuda et al. (1998) concluded that no difference in Cc and σ`p
were observed for the vertical and horizontal specimens of muck soil samples
have loss on ignition in the range of 30-70% with natural water content in the
range 350-550%. The values of preconsolidation pressure σ`p from EOP εv -log
σ`v compression curves for specimens that were cut with their axes
perpendicular or parallel to the vertical direction were more or less equal which
indicate that the fabric of encountered organic soils is isotropic. This was
confirmed by SEM images which indicated that there are no significant
differences in the fabric of the organic soil encountered, and it is highly porous
in both vertical and horizontal directions.
Figure (6.17) shows the variation of constrained modulus (D = 1/ mv) with
vertical effective stress. The data for the three stratums are very consistent.
Unexpectedlly, constrained modulus D decreases in the recompression range,
up to σ`p, and then, as expected, increases continuously by more than one order
of magnitude in the normally consolidation range. The same trend was noted by
Santagata et al. (2008).
Due to the S-shape of the compression curve, the compression index Cc and
similarly compression ratio of the soil Cc` = Cc /(1+eo) changes significantly
over the stress range investigated. Figure (6.18) shows the variation of Cc` with
normalized effective applied pressures (σ`v/σ`p). Cc` increases gradually within
the recompression range till approximately 0.4σ`p for RU, RL, and D stratum,
then increases sharply till 2σ`p (in the normally consolidation region), then
decrease continuously with increasing vertical effective stress for RL and D
stratums, while be almost constant for RU stratum then beyond approximately
5σ`p-10σ`p decrease continuously with increasing vertical effective stress.
268
RU
Constrained Modulus D=1/mv (kPa
1000
100
RU-15-(IL+Long S)
10
RU-3-(IL+Late LS)
RU-13-(IL+FS)V+Perm
RU-9-(IL+FS+Loop)
RU-8-(IL+Sec)
RU-13-(IL+FS+Perm )H
1
10
100
1000
10000
Vertical Effective Stress, σ' v ( kPa)
RL
Constrained Modulus D=1/mv (kPa
1000
100
RL-5-(IL+Sec)-m od
RL-6-(IL+Long Sec.)
10
RL-18-(IL+Late LS)
RL-17-(IL+FS)V
RL-17-(IL+FS)H
RL-11-(IL+FS+Loop)
RL-12-(IL+FS+Perm )V
RL-12-(IL+FS+Perm )H
1
10
100
1000
10000
Vertical Effective Stress, σ'v ( kPa)
D
Constrained Modulus D=1/mv (kPa
1000
100
D-7-(IL+FS)V
D-7-(IL+FS)H
10
D-2-(IL+Long S)
D-13-(IL+Late LS)
D-5-(IL+FS)V+Perm
D-5-(IL+FS)H+Perm
D-8-(IL+FS+Loop)
D-11-(IL+Sec)
1
10
100
1000
10000
Vertical Effective Stress, σ'v ( kPa)
Fig. (6.17): The variation of constrained modulus (D = 1/ mv) with vertical
effective stress for RU, RL, and D stratums.
269
0.7
RU-15-(IL+Long S)
0.6
RU-3-(IL+Late LS)
0.5
RU-9-(IL+FS+Loop)
0.4
RU-9-(IL+FS+P erm)H
RU-13-(IL+FS)V+P erm
RU
Cc/(1+eo)
RU-8-(IL+Sec)
0.3
0.2
0.1
0
0.01
0.1
1
10
100
Normalized Applied Pressure, (σ'v/σ'p)
0.7
RL-12-(IL+FS)V+P erm
RL-5-(IL+Sec)-mod
0.6
RL-17-(IL+FS)V
Rl-17-(IL+FS)H
RL
Cc/(1+eo)
0.5
RL-11-(IL+FS+Loop)
RL-6-(IL+Long Sec)
0.4
RL-18-(IL+Late LS)
RL-12-(IL+FS+P erm)H
0.3
0.2
0.1
0.0
0.01
0.1
1
10
100
Normalized Applied Pressure, (σ'v/σ'p)
0.7
0.6
D-11-(IL+Sec)
D-7-(IL+FS)V
D-2-(IL+Long S)
0.5
D-18-(IL+Late LS)
0.4
D-8-(IL+FS+Loop)
0.3
D-5-(IL+FS+P erm)
D
Cc/(1+eo)
D-5-(IL+FS)V+P erm
D-7-(IL+FS)H
0.2
0.1
0
0.01
0.1
1
10
100
Normalized Applied Pressure, (σ' v/σ' p)
Fig. (6.18): The variation of Cc` with normalized effective applied pressures
(σ'v/σ'p) for RU, RL, and D stratums.
270
It was found that mean value of Cc for RU, RL, and D stratums is 1.05, 2.86,
and 3.631 respectively. That is, Cc increases as mean organic content increases.
On the other hand, it is evident that Cc for RU stratum, classified as highly
organic silty clay, is slightly higher than that reported for soft clay and silt
deposits, while Cc for RL and D stratums, classified as peaty muck, is
comparable to that reported by Santagata et al., (2008) for peaty muck (Ave.
OC = 59.5%) of 2.62 and still in the lower limits reported for peat 2-15
(Lefebvre, et al, 1984).
In this research, one sample of every stratum RU, RL, and D was unloaded just
after σ`p incrementally to less than σ`vo then subsequently reloaded
incrementally to the desired effective stress. Values of recompression index Cr
for RU, RL, and D stratums, measured from the developed loops, are 0.165,
0.458, and 0.574 respectively. Corresponding values of Cr /Cc ratio are 0.13,
0.144, and 0.198 respectively and comparable to that reported for inorganic silt
and clay of 0.1-0.2 and is still in the lower limits reported for fibrous peat of
0.1-0.3 (Mesri and Ajlouni, 2007). On the other hand, for the other samples Cr`
was measured as the slope of the tangent at σ`vo of EOP εv -log σ`v curves, and
Cr = Cr`(1+eo) was computed in a trial to estimate an approximate value of
recompression index Cr and illustrated in Figure (6.19). This method
overestimated Cr for RU samples, while underestimated Cr values for D
samples.
For the above three samples, two rebound cycles were performed in every test.
The first cycle was at pressure 200 kPa, 400 kPa, and 200 kPa for RU, RL, and
D samples respectively. The swelling index, Cs = ∆e/∆ log σ`v, with mean
values of 0.16, 0.51, and 0.7 for RU, RL, and D samples respectively, increases
with overconsolidation ratio [OCR = σ`v(max)/ σ`v]. The second cycle was at
the end of the scheduled load sequence, 3200 kPa. The swelling index, Cs, with
mean values of 0.22, 0.35, and 0.49 for RU, RL, and D samples respectively,
decreases with overconsolidation ratio [OCR = σ`v(max)/ σ`v].
271
0.14
Cr = 0.130Cc
RU
Cr/(1+eo)
0.12
0.10
0.08
0.06
0.04
0.02
0.00
0.0
0.2
0.4
0.6
0.8
1.0
Cc/(1+e o )
0.16
0.14
Cr = 0.144Cc
RL
Cr/(1+eo)
0.12
0.10
0.08
0.06
0.04
0.02
0.00
0.0
0.2
0.4
0.6
0.8
1.0
Cc/(1+e o)
0.16
0.14
D
Cr/(1+eo)
0.12
Cr = 0.198Cc
0.10
0.08
0.06
0.04
0.02
0.00
0.0
0.2
0.4
0.6
0.8
1.0
Cc/(1+e o )
Fig. (6.19): Relationship between Cr` and Cc` for RU, RL, and D stratums.
272
It is evident that the swelling index, Cs, increases as organic content increases.
Also, it increases slightly with the decrease in σ`v(max) from which unloading
takes place for the highly organic samples (RL & D), as concluded by Mesri et
al., (1997), while decreases for lower organic sample (RU). Furthermore, the
swelling index values for RL, and D samples (highly organic samples) are
within the range of 0.3-0.9 reported by Mesri et al., (1997) for Middleton
fibrous peat, while that for RU (organic silt and clay) is lower than that range.
Moreover, the values of Cs/Cc increases from 0.13, 0.20, and 0.27 at low
pressure rebound to 0.24, 0.34, and 0.47 at high pressure rebound for RU, RL,
and D samples respectively mainly because of significant decrease in Cc with
σ`v.
According to Terzaghi et al., (1996), coefficient of consolidation, cv, for soft
clay and silt deposits is more or less constant in the compression range from σ`p
to 5 σ`p, and has values in the range of 0.5 to 5 m²/year; in the recompression
range it is typically 10 times larger. Compared with mineral soils, Mesri and
Ajlouni, (2007) indicated that the cv of fibrous peats can be as high as 500
m²⁄year in the recompression range, the values of cv at σ`p for fibrous peats are
at 20 to 100 m²⁄year and decrease by a factor of 10 to 20 as consolidation
pressure increases to 5 σ`p. It is evident that the coefficient of consolidation cv,
of organic-rich soils, varies as a function of stress level; it decreases with an
increase in effective stress. This decrease is more marked in the soils which
have higher organic content (Farrell et al., 1994) as shown in Figure (3.31).
To facilitate comparison of coefficient of consolidation data of the encountered
organic soils with that reported in literature, it is better to plot coefficient of
consolidation versus normalized effective vertical stress σ`v/σ`p to account for
different stress histories of the soils as shown in Figures (6.20 & 6.21).
273
RU-15-(IL+Long S)
RU-3-(IL+Late LS)
2
Coeffcient of Consolidation, cv cm /sec
1.E+00
RU
1.E-01
RU-13-(IL+FS)V+Perm
RU-9-(IL+FS+Loop)
1.E-02
RU-8-(IL+Sec)
1.E-03
1.E-04
1.E-05
0.01
0.1
1
10
100
Normalized Applied Pressure, (σ' v/σ'p)
RL-5-(IL+Sec)
RL-17-(IL+FS)V
2
Coeffcient of Consolidation, cv cm /sec
1.E+00
RL
1.E-01
RL-6-(IL+Long S)
RL-18-(IL+Late LS)
RL-12-(IL+FS)V+Perm
1.E-02
RL-11-(IL+FS+Loop)
1.E-03
1.E-04
1.E-05
0.01
0.1
1
10
100
Normalized Applied Pressure, (σ' v/σ'p)
2
Coeffcient of Consolidation, cv cm /sec)
1.E+00
D
D-11-(IL+Sec)
D-7-(IL+FS)V
1.E-01
D-2-(IL+Long S)
D-18-(IL+Late LS)
D-5-(IL+FS)V+Perm
1.E-02
D-8-(IL+FS+Loop)
1.E-03
1.E-04
1.E-05
0.01
0.1
1
10
100
Normalized Applied Pressure, (σ' v/σ'p)
Fig. (6.20): The variation of coefficient of consolidation in the vertical
direction cv with vertical effective stress for RU, RL, and D stratums.
274
RU-13-(IL+FS)H+Perm
2
Coeffcient of Consolidation, cv cm /sec
1.E+00
RU
1.E-01
1.E-02
1.E-03
1.E-04
1.E-05
0.01
0.1
1
10
100
1.E+00
RL-17-(IL+FS)H
2
Coeffcient of Consolidation, c v cm /sec
Normalized Applied Pressure, (σ'v/σ'p)
RL
RL-12-(IL+FS)H+Perm
1.E-01
1.E-02
1.E-03
1.E-04
1.E-05
0.01
0.1
1
10
100
Normalized Applied Pressure, (σ' v/σ'p)
2
Coeffcient of Consolidation, cv cm /sec)
1.E+00
D
D-7-(IL+FS)H
D-5-(IL+FS)H+Perm
1.E-01
1.E-02
1.E-03
1.E-04
1.E-05
0.01
0.1
1
10
100
Normalized Applied Pressure, (σ' v/σ'p)
Fig. (6.21): The variation of coefficient of consolidation in the vertical
direction ch with vertical effective stress for RU, RL, and D stratums.
275
Figure (6.20) shows the variation of the coefficient of consolidation in the
vertical direction (cv) with normalized effective vertical stress for the
encountered organic soils. The average vertical coefficients of consolidation in
the recompression range (σ`vo- σ`p) are in the ranges 1.0 to 3.3 m²/year, 10.4 to
26.0 m²/year, and 5.5 – 34.7 m²/year for RU, RL, and D stratums respectively.
