COMPRESSIBILTY CHARACTERISTICS OF FIBROUS PEAT SOIL
YULINDASARI
A thesis submitted in fulfilment of the
requirements for the award of the degree of
Master of Engineering (Geotechnics)
Faculty of Civil Engineering
Universiti Teknologi Malaysia
OCTOBER 2006
iii
To my beloved Father (Sutedjo), to my beloved mother (Rohyati), to my beloved
brother (Andi Kurniawan, SE.), and to my beloved sister (Melya Kurniati, SE.).
There's nothing in life that makes me happier than loving all of you.
iv
ACKNOWLEDGEMENTS
I would like to deeply praise the ALLAH SWT for allowing me passing all
of this moment. I also would like to take this opportunity to express my sincere
gratitude to all those who have contributed in completing this project.
First of all, I would like to thank with my supervisor, Dr. Nurly Gofar for
guiding me through the research process in the writing of this thesis. Her personal
kindness, skill, patience and guidance are highly appreciated. As my supervisor, she
also becomes my foster parent. This research is partly funded by UTM Fundamental
Research Grant Vot No. 75137 head by Dr. Nurly Gofar. Special acknowledgement
is also extended to Assoc. Prof. Dr. Khairul Anuar Kassim and Dr. Kamarudin
Ahmad for their help during the course of this study.
Beside that, I would like to say thank you to my parents and my family for
their support and encouragement. Their encouragements provide the energy for me
to concentrate on my Master study.
I would also like to express my sincere gratitude to Wong Leong Sing for
sharing research data and for friendship.
Sincere gratitude also goes to all
technicians in UTM Geotechnical Laboratory, especially En. Zulkiflee Wahid for his
assistance in my laboratory work. Without their help, this research and thesis will
not be a success.
A special thank to En. Azman Kassim and Lee Min Lee and also to
undergraduate student: Eng Chun Wei, Bong Ting Ting, and Ushaa Nair.
Lastly, I am very thankful to my friends in KTHO-L12 for their support and
motivation especially Vivi, Kak Jati, Kak Isal, Yuk Mala, Kak Hilma, Farah, Ika,
Sylvia, Aliya, Lilian, Ema, Aina and Uliya.
v
ABSTRACT
Peat has been identified as one of major groups of soils found in Malaysia.
Peat deposit covers large area of West Johore especially Pontian, Batu Pahat, and
Muar. Despite of this fact, not much research has been focused on the compression
behavior of peat. This study is focused on the compressibility characteristics of
fibrous peat based on time-compression curves derived from consolidation tests. The
peat samples were collected from Kampung Bahru, Pontian, West Johore by block
sampling method. The laboratory testing program included the standard laboratory
testing for identification and classification purposes, i.e., Scanning Electron
Micrograph (SEM), shear box test, constant head permeability test, Oedometer tests,
and large strain consolidation test using Rowe cell. The results of the study show
that the peat soil can be classified as fibrous peat with low to medium degree of
decomposition (H4 in von Post scale) and of very high organic content (97 %) and
fiber content (90 %).
The natural water content of the peat is 608 % which
corresponds to initial void ratio of about 9. The undrained shear strength of peat is
10.10 kPa, with sensitivity of 5.64. The initial permeability is high, but it decreases
significantly with applied pressures. The fibrous peat has a high compressibility with
significant secondary compression stage, which is not constant with the logarithmic
of time. Eventhough the duration of the primary consolidation was short, but the
settlement was high. This is due to high initial void ratio. Besides, the magnitude of
the secondary compression of fibrous peat is also significant with respect to the
design life of a structure. The comparison between the results of the consolidation
test using Rowe and Oedometer cells show that the use of Rowe cell for the
evaluation of the consolidation characteristics of soil exhibiting secondary
compression is advantageous because it enables the observation of the large
deformation. The compression index (cc) obtained from consolidation test on Rowe
cell was 3.128, while the coefficient of secondary compression (cα) range from 0.102
to 0.304.
The settlement analysis performed for the hypothetical case of an
embankment on peat deposit showed that the compression of fibrous peat deposit can
be estimated based on the time-compression and the time-excess pore water pressure
curves.
vi
ABSTRAK
Tanah gambut dikenalpasti sebagai salah satu kumpulan utama tanah di
Malaysia. Kawasan tanah gambut terdapat di Johor bahagian barat terutamanya
Pontian, Batu Pahat, dan Muar. Walaupun demikian, tidak banyak penyelidikan
tertumpu pada kelakuan pemampatan tanah gambut.
Kajian ini tertumpu pada
analisis sifat kebolehmampatan tanah gambut berdasarkan lengkung masapemampatan yang diperoleh daripada ujian pengukuhan. Sampel tanah gambut dari
Kampung Bahru, Pontian, Johor barat dengan kaedah pensampelan blok. Program
ujian makmal termasuk ujian piawaian makmal digunakan bagi tujuan mengenalpasti
dan pengkelasan, iaitu mikrograf elektron imbasan (SEM), ujian kotak ricih, ujian
kebolehtelapan turus malar, ujian pengukuhan Oedometer, dan ujian terikan tinggi
menggunakan sel Rowe.
Keputusan kajian menunjukkan bahawa tanah gambut
boleh dikelaskan sebagai gambut gentian dengan darjah penguraian rendah ke
sederhana (H4 pada skala von Post) dan kandungan organik (97 %) dan kandungan
gentian (90 %) yang tinggi. Kandungan lembapan asli bagi tanah gambut tersebut
adalah 608 % dengan nisbah lompang mula 9. Kekuatan ricih tak bersalir gambut
adalah 10.10 kPa dengan kepekaan 5.64. Kebolehtelapan mula adalah tinggi, tetapi
nilainya
berkurangan
dengan
tekanan.
Gambut
gentian
mempunyai
kebolehmampatan yang tinggi pada peringkat mampatan sekunder dan tidak malar
dengan logaritma masa. Walaupun, tempoh pengukuhan utama adalah pendek tetapi
enapan adalah tinggi. Ini disebabkan nisbah lompang mula yang tinggi. Selain itu,
magnitud mampatan kedua bagi gambut gentian adalah penting juga untuk hayat
rekabentuk sesuatu struktur. Perbandingan antara keputusan ujian pengukuhan sel
Rowe dan sel Oedometer menunjukkan penggunaan sel Rowe untuk penaksiran sifat
pengukuhan pada mampatan punya kelebihan kerana ia boleh meninjau ubah bentuk
yang besar. Indeks mampatan (cc) yang diperoleh dari ujian pengukuhan pada sel
Rowe ialah 3.128, manakala julat mampatan sekunder (cα) ialah 0.102 hingga 0.304.
Analisis enapan yang dilakukan untuk kes hipotesis benteng di kawasan tanah
gambut menunjukkan bahawa kebolehmampatan gambut gentian dapat dianggar
berdasarkan kepada lengkung masa-pemampatan dan lengkung masa-tekanan air
liang lebihan.
vii
TABLE OF CONTENTS
CHAPTER
1
2
TITLE
PAGE
THESIS TITLE
i
DECLARATION
ii
DEDICATION
iii
ACKNOWLEDGEMENTS
iv
ABSTRACT
v
ABSTRAK
vi
TABLE OF CONTENTS
vii
LIST OF TABLES
xi
LIST OF FIGURES
xiii
LIST OF SYMBOLS
xx
LIST OF APPENDICES
xxv
INTRODUCTION
1
1.1
Background
1
1.2
Problem Statement
4
1.3
Objectives
4
1.4
Scopes
5
1.5
Significance of the Study
6
1.6
Thesis Structure
6
LITERATURE REVIEW
8
2.1
Fibrous Peat
8
2.1.1
Definition
8
2.1.2
Sampling of Peat
9
2.1.3
Structural Arrangement
11
viii
2.1.4
Physical and Chemical Properties
14
2.1.5
Classification
18
2.1.6
Shear Strength
21
2.1.7
Compressibility
22
2.1.8
Permeability
25
Soil Compressibility
25
2.2.1
Primary Consolidation
26
2.2.2
Secondary Compression
34
2.3
Compressibility of Fibrous Peat
36
2.4
Consolidation Test
40
2.4.1
Problems Related to Conventional Test
40
2.4.2
Large Strain Consolidation Tests (Rowe Cell)
42
2.2
2.5
Evaluation of Compression Curves derived
45
from Consolidation Test
3
2.5.1
Time-Compression Curve
48
2.5.2
The e-log p’ Curve
56
METHODOLOGY
58
3.1
Introduction
58
3.2
Sampling of Peat
60
3.3
Preliminary Tests
62
3.3.1
Physical Properties and Classification
62
3.3.2
Classification
62
3.3.3
Fiber Content and Fiber Orientation
63
3.3.4
Shear Strength
64
3.3.5
Permeability
65
3.3.6
Standard Consolidation Test
66
3.4
Large Strain Consolidation Tests (Rowe Cell)
67
3.4.1
Calibration
71
3.4.2
Cell Assembly and Connections
75
3.4.3
Consolidation Test
80
3.4.3.1 Preliminaries
81
ix
3.5
4
3.4.3.2 Saturation
81
3.4.3.3 Loading Stage
81
3.4.3.4 Consolidation Stage
82
3.4.3.5 Further Load Increments
82
3.4.3.6 Unloading
82
3.4.3.7 Conclusion of Test
83
3.4.3.8 Measurement and Removal of Sample
83
3.4.4
Consolidation Test with Horizontal Drainage
84
3.4.5
Permeability Tests
87
3.4.6
Permeability Test for Horizontal Drainage
91
Data Analysis
92
3.5.1
Time-Compression Curve
93
3.5.2
The e-log p’ Curve
93
3.5.3
Settlement Analysis
94
GENERAL CHARACTERISTICS
95
4.1
Soil Identification
95
4.2
Classification
98
4.3
Fiber Orientation
100
4.4
Shear Strength
101
4.5
Initial Permeability
103
4.6
Compressibility
104
4.6.1
Analysis of Time-Compression Curve
105
4.6.2
Analysis of the e-log p’ Curve
110
4.6.3
Coefficient of Permeability based on
114
the Standard Consolidation Test
4.6.4
5
Summary
115
COMPRESSIBILITY CHARACTERISTICS
116
5.1
Introduction
116
5.2
Test Results and Analysis
117
5.2.1
117
Analysis of Time-Compression Curve
x
5.2.2
Analysis of the e-log p’ Curve
127
5.2.3
Evaluation of Permeability
131
5.2.4
Summary
132
5.3
Comparison with Oedometer Data
133
5.4
Comparison with Published Data
139
5.5
Effect of fiber
143
5.6
Settlement Estimation
147
5.6.1
Introduction
147
5.6.2
Hypothetical Problem
148
5.6.3
Settlement Analysis by Cassagrande (1936)
Method
Settlement Analysis by Robinson (2003)
Method
Discussion
150
5.6.4
5.6.5
6
152
155
SUMMARY, CONCLUSION, AND
RECOMMENDATION
157
6.1
Summary
157
6.2
Conclusion
158
6.3
Recommendation
160
REFERENCES
162
Appendices A-H
170-212
xi
LIST OF TABLES
TABLE NO.
TITLE
PAGE
2.1
Physical properties of peat based on location (Huat,
2004)
15
2.2
Important physical and chemical properties for
some peat deposits (Ajlouni, 2000)
16
2.3
Classification of peat based on degree of
decomposition (von Post, 1922)
19
2.4
Classification of peat based on organic and fiber
content
20
2.5
Compressibility characteristics of some peat
deposit (Ajlouni, 2000)
23
2.6
Curve fitting data for evaluation of coefficient of
rate of consolidation (Head, 1986)
47
4.1
The summary of index properties of peat soil in
West Malaysia
96
4.2
The summary classification test results in West
Malaysia peat
100
4.3
Compressibility parameters obtained from
consolidation curves
107
4.4
The average coefficient of volume compressibility
114
4.5
Average coefficient of permeability for each
consolidation pressure
114
4.6
The summary of data obtained from Oedometer test
115
5.1
Average time for end of primary consolidation
(t100) and the beginning of secondary compression
(tp) obtained from Rowe test results
123
5.2
Average coefficient of rate of consolidation for
each pressure
125
xii
5.3
Average coefficient of secondary compression
126
5.4
Average time of secondary compression
127
5.5
The average coefficient of volume compressibility
130
5.6
Vertical coefficient of permeability based on large
strain consolidation test
132
5.7
The summary of large strain consolidation data
132
5.8
Compressibility parameters obtained from Rowe
cell and Oedometer tests
136
5.9
Comparison of the data obtained from the analysis
of data obtained in the present study with published
data
143
5.10
Coefficient of volume compressibility and
coefficient of permeability based on large strain
consolidation test
145
5.11
Effect of consolidation pressure on coefficient of
permeability
145
5.12
The properties of fibrous peat deposit obtained from
large strain consolidation test and Oedometer test for
consolidation pressure 50 kPa
149
5.13
The results of settlement calculated based on Rowe
consolidation test
151
5.14
The results of settlement calculated based on
Robinson’s method
154
xiii
LIST OF FIGURES
FIGURE NO.
TITLE
PAGE
2.1
Schematic diagram of (a) deposition and (b)
multi-phase system of fibrous peat (Kogure et
al., 1993)
12
2.2
Scanning Electron Micrographs of Middleton
fibrous peat; (a) horizontal plane, (b) vertical
plane (Fox and Edil, 1996)
13
2.3
Plot of Void ratio versus pressure in linear scale
(Nurly Gofar and Khairul Anuar Kassim, 2005)
27
2.4
Plot of void ratio versus pressure in logarithmic
scale (Nurly Gofar and Khairul Anuar Kassim,
2005)
27
2.5
Consolidation curve
drainage (Head, 1982)
vertical
31
2.6
Determination of coefficient of rate of
consolidation by Cassagrande’s method (Nurly
Gofar and Khairul Anuar Kassim, 2005)
33
2.7
Determination of coefficient of rate of
consolidation by Taylor method (Nurly Gofar
and Khairul Anuar Kassim, 2005)
34
2.8
Determination of the coefficient of rate of
secondary compression from consolidation curve
(Cassagrande’s method) (Nurly Gofar and
Khairul Anuar Kassim, 2005)
35
2.9
Rheological model used for soil undergoing
secondary compression
39
2.10
Schematic diagram of Oedometer cell (Bardet,
1997)
41
2.11
Schematic diagram of Rowe consolidation cell
(Head, 1986)
43
for
two-way
xiv
2.12
Drainage
and
loading
conditions
for
consolidations tests in Rowe cell: (a), (c), (e), (g)
with ‘free strain’ loading, (b), (d), (f), (h) with
‘equal strain’ loading (Head, 1986)
46
2.13
Types of compression versus logarithmic of time
curve derived from consolidation test (Leonards
and Girault, 1961)
48
2.14
Vertical strain versus logarithmic of time curve
of fibrous peat for one-dimensional consolidation
(Dhowian and Edil, 1980)
49
2.15
Sridharan and Prakash log δ log t curve
(Sridharan and Prakash, 1998)
50
2.16
(a) Compression-time curves, and (b) Degree of
consolidation-time from the measured pore water
pressure dissipation curves for peat (Robinson,
2003)
52
2.17
Degree of consolidation from the pore water
pressure dissipation curves plotted against
compression for several consolidation data for
peat (Robinson, 2003)
53
2.18
(a) Total settlement-time curves for peat and (b)
Primary settlement-time curve after removing the
secondary compression (Robinson, 2003)
55
2.19
Secondary compression versus logarithmic of
time curve for evaluation of coefficient of
secondary compression (Robinson, 2003)
56
2.20
Typical Laboratory consolidation curve (Fox,
2003)
57
3.1
Flow chart of the study
59
3.2
Sampling methods (a) block sample, (b) piston
sample
61
3.3
The equipment for the Scanning Electron
Microscope (SEM)
63
3.4
Shear strength tests (a) Vane shear test carried
out at site (b) Shear box apparatus
64
3.5
Constan Head permeability test
65
xv
3.6
Piston sampler (a) pushed in vertical direction
(b) pushed in horizontal direction
66
3.7
Standard consolidation test (a) Oedometer cell
(b) Assembly of all components of Oedometer
test
67
3.8
Rowe consolidation cell
67
3.9
50 mm Linear
Transducer (LVDT)
3.10
1500 kPa Pressure transducer
68
3.11
Main page of the GDSLAB v 2.0.6 program for
collecting data system
69
3.12
Serial pad 1
70
3.13
The schematic arrangement of control system for
the Rowe consolidation tests
70
3.14
Linear Displacement
calibration process
71
3.15
The transducer object
73
3.16
The advanced tab for the transducer