The average vertical coefficients of consolidation in the compression range (σ`p
- 3 σ`p) are in the ranges 0.4 to 1.2 m²/year, 1.1 to 8.5 m²/year, and 0.7 to 16.1
m²/year which decreases by a factor of 3, 3 to 10, and 2 to 8 for RU, RL, and D
stratums respectively.
Figure (6.21), shows the variation of the coefficient of consolidation, in the
horizontal direction, (ch) with normalized effective horizontal stress. The
average horizontal coefficient of consolidation in the recompression range
(σ`vo- σ`p) is 0.7m²/year, 37.9 to 55.2 m²/year, and 5.5 to 17.4 m²/year for RU,
RL, and D stratums respectively. The average horizontal coefficient of
consolidation in the compression range (σ`p - 3 σ`p) are 0.4 m²/year, 3.6 to 20.8
m²/year, and 0.9 to 4.4 m²/year which decreases by a factor of 2, 3 to 10, and 4
to 6 for RU, RL, and D stratums respectively.
It is evident that the data of RU stratum is comparable to that of soft clay and
silt deposits, while the data of RL and D stratums (muck) lies in between that
of fibrous peat and that of organic silt and clay (RU). Also, as effective stress
increases permeability decreases as shown in Figure (5.17) and necessarily
coefficient of consolidation decreases continuously, especially once the soil is
loaded well into NC region, both in the vertical direction and in horizontal
direction. Furthermore, the data for the three stratums are consistent except
that, as concluded by Farrell et al., (1994), for RU stratum (Organic silty clay)
of lower organic content the decrease in coefficient of consolidation is less in
magnitude than that for RL and D stratums (peaty muck) by one order of
magnitude. Moreover, the variability of organic content and fabric (constituent
arrangement) of the different specimens explains the scatter of data.
276
Figure (3.30) illustrates a comparison of the coefficient of consolidation data
for three soils; Boston blue soft inorganic clay, peaty muck from West
Lafayette (Indiana), and Middleton fibrous peat (Wisconsin), by plotting cv
versus σ`v ⁄σ`p. It is revealed that the key difference between the inorganic clay
and the organic soils is the change in cv in the NC region, which reflects both
changes in compressibility and in permeability. Figure (6.22) shows the
envelope diagrams drawn for the coefficient of consolidation data of peaty
muck from West Lafayette (Santagata et al., 2008), and Middleton fibrous peat
(Ajlouni, 2000).
Figure (6.23) shows the data of coefficient of vertical consolidation of the
current study compared with these data which confirms that the data of highly
organic soils encountered (RL & D stratums) is comparable to those data. On
the other hand, the initial coefficient of consolidation for organic silty clay (RU
stratum) is lower than that of highly organic soils (RL & D stratums, muck
from West Lafayette, and Middleton fibrous peat) and decreasing by lower rate
than those soils because of its low hydraulic conductivity and low
compressibility compared of these soils.
Ajlouni Env., 2000
Santagata, et. al., 2008
2
Coeffcient of Consolidation, cv cm /sec
1.E+00
1.E-01
1.E-02
1.E-03
1.E-04
1.E-05
0.1
1
10
100
Normalized Applied Pressure, (σ'v/σ'p)
Fig. (6.22): The envelope diagrams drawn for the coefficient of consolidation data of
peaty muck from West Lafayette (Santagata et al., 2008), and Middleton fibrous peat
(Ajlouni, 2000).
277
2
Coeffcient of Consolidation, cv (cm /sec)
1.E+00
RU
RU-15-(IL+Long S)
RU-3-(IL+Late LS)
RU-13-(IL+FS)V+Perm
1.E-01
RU-9-(IL+FS+Loop)
RU-8-(IL+Sec)
Aj louni Env.
1.E-02
Santagata, 2008
1.E-03
1.E-04
1.E-05
0.1
1
10
100
Normalized Applied Pressure, (σ' v/σ' p)
2
Coeffcient of Consolidation, cv ( cm /sec)
1.E+00
RL
RL-5-(IL+Sec)
RL-17-(IL+FS)V
RL-6-(IL+Long S)
1.E-01
RL-18-(IL+Late LS)
RL-12-(IL+FS)V+Perm
RL-11-(IL+FS+Loop)
1.E-02
Aj louni Env.
Santagata, 2008
1.E-03
1.E-04
1.E-05
0.1
1
10
100
Normalized Applied Pressure, (σ' v/σ'p)
2
Coeffcient of Consolidation, cv ( cm /sec)
1.E+00
D
D-11-(IL+Sec)
D-7-(IL+FS)V
D-2-(IL+Long S)
1.E-01
D-18-(IL+Late LS)
D-5-(IL+FS)V+Perm
D-8-(IL+FS+Loop)
1.E-02
Aj louni Env.
Santagata
1.E-03
1.E-04
1.E-05
0.1
1
10
100
Normalized Applied Pressure, (σ' v/σ' p)
Fig. (6.23): The variation of vertical coefficient of consolidation (cv) with
normalized applied pressure (σ`v ⁄σ`p) for RU, RL, and D stratums.
278
The Ck /Cc ratio measures the decrease in k deriving from the increase in the
effective stress. According to Mesri and Ajlouni (2007), the dramatic decrease
in the coefficient of consolidation of highly organic soils, which severely affect
the rate of primary consolidation, is expected due to small value of Ck /Cc = 0.30.4 compared with Ck /Cc = 1.0 for a large number of soft clay and silt deposits,
which means that a significant decrease in permeability is associated with any
increase in effective stress.
For the highly organic soil investigated in this study, RL and D stratums, Ck /Cc
is 0.18 and 0.36 respectively in the vertical direction and 0.23 and 0.45
respectively in the horizontal direction which is comparable to that of fibrous
peat, while slightly lower than that reported for peaty muck (0.4-0.6) by
Santagata et al., (2008) . For organic silty clay, RU stratum, Ck /Cc is 0.66 in the
vertical direction and equals 0.8 in the horizontal direction which is
intermediate between that of fibrous peat and muck and that of soft clay and silt
deposits. That is, as organic content increase the natural water content wo and
compression index Cc increase and Ck /Cc decrease.
The low Ck /Cc value during consolidation of organic soil occurs because the
amount of water to be expelled remains high (high Cc), while the flow channels
that discharge the water become rapidly constrained (low Ck). This was
revealed from Figures (5.9, 5.11, & 5.13) which show the thick relatively stiff
mats formed after consolidation under 3200 kPa in the horizontal and vertical
sections.
Therefore, the duration of primary consolidation (tp=t100) increased
significantly throughout the tests from increment to increment as effective
stress increase. The duration of primary consolidation (tp=t100) vary from less
than a minute in the early increments to several hours at the maximum stress as
shown in Figure (5.28).
279
At the planning stage before a detailed subsurface investigation is carried out,
preconsolidation pressure σ`p and compression index Cc may be estimated from
empirical correlations. Mesri and Ajlouni (2007) concluded that a direct
relationship between the compression index and natural water content should
exist for saturated soils because both are controlled by the composition and the
structure of the soil. Composition and structure control both the in-situ void
ratio at which a soil comes to equilibrium and the compressibility after the soil
structure yields at the preconsolidation pressure (Hanrahan 1954; Landva and
La Rochelle 1983).
Mesri and Ajlouni (2007), introduced a plot of Cc evaluated from EOP e versus
log σ`v relationship in the consolidation pressure range of σ`p to 2σ`p versus
natural water content wo (Figure 3.29). It includes Cc data for Middleton peat,
James Bay peat, and peats from the literature, including from Canada, Japan
and the United States, as well as for reference, for soft clay and silt deposits.
The plot suggests that preliminary estimate of compression index may be
obtained using Cc = wo/100. Figure (6.24) shows the data of the current study,
within an envelope diagram, drawn for the above described data which
compiled by Mesri and Ajlouni (2007) for organic soils and peats. The figure
indicates that the data is comparable, taking into account that samples suffered
some drying during storage as mentioned earlier.
Kogure and Ohira (1977) developed an empirical correlation between σ`p and
in situ void ratio eo for surficial peat deposits. Mesri and Ajlouni (2007)
introduced a correlation between σ`p and eo for peats based on compression data
of Middleton peat and James Bay peat, together with Kogure and Ohira (1977)
data for surficial peat deposits (Figure 3.27). Figure (6.25) shows a correlation
between σ`p and eo for the encountered buried organic soils. In the absence of
undisturbed sampling and oedometer testing, σ`p may be estimated from this
empirical correlation or correlations with undrained shear strength described
earlier.
280
Compression Index, Cc
100
10
Cc = wo /100
RU
1
RL
D
Organic Soil Env. (Mesri
and Ajlouni, 2007)
0.1
10
100
1000
Natural Water Content, w o (% )
10000
Fig. (6.24): Correlation between Cc and wo for RU, RL, and D samples
1000
σ'p =
1
σ'p =
7
40 0
/
eo
00/
e
σ'p, kPa
o
100
σ'p=
3
00 /
e
o
σ' p=
1
RU
50 /
e
o
RL
D
10
1
10
Void Ratio, eo
Fig. (6.25): Correlation between σ`p and eo for RU, RL, and D samples
281
6.3.3.2 Secondary Compression
As can be seen from above, the primary consolidation of highly organic soil
(RL & D stratums) is very rapid and relatively rapid for organic silt and clay
(RU Stratum) in laboratory compared to inorganic silt and clay. This was
confirmed in the field; for most organic-rich deposits rapid dissipation of water
pressure are completed within a few weeks or months (Mesri et al., 1997). This
suggests that significant settlement occurs due to secondary compression under
a constant effective stress. On the other hand, this long-term compression has
no end within the time of engineering interest i.e. design life of structures
(Berry and Poskitt, 1972, Mesri et al. 1997, Mesri and Vardhanabhuti, 2005).
Typical logarithm of time-compression curves, for load increments that were
allowed to undergo secondary compression for one week in the recompression
and compression ranges, are shown in Figures (6.26, 6.27). For these pressure
increments, it is possible to define the end of primary consolidation, tp, and to
examine the shape of the secondary compression curve as explained in Figure
(6.28). In Figure (6.26), the pressure increments are in the recompression
range; primary consolidation is completed in 8, 4 and 0.5 min for RU, RL and
D samples respectively, and significant secondary compression follows
thereafter. In Figure (6.27), the pressure increments are in the compression
range (σ`p -2σ`p); primary consolidation is completed in 90, 100 and 16 min for
RU, RL and D samples respectively, then followed by secondary compression.
It can be observed that the curves exhibited significant variations in Cα with
time in the recompression and compression ranges. In their classical paper,
Mesri and Godlewski (1977) postulated that the magnitude and behavior of Cα
with time is directly related to the magnitude and behavior of Cc with effective
vertical stress, σ`v. In general Cα remains constant, decreases or increases with
time, in the range of σ`v at which Cc remains constant, decreases or increases
with σ`v respectively. It is obvious that the load increments where in ranges
such that Cc increases with σ`v, therefore Cα increases with time.
282
10.03
10.02
tp = 8 min
RU
Deformation, mm
10.01
10.00
9.99
9.98
9.97
9.96
9.95
0.1
1
10
100
Time (minutes)
1000
10000
100000
1000
10000
100000
1000
10000
100000
8.60
tp = 4 min
8.40
RL
Deformation, mm
8.20
8.00
7.80
7.60
7.40
7.20
7.00
6.80
0.1
1
10
100
Time (minutes)
10.10
tp = 0.5 min
D
Deformation, mm
10.00
9.90
9.80
9.70
9.60
9.50
0.1
1
10
100
Time (minutes)
Typical logarithm of time-compression curves in the
recompression range.
Fig.