73
3.17
The transducer calibrations (a) The calibration
detail tab (b) The results of
transducer
calibrations
74
3.18
Cutting rings containing soil sample are fitted on
top of the Rowe cell
76
3.19
A porous disc is used to slowly and steadily push
the soil sample vertically downward into the
Rowe cell body
76
3.20
Schematic diagram of filling of distilled water
into the diaphragm (Head, 1986)
77
3.21
Realistic view of filling of distilled water into the
diaphragm
77
3.22
Diaphragm inserted into Rowe cell body (Head,
1986)
78
3.23
Diaphragm is correctly seated (Head, 1986)
79
Variable
Displacement
Transducer
(LVDT)
68
xvi
3.24
Arrangement of Rowe cell for consolidation test
with two-way vertical drainage (Head, 1986)
80
3.25
Arrangement of Rowe cell for consolidation test
with horizontal drainage to periphery; excess
pore pressure measurement from centre of base
of sample (Head, 1986)
84
3.26
Fitting porous plastic liner in Rowe cell: (a)
initial fitting and marking, (b) locating line of
cut, (c) final fitting (Head, 1986)
85
3.27
Peripheral drain fitted into the Rowe cell body
86
3.28
Arrangement of Rowe cell for permeability test
with horizontal outward drainage (Head, 1986)
88
3.29
Downward vertical flow condition
permeability test in Rowe cell (Head, 1986)
for
88
3.30
Arrangement for vertical permeability test using
one back pressure system for downward flow
(Head, 1986)
90
3.31
Arrangement of Rowe cell for permeability test
with horizontal outward drainage (Head, 1986)
91
3.32
Hypothetical problem for analysis of settlement
94
4.1
Correlation of bulk density, water content,
specific gravity, and degree of saturation of
fibrous peat (Hobbs, 1986)
97
4.2
Correlation of dry density and natural water
content for West Malaysian peat (Al-Raziqi et
al., 2003)
97
4.3
The range of organic content of fibrous peat
based on specific gravity (Lechowicz et al.,
1996)
99
4.4
The range of organic content of fibrous peat
based on water content (Al- Raziqi et al., 2003)
99
4.5
The Scanning Electron Microphotographs (SEM)
of fibrous peat samples at initial state (a)
horizontal section x 400, (b) vertical section x
400
102
xvii
4.6
The Scanning Electron Microphotographs (SEM)
of fibrous peat samples under consolidation
pressure of 200 kPa (a) horizontal section x 400
(b) vertical section x 400
102
4.7
Results of the shear box test
103
4.8
Effect of initial void ratio (eo) on the initial
permeability of soil (Hobbs, 1986)
104
4.9
Typical compression versus logarithmic of time
curves from Oedometer test
106
4.10
Analysis of compression versus logarithmic of
time curves from Oedometer test
107
4.11
Variation of the time of completion of primary
consolidation with consolidation pressure
108
4.12
Variation of the time of completion of secondary
compression versus consolidation pressure
109
4.13
Variation of the coefficient of rate
consolidation with consolidation pressure
of
109
4.14
Variation coefficient of secondary compression
with consolidation pressure
110
4.15
The e-log p curves obtained from the standard
consolidation test on Oedometer cell
111
4.16
Relationship between pre-consolidation pressure
and in-situ void ratio (Kogure and Ohira, 1977)
112
4.17
Relationship between compression index and
natural water content (Kogure and Ohira, 1977)
113
5.1
The compression versus logarithmic of time
curve obtained from large strain consolidation
tests on Rowe cell
118
5.2
Compression versus logarithmic of time curves
for Test 4
121
5.3
Excess pore water pressure versus logarithmic of
time curves for Test 4
121
5.4
Typical compression versus degree of
consolidation
curve
from
large
strain
consolidation test with two-way vertical drainage
122
xviii
5.5
Average time of completion of primary
consolidation versus consolidation pressure
124
5.6
Variation of the beginning of secondary
compression with consolidation pressure for
sample tested under vertical consolidation
124
5.7
Variation coefficient of rate of consolidation
with consolidation pressure
125
5.8
Variation coefficient of secondary compression
versus consolidation pressure
126
5.9
The consolidation curve from large strain
consolidation test on Rowe cell based on primary
and total settlement (a) typical e-p’ curve, (b)
typical e-log p’ curve
128
5.10
The void ratio versus logarithmic of
consolidation pressure curve of large strain
consolidation test on Rowe cell based on primary
settlement
129
5.11
Variation
of
coefficient
of
volume
compressibility versus consolidation pressure
131
5.12
The typical strain versus logarithmic of time
curve from Rowe cell and Oedometer test
133
5.13
Void ratio versus consolidation pressure curve
from Rowe cell and Oedometer test (a) typical ep’ curve, (b) typical e-log p’ curve
135
5.14
Strain versus logarithmic of time curves
139
5.15
Excess pore water pressure versus logarithmic of
time curves
141
5.16
Void ratio versus
(logarithmic scale)
142
5.17
Void ratio versus consolidation pressure
142
5.18
The relationship between the void ratio and the
coefficient of permeability in horizontal and
vertical direction
146
5.19
Geometry and soil properties for the hypothetical
problem
149
consolidation
pressure
xix
5.20
The curve of settlement with time based on
Rowe consolidation test
152
5.21
Settlement versus logarithmic of time curve
based on Robinson’s method (2003)
155
xx
LIST OF SYMBOLS
A
-
Area of sample
a
-
Primary compressibility (based on Rheological model)
AC
-
Ash content
av
Coefficient of axial compressibility, Coefficient of volume
compressibility
B
-
Pore pressure parameter
b
-
Coefficient of secondary compressibility (based on Rheological
model)
c’
-
Effective cohesion
cu
-
Undrained shear strength
cc
-
Compression index
cr
-
Recompression index
cv
-
Coefficient of rate of consolidation
cvo
-
Coefficient of rate of consolidation
cα
-
Rate of secondary compression; Slope, Coefficient of
secondary compression
cα1
-
Coefficient of secondary compression
cα2
-
Coefficient of tertiary compression
D
-
Diameter of sample
Do
-
Initial reading; Deformation
D100
-
Deformation corresponds to U = 100 %
dz
-
Elemental layer of thickness at depth z
e
-
Void ratio
xxi
eo
-
Initial void ratio
eop
-
Void ratio at the beginning of secondary compression
ec
-
Corrected void ratio
em
-
Measured void ratio
e1
-
Void ratio of the compressible soil layer corresponding to
compression δ1 at time t1
e2
-
Void ratio of the compressible soil layer corresponding to
compression δ2 at time t2
FC
-
Fiber content
Gs
-
Specific gravity
H
-
Thickness of consolidation soil layers; Initial thickness
Hd
-
Length of drainage path for a particular pressure increment
h
-
Height from the top of the sample to the level of water in the
header tank; Head loss due to the height of water in the burette
i
-
Hydraulic gradient
k
-
Coefficient of permeability
kv
-
Vertical coefficient of permeability
kvo
-
Vertical coefficient of permeability
kh
-
Horizontal coefficient of permeability
L
-
Longest drainage path in consolidating soil layer; equal to half
of H with top and bottom drainage; and equal to H with top
drainage only
LIR
-
Load increment ratio
m
-
Secondary compression factor
mv
-
Coefficient of volume compressibility
OC
-
Organic content
pH
p’
Acidity
-
Consolidation pressure
xxii
po
-
Initial pressure; Seating pressure
p1
-
Inlet pressure
p2
-
Outlet pressure
Q
-
Cumulative flow
q
-
Rate of flow
qv
-
Rate of vertical flow
qh
-
Rate of horizontal flow
r
-
Radius of sample
St
-
Sensitivity
Sc
-
Consolidation settlement
Ss
-
Secondary compression
T
-
Time
Tv
-
Vertical theoretical time factor, Time factor
Tc, Tro,Tr
-
Theoretical time factors
T50, T90
-
Theoretical time factors
t0.5 , t0.465
-
Time function
t
-
Time
to
-
Beginning of secondary compression
tp
-
Beginning of secondary compression; End of primary
consolidation; Time for primary consolidation; Time of the
completion of primary consolidation
ts
-
Time of secondary
compression
tf
-
Time for the secondary compression settlement
t100
-
End of primary consolidation; Time of the completion of
primary consolidation
Uh
-
Average degree of consolidation due to horizontal drainage
compression;
End
of
secondary
xxiii
Uv
-
Average degree of consolidation due to vertical drainage
u
-
Excess pore water pressure at any point and any time
uo
-
Initial excess pore water pressure
ue
-
Excess pore water pressure
uavg
-
Average degree of consolidation
µe
-
Excess pore water pressure
ωo; ω
-
Natural water content
x
-
Difference in the dial reading
∆e
-
Change of void ratio from tp to tf
∆H
-
Consolidation settlement
∆V
-
Change in volume
∆p
-
Pressure difference
∆σ’
-
Additional stress, The change in the effective in e-p’ curve
β
-
Degree of compression
εi
-
Instantaneous strain
εp
-
Primary strain
εs
-
Secondary
sampling
εt
-
Tertiary strain
γ
-
Unit weight
γw
-
Unit weight of water
σ
-
Effective stress
σ'v
-
Effective vertical stress
σ'o
-
Existing overburden pressure
σ’p
-
In-situ effective stress
σc’
-
Pre-consolidation pressure
strain; Measured compression strain during
xxiv
τ’f
-
Shear strength
φ'
-
Effective internal friction, Friction angle
δ
-
Total compression
δp
-
Primary consolidation settlement
δs
-
Secondary compression
Z
-
Geometry factor, Depth
†
-
Drain ratio 1/20
xxv
LIST OF APPENDICES
APPENDIX
TITLE
PAGE
A
Sampling procedure
170
B
Index tests data
174
C
Soil fabric
178
D
Shear strength
183
E
Initial permeability test
186
F
Standard consolidation tests
191
G
System calibration for consolidation test on
Rowe cell
196
H
Large strain consolidation and permeability
(Rowe cell)
203
CHAPTER 1
INTRODUCTION
1.1
Background
Peat has been identified as one of the major groups of soils found in
Malaysia. Three million hectares or 8 % of the area is covered with peat (Huat,
2004). Some 6300 Hectares of the peat-land is found in Pontian, Batu Pahat and
Muar, West Johore area.
On the west coast of Malaysian peninsular, the peat
deposits are formed in depressions consisting predominantly of marine clay deposits
or a mixture of marine and river deposits especially in areas along river courses.
There are two types of peat deposit, the shallow deposit usually less than 3 m thick
while the thickness of deep peat deposit in Malaysia exceeds 5 m. The underlying
materials is usually consists of marine clay (Muttalib et al., 1991).
Recently, the utilization of peat-land in Malaysia is quite low although
construction on marginal land such as peat has become increasingly necessary for
economic reasons. Engineers are reluctant to construct on peat because of difficulty
to access the site and other problems related to unique characteristics of peat.
Therefore, not much research has been focused on the behavior of peat and the
development of soil improvement method for construction on peat soil area.
2
Replacing the peat with good quality soil is still a common practice when
construction has to take place on peat deposit even though most probably this effort
will lead to uneconomic design. Approaches have been developed to address the
problems associated with construction over peat deposits (Lea and Brawer, 1963;
Berry, 1983; Hansbo, 1991). Alternative construction and stabilization methods such
as surface reinforcement, preloading, chemical stabilization, sand or stone column,
pre-fabricated vertical drains, and the use of piles were discussed in literatures (Noto,
1991; Hartlen and Wolsky, 1996; Huat, 2004, and others). The selection of the most
appropriate method should be based on the examination of the index and engineering
characteristics of the soil. The knowledge on the shear strength and compression
behavior is essential as it enables designers to understand the response of the soil to
load and to suggest proper engineering solutions to overcome the problem.
There are two types of peat: amorphous peat and fibrous peat (ASTM
D4427). The compressibility behavior of the amorphous peat is known to be similar
with clay soil which can be evaluated based on Terzaghi’s theory of consolidation.
Fibrous peat is peat with high organic and fiber content with low degree of
humification. The behavior of fibrous peat is different from mineral soil because of
different phase properties and microstructure (Edil, 2003), thus Terzaghi’s theory of
consolidation cannot be applied to predict the compression behavior of fibrous peat.
The compression behavior of fibrous peat consists of two phases i.e.: primary
consolidation and secondary compression. The primary consolidation of fibrous peat
is much larger than that of other soils due to high initial water content, while the
secondary compression occurs due to not only compression of solid particles, but
also the plastic yielding (buckling, bending, and squeezing) of the particles (Samson
and La Rochelle, 1972).
The magnitude of secondary compression takes more
significant part of the compression of peat and plays an important role in determining
the total settlement of the peat because the secondary compression occurs during the
design life of a structure after the rapid primary consolidation. Tertiary compression
was reported by several researchers (e.g. Candler and Chartres, 1988; Fox et al.,
1992; Mitchell, 1993), but other researchers (e.g. 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.
3
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 void, causing a significant reduction in the
vertical permeability. Moreover, 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.
In order to develop a visual appreciation of the fiber content and
orientation, the microstructure of the peat was examined under a Scanning Electron
Microscope (SEM).
Many researchers (Berry and Poskitt, 1972; Ajlouni, 2000; Robinson, 2003)
have examined fibrous peat from different parts of the world and their findings are
quite different from one and another due to different content of peat soils. The
properties of peat soils such as natural water content, acidity, degree of humification,
fiber content, shear strength, and compressibility is affected by the formation of peat
deposit. This indicates that in term of content, fibrous peat is different from one
location to another location and detailed soil investigations need to be conducted for
fibrous peat at a particular site where a building is intended to be constructed. The
difference becomes particularly apparent especially under low vertical stresses or
shallow depth. Thus assessment on the response of peat deposit to loading should be
made before any construction has to take place at a particular site.
Most of the methods to predict compressibility characteristics of soil are
developed based on the results of laboratory consolidation test. Several test methods
have been used to study the compressibility of different type of soil including peat.
The oldest and the most popular one is the conventional Oedometer test. This test is
still used as a standard consolidation test method in Malaysia as well as in many
parts of the world. More advanced testing methods have also been developed such as
for example the Rowe cell or large strain consolidometer, and constant rate of strain
(CRS) test. Among these testing methods, Rowe cell has the capability of testing
large diameter sample to provide more reliable data for settlement analysis (Head,
1986).
4
1.2
Problem Statement
The compressibility behavior of fibrous peat is different from that of clay
soil. The behavior is controlled by several factors including the initial water content,
fiber arrangement, and fiber content. The condition in which the fibrous peat is
deposited is also an important factor to be considered.
The large compressibility of peat results in a large deformations and strains.
Accordingly, equipment capable of measuring large strain consolidometer is needed
to study the compressibility characteristics of peat. Several consolidation parameters
of the peat under study will be determined. The results are useful for identification
of the compressibility characteristics and predicting the compression behavior of
fibrous peat.
1.3
Objectives
Based on the uniqueness of the properties of fibrous peat and the importance
of compressibility of the peat in the evaluation of its response to loading, the
following objectives were set forth:
1.
To identify the type and engineering properties of peat found in Kampung
Bahru, Pontian, West Johore.
2.
To study the compressibility characteristics of the fibrous peat based on
the results of consolidation test using large strain consolidometer (Rowe
Cell).
3.
To investigate the suitable method for predicting compression behavior of
fibrous peat and estimating settlement based on the time-compression
curve derived from the test.