(6.26):
283
8.80
8.70
8.60
RU
Deformation, mm
8.50
tp = 90 min
8.40
8.30
8.20
8.10
8.00
7.90
7.80
7.70
7.60
0.1
1
10
100
1000
10000
100000
RL
Deformation, mm
Time (minutes)
8.60
8.40
8.20
8.00
7.80
7.60
7.40
7.20
7.00
6.80
6.60
6.40
6.20
6.00
5.80
5.60
5.40
5.20
tp = 100 min
0.1
1
10
100
1000
10000
100000
1000
10000
100000
Time (minutes)
9.00
8.80
tp = 16 min
8.60
D
Deformation, mm
8.40
8.20
8.00
7.80
7.60
7.40
7.20
7.00
6.80
6.60
6.40
0.1
1
10
100
Time (minutes)
Fig. (6.27): Typical logarithm of time-compression curves, in the compression
ranges
284
8.6000
8.4000
tp = 4 min
8.2000
Deformation (mm
8.0000
7.8000
7.6000
7.4000
7.2000
7.0000
6.8000
0.1
1
10
100
Time (minutes)
1000
10000
100000
Fig. (6.28): A sketch explains obviously the possiblilty of using Casagrande
method to define the time to the end of primary consolidation, tp, for typical
logarithm of time-compression curve with proper scale (Fig. 6.26-RL)
Figure (6.29) illustrates the variation of Cα`= Cα/(1+eo) with normalized applied
pressure for RU, RL, and D samples of different organic content. The values of
the secondary compression ratio Cα` derived from the IL tests varied
considerably as a function of organic content of the soil and the stress level. Cα`
increases gradually within the recompression range till approximately 0.4σ`p for
RU, RL, and D stratum, then increases sharply till 2σ`p (in the normally
consolidation region), then decrease continuously with increasing vertical
effective stress for RL and D stratums, while be almost constant for RU stratum
then beyond approximately 5σ`p-10σ`p decrease continuously with increasing
vertical effective stress. As can be seen, it is typically the same behavior as Cc`.
285
0.05
RU-15-(IL+Long S)
RU-3-(IL+Late LS)
0.04
RU-13-(IL+FS)V+Perm
RU
Cα /(1+eo )
RU-9-(IL+FS+Loop)
RU-8-(IL+Sec)
0.03
RU-9-(IL+FS+Perm)H
0.02
0.01
0
0.01
0.1
1
10
100
Normalized Applied Pressure, (σ'v/σ'p)
0.05
RL-12-(IL+FS)V+ Perm
RL-5-(IL+Sec)
0.04
RL-17-(IL+FS)V
RL
Cα /(1+eo )
RL-17-(IL+FS)H
0.03
RL-11-(IL+FS+Loop)
RL-6-(IL+Long Sec)
RL-12-(IL+FS+Perm)H
0.02
0.01
0
0.01
0.1
1
10
100
Normalized Applied Pressure, (σ'v/σ'p)
0.05
D-11-(IL+Sec)
D-7-(IL+FS)V
0.04
D-2-(IL+Long S)
D-18-(IL+Late LS)
D
Cα /(1+eo )
D-5-(IL+FS)V+Perm
0.03
D-8-(IL+FS+Loop)
D-7-(IL+FS)H
D-5-(IL+FS+Perm)-H
0.02
0.01
0
0.01
0.1
1
10
100
Normalized Applied Pressure, (σ' v/σ' p)
Fig. (6.29): The variation of Cα` with normalized effective applied pressures
(σ'v/σ'p) for RU, RL, and D stratums.
286
However, Mesri and Godlewski (1977) concluded that for any given soil a
unique relationship exists between Cα = ∆e/∆ log t and Cc = ∆e/∆ log σ`v
throughout the secondary compression stage, and for all pressures in the
recompression and compression range. According to recommendations made
by Mesri and Castro (1987), at any given effective stress, the value of Cα from
the first log cycle of secondary compression and the corresponding Cc value
computed from the EOP e-log σ`v curve are used to define the relationship
between Cα and Cc. Moreover, the values of Cα /Cc for all geotechnical
materials are in the narrow range of 0.01 to 0.07. For organic clays and silts,
Cα/Cc = 0.05±0.01, while for fibrous peat deposits Cα/Cc = 0.06±0.01, which
display the highest values of Cα /Cc.
The procedure outlined above to develop the unique Cα /Cc was successfully
employed for organic soils encountered RU, RL, and D stratums, and the
results are illustrated in Figure (6.30). The values of Cα /Cc ratio are 0.041,
0.049, and 0.05 for RU, RL, and D stratums respectively. Again, as organic
content increase Cα /Cc ratio increase, but still in the lower range of peat and
coincide the range reported for organic silt and clay of 0.04-0.06.
287
0.05
Cα/(1+eo)
0.04
RU
0.03
0.02
Cα /Cc = 0.041
0.01
0.00
0.0
0.2
0.4
0.6
0.8
1.0
Cc/(1+eo )
0.05
RL
Cα/(1+eo)
0.04
0.03
Cα /Cc = 0.049
0.02
0.01
0.00
0.0
0.2
0.4
0.6
0.8
1.0
Cc/(1+eo )
0.05
D
Cα /(1+eo)
0.04
Cα /Cc = 0.05
0.03
0.02
0.01
0.00
0.0
0.2
0.4
0.6
0.8
1.0
Cc/(1+eo )
Fig. (6.30): The variation of Cα` with normalized effective applied pressures
(σ'v/σ'p) for RU, RL, and D stratums.
288
CHAPTER 7
SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS
7.1
Summary
A comprehensive laboratory testing program was carried out on undisturbed
organic soil samples obtained from two sites located at West-Delta, the thickest
and most extensive organic deposits found in Egypt, for evaluating and
outlining the distinct compressibility behaviors of the organic soils
encountered. One site, located at Robaomaah village- Mahmoudia - Bohira
governorate, has two stratums: shallow one (RU) at depth 4.0-6.5 m and deep
one (RL) at depth 10.0-14.0 m. The other, located at Ezbet El-Domyati Motoubes - Kafr -Elsheikh governorate, has a deep stratum (D) at depth 8.012.0 m. In addition, a comprehensive literature review was conducted to
provide rationale of the research, and to gather sufficient background
information on the general and engineering characteristics of organic soils. The
background was used to develop the concept adopted for this research, i.e. the
behavior and properties of both inorganic soils and organic soils follow the
same fundamental mechanisms and factors determine.
The undisturbed organic soil samples were obtained using a specially-designed
thick-walled open-drive 100 mm diameter sampler. The general characteristics
of the encountered organic soils were identified through the determination of
physical, index, and chemical properties of the soils. X-Ray diffraction analysis
was performed to identify the different minerals constituting the inorganic
portion of the organic soils. In addition, the differences in the fiber contents,
the pore spaces, and the perforated plant structure of the organic soil samples,
in its initial state and after compression under high stress, were observed using
the Scanning Electron Microscope (SEM). Organic soil samples were classified
based on “Tentative ASTM Standard”, recommended in this research, as a
suitable classification system to be used in Egypt. The test results were
289
compared with published data and correlations between different index
properties were investigated.
Engineering characteristics evaluated in this research include permeability,
undrained shear strength, and compressibility. Undrained shear strength was
determined via unconfined compression test by two methods: in compression
device and using pocket penetrometer. Permeability measurements were
determined through falling-head flow measurements during the secondary
compression stage of IL oedometer tests. The test results were analyzed and
compared with published data.
Predicting and dealing with settlements of organic soils has been a problem for
highway and foundation engineers. Therefore, the study focused on evaluating
the distinct compressibility characteristics in order to devise suitable design
compression parameters for settlement analysis. The test results were analyzed
and also compared with published data.
7.2 Conclusions
The following are the conclusions derived based on the results obtained from
the current study to characterize the general and engineering properties of
organic soils obtained from West-Delta, and the data compiled from literature:
Genaral Characteristics
1. The sampler developed during this research was successful in retrieving
samples with "good to very good" quality according to Sample Quality
Designation (SQD) scale (εvo = 1- 4%). This technique could be
recommended for organic soils sampling in Egypt.
2. Organic soils in Egypt are occurring as deposits buried under alluvial soils
thousands of years ago (Geotechnical Encyclopedia of Egypt, 2002). Thus
the organic soils encountered are subject to moderate degree of
290
decomposition and preservation processes long time ago, which explains
the disappearance of fibrous structure as observed from SEM images (Figs.
5.8 – 5.13) and that detritus gradually becomes finer as indicated from
particle size distribution (Fig. 5.4). Therefore, the organic soils encountered
consist mainly of partly decomposed plant material has an amorphous
granular appearance, a sponge-like fabric, and dark in color.
3. The "Tentative ASTM Standard classification system" for organic soils and
peat was recommended, in this research, to be used in Egypt as a standard
definition (Tables 2.8 & 2.10). This classification system distinguishes
between various types of organic soils based on their organic content, and
differentiates between various types of peats based on their physical, index
and chemical properties. It correlates well different types of organic soils
and peats and their index and engineering properties (Tables 6.1, 6.7, 6.8 &
6.11-6.13) so that the described behavior can be related to the proper
material. In addition, it could be integrated with the USCS to bridge the
gap between peat as purely vegetable matter, and purely inorganic silts and
clays.
4. The three stratums examined in this study were classified as peaty muck for
RL & D and as highly organic silty clay for RU. On the other hand, the
organic soil samples ranged from slightly organic silty clay to amorphous
peat, with some specimens obtained from the same tube, within 10 cm of
each other, exhibited vastly different organic contents with index properties
varying over a fairly wide range, (Tables 5-7, 5-8, & 5-9) which confirm the
high variability of organic soil within each deposit and that the
characteristics of the organic soils are the product of morphology of peat
land.
5. The morphological differences between the three stratums encountered in
this study aroused from the circumstances surrounded their formation and
291
the plant types constituted the organic soil. These differences extended to
structure, fabric, degree of decomposition, and proportion of mineral
material, which had a considerable influence on the plasticity, permeability,
compressibility and strength of organic soils encountered and so on
different engineering behavior.
6. A very high concentration of sodium chloride and sulpher trioxide exists in
both organic soil and groundwater, such that they are highly aggressive with
regarding to concrete and steel reinforcement.
Engineering Characteristics
7. The study confirmed that although the engineering properties of the organic
soils are significantly different from those of inorganic soils, the same
fundamental mechanisms and factors determine the behavior and properties
of both inorganic and organic soils. In this study, the engineering properties
were measured using the same methods used for inorganic silt and clay.
8. The organic soils encountered can be considered as a transitional material
between soft inorganic silts and clays and fibrous peat regarding their
mechanical behavior. Many of their engineering properties, including
undrained shear strength, permeability, and compressibility fall in between
those characteristics of soft inorganic silts and clays and those typical of
fibrous peat.
Compressibililty Characteristics
9. Because of their buried nature, the preconsolidation pressure σ`p of the three
stratums encountered in this study was easily determined from EOP εv -log
σ`v compression curves which were characterized by the S-shape and clear
break at the preconsolidation pressure, compared with surficial organic
deposits, which generally have small to medium past pressures because of
their typical locations, i.e., near the surface. The mean in-situ
292
overconsolidation ratio (OCR) was computed as 1.91, 2.68, and 2.19 for
RU, RL, and D, respectively. However, aging is not the only reason for the
overconsolidation of the encountered organic soils.
10. Organic soils encountered display high compressibility; the mean value of
Cc right after the preconsolidation pressure is 1.05, 2.86, and 3.631 for RU,
RL, and D stratums, respectively. The value of Cc increases as mean
organic content increases. These values of Cc for organic silt and clay
encountered (RU stratum) is equal 2 to 3 times the corresponding
compressibility of typical Egyptian soft clay and silt deposits, while that for
highly organic soils encountered (RL & D stratums) are as high as 4 to 10
times of those soils. On the other hand, it is evident that Cc for RL and D
stratums, classified as peaty muck, are comparable to that reported for peaty
muck and still in the lower limits reported for peat. Also, due to the S-shape
of the compression curve, the compression index Cc changes significantly
over the stress range investigated.
11. Values of recompression index Cr for RU, RL, and D stratums are 0.165,
0.458, and 0.574, respectively. Corresponding values of Cr /Cc ratio are
0.13, 0.144, and 0.198, respectively. These ratios, for the three stratums, are
comparable to that reported for inorganic silt and clay of 0.1-0.2 and are
still in the lower limits reported for fibrous peat of 0.1-0.3.