5
1.4
Scopes
The study focuses on the compressibility characteristics of peat soil found
in Kampung Bahru, Pontian, West Johore. Therefore, the interpretation of the
results of the study was limited as indicated in the followings:
1.
Peat soil found in Kampung Bahru, Pontian, West Johore.
2.
Samples were obtained using block sampling method (procedure outlined
in Appendix A).
3.
Identification of index properties of soil includes: water content, specific
gravity, sieve analysis, and acidity.
4.
Classification of peat was made based on degree of humification (von Post)
as well as the fiber and organic content.
5.
Evaluation of shear strength of the peat was made by vane shear (field) and
shear box tests (laboratory).
6.
Evaluation of permeability based on constant head permeability test.
7.
The use of the standard consolidation test (Oedometer) data to determine
the range of pressure and estimate the length of primary consolidation to
be applied in large strain consolidation test (Rowe Cell).
8.
Evaluation of compressibility characteristics was made based on the results
of large strain consolidation test (Rowe Cell)
9.
Comparison of the data obtained from large strain consolidation test with
those obtained from the standard consolidation test.
10.
Evaluation of the effect of fabric on the compressibility characteristics
based on Scanning Electron Micrograph (SEM) and consolidation test
done with horizontal drainage.
11.
Evaluation of the settlement was made on a hypothetical problem.
6
1.5
Significance of Study
This research will enrich the knowledge on the characteristics of peat soil
and the results will be used in the development of suitable soil improvement for
fibrous peat in Kampung Bahru, Pontian, West Johore as foundation as well as
construction material.
1.6
Thesis Structure
The thesis is composed of six chapters.
Chapter 1 presents general
information regarding background, problem statement, objectives, scope, and
significance of the study, and thesis structure. Chapter 2 provides the background of
the study on different topics related to the research.
This chapter outlines
information on the general characteristics of fibrous peat, the theory of consolidation,
the compressibility of fibrous peat, and the theories and models developed by
researchers for the study of the compressibility of peat. Chapter 2 also covers review
on the standard consolidation test as well as the large strain consolidation test on
Rowe cell.
Chapter 3 provides the overall experimental program including laboratory
tests and data analysis. The experimental program includes sampling of peat and
laboratory soil tests performed to classify the soil and to determine the engineering
properties of peats. This chapter also discuss the detail set up and procedures of
large strain consolidation test on Rowe cell and analysis of the data obtained from
the test.
Chapter 4 presents general characteristics of the peat derived from the results
of preliminary test.
These include soil identification, soil classification, fiber
content, shear strength, initial permeability, and compressibility data obtained from
the standard consolidation test on Oedometer cell.
7
Chapter 5 presents the results obtained from large strain consolidation test on
Rowe cell. Analysis of the test data for determining the compressibility parameters
are presented and discussed in detail in this chapter. Comparisons of the results of
large strain consolidation test with data obtained from the standard consolidation test
on Oedometer cell are also presented.
Furthermore, the compression behavior
obtained from Rowe consolidation test were compared to published data in terms of
time-compression curve, consolidation curve, and the range of compressibility
parameters. Effect of fiber on the compressibility of the soil is also highlighted in
this chapter. Finally, the applications of consolidation parameters from large strain
consolidation test for settlement analysis based on hypothetical problem are also
discussed in Chapter 5.
Chapter 6 presents the summary and conclusions of major findings of this
research and recommendation for future work on the topic related to the present
study.
CHAPTER 2
LITERATURE REVIEW
2.1
Fibrous Peat
Peat is usually found as an extremely loose, wet, unconsolidated surface
deposit which forms as an integral part of a wetland system, therefore access to the
peat deposit is usually very difficult as the water table exists at, near, or above the
ground surface. The peat deposit is generally found in thick layers on limited areas.
In tropical region such as Sarawak, Malaysian Peninsular and Sumatera, the peat
form a doomed deposit consists of two layers: the top layer consist of fibrous peat
containing long and slender roots and rootlets, while the bottom is a dense woody
peat derived from the decomposition of the vegetation (Cameron, 1989). The peat
deposit is usually underlined by thick clay layer.
2.1.1 Definition
Peat is a mixture of fragmented organic material formed in wetlands under
appropriate climatic and topographic conditions. The peat soil is known for its low
shear strength and high compressibility, which often results in difficulties when
9
construction work has to take place on peat deposit. The low strength often causes
stability problem and consequently the applied load is limited or the load 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.
In general, peat is grouped into two categories; amorphous peat and fibrous
peat. Amorphous peat is the peat soil with fiber content less than 20 % (ASTM
D4427). It contains mostly particles of colloidal size (less than 2 microns), and the
pore water is absorbed around the particle surface. Previous researches (Berry and
Poskitt, 1972; Edil and Dhowian, 1979; Edil and Dhowian, 1981) have found that the
behavior of amorphous peat is similar to clay soil, thus evaluation of its
compressibility characteristics can be made based on Terzaghi one-dimensional
theory of consolidation. Fibrous peat is the one that consists of fiber content more
than 20 % (ASTM D4427). The behavior of fibrous peat is very different from clay
due to the existence of the fiber in the soil. The fibrous peat has many void spaces
existing between the solid grains. Due to the irregular shape of individual particles,
fibrous peat deposits are porous and the soil is considered as a permeable material.
Therefore the rate of consolidation of fibrous peat is high but the rate decreases
significantly due to consolidation.
2.1.2
Sampling of Peat
Sampling of fibrous peat involves a lot of difficulties related to the high water
table and the nature of the fiber. Sampling methods vary with the peat texture, water
content, and the expected use of samples. In general, there are two types of samples;
disturbed and undisturbed samples.
Disturbed samples can be used for identification purpose. Block sampling
and piston sampler can be used to obtain samples at shallow depth (Noto, 1991). For
deeper elevation, screw augers, and split spoon sampler can provide disturbed
sample. The success rate of samplers in the standard penetration test (split spoon
10
sampler) or Raymond sampler is about 90 % for peat containing some clay, but can
be as low as 68-89 % for typical peat (Noto, 1991).
It is virtually impossible to obtain undisturbed samples of any type of soil,
including peat. Both physical intrusions of the sampler and the removal of in-situ
stresses can cause disturbance.
However, disturbance can be minimized using
certain sampling techniques. There was a reasonably well-established understanding
of the causes of disturbance during sampling, transport, and handling of inorganic
clays and corresponding accepted practices for sampling of soils. However, for
sampling of 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 must be
considered.
Kogure and Ohira (1977) pointed out the difficulties associated with the use
of most standard soil samplers because of the presence of fibers in peat. During
sampling, 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. Additional disturbance takes place from
water drainage while extracting the peat sample, thus extraction of sample should be
executed with extra care to minimize the loss of water.
Undisturbed samples can be obtained at shallow depth by block sampling
method, while large diameter tube sampler modified by adding sharp cutting edge
may be used to obtain sample at depth. Lefebvre (1984) claimed that both methods
give good quality samples for obtaining engineering characteristics of peat.
For block sampling method, 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.
Landva et al. (1983) attributed the disturbance during sampling to the loss of
volume with the presence of gas, the loss of moisture, and the deformation of the
peat structure. Large block samples (250 mm-square) can be obtained from below
11
the ground and groundwater surface or down to a depth of 175 mm using a block
sampler for peat. Large-size down-hole block samplers such as Sherbrooke sampler
(250-mm. in diameter) and Laval sampler (200-mm in diameter) that have been
developed for sampling clays can also be used for organic soils and probably for
peat. They also suggested that large diameter (more than 100 mm) thin walled fixed
piston sampler can be used in the same way as in soft clay when obtaining
undisturbed peat sampler. This is especially useful for obtaining deeper sample.
Recovery ratio is above 95 % except for fibrous peat containing tough fibers (Noto,
1991).
Hobbs (1986) stated 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 and applying a
correction to the void ratio as follows:
ec = eo = (em+εs) x (1-εs)
(2.1)
where ec = corrected void ratio,
εs = the measured compression strain during sampling, and
em = the measured void ratio.
It is not easy, however, to measure the compression strain during sampling.
Hence, the use of block sampling method is preferred for practical depth.
2.1.3
Structural Arrangement
The structural arrangement or texture of peat highly influences its
engineering properties.
The different textures are woody, fibrous, and granular
amorphous. They are dependent on the forming plant, the conditions on which the
peat accumulated and deposited, and the degree of decomposition.
12
According to Berry and Poskitt (1972), the mechanical properties of peat vary
considerably with the difference of their structure. The presence of fiber alters the
consolidation process of peat from that of clay and amorphous granular peat. The
texture of fibrous peat is coarser when compared to clay. This condition give an
implication on the geotechnical properties of peat related to the particle size and
compressibility behavior of peat.
The fibrous peat has essentially an open structure with interstices filled with a
secondary structural arrangement of non-woody, fine fibrous material (Dhowian and
Edil, 1980), thus physical properties of fibrous peat differ markedly from those of
mineral soils. The fibrous peat has many void spaces existing between the solid
grains. Due to the irregular shape of individual particles, fibrous peat deposits are
porous and the soil is considered as a permeable material.
Kogure et al. (1993) presented the idea of multi-phase system of fibrous peat,
which consists of organic bodies and organic space. The organic body consists of
organic matter and water in inner voids, while the organic space consists of water in
outer voids and the soil particles. The solid organic matter can be drained under
consolidation pressure. The cross section of deposition and diagram of the multiphase system of fibrous peat are schematically shown in Figure 2.1(a) and (b).
Organic spaces
Organic particle
Organic bodies
Organic
matters
(Solids)
Water
(Inner
voids)
Water
(Outer
voids)
Soil
particles
(Solids)
(b)
Figure 2.1: Schematic diagram of (a) deposition and (b) multi-phase system of
(a)
fibrous peat (Kogure et al., 1993)
13
It can be observed from Figure 2.1(a) that organic particles consist of solid
organic matter and inner voids. The solid organic matter is flexible with the inner
voids, which are filled with water that can be drained under consolidation pressure.
The spaces between the organic bodies, called outer voids, are filled with solid
particles and water.
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 differences in fiber content can be observed in the micrographs through
the Scanning Electron Micrograph (SEM). The higher the fiber content, the more the
peat will differ from an inorganic soil in its behavior. Figure 2.2 shows a Scanning
Electron Micrograph of Middleton fibrous peat specimen under 400 kPa vertical
consolidation pressures (Fox and Edil, 1996). The photograph was taken in vertical
and horizontal planes.
Figure 2.2: Scanning Electron Micrographs of Middleton fibrous peat; (a) horizontal
plane, (b) vertical plane (Fox and Edil, 1996)
14
Comparison of the two micrographs in Figure 2.2 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.
Individual microstructures remained essentially intact after
compression under high-stress conditions. This implies that for the fibrous peat,
horizontal rates of permeability and consolidation are larger than their respective
vertical rates of permeability and consolidation (Fox and Edil, 1996).
2.1.4
Physical and Chemical Properties
Variability of peat is extreme both horizontally and vertically. The variability
results in a wide range of physical properties such as texture, color, water content,
density, and specific gravity. The results of previous researches on the physical
properties of peat around the world are presented in Table 2.1 and 2.2.
Fibrous peat generally has very high natural water content due to its natural
water-holding capacity. Soil fabric, characterized by organic coarse particles, holds
a considerable amount of water because the coarse particles are generally very loose,
and the organic particle itself is hollow and largely full of water. Previous researches
have indicated that the average water content of fibrous peat is about 600 %. High
water content results in high buoyancy and high pore volume leading to low bulk
density and low bearing capacity. The water content of peat researched in West
Malaysia ranges from 200 to 700 % (Huat, 2004).
Unit weight of peat is typically lower compared to inorganic soils. The
average unit weight of fibrous peat is about equal to or slightly higher than the unit
weight of water. Sharp reduction of unit weight was identified with increasing of
water content. Previous researches suggested that for peat water content about 500
%, the unit weight ranges from 10 to 13 kN/m3. Based on his research, Berry (1983)
pointed out that the average unit weight of fibrous peat is about 10.5 kN/m3. A range
of 8.3-11.5 kN/m3 is common for unit weight of fibrous peat in West Malaysia (Huat,
2004).
15
Table 2.1: Physical properties of peat based on location (Huat, 2004)
Soil
deposits
Fibrous peat
Quebec
Fibrous peat,
Antoniny Poland
Fibrous peat,
Co. Offaly Ireland
Amorphous peat,
Cork, Ireland
Cranberry bog peat,
Massachusetts
Peat
Austria
Peat
Japan
Peat
Italy
Peat
America
Peat
Canada
Peat
Hokkaido
Peat
West Malaysia
Peat
East Malaysia
Peat
Central Kalimantan
Natural
water
content
(ωo, %)
Unit
weight
γ
(kΝ/m3)
Specific
gravity
(Gs)
Organic
content
(%)
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
200-800
9.8-13.0
-
-
334-1320
-
-
20-98
200-300
10.2-14.3
-
70-80
178-600
-
-
-
223-1040
-
-
17-80
115-1150
9.5-11.2
-
20-98
200-700
8.3-11.5
1.38-1.70
65-97
200-2207
8.0-12.0
-
76-98
467-1224
8.0-14.0
1.50-1.77
41-99
16
Table 2.2: Important physical and chemical properties for some peat deposits
(Ajlouni, 2000)
Peat
type
Natural
water
content
(ωo, %)
Bulk
density
Mg/m3
Specific
Gravity
Gs
Acidity
pH
Ash
content
%
Fibrouswoody
484-909
-
-
-
17
Fibrous
850
0.95-1.03
1.1-1.8
-
-
Peat
520
-
-
-
-
Amorphous
and fibrous
500-1500
0.88-1.22
1.5-1.6
-
-
200-600
355-425
-
1.62
1.73
4.8-6.3
6.7
12.2-22.5
15.9
Amorphous
To fibrous
850
-
1.5
-
14
Fibrous
605-1290
0.87-1.04
1.41-1.7
-
4.6-15.8
Coarse
Fibrous
613-886
1.04
1.5
4.1
9.4
350
-
-
4.3
4.8
778
-
-
3.3
1
202-1159
1.05
1.5
4.17
14.3
660
1.05
1.58
6.9
23.9
418
1.05
1.73
6.9
9.4
336
1.05
1.72
7.3
19.5
600
0.96
1.72
7.3
19.5
460
0.96
1.68
6.2
15
510
0.91
1.41
7
12
173-757
0.84
1.56
6.4
6.9-8.4
660-1590
-
1.53-1.68
-
0.1-32.0
660-890
0.94-1.15
-
-
-
200-875
1.04-1.23
-
-
-
Peat
125-375
0
1.55-1.63
5-7
22-45
Peat
419
1
1.61
-
22-45
Peat
490-1250
-
1.45
-
20-33
Peat
630-1200
-
1.58-1.71
-
22-35
Peat
Fibrous
Peat
(Netherlands)
Fibrous
(Middleton)
Fibrous
(James Bay)
400-1100
700-800
0.99-1.1
~1.00
1.47
-
4.2
-
5-15
-
669
0.97
1.52
-
20.8
510-850
0.99-1.1
1.47-1.64
4.2
5-7
1000-1340
0.85-1.02
1.37-1.55
5.3
4.1
Fibrous
sedge
Fibrous
Sphagnum
Coarse
Fibrous
Fine
Fibrous
Fine
Fibrous
Amorphous
Granular
Peat
Portage
Peat
Waupaca
Fibrous Peat
Middleton
Fibrous Peat
Noblesville
Fibrous
Fibrous
Peat
Amorphous
Peat
Reference
Colley
1950
Hanrahan 1954
Lewis 1956
Lea and Browner
1963
Adams
1965
Keene and
Zawodniak 1968
Samson and
LaRochell 1972
Berry and
Vickers
1975
Levesque et al.
1980
Berry 1983
NG and Eischen
1983
Edil and Mochtar
1984
Lefebvre et al.
1984
Olson
1970
Yamaguchi et al.
1985
Jones et al. 1986
Yamaguchi et al.