12. Considering the buried nature of the highly organic soils encountered (RL
& D stratums), and the high OCR measured, when construction has to take
place on such deposits, the expected stresses from surface loads will be in
the recompression range (Cr = 0.4-0.6), which is comparable to Cc values of
typical Egyptian soft clay and silt deposits. This indicates that highly
organic soils encountered is as problematic as typical Egyptian soft clay and
silt deposits regarding the usually loading scenarios
293
13. The coefficient of consolidation data for RU stratum (organic silty clay) is
comparable to that of soft clay and silt deposits, while that of RL and D
stratums (peaty muck) is lying in between that of fibrous peat and that of
organic silt and clay (RU). In addition, the decrease in coefficient of
consolidation for RU stratum of lower organic content is less in magnitude
than that for RL and D stratums of high organic content typically as
reported in literature.
14. Secondary compression is significant for organic soils because they display
high values of Cc and Ca/Cc and low values of tp. The values of Cα /Cc ratio
are 0.041, 0.049, and 0.05 for RU, RL, and D stratums, respectively, but
still in the lower range of fibrous peat deposits (Cα/Cc = 0.06±0.01) and
coincide the range reported for organic silt and clay (Cα/Cc = 0.05±0.01).
Permeability Characteristics
15. The peaty muck deposits encountered (RL & D) possess medium initial
permeability (ko); just below the range reported for peats and around 100 to
1000 times the initial permeability of soft clay and silt deposits. This is
because highly colloidal, mostly decomposed, and amorphous-granular soils
tend to be less permeable than well preserved fibrous soils. On the other
hand, initial permeability (ko) for highly organic silty clay soil (RU stratum)
exists just at the upper limit of soft clays and silts deposits, which indicates
that organic matter encountered in soil matrix promotes loose and open
fabric.
16. Upon compression, the permeability of the organic soils encountered
decreases dramatically as they compress under loads because of their high
compressibility. This was evident from post consolidation micrographs
shown in Figs. (5-9, 5-11, & 5-13) which revealed the thick relatively stiff
mats formed after consolidation under 3200 kPa in the horizontal and
vertical sections. Ck /eo was estimated as 0.30, 0.14, and 0.24 for RU, RL,
294
and D stratums respectively compared to 0.25 for fibrous peat and 0.5 for
soft clays and silts deposits.
Shear Strength Characteristics
17. The mean undrained shear strength values obtained through pocket
penetrometer tests (Sup) were twice or more the mean undrained shear
strength values obtained through unconfined compression tests (Sun).
However, the undrained shear strength values obtained through pocket
penetrometer tests (Sup) were more reasonable, since they increase with
depth and OCR ratio. Therefore, the mean normalized undrained shear
strength (Sup/σ`p) values were more reasonable and comparable to that
reported in literature. The Sup/σ`p = 0.25 for RU is comparable to that of
inorganic soft clay and silt deposits, and Sup/σ`p = 0.45 and 0.46 for RL and
D, respectively are comparable to that of highly organic soils taking into
account the disappearing of fibrous structure of the encountered organic
soils as concluded by Mesri and Ajlouni (2007).
18. The friable nature and less cohesion of highly organic soils, encountered in
this study (RL & D), severly underestimated the undrained shear strength of
this soil, obtained through unconfined compression tests, due to inevitable
change of effective stress and mechanical disturbances during the process
from sampling to laboratory testing. Therefore, it is not preferable, for
highly organic soils, to determine the undrained shear strength based on the
result of uconfined compression tests only.
7.3 Recommendation for Future Work
Based on the results of this research, the following topics are suggested for
further investigation:
295
1. The use of Piezocone Penetration test (CPTu) need to be evaluated, as
reliable and efficient tool, for on-site estimation of organic soil engineering
properties. The undrained shear strength (Su), the undrained deformation
modulus (Eu), constrained modulus ((D = 1/mv)) and compression index
(Cc) can be determined based on interpretation of Piezocone Penetration test
results, and coefficient of consolidation (ch then cv) can be determined from
concurrent porewater dissipation experiments data, derived from Piezocone
Penetration test (CPTu), based on the work introduced by Lunne et al.,
(1997) and Abu-Farsakh et al., (2005).
2. The capability of preloading and surcharging to reduce postconstruction
secondary settlements to acceptable limits, need to be investigated for the
subsurface conditions of Egypt where organic soil deposits encountered
usually interlayered and underlined by soft clay layers.
3. Proper monitoring of buildings constructed or embankments instumented
and well characterized, on organic soil deposits, is needed to further
examine the Terzaghi consolidation theory for organic soils found in Egypt.
4. Organic contents were measured using loss on ignition method. Specified
time required for complete firing needs to be more investigated, so that only
organic matter may burn off.
296
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321
APPENDIX - A
322
Table (A.1): Important index and chemical properties for some fibrous peat
deposits
Ash
content
%
Natural
water
content
(ωo, %)
Fibrouswoody
17
484-909
-
-
-
Colley, 1950
Fibrous
-
850
0.95-1.03
1.1-1.8
-
Hanrahan 1954
Peat
-
520
-
-
-
Lewis 1956
Amorphous
and fibrous
-
500-1500
0.88-1.22
1.5-1.6
-
12.2-22.5
200-600
-
1.62
4.8-6.3
Amorphous
To fibrous
14
850
-
1.5
-
Fibrous Peat
-
660-890
0.94-1.15
-
-
17.5 -25
311-392
-
1.53-1.59
-
1.25
560-890
-
1.41-1.47
-
12.1
530
-
-
-
4.6-15.8
605-1290
0.87-1.04
1.41-1.7
-
9.4
613-886
1.04
1.5
4.1
4.8
350
-
-
4.3
1
778
-
-
3.3
Levesque et al.
1980
14.3
202-1159
1.05
1.5
4.17
Berry 1983
23.9
660
1.05
1.58
6.9
9.4
19.5
15
418
600
460
1.05
0.96
0.96
1.73
1.72
1.68
6.9
7.3
6.2
NG and Eischen
1983
12
510
0.91
1.41
7
6.9-8.4
173-757
0.84
1.56
6.4
0.1-32.0
21- 25
5-15
-
660-1590
858-1186
400-1100
700-800
0.99-1.1
~1.00
1.53-1.68
1.56-1.67
1.47
-
5-7
4.2
-
20.8
669
0.97
1.52
-
Bog peat
13.8-24.4
662-965
1.01-1.14
1.51-1.59
3.8-4.4
Fibrous
Fibrous
(Middleton)
Fibrous
(James Bay)
Fibrous
(Florida)
Fibrous
(Florida)
20 - 21
800-900
1.1 -1.2
1.9
-
5-7
510-850
0.99-1.1
1.47-1.64
4.2
4.1
1000-1340
0.85-1.02
1.37-1.55
5.3
8.5-19.9
420-582
0.97-1.06
1.1-1.39
6.7-8.8
7.7-24
370-652
1.0-1.12
Peat
type
Fibrous
Fine fibrous
Fibrous
Coarse
Fibrous
Fibrous sedge
Fibrous
Sphagnum
Coarse
Fibrous
Fine Fibrous
Fine Fibrous
Peat Portage
Peat Waupaca
Fibrous Peat
Middleton
Fibrous Peat
Noblesville
Fibrous
Peat
Peat
Fibrous
Peat
(Netherlands)
Specific
Gravity
Bulk
Density
Mg/m
Gs
3
323
Acidity
pH
Reference
Lea and Browner
1963
Adams
1965
Keene and
Zawodniak 1968
Olson
1970
Skempton &Petly
1970
Samson and
LaRochell 1972
Berry and Vickers
1975
Edil and Mochtar
1984
Lefebvre et al. 1984
Yamaguchi 1985
Yamaguchi 1990
Hansbo 1991
Termatt and
Topolnicki 1994
Nichol & Farmer,
1998
Farrell & Hebib 1998
Ajlouni, 2000
MacVay & Nagyen
2004
Sobhan, 2007
Table (A.2): Important index and chemical properties for some amorphous
peat deposits
Ash
content
Bulk
Density
%
Natural
water
content
(ωo, %)
Mg/m
Gs
15.9
355-425
-
1.73
6.7
12.25
330-375
-
1.63
6.2
Canada
25
600
-
2.0
Amorphous
Peat
-
200-875
1.04-1.23
-
-
Olson 1970
King's Lynn UK
21.7
317
-
1.59
-
Skempton &Petly
1970
Amorphous
Granular
19.5
336
1.05
1.72
7.3
NG and Eischen
1983
Italy
20-30
250-400
0.92-1.12
-
4.1-6
Colleselli &
Cortellazzo, 1998
Cork - Irland
20
450
1.04
Peat
type
3
Specific
Gravity
Acidity
pH
Canada
324
Reference
Adams
1965
Wilson et al., 1965
Huat, 2004
Table (A.3): Important index and chemical properties for some Peaty muck
deposits
Location
Organic
content
%
Natural
water
content
(ωo, %)
Bulk
Density
3
Mg/m
Specific
Gravity
Acidity
pH
Reference
Gs
Avonmouth UK
53-75
335-391
-
1.62-1.83
-
Skempton
&Petly
1970
Chicago - USA
71.6
692
-
1.69
-
Franklin et al.,
1973
Fond du Lac
County, WiscUSA
60.2
240
1.04
1.94
6.24
Edil &
Dhowian 1981
55-78
125-375
-
1.55-1.63
5-7
52 - 75
655 - 924
-
1.64-1.89
5-7
Peat
55-78
419
1.0
1.61
-
Jones et al.
1986
Dakahlia Egypt
51-72
312-625
1.04-1.13
1.48-1.75
6-7.15
Zayed, 1989
Plovdiv Bulgaria
58.1
154
1.23
1.87
-
Kirov, 1998
Sherman Island
– California USA
56-65
152-240
1.13-1.2
-
-
Boulanger et
al., 1998
North Wales UK
61.5-69
561-971
1.03-1.18
1.65-1.73
3.8-4.4
Nichol &
Farmer, 1998
Kamedago Japan
57.5
506
-
1.764
-
Kanmuri et al,
1998
Akita - Japan
60-70
560-680
-
1.69-1.75
-
Mitachi et al.,
2001
SacramentoSan JoaquinCalifornia
51
334
1.102
-
-
52
588
1.066
-
-
Florida - USA
67.6
300
1.15
1.55
-
MacVay &
Nagyen
2004
Florida - USA
60-61.5
205-368
1.06-1.13
-
-
Sobhan, 2007
Celory bog,
Indiana - USA
41-65
264-314
-
-
5.6-5.9
Santagata,
2008
Japan
325
Yamaguchi et
al. 1985
Wehling et al.,
2003
Table (A.4): Important index and chemical properties for some Silty and/or
Clayey muck deposits
Organic
content
Bulk
Density
%
Natural
water
content
(ωo, %)
Mg/m
Gs
26 -44
146-239
-
40
414
43.6
Ohmiya - Japan
Specific
Gravity
Acidity
pH
Reference
1.88-2.27
-
Skempton
&Petly
1970
-
2.04
-
150
-
2.17
-
30 - 45
315 - 524
-
2.0 - 2.26
-
Yamaguchi et
al. 1985
Dakahlia Egypt
25 -42.2
108-374
1.09-1.31
1.62-2.07
7-7.35
Zayed, 1989
Sherman Island
– California USA
44-46
180-200
1.15-1.17
-
-
Boulanger et
al., 1998
Japan
45.9
197-217
-
2.15
2.8
Kamao &
Yamada 1998
SacramentoSan JoaquinCalifornia
31-48
171-409
1.08-1.24
-
-
34-43
430-512
1.06-1.15
-
-
Location
Avonmouth UK
3
Chicago - USA
Franklin et al.,
1973
Wehling et al.,
2003
Florida - USA
27.8-47.1
105-233
1.07-1.29
1.43-2.13
-
MacVay &
Nagyen
2004
Florida - USA
28.7-39.3
205-368
1.06-1.13
-
-
Sobhan, 2007
Celory bog,
Indiana - USA
36-45.2
157-219
-
1.95-2.2
5.6-5.9
Santagata,
2008
326
Table (A.5): Important index and chemical properties for some organic silt
and/or clay deposits
Organic
content
Bulk
Density
%
Natural
water
content
(ωo, %)
Mg/m
Gs
1.9-9.9
22-72
-
2.54-2.69
-
10.5-22.5
76-
-
2.24-2.47
-
16.2
122
-
2.3
-
3-9
61-155
-
-
-
13 -15
234
-
-
-
Ohmiya Japan
10
102
Dakahlia Egypt
16-22.5
82-120
1.35-1.46
2.0-2.24
7-7.9
Zayed, 1989
Plovdiv Bulgaria
17.2
29
1.94
2.67
-
Kirov, 1998
4.8-9.8
25-70
1.48-1.84
1.88-2.55
8-8.5
12.6-24.7
51-119
1.26-1.6
1.9-2.3
8.6
MacVay &
Nagyen
2004
21.3
186.6
1.16
-
-
Location
Avonmouth UK
King's Lynn UK
3
Specific
Gravity
Acidity
pH
Finland
Florida USA
Florida USA
Reference
Skempton
&Petly
1970
Slunga
&Helander
1985
Yamaguchi et
al. 1985
2.56
Sobhan, 2007
13.5
73.1
-
327
-
‫اﻟﺨﻮاص اﻹﻧﻀﻐﺎﻃﻴﺔ ﻟﻠﺘﺮﺑﺔ اﻟﻌﻀﻮﻳﺔ ﻓﻰ ﻣﺼﺮ‬
‫اﻋﺪاد‬
‫ﻣﻬﻨﺪس‪ /‬ﺣﺴﺎم إﺑﺮاهﻴﻢ ﻋﺒﺪ اﻟﻘﺎدر‬
‫ﺑﻜﺎﻟﻮرﻳﻮس اﻟﻬﻨﺪﺳﺔ اﻟﻤﺪﻧﻴﺔ ‪١٩٨٥‬‬
‫دﺑﻠﻮم ﻣﻴﻜﺎﻧﻴﻜﺎ اﻟﺘﺮﺑﺔ و هﻨﺪﺳﺔ اﻷﺳﺎﺳﺎت ‪١٩٩٠‬‬
‫دﺑﻠﻮم إدارة و هﻨﺪﺳﺔ اﻟﺘﺸﻴﻴﺪ ‪٢٠٠٢‬‬
‫رﺳﺎﻟﺔ ﻣﻘﺪﻣﺔ إﻟﻰ آﻠﻴﺔ اﻟﻬﻨﺪﺳﺔ‪ ،‬ﺟﺎﻣﻌﺔ اﻟﻘﺎهﺮة‬
‫آﺠﺰء ﻣﻦ ﻣﺘﻄﻠﺒﺎت اﻟﺤﺼﻮل ﻋﻠﻰ درﺟﺔ اﻟﻤﺎﺟﺴﺘﻴﺮ‬