1987
Nakayama et al.
1990
Yamaguchi 1990
Hansbo 1991
Termatt and
Topolnicki 1994
Ajlouni, 2000
17
Specific gravity of peat depends greatly on its composition and percentage of
the organic content. For an organic content greater than 75 %, the specific gravity of
peat ranges between 1.3 and 1.8 with an average of 1.5 (Davis, 1997). The lower
specific gravity indicates a lower degree of decomposition and low mineral content.
Natural void ratio of peat is generally higher than that of inorganic soils
indicating their higher capacity for compression.
Natural void ratio of 5-15 is
common and a value as high as 25 have been reported for fibrous peat (Hanrahan,
1954).
Peat will shrink extensively when dried. The shrinkage could reach 50 % of
the initial volume, but the dried peat will not swell up upon re-saturation because
dried peat cannot absorb water as much as initial condition; only 33 % to 55 % of the
water can be reabsorbed (Mochtar, 1997).
Generally, peat soils are very acidic with low pH values, often lies between 4
and 7 (Lea, 1956). Peat in Peninsular Malaysia is known to have very low pH values
ranging from 3.0 to 4.5, the acidity tends to decrease with depth, and the decrease
may be large near the bottom layer depending on the type of the underlying soil
(Muttalib et al., 1991).
Chemically, peat consists of carbon, hydrogen, oxygen, and small amount of
nitrogen.
Previous researches (Soper and Obson, 1922; Chynoweth, 1983;
Schelkoph et al., 1983; Cameron et al., 1989) showed that the percentage hydrogen,
oxygen, and small amount of nitrogen are in the ranges of 40-60 %, 20-40 %, 4-6 %,
and 0-5 % respectively.
The composition is greatly related to the degree of
decomposition, the more the peat is decomposed, the less the percentage of the
carbon is produced.
The submerged organic component of peat 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. Gas content affects all
physical properties measured and field performance that relates to compression and
water flow. The gas content is difficult to determine and no widely recognized
18
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).
2.1.5
Classification
The physical, chemical, and geotechnical characteristic commonly used for
classification of inorganic soil may not be applicable to the characterization of peat.
On the other hand, properties which are not pertinent to inorganic soil may be
important for classification of peat. Furthermore, the ranges of values applied for
some properties of inorganic soil may not be relevant for peat soil. Generally, the
classification of peat soil is developed based on the decomposition of fiber, the
vegetation forming the organic content, organic content, and fiber content.
The classification based on the degree of decomposition was proposed by von
Post (1922) in which the degree of decomposition is grouped into H1 to H10: the
higher the number, the higher the degree of decomposition (Table 2.3). The test was
conducted by taking a handful of peat and when pressed in the hand, gives off
marked muddy water. The pressed residue is some-what thick and the material
remaining in the hand has fibrous structure. Fibrous peat with more than 60 % fiber
content is usually in the range of H1 to H4 (Hartlen and Wolski, 1996).
The most widely used classification system in engineering practice is based
on organic content. A soil with organic content of more than 75 % is classified as
peat (Lechowicz et al., 1996). Ash content is the percentage of ash to the weight of
dried peat. The ash content in most of the peat of the west coast of Peninsular
Malaysia is less than 10 %, showing a very high content of organic matter. This is
indicated by a loss of ignition value exceeding 90 % (Muttalib et al., 1991). The peat
is further classified based on fiber content because the presence of fiber alters the
consolidation process of fibrous peat from that of organic soil or amorphous peat.
Amorphous peat is the peat soil with fiber content less than 20 % (ASTM D4427). It
contains mostly particles of colloidal size (less than 2 microns), and the pore water is
absorbed around the particle surface. The behavior of amorphous granular peat is
19
similar to clay soil. Fibrous peat is the one having fiber content more than 20 %
(ASTM D4427) and posses two types of pore i.e.: macro-pores (pores between the
fibers) micro-pores (pores inside the fiber itself). Table 2.4 shows the classification
of peat based on organic and fiber content.
Table 2.3: Classification of peat based on degree of decomposition (von Post, 1922)
Condition of peat before squeezing
Condition of peat on sequeezing
Degree
of
Humifi
cation
Soil
color
H1
White
or
yellow
Very
pale
brown
Pale
brown
None
Easily
identified
Clear, colorless water
Insignificant
Easily
identified
Nothing
Not
pasty
Very slight
Still
identified
Nothing
Not
pasty
H4
Pale
brown
Slight
Not easily
identified
Some peat
H5
Brown
Moderate
Recognizabl
e but vague
Some
what
pasty
Strongly
pasty
H6
Brown
Moderately
strong
About onethird of peat
squeezed out
Very
strongly
pasty
H7
Dark
brown
Strong
Indistinct
(more
distinct after
squeezing)
Faintly
recognizable
Yellowish
water/pale
brown-yellow
Dark brown,
muddy water
not peat
Very dark
brown muddy
water
Very dark
brown muddy
water
Very dark
brown muddy
water
H8
Dark
brown
Very strong
Very
indistinct
H9
Very
dark
brown
Nearly
complete
Almost
recognizable
Very dark
brown muddy
water
Very dark
brown pasty
water
Very dark
brown muddy
water
Very
strongly
pasty
Very
strongly
pasty
Very
strongly
pasty
H10
Black
Complete
Not
discernible
About onehalf of peat
squeezed out
About twothird
squeezed out
Nearly all the
peat squeezed
out as fairly
uniform paste
All the peat
passes
between the
fingers; no
free water
visible
H2
H3
Degree of
decomposition
Plant
structure
Squeezed
solution
Very dark
brown muddy
paste
Material
extruded
(passing
between
fingers)
Nothing
Some peat
Nature
of
Residue
Not
pasty
N/A
20
Table 2.4: Classification of peat based on organic and fiber content
Classification peat soil based on ASTM standards
Fiber Content
(ASTM D1997)
Ash Content
(ASTM D2974)
Fibric : Peat with greater than 67 % fibers
Hemic : Peat with between 33 % and 67 % fibers
Sapric : Peat with less than 33 % fibers
Low Ash : Peat with less than 5 % ash
Medium Ash : Peat with between 5% and 15 % ash
High Ash : Peat with more than 15 % ash
Highly Acidic : Peat with a pH less than 4.5
Acidity
Moderately Acidic : Peat with a pH between 4.5 and 5.5
(ASTM D2976)
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
The classification based on the vegetation forming the organic material is not
usually adopted in engineering practice even though researches have indicated that
the fiber content or the type of plant forming the peat soil, and degree of
decomposition significantly affects the behavior of fibrous peat.
Based on the
botanical composition, peat is classified as Moss peat, Sedge peat, and Wood peat.
Concerning the degree of decomposition, peat is also grouped as fibric (weakly
decomposed peat), hemic (medium decomposed peat), and sapric (strongly
decomposed peat). In terms of texture, the peat is classified as woody, fibrous,
sedimentary, and granular peat (Davis, 1997).
Consistency or Atterberg limit is not generally used for classification of peat
because plasticity gives little indication of the characteristics of peat (Hobbs, 1986),
and the existence of fiber makes it difficult or impossible to carry out the test for
determination of liquid limit and plastic limit of most peat. Nevertheless, some
researchers have reported the liquid limit and plastic limit of fibrous peat soil (Huat,
2004). The presence of fibers makes both liquid limit and plastic limit measurement
difficult so the determination of Atterberg limit for amorphous or granular peat may
be possible (MacFarlane, 1969).
21
2.1.6
Shear Strength
The shear strength of peat soil is very low; however, the strength could
increase significantly upon consolidation. The rate of strength increase is almost
equal to the increase in the consolidation pressure compared to soft clay with a rate
of strength increase of 0.3 (Noto, 1991). The shear strength of these soils is also
associated with several variables namely origin of soil, water content, organic
content, and degree of decomposition.
Most peat is considered frictional or non-cohesive material (Adam, 1965) due
to the fiber content, thus the shear strength of peat is determined based on drained
condition as:
τ’f = σ’ tan φ’
(2.2)
where τ’f = shear strength,
σ’ = effective stress, and
φ ’= friction angle.