‫ﻓﻰ اﻷﺷﻐﺎل اﻟﻌﺎﻣﺔ‬
‫آﻠﻴﺔ اﻟﻬﻨﺪﺳﺔ‪ ،‬ﺟﺎﻣﻌﺔ اﻟﻘﺎهﺮة‬
‫اﻟﺠﻴﺰة‪ ،‬ﺟﻤﻬﻮرﻳﺔ ﻣﺼﺮ اﻟﻌﺮﺑﻴﺔ‬
‫ﺳﺒﺘﻤﺒﺮ ‪٢٠١٠‬‬
‫اﻟﺨﻮاص اﻹﻧﻀﻐﺎﻃﻴﺔ ﻟﻠﺘﺮﺑﺔ اﻟﻌﻀﻮﻳﺔ ﻓﻰ ﻣﺼﺮ‬
‫اﻋﺪاد‬
‫ﻣﻬﻨﺪس‪ /‬ﺣﺴﺎم إﺑﺮاهﻴﻢ ﻋﺒﺪ اﻟﻘﺎدر‬
‫ﺑﻜﺎﻟﻮرﻳﻮس اﻟﻬﻨﺪﺳﺔ اﻟﻤﺪﻧﻴﺔ ‪١٩٨٥‬‬
‫دﺑﻠﻮم ﻣﻴﻜﺎﻧﻴﻜﺎ اﻟﺘﺮﺑﺔ و هﻨﺪﺳﺔ اﻷﺳﺎﺳﺎت ‪١٩٩٠‬‬
‫دﺑﻠﻮم إدارة و هﻨﺪﺳﺔ اﻟﺘﺸﻴﻴﺪ ‪٢٠٠٢‬‬
‫رﺳﺎﻟﺔ ﻣﻘﺪﻣﺔ إﻟﻰ آﻠﻴﺔ اﻟﻬﻨﺪﺳﺔ‪ ،‬ﺟﺎﻣﻌﺔ اﻟﻘﺎهﺮة‬
‫آﺠﺰء ﻣﻦ ﻣﺘﻄﻠﺒﺎت اﻟﺤﺼﻮل ﻋﻠﻰ درﺟﺔ اﻟﻤﺎﺟﺴﺘﻴﺮ‬
‫ﻓﻰ اﻷﺷﻐﺎل اﻟﻌﺎﻣﺔ‬
‫ﺗﺤﺖ إﺷﺮاف‬
‫أ‪ .‬د‪ .‬ﻣﺼﻄﻔﻰ اﻟﺴﻴﺪ ﻣﺴﻌﺪ‬
‫أﺳﺘﺎذ ﻣﻴﻜﺎﻧﻴﻜﺎ اﻟﺘﺮﺑﺔ و هﻨﺪﺳﺔ اﻷﺳﺎﺳﺎت‬
‫آﻠﻴﺔ اﻟﻬﻨﺪﺳﺔ – ﺟﺎﻣﻌﺔ اﻟﻘﺎهﺮة‬
‫د‪ .‬ﻣﺮوان ﻣﻐﺎورى ﺷﺎهﻴﻦ‬
‫أﺳﺘﺎذ ﻣﺴﺎﻋﺪ ﻣﻴﻜﺎﻧﻴﻜﺎ اﻟﺘﺮﺑﺔ و هﻨﺪﺳﺔ اﻷﺳﺎﺳﺎت‬
‫آﻠﻴﺔ اﻟﻬﻨﺪﺳﺔ – ﺟﺎﻣﻌﺔ ﻃﻨﻄﺎ‬
‫آﻠﻴﺔ اﻟﻬﻨﺪﺳﺔ‪ ،‬ﺟﺎﻣﻌﺔ اﻟﻘﺎهﺮة‬
‫اﻟﺠﻴﺰة‪ ،‬ﺟﻤﻬﻮرﻳﺔ ﻣﺼﺮ اﻟﻌﺮﺑﻴﺔ‬
‫ﺳﺒﺘﻤﺒﺮ ‪٢٠١٠‬‬
‫اﻟﺨﻮاص اﻹﻧﻀﻐﺎﻃﻴﺔ ﻟﻠﺘﺮﺑﺔ اﻟﻌﻀﻮﻳﺔ ﻓﻰ ﻣﺼﺮ‬
‫اﻋﺪاد‬
‫ﻣﻬﻨﺪس‪ /‬ﺣﺴﺎم إﺑﺮاهﻴﻢ ﻋﺒﺪ اﻟﻘﺎدر‬
‫ﺑﻜﺎﻟﻮرﻳﻮس اﻟﻬﻨﺪﺳﺔ اﻟﻤﺪﻧﻴﺔ ‪١٩٨٥‬‬
‫دﺑﻠﻮم ﻣﻴﻜﺎﻧﻴﻜﺎ اﻟﺘﺮﺑﺔ و هﻨﺪﺳﺔ اﻷﺳﺎﺳﺎت ‪١٩٩٠‬‬
‫دﺑﻠﻮم إدارة و هﻨﺪﺳﺔ اﻟﺘﺸﻴﻴﺪ ‪٢٠٠٢‬‬
‫رﺳﺎﻟﺔ ﻣﻘﺪﻣﺔ إﻟﻰ آﻠﻴﺔ اﻟﻬﻨﺪﺳﺔ‪ ،‬ﺟﺎﻣﻌﺔ اﻟﻘﺎهﺮة‬
‫آﺠﺰء ﻣﻦ ﻣﺘﻄﻠﺒﺎت اﻟﺤﺼﻮل ﻋﻠﻰ درﺟﺔ اﻟﻤﺎﺟﺴﺘﻴﺮ‬
‫ﻓﻰ اﻷﺷﻐﺎل اﻟﻌﺎﻣﺔ‬
‫ﻳﻌﺘﻤﺪ ﻣﻦ ﻟﺠﻨﺔ اﻟﻤﻤﺘﺤﻨﻴﻦ ‪:‬‬
‫اﻷﺳﺘﺎذ اﻟﺪآﺘﻮر ‪ /‬ﻣﺤﻤﺪ ﻣﻤﺪوح ﻋﻠﻰ ﺻﺒﺮى‬
‫ﻋﻀﻮا‬
‫اﻷﺳﺘﺎذ اﻟﺪآﺘﻮر ‪ /‬ﻓﺘﺢ اﷲ ﻣﺤﻤﺪ اﻟﻨﺤﺎس‬
‫ﻋﻀﻮا‬
‫اﻷﺳﺘﺎذ اﻟﺪآﺘﻮر ‪ /‬ﻣﺼﻄﻔﻰ اﻟﺴﻴﺪ ﻣﺴﻌﺪ‬
‫اﻟﻤﺸﺮف اﻟﺮﺋﻴﺴﻰ‬
‫اﻟﺪآﺘﻮر ‪ /‬ﻣﺮوان ﻣﻐﺎورى ﺷﺎهﻴﻦ‬
‫ﻣﺸﺮﻓﺎ‬
‫آﻠﻴﺔ اﻟﻬﻨﺪﺳﺔ‪ ،‬ﺟﺎﻣﻌﺔ اﻟﻘﺎهﺮة‬
‫اﻟﺠﻴﺰة‪ ،‬ﺟﻤﻬﻮرﻳﺔ ﻣﺼﺮ اﻟﻌﺮﺑﻴﺔ‬
‫ﺳﺒﺘﻤﺒﺮ ‪٢٠١٠‬‬
‫اﻟﺨﻮاص اﻹﻧﻀﻐﺎﻃﻴﺔ ﻟﻠﺘﺮﺑﺔ اﻟﻌﻀﻮﻳﺔ ﻓﻰ ﻣﺼﺮ‬
‫ﻣﻠﺨﺺ اﻟﺮﺳﺎﻟﺔ‬
‫ﺗﺘﻮاﺟﺪ اﻟﺘﺮﺑﺔ اﻟﻌﻀﻮﻳﺔ ﺑﻤﺼﺮ ﻓﻰ ﻣﺴﺎﺣﺎت آﺒﻴﺮة ﻣﻦ ﺷﻤﺎل اﻟﺪﻟﺘﺎ‪ ،‬ﻓﻰ ﺻﻮرة ﺣﺰام ﺗﺤﺖ ﺳﻄﺤﻰ ﻳﻤﺘﺪ ﻣﻦ‬
‫اﻟ ﺸﺮق اﻟ ﻰ اﻟﻐ ﺮب‪ ،‬ﺣ ﻮل و ﺑ ﻴﻦ ﻓﺮﻋ ﻰ ﻧﻬ ﺮ اﻟﻨﻴ ﻞ رﺷ ﻴﺪ و دﻣﻴ ﺎط‪ .‬ه ﺬا و ﺗﺘﺒ ﺎﻳﻦ اﻟﺨ ﻮاص اﻟﻔﻴﺰﻳﺎﺋﻴ ﺔ و‬
‫اﻟﻬﻨﺪﺳﻴﺔ ﻟﻠﺘﺮﺑﺔ اﻟﻌﻀﻮﻳﺔ ﺗﺒﻌﺎ ﻟﻜﻤﻴﺔ و ﻧﻮﻋﻴﺔ و درﺟﺔ ﺗﺤﻠﻞ اﻟﻤﺎدة اﻟﻌﻀﻮﻳﺔ اﻟﻤﺤﺘﻮاﻩ ﺑﺎﻟﺘﺮﺑﺔ )ﻧ ﻮع اﻟﺘﺮﺑ ﺔ(‬
‫و آﺬﻟﻚ ﻃﺒﻴﻌﺔ اﻹﺟﻬﺎدات اﻟﻤﺆﺛﺮة ﻋﻠﻴﻬﺎ ‪ .‬و ﻋﻠﻰ اﻟﺮﻏﻢ ﻣﻦ ذﻟﻚ ﻓﺈن هﺬﻩ اﻟﻨﻮﻋﻴﺔ ﻣﻦ اﻟﺘﺮﺑﺔ ﻓ ﻰ ﻣ ﺼﺮ ﻟ ﻢ‬
‫ﺗﺤﻈﻰ ﺑﺎﻟﻘﺪر اﻟﻜﺎﻓﻰ ﻣﻦ اﻟﺪراﺳﺔ و هﻮ ﻣﺎ ﻳﺘﻀﺢ ﻣﻦ ﻧﺪرة اﻟﺒﺤﻮث و اﻟﺪراﺳﺎت اﻟﺘﻰ ﺗﻢ ﻧﺸﺮهﺎ ﻓﻴﻤ ﺎ ﻳﺨ ﺺ‬
‫اﻟﺨﻮاص اﻟﻌﺎﻣﺔ و اﻟﺨﻮاص اﻟﻬﻨﺪﺳﻴﺔ ﻟﻬﺬﻩ اﻟﺘﺮﺑﺔ‪ ،‬آﻤﺎ اﻧﻌﻜﺲ ذﻟﻚ ﻋﻠﻰ اﻟﻄﺮﻳﻘ ﺔ اﻟﺘ ﻰ ﻳ ﺘﻢ ﺑﻬ ﺎ ﺗﻌﺮﻳ ﻒ ه ﺬﻩ‬
‫اﻟﻨﻮﻋﻴﺔ ﻣﻦ اﻟﺘﺮﺑﺔ ﻓﻰ اﻟﻜﻮد اﻟﻤﺼﺮى ﻟﻸﺳﺎﺳﺎت آﺄﺣﺪ أﻧﻮاع اﻟﺘﺮﺑﺔ اﻟﻄﻴﻨﻴﺔ اﻟﻠﻴﻨﺔ ذات اﻟﻤﺸﺎآﻞ )!(‪.‬‬
‫ﻓﻰ هﺬا اﻟﺒﺤﺚ ﺗﻢ دراﺳﺔ اﻟﺨﻮاص اﻟﻔﻴﺰﻳﺎﺋﻴﺔ و اﻟﻜﻴﻤﻴﺎﺋﻴ ﺔ و اﻟﻤﻮرﻓﻮﻟﻮﺟﻴ ﺔ و اﻟﻬﻨﺪﺳ ﻴﺔ ﻟﻸﻧ ﻮاع اﻟﻤﺨﺘﻠﻔ ﺔ ﻣ ﻦ‬
‫اﻟﺘﺮﺑﺔ اﻟﻌﻀﻮﻳﺔ ﻓﻰ ﻣﻨﻄﻘﺔ ﻏﺮب اﻟﺪﻟﺘﺎ‪ ،‬اﻟﺘﻰ ﺗﺘﻤﻴﺰ ﺑﻮﺟﻮد اﻟﻄﺒﻘﺎت اﻷآﺒﺮ ﺳﻤﻜﺎ ﻟﻠﺘﺮﺑﺔ اﻟﻌﻀﻮﻳﺔ ﻓﻰ ﻣﺼﺮ‬
‫آﻤ ﺎ ﺗﺘﻤﻴ ﺰ ﺑﺎﻣﻜﺎﻧﻴ ﺔ ﻇﻬ ﻮر أآﺜ ﺮ ﻣ ﻦ ﻃﺒﻘ ﺔ ﻣﻨﻬ ﺎ‪ ،‬ﻟﻌﻴﻨ ﺎت ﺗ ﻢ اﺳﺘﺨﻼﺻ ﻬﺎ ﺑﺤﺎﻟﺘﻬ ﺎ اﻟﻄﺒﻴﻌﻴ ﺔ ﻏﻴ ﺮ اﻟﻤﻘﻠﻘﻠ ﺔ‬
‫ﺑﻮاﺳﻄﺔ ﺟﻬﺎز ﺧﺎص ﺗﻢ ﺗﺼﻤﻴﻤﻪ و ﺗﺼﻨﻴﻌﻪ ﺧﺼﻴﺼﺎ ﻟﻬﺬا اﻟﻐﺮض‪ ،‬ﻣ ﻦ ﻣ ﻮﻗﻌﻴﻦ ﻣﺨﺘﻠﻔ ﻴﻦ ﺑﻤﺤ ﺎﻓﻈﺘﻰ آﻔ ﺮ‬
‫اﻟﺸﻴﺦ و اﻟﺒﺤﻴﺮة‪ .‬ﻳﺮآﺰاﻟﺒﺤﺚ ﻋﻠﻰ دراﺳﺔ اﻟﺨﻮاص اﻹﻧﻀﻐﺎﻃﻴﺔ ﻟﻠﺘﺮﺑ ﺔ اﻟﻌ ﻀﻮﻳﺔ ﻓ ﻰ ﻣﺮﺣﻠﺘ ﻰ اﻹﻧ ﻀﻐﺎط‬
‫اﻷﺳﺎﺳﻰ و اﻹﻧﻀﻐﺎط اﻟﺜﺎﻧﻮى‪ ،‬ﺣﻴﺚ ﺗ ﻢ ﺗﺤﺪﻳ ﺪ ﻣﻌ ﺎﻣﻼت اﻹﻧ ﻀﻐﺎط اﻷﺳﺎﺳ ﻰ و اﻹﻧ ﻀﻐﺎط اﻟﺜ ﺎﻧﻮى ﻟﻬ ﺬﻩ‬
‫اﻟﺘﺮﺑﺔ‪ -‬ﻓ ﻰ ﺻ ﻮرهﺎ اﻟﻤﺨﺘﻠﻔ ﺔ‪ -‬و ذﻟ ﻚ ﻓ ﻰ اﻹﺗﺠ ﺎهﻴﻦ اﻟﺮأﺳ ﻰ و اﻷﻓﻘ ﻰ ودراﺳ ﺔ ﻣ ﺪى ﺗﻐﻴﺮه ﺎ ﺗﺤ ﺖ ﺗ ﺄﺛﻴﺮ‬
‫ﺣﺎﻻت اﻟﺘﺤﻤﻴﻞ اﻟﻤﺨﺘﻠﻔﺔ و اﻹﺟﻬﺎدات اﻟﻤﺨﺘﻠﻔﺔ و ﻣﻊ اﻟﺰﻣﻦ‪ .‬آﺬﻟﻚ ﺗﻢ ﺗﺤﺪﻳﺪ ﻣﻌﺎﻣﻞ اﻟﻨﻔﺎدﻳﺔ ﻟﻬ ﺬﻩ اﻟﺘﺮﺑ ﺔ ﻓ ﻰ‬
‫اﻹﺗﺠﺎهﻴﻦ اﻟﺮأﺳﻰ و اﻷﻓﻘﻰ ﻓﻰ ﺣﺎﻟﺘﻬﺎ اﻟﻄﺒﻴﻌﻴﺔ و رﺻﺪ اﻟﺘﻐﻴ ﺮ ﻓ ﻰ ﻣﻌﺎﻣ ﻞ اﻟﻨﻔﺎدﻳ ﺔ ﺗﺤ ﺖ ﺗ ﺄﺛﻴﺮ اﻹﺟﻬ ﺎدات‬
‫اﻟﻤﺨﺘﻠﻔﺔ‪ .‬آﺬﻟﻚ ﺗﻢ ﺗﻌﻴﻴﻦ ﻣﻌﺎﻣﻞ ﻣﻘﺎوﻣﺔ اﻟﻘﺺ ﺑﻄﺮﻳﻘﺔ اﻟﻀﻐﻂ اﻟﻤﺤ ﻮرى ﻏﻴ ﺮ اﻟﻤﺤ ﺎط‪ ،‬و أﻳ ﻀﺎ ﺑﺎﺳ ﺘﺨﺪام‬
‫ﺟﻬﺎز اﻟﻐﺰ اﻟﺠﻴﺒﻰ‪ .‬آﻤﺎ ﺗﻢ رﺻﺪ اﻟﺘﻐﻴﺮ اﻟﺤﺎدث ﻓ ﻰ ﻧ ﺴﻴﺞ و ﻣ ﺴﺎﻣﻴﺔ اﻟﺘﺮﺑ ﺔ ﻧﺘﻴﺠ ﺔ ﻹﻧ ﻀﻐﺎﻃﻬﺎ ﻣ ﻦ دراﺳ ﺔ‬
‫ﺻﻮر اﻟﻤﻴﻜﺮوﺳﻜﻮب اﻹﻟﻴﻜﺘﺮوﻧﻰ ﻟﻌﻴﻨﺎت اﻟﺘﺮﺑﺔ ﻓﻰ اﻹﺗﺠﺎهﻴﻦ اﻟﺮأﺳﻰ و اﻷﻓﻘﻰ ﻓﻰ ﺣﺎﻟﺘﻬﺎ اﻟﻄﺒﻴﻌﻴﺔ و ﺑﻌ ﺪ‬
‫اﻧﻀﻐﺎﻃﻬﺎ‪ ،‬و ﻗﺪ ﺗﻢ ﻣﻘﺎرﻧ ﺔ اﻟﻨﺘ ﺎﺋﺞ اﻟﻤ ﺴﺘﺨﺮﺟﺔ ﻣ ﻦ اﻟﺒﺤ ﺚ ﻣ ﻊ ﻧﻈﺎﺋﺮه ﺎ اﻟﻤﻨ ﺸﻮرة ﺑ ﺎﻟﻤﺮاﺟﻊ و اﻟ ﺪورﻳﺎت‬
‫اﻟﻤﺨﺘﻠﻔﺔ‪ .‬آﺬﻟﻚ ﺗﻢ اﻗﺘﺮاح ﻧﻈﺎم ﻟﺘﻌﺮﻳﻒ و ﺗﺼﻨﻴﻒ اﻷﻧﻮاع اﻟﻤﺨﺘﻠﻔﺔ ﻟﻠﺘﺮﺑﺔ اﻟﻌﻀﻮﻳﺔ ﻓﻰ ﻣﺼﺮ‪.‬‬
‫و ﻗﺪ أﻇﻬﺮت اﻟﺪراﺳﺔ أن ﻗﻴﻢ اﻟﺨﻮاص اﻟﻬﻨﺪﺳﻴﺔ ﻟﻠﺘﺮﺑﺔ اﻟﻌﻀﻮﻳﺔ اﻟﺘ ﻰ ﺗ ﻢ دراﺳ ﺘﻬﺎ ﻓ ﻰ ه ﺬا اﻟﺒﺤ ﺚ و ﺗ ﺸﻤﻞ‬
‫اﻟﻨﻔﺎذﻳﺔ و ﻣﻘﺎوﻣ ﺔ اﻟﻘ ﺺ ﺑﻄﺮﻳﻘ ﺔ اﻟ ﻀﻐﻂ اﻟﻤﺤ ﻮرى ﻏﻴ ﺮ اﻟﻤﺤ ﺎط و اﻟﺨ ﻮاص اﻹﻧ ﻀﻐﺎﻃﻴﺔ ﺗﻘ ﻊ ﺑ ﻴﻦ ﺗﻠ ﻚ‬
‫اﻟﻤﻤﻴﺰة ﻟﻠﺘﺮﺑﺔ اﻟﻠﻴﻔﻴﺔ اﻟﻌﻀﻮﻳﺔ ﺷﺪﻳﺪة اﻟﻤ ﺸﺎآﻞ و اﻟﺘﺮﺑ ﺔ اﻟﻄﻴﻨﻴ ﺔ اﻟﻄﻤﻴﻴ ﺔ اﻟﻠﻴﻨ ﺔ‪ .‬آﻤ ﺎ أوﺿ ﺤﺖ اﻟﺪراﺳ ﺔ أن‬
‫إﺷﻜﺎﻟﻴﺔ اﻟﺘﺄﺳﻴﺲ ﻓﻮﻗﻬﺎ‪ ،‬ﺗﺤﺖ ﺗﺄﺛﻴﺮ اﻹﺟﻬ ﺎدات اﻟﻤﻌﺘ ﺎدة‪ ،‬ﻣﻨ ﺎﻇﺮة ﻹﺷ ﻜﺎﻟﻴﺔ اﻟﺘﺄﺳ ﻴﺲ ﻋﻠ ﻰ اﻟﻄ ﻴﻦ اﻟﻠ ﻴﻦ ﻓ ﻰ‬
‫ﻣﺼﺮ‪ ،‬ﻣﻦ ﻧﺎﺣﻴﺔ ﻗﺎﺑﻠﻴﺘﻬﺎ ﻟﻺﻧﻀﻐﺎط‪ ،‬ﻧﻈﺮا ﻟﻄﺒﻴﻌﺘﻬﺎ اﻟﺘﺤﺖ ﺳﻄﺤﻴﺔ و ارﺗﻔﺎع ﻗﻴﻢ ﻧﺴﺒﺔ ﺳﺒﻖ اﻟﺘﺪﻋﻴﻢ ﻟﻬﺎ‪.‬‬
‫و ﻳﺸﺘﻤﻞ اﻟﺒﺤﺚ ﻋﻠﻰ ﺳﺒﻌﺔ ﻓﺼﻮل ﺑﻴﺎﻧﻬﺎ آﺎﻟﺘﺎﻟﻰ‬
‫اﻟﻔﺼﻞ اﻷول‬
‫ﻳﺘﻨ ﺎول ﻣﻘﺪﻣ ﺔ ﻋﺎﻣ ﺔ ﻋ ﻦ اﻟﺒﺤ ﺚ و اﻷه ﺪاف اﻟﻤﺘﻮﺧ ﺎﻩ ﻣﻨ ﻪ و اﻟﻤ ﻨﻬﺞ اﻟﻤﺘﺒ ﻊ ﻟﺘﺤﻘﻴ ﻖ ه ﺬﻩ‬
‫اﻷهﺪاف ﻣﻊ ﻋﺮض ﻟﻤﺤﺘﻮﻳﺎت ﻓﺼﻮل اﻟﺮﺳﺎﻟﺔ‪.‬‬
‫اﻟﻔﺼﻞ اﻟﺜﺎﻧﻰ‬
‫ﻳﻌﺮض هﺬا اﻟﻔﺼﻞ ﻣﻠﺨﺺ ﻟﻤﺎ ﺳﺒﻖ ﻧﺸﺮﻩ ﻋﻦ أﻣﺎآﻦ ﺗﻮاﺟﺪ اﻟﺘﺮﺑﺔ اﻟﻌﻀﻮﻳﺔ ﻓﻰ ﻣﺼﺮ و‬
‫ﻋﻠﻰ ﻣ ﺴﺘﻮى اﻟﻌ ﺎﻟﻢ و اﻟﺨ ﻮاص اﻟﻌﺎﻣ ﺔ ﻟﻬ ﺎ و اﻟﺘ ﻰ ﺗﺘ ﻀﻤﻦ ﻃﺒﻴﻌﺘﻬ ﺎ و ﻃﺮﻳﻘ ﺔ ﺗﻜﻮﻧﻬ ﺎ و‬
‫اﻟﻤﺮاﺣ ﻞ اﻟﺘ ﻰ ﺗﻤ ﺮ ﺑﻬ ﺎ ﻋﻤﻠﻴ ﺔ اﻟﺘﻜ ﻮﻳﻦ و اﻟﺨ ﻮاص اﻟﻤﻮرﻓﻮﻟﻮﺟﻴ ﺔ ﻟﻬ ﺎ و اﻟﺘﻐﻴ ﺮات اﻟﺘ ﻰ‬
‫ﺗﺠ ﺮى ﻋﻠﻴﻬ ﺎ ﺑﻤ ﺮور اﻟ ﺰﻣﻦ و ﺑﻔﻌ ﻞ اﻟﻌﻮاﻣ ﻞ اﻟﺒﻴﺌﻴ ﺔ اﻟﻤﺤﻴﻄ ﺔ‪ ،‬اﻷﻧ ﻮاع اﻟﻤﺨﺘﻠﻔ ﺔ ﻟﻠﺘﺮﺑ ﺔ‬
‫اﻟﻌﻀﻮﻳﺔ واﻷﻧﻈﻤ ﺔ اﻟﻤﺨﺘﻠﻔ ﺔ ﻟﺘ ﺼﻨﻴﻔﻬﺎ ﻣ ﻊ اﻟﻤﻘﺎرﻧ ﺔ ﺑﻴﻨﻬ ﺎ‪ .‬آﻤ ﺎ ﺗ ﻢ اﺳ ﺘﻌﺮاض اﻷﺳ ﺎﻟﻴﺐ‬
‫اﻟﻤﺘﺒﻌﺔ ﻹﺳﺘﺨﺮاج ﻋﻴﻨﺎت ﻣﻤﺜﻠﺔ ﻣﻨﻬﺎ و اﻹﺧﺘﺒ ﺎرات اﻟﺤﻘﻠﻴ ﺔ اﻟﻤﻨﺎﺳ ﺒﺔ ﻟﺘﺤﺪﻳ ﺪ اﻟﻤﻌ ﺎﻣﻼت‬
‫اﻟﻬﻨﺪﺳﻴﺔ ﻟﻬﺎ‪ ،‬و آﺬﻟﻚ اﻟﺨﻮاص اﻟﻔﻴﺰﻳﺎﺋﻴﺔ و اﻟﻜﻴﻤﻴﺎﺋﻴﺔ ﻟﻬﺎ و آﻴﻔﻴﺔ ﺗﺤﺪﻳﺪهﺎ‪.‬‬
‫اﻟﻔﺼﻞ اﻟﺜﺎﻟﺚ‬
‫ﻳﻌﺮض ه ﺬا اﻟﻔ ﺼﻞ ﻣﻠﺨ ﺺ ﻟﻤ ﺎ ﺳ ﺒﻖ ﻧ ﺸﺮﻩ ﻋ ﻦ اﻟﺨ ﻮاص اﻟﻬﻨﺪﺳ ﻴﺔ ﻟﻠﺘﺮﺑ ﺔ اﻟﻌ ﻀﻮﻳﺔ و‬
‫اﻟﺘﻰ ﺗﺸﻤﻞ ﺧﻮاص اﻟﻨﻔﺎذﻳﺔ و ﻣﻘﺎوﻣﺔ اﻟﻘﺺ و اﻹﻧﻀﻐﺎط ﻟﻠﺘﺮﺑﺔ و آﻴﻔﻴﺔ ﺗﺤﺪﻳﺪهﺎ ﺑﺎﻟﻤﻌﻤﻞ‬
‫و ﻓﻰ اﻟﻄﺒﻴﻌﺔ‪ ،‬ﻣﻊ اﺳﺘﻌﺮاض ﻟﻤﺨﺘﻠﻒ اﻟﻌﻮاﻣﻞ اﻟﺘﻰ ﺗﺆﺛﺮ ﻋﻠﻰ هﺬﻩ اﻟﺨﻮاص ﻟﻠﺘﺮﺑﺔ‪ .‬آﻤ ﺎ‬
‫ﻳﺘﻨ ﺎول ه ﺬا اﻟﻔ ﺼﻞ ﺑﺎﻟ ﺸﺮح و اﻟﻤﻨﺎﻗ ﺸﺔ أوﺟ ﻪ اﺧ ﺘﻼف اﻟ ﺴﻠﻮك اﻹﻧ ﻀﻐﺎﻃﻰ ﻟﺘﻜﻮﻳﻨ ﺎت‬
‫اﻟﺘﺮﺑﺔ اﻟﻌﻀﻮﻳﺔ ﻓﻰ ﻣﺮﺣﻠﺘﻴﻪ اﻷﺳﺎﺳ ﻰ و اﻟﺜ ﺎﻧﻮى ﻋ ﻦ ﻧﻈﻴﺮﻳﻬﻤ ﺎ ﻟﻠﺘﺮﺑ ﺔ اﻟﻄﻴﻨﻴ ﺔ اﻟﻤﻌﺘ ﺎدة‬
‫ﻓ ﻰ اﻟﻄﺒﻴﻌ ﺔ و آ ﺬﻟﻚ ﻓﻴﻤ ﺎ ﻳﺨ ﺺ ﻧﺘ ﺎﺋﺞ اﺧﺘﺒ ﺎرات اﻹﻧ ﻀﻐﺎط أﺣ ﺎدى اﻹﺗﺠ ﺎﻩ ﻣ ﻦ ﺣﻴ ﺚ‬
‫ﻣﻨﺤﻨﻴﺎت اﻹﻧﻀﻐﺎط اﻟﻤﺘﻮﻟﺪة ﻋﻨﻬ ﺎ و ﻣﻌ ﺎﻣﻼت اﻹﻧ ﻀﻐﺎط ﻟﻬ ﺎ‪ .‬آﻤ ﺎ ﻳﻌ ﺮض ه ﺬا اﻟﻔ ﺼﻞ‬
‫ﻟﻠﻨﻈﺮﻳﺎت اﻟﺘﻰ وﺿﻌﺖ ﻟﺘﻔﺴﻴﺮ هﺬﻩ اﻟﺨﻮاص و ﻓﺮوﺿﻬﺎ و و ﻣ ﺪى اﺗ ﺴﺎﻗﻬﺎ ﻣ ﻊ ﻧﻈﻴﺮﺗﻬ ﺎ‬
‫ﻟﻠﺘﺮﺑﺔ اﻟﻄﻴﻨﻴﺔ‪.‬‬
‫اﻟﻔﺼﻞ اﻟﺮاﺑﻊ‬
‫و ﻳﺘﻨ ﺎول ه ﺬا اﻟﻔ ﺼﻞ ﺑﺮﻧ ﺎﻣﺞ اﻟﺒﺤ ﺚ و ﺷ ﺮح ﻷهﺪاﻓ ﻪ ﺑﺎﻟﺘﻔ ﺼﻴﻞ و اﻷﺳ ﺎﻟﻴﺐ اﻟﺘ ﻰ ﺗ ﻢ‬
‫اﺗﺒﺎﻋﻬﺎ ﻟﺘﺤﻘﻴﻖ هﺬﻩ اﻷهﺪاف‪ ،‬ﺣﻴﺚ ﻳﺘﻨﺎول ﺑﺎﻟﺸﺮح اﻟﻤﻌﺎﻳﻴﺮ اﻟﺘﻰ ﺗﻢ ﻋﻠﻰ أﺳﺎﺳﻬﺎ اﺧﺘﻴ ﺎر‬
‫اﻟﻤﻮاﻗﻊ اﻟﻤﻨﺎﺳﺒﺔ ﻹﺳ ﺘﺨﺮاج اﻟﻌﻴﻨ ﺎت اﻟﻼزﻣ ﺔ ﻟﻠﺒﺤ ﺚ‪ ،‬و اﻟﻄﺮﻳﻘ ﺔ اﻟﻤ ﺴﺘﺨﺪﻣﺔ ﻹﺳ ﺘﺨﺮاج‬
‫اﻟﻌﻴﻨ ﺎت ﺑﺤﺎﻟﺘﻬ ﺎ اﻟﻄﺒﻴﻌﻴ ﺔ اﻟﻐﻴ ﺮ اﻟﻤﻘﻠﻘﻠ ﺔ‪ ،‬و اﻟﻄ ﺮق اﻟﺘ ﻰ اﺳ ﺘﺨﺪاﻣﻬﺎ ﻟﺘﻌﻴ ﻴﻦ اﻟﺨ ﻮاص‬
‫اﻟﻔﻴﺰﻳﺎﺋﻴ ﺔ و اﻟﻜﻴﻤﻴﺎﺋﻴ ﺔ ﻟﻠﺘﺮﺑ ﺔ اﻟﻌ ﻀﻮﻳﺔ اﻟﻤ ﺴﺘﺨﺮﺟﺔ‪ ،‬آﻤ ﺎ ﺗ ﻢ اﻗﺘ ﺮاح ﻧﻈ ﺎم ﻟﺘﻌﺮﻳ ﻒ و‬
‫ﺗ ﺼﻨﻴﻒ اﻟﺘﺮﺑ ﺔ اﻟﻌ ﻀﻮﻳﺔ ﻓ ﻰ ﻣ ﺼﺮ و اﻟ ﺬى ﺗ ﻢ اﺗﺒﺎﻋ ﻪ ﻟﺘ ﺼﻨﻴﻒ ﻋﻴﻨ ﺎت اﻟﺘﺮﺑ ﺔ‪ ،‬آﻤ ﺎ‬
‫ﻳﻌﺮض ﺑﺎﻟﺘﻔﺼﻴﻞ ﻟﻠﺒﺮوﺗﻮآﻮل اﻟﺬى ﺗﻢ وﺿ ﻌﻪ ﻟﺘﺤﺪﻳ ﺪ اﻟﺨ ﻮاص اﻟﻬﻨﺪﺳ ﻴﺔ ﻟﻠﺘﺮﺑ ﺔ و اﻟﺘ ﻰ‬
‫ﺗ ﺸﻤﻞ ﺗﻌﻴ ﻴﻦ ﻣﻌﺎﻣ ﻞ اﻟﻨﻔﺎذﻳ ﺔ و ﻣﻌﺎﻣ ﻞ ﻣﻘﺎوﻣ ﺔ اﻟﻘ ﺺ ﺑﻄﺮﻳﻘ ﺔ اﻟ ﻀﻐﻂ اﻟﻤﺤ ﻮرى ﺑ ﺪون‬
‫ﺿﻐﻂ ﻣﺤﻴﻂ و ﻣﻌ ﺎﻣﻼت اﻹﻧ ﻀﻐﺎط ﻟﻬ ﺎ و اﻷﺟﻬ ﺰة اﻟﻤ ﺴﺘﺨﺪﻣﺔ و اﻟﻤﻮاﺻ ﻔﺎت اﻟﺘ ﻰ ﺗ ﻢ‬
‫اﺗﺒﺎﻋﻬﺎ ﻟﺘﺤﺪﻳﺪ هﺬﻩ اﻟﺨﻮاص‪.‬‬
‫اﻟﻔﺼﻞ اﻟﺨﺎﻣﺲ ﻳﻌ ﺮض ه ﺬا اﻟﻔ ﺼﻞ وﺻ ﻔﺎ ﺗﻔ ﺼﻴﻠﻴﺎ ﻟﻄﺒﻴﻌ ﺔ اﻟﺘﺮﺑ ﺔ اﻟﺘ ﻰ ﺗ ﻢ اﺳ ﺘﺨﺮاﺟﻬﺎ ﻣ ﻦ ﻣ ﻮﻗﻌﻲ‬
‫ﻣﺪرﺳ ﺘﻴﻦ ﺗﺤ ﺖ اﻹﻧ ﺸﺎء ﺑﻤﻨﻄﻘ ﺔ ﻏ ﺮب اﻟ ﺪﻟﺘﺎ ﻓ ﻰ ﺣﺎﻟﺘﻬ ﺎ اﻟﻄﺒﻴﻌﻴ ﺔ‪ ،‬آﻤ ﺎ ﻳﻌ ﺮض ﻧﺘ ﺎﺋﺞ‬
‫اﻷﺧﺘﺒ ﺎرات اﻟﻤﻌﻤﻠﻴ ﺔ اﻟﺘ ﻰ ﺗ ﻢ اﺟﺮاﺋﻬ ﺎ ﻋﻠ ﻰ ﻋﻴﻨ ﺎت اﻟﺘﺮﺑ ﺔ اﻟﻌ ﻀﻮﻳﺔ ﻟﺘﺤﺪﻳ ﺪ اﻟﺨ ﻮاص‬
‫اﻟﻌﺎﻣ ﺔ ﻟﻬ ﺎ و اﻟﺘ ﻰ ﺗﺘ ﻀﻤﻦ اﻟﺨ ﻮاص اﻟﻔﻴﺰﻳﺎﺋﻴ ﺔ و اﻟﻜﻴﻤﻴﺎﺋﻴ ﺔ ﺑﺎﻹﺿ ﺎﻓﺔ اﻟ ﻰ ﺻ ﻮر‬
‫اﻟﻤﻴﻜﺮوﺳ ﻜﻮب اﻹﻟﻴﻜﺘﺮوﻧ ﻰ ﻟﻌﻴﻨ ﺎت اﻟﺘﺮﺑ ﺔ ﻓ ﻰ اﻹﺗﺠ ﺎهﻴﻦ اﻟﺮأﺳ ﻰ و اﻷﻓﻘ ﻰ ﻓ ﻰ ﺣﺎﻟﺘﻬ ﺎ‬
‫اﻟﻄﺒﻴﻌﻴﺔ و ﺑﻌﺪ اﻧﻀﻐﺎﻃﻬﺎ‪ ،‬و آﺬﻟﻚ ﺗﺼﻨﻴﻒ اﻟﺘﺮﺑﺔ ﻃﺒﻘﺎ ﻟﻨﻈﺎم اﻟﺘﺼﻨﻴﻒ اﻟﺬى ﺗﻢ اﻗﺘﺮاﺣﻪ‬
‫ﻓﻰ هﺬا اﻟﺒﺤﺚ‪ .‬آﻤﺎ ﻳﻌﺮض ﻧﺘﺎﺋﺞ اﻷﺧﺘﺒﺎرات اﻟﻤﻌﻤﻠﻴﺔ اﻟﺘﻰ ﺗﻢ اﺟﺮاﺋﻬ ﺎ ﻟﺘﺤﺪﻳ ﺪ اﻟﺨ ﻮاص‬
‫اﻟﻬﻨﺪﺳﻴﺔ ﻟﻬﺎ و اﻟﺘﻰ ﺗﺘﻀﻤﻦ ﺗﺤﺪﻳ ﺪ ﻣﻌﺎﻣ ﻞ اﻟﻨﻔﺎدﻳ ﺔ ﻟﻬ ﺬﻩ اﻟﺘﺮﺑ ﺔ ﻓ ﻰ اﻹﺗﺠ ﺎهﻴﻦ اﻟﺮأﺳ ﻰ و‬
‫اﻷﻓﻘﻰ ﻓﻰ ﺣﺎﻟﺘﻬﺎ اﻟﻄﺒﻴﻌﻴﺔ و اﻟﺘﻐﻴﺮ ﻓﻰ ﻣﻌﺎﻣﻞ اﻟﻨﻔﺎدﻳ ﺔ ﺗﺤ ﺖ ﺗ ﺄﺛﻴﺮ اﻹﺟﻬ ﺎدات اﻟﻤﺨﺘﻠﻔ ﺔ‪،‬‬
‫آﺬﻟﻚ ﻣﻌﺎﻣﻼت ﻣﻘﺎوﻣﺔ اﻟﻘﺺ ﺑﻄﺮﻳﻘ ﺔ اﻟ ﻀﻐﻂ اﻟﻤﺤ ﻮرى ﺑ ﺪون ﺿ ﻐﻂ ﻣﺤ ﻴﻂ ‪ ،‬و أﻳ ﻀﺎ‬
‫ﺑﺎﺳ ﺘﺨﺪام ﺟﻬ ﺎز اﻟﻐ ﺰ اﻟﺠﻴﺒ ﻰ‪ ،‬و آ ﺬﻟﻚ ﻣﻌ ﺎﻣﻼت اﻹﻧ ﻀﻐﺎط اﻷﺳﺎﺳ ﻰ و اﻹﻧ ﻀﻐﺎط‬
‫اﻟﺜﺎﻧﻮى ﻟﻬﺬﻩ اﻟﺘﺮﺑﺔ و ذﻟﻚ ﻓﻰ اﻹﺗﺠﺎهﻴﻦ اﻟﺮأﺳﻰ و اﻷﻓﻘﻰ و ﺗﺤﺖ ﺗ ﺄﺛﻴﺮ ﺣ ﺎﻻت اﻟﺘﺤﻤﻴ ﻞ‬
‫اﻟﻤﺨﺘﻠﻔﺔ و اﻹﺟﻬﺎدات اﻟﻤﺨﺘﻠﻔﺔ و ﻣﻊ اﻟﺰﻣﻦ‪.‬‬
‫اﻟﻔﺼﻞ اﻟﺴﺎدس ﻳﻨﺎﻗﺶ هﺬا اﻟﻔﺼﻞ اﻟﺨﻮاص اﻟﻌﺎﻣﺔ و اﻟﺨﻮاص اﻟﻬﻨﺪﺳﻴﺔ ﻟﻠﺘﺮﺑﺔ اﻟﻌﻀﻮﻳﺔ اﻟﻤﺴﺘﺨﺮﺟﺔ ﻣ ﻦ‬
‫اﻟﻤﻮﻗﻌﻴﻦ اﻟﺴﺎﺑﻖ اﻹﺷﺎرة اﻟﻴﻬﻤ ﺎ آﻤ ﺎ ﻳﻮﺿ ﺢ أوﺟ ﻪ اﻹﺧ ﺘﻼف و اﻹﺗﻔ ﺎق ﺑ ﻴﻦ ﻃﺒﻴﻌ ﺔ ه ﺬﻩ‬
‫اﻟﺘﺮﺑﺔ ﻓﻰ ﻣﺼﺮ و ﻧﻈﻴﺮﺗﻬﺎ ﻓﻰ أرﺟﺎء اﻟﻌﺎﻟﻢ ﺣﻴﺚ ﺗﻢ ﻣﻘﺎرﻧﺔ اﻟﻨﺘﺎﺋﺞ اﻟﺘﻰ ﺗﻢ اﻟﺘﻮﺻﻞ اﻟﻴﻬﺎ‬
‫ﻓ ﻰ ه ﺬا اﻟﺒﺤ ﺚ ﻣ ﻊ ﻧﻈﻴﺮﺗﻬ ﺎ اﻟﻤﻨ ﺸﻮرة ﺑ ﺎﻟﻤﺮاﺟﻊ و اﻟ ﺪورﻳﺎت‪ ،‬و اﻟﺘ ﻰ ﺗ ﻢ ﺗﺠﻤﻴﻌﻬ ﺎ و‬
‫ﺗﺼﻨﻴﻔﻬﺎ ﺧﻼل هﺬا اﻟﺒﺤﺚ ﻃﺒﻘﺎ ﻟﻨﻈﺎم اﻟﺘﺼﻨﻴﻒ اﻟﺬى ﺗﻢ اﻋﺘﻤﺎدﻩ ﻟﺘﺼﻨﻴﻒ ﻋﻴﻨﺎت اﻟﺘﺮﺑﺔ ‪،‬‬
‫و آﺬﻟﻚ ﺗﻢ اﺳﺘﻨﺒﺎط ﺑﻌﺾ اﻟﻌﻼﻗﺎت ﺑﻴﻦ اﻟﺨﻮاص اﻟﻔﻴﺰﻳﺎﺋﻴﺔ و اﻟﺨﻮاص اﻟﻬﻨﺪﺳﻴﺔ ﻟﻬﺎ‪.‬‬
‫اﻟﻔﺼﻞ ااﻟﺴﺎﺑﻊ‬
‫ﻳﻌﺮض هﺬا اﻟﻔﺼﻞ اهﻢ اﻟﻨﺘ ﺎﺋﺞ و اﻟﺘﻮﺻ ﻴﺎت اﻟﺘ ﻰ ﺗ ﻢ اﻟﻮﺻ ﻮل اﻟﻴﻬ ﺎ ﺧ ﻼل ه ﺬا اﻟﺒﺤ ﺚ‪،‬‬
‫آﻤﺎ ﻳﻌﺮض ﺑﻌﺾ ﻣﺠﺎﻻت اﻟﺒﺤﺚ اﻟﻤﺤﺘﻤﻠﺔ ﻟﻨﻔﺲ اﻟﻤﻮﺿﻮع ‪.‬‬