However, the friction is mostly due to the fiber and the fiber is not always
solid because it is usually filled with water and gas. Thus, the high friction angle
does not actually reflect the high shear strength of the soil.
Shear box and triaxial equipment have been used to determine the drained
shear strength of peat soil although the results of triaxial test on fibrous peat are
difficult to interpret because fiber often act as horizontal reinforcement, so failure is
seldom obtained in a drained test. In addition, triaxial test in drained condition may
take several weeks for peat with low permeability. Shear box is the most common
test for determining the drained shear strength of fibrous peat while triaxial test under
consolidated-undrained condition is common for laboratory evaluation of undrained
shear strength of peat (Noto, 1991).
22
Previous studies indicated that the effective internal friction φ' of peat is
generally higher than inorganic soil i.e: 50o for amorphous granular peat and in the
range of 53o-57o for fibrous peat (Edil and Dhowian, 1981). Landva (1983) indicated
the range of undrained friction angle of 27o-32o under a normal pressure of 3 to 50
kPa. The range of undrained friction angle of peat in West Malaysia is 3o-25o (Huat,
2004).
Considering the presence of peat soil is almost always below the groundwater
level, the determination of undrained shear strength is also important. This is usually
done in-situ because sampling of peat for laboratory evaluation of undrained shear
strength of fibrous peat is almost impossible. Some approaches to in-situ testing in
peat deposits are: vane shear test, cone penetration test, pressure-meter test,
dilatometer test, plate load test, and screw plate load tests (Edil, 2001). Among
them, the vane shear test is the most commonly used; however, the interpretation of
the test results must be handled with caution. An undrained shear strength of peat
soil (cu) obtained by vane shear test was in range of 3-15 kPa, which is much lower
than that of the mineral soils. A correction factor of 0.5 is suggested for the test
results on organic soil with a liquid limit of more than 200 % (Hartlen and Wolsky,
1996).
2.1.7
Compressibility
The compression behavior of fibrous peat is different from that of clay soil.
The compressibility of fibrous peat consists of two stages: primary consolidation and
secondary compression. The primary consolidation of the fibrous peat is very rapid,
and large secondary compression, even tertiary compression is observed. Secondary
compression is generally found as the more significant part of compression because
the time rate is much slower than the primary consolidation. Subsequently the
formula used to estimate the amount of compression is different from that of clay
soil. The dominant factors controlling the compressibility characteristics of peat
include the fiber content, natural water content, void ratio, and initial permeability.
Published data on the compressibility properties of peat are given in Table 2.5.
23
Table 2.5: Compressibility characteristics of some peat deposit (Ajlouni, 2000)
Peat
Fibrous
peat
Peat
Vertical
coefficient of
permea-
Coefficient
Natutal water
content ωo %
or Initial void
ratio eo
bility
dation
kvo (m/s)
cvo (m2/year)
cc
cα/cc
850
4x10-6
-
10
0.060.1
Hanrahan
1954
520
-
-
-
0.0610.078
Lewis 1956
14-17
2.5-5
0.0350.083
Lea and
Browner
1963
-
-
0.090.1
Adams
1965
Keene and
Zawodniak
1968
Amorphous
and fibrous
peat
500-1500
10 -10
Canadian
muskeg
200-600
10-5
-7
-6
of rate of
consoli-
Compress-
Ratio
ion index
Reference
Amorphous
to fibrous
peat
705
-
55.6
4.7-10.3
0.0730.091
Peat
400-750
10-5
-
-
0.0750.085
Weber
1969
Amorphous
granular
peat
eo=7
4x10-7
64
2.6
0.05
Berry and
Poskitt
1972
eo=11
8x10-7
16.1
4.4
0.05
Fibrous
peat
Fibrous
Samson
and
LaRochelle
1972
Berry and
Vickers
1975
Dhowian
and Edil
1980
605-1290
10
-
-
0.0520.072
Fibrous peat
613-886
10-6-10-5
9.1
-
0.060.085
Fibrous peat
600
10-6
-
-
0.0420.083
Coarse
fibrous
202-1159
1.1x10-6
-
6.4
0.0550.064
Berry 1983
Fibrous peat
660-1590
5x10-6-5x10-5
-
4.5-15
0.06
Lefebvre et
al. 1984
Fibrous peat
200-875
-
27.2
-
-
Amorphous
peat
125-375
-
3.79
-
-
Peat
419
3x10-8
>6.4
-
-
Jones et al.
1986
Fibrous peat
700-800
10-6
3-6
-
0.0420.083
Hansbo
1991
Fibrous peat
370
1.4x10-12
-
-
0.06
Fibrous peat
610-850
6.8x10-8 - x10-7
-
-
0.052
peat
(Middleton)
510-850
3x10-8-10-6
20-150
6-9
0.053
Fibrous peat
1000-1340
4x10-7- 7x10-6
30-300
10-12
0.059
peat
-6
Olson
1970
den Haan
1994
Mesri et al.
1997
Fibrous
Ajlouni
2000
24
The unit weight of peat is close to that of water. Thus, the in-situ effective
stress (σ’c) is very small and sometimes cannot be detected from the results of
consolidation test (Mesri et al., 1997). It is also very difficult to obtain the beginning
of secondary compression (tp) from the consolidation curve because the preliminary
consolidation occurs rapidly. The natural void ratio (eo) is very high due to large
pores and high initial water content.
The e-log p’ curves show a steep slope
indicating a high value of compression index (cc). Published data on cc ranges from
2-15 (Lefebvre et al., 1984).
Furthermore, there is possibility that secondary
compression start before the dissipation of excess pore water pressure is completed
(Leonards and Girault, 1961).
Compression of fibrous peat continues at a gradually decreasing rate under
constant effective stress, and this is termed as the secondary compression. The
secondary compression of peat is thought to be due to further decomposition of fiber
which is conveniently assumed to occur at a slower rate after the end of primary
consolidation (Mesri et al., 1997).
The rate of secondary compression is
conveniently defined by the slope of the final part of the void ratio versus
logarithmic of time curve (cα). This estimate is based on assumptions that cα is
independent of time, thickness of compressible layer, and applied pressure. Ratio of
cα/cc has been used widely to study the behavior of peat (Dhowian and Edil, 1980;
den Haan, 1994; Mesri et.al., 1997). The ranges of cα/cc ratio obtained by previous
researchers are summarized in Table 2.5.
The rate of primary consolidation of fibrous peat is very high; however it
decreases with the application of consolidation pressure. Lea and Browner (1963)
indicated a significant decrease of coefficient of rate 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.
25
2.1.8
Permeability
Permeability is one of the most important properties of peat because it
controls the rate of consolidation and increase in the shear strength of the soil
(Hobbs, 1986). The permeability of peat depends on the void ratio, mineral content,
degree of decomposition of the peat, chemistry, and the presence of gas. Previous
studies on physical and hydraulic properties of fibrous peat indicated that the peat is
averagely porous, and this certifies the fact that fibrous peat has a medium degree of
permeability (MacFarlane, 1969; Lishtvan, 1981; Lefebvre et al., 1984, Hobbs,
1986). In its natural state, the hydraulic conductivity of fibrous peat is as high as
sand, i.e., 10-5 to 10-4 m/s (Colleselli et. al., 2000). Thus, constant head permeability
tests have been used to determine the vertical and horizontal coefficient of
permeability of fibrous peat.
The change in permeability as a result of compression is drastic for fibrous
peat (Dhowian and Edil, 1980). Research on Portage fibrous peat shows the soil
initially has a relatively high permeability comparable to fine sand or silty sand;
however, as compression proceeds and void ratio decreases rapidly, permeability is
greatly reduced to a value comparable to that of clay i.e. about 10-8 to 10-9 m/s (Hillis
and Browner, 1961; Lea and Browner, 1963; Dhowian and Edil, 1980). The findings
showed that the rate of decrease of hydraulic conductivity with decreasing void ratio
is usually higher than that in clays (Edil, 2003).
2.2
Soil Compressibility
In general, the compressibility of a soil consists of three stages, namely initial
compression, primary consolidation, and secondary compression.
While initial
compression occurs instantaneously after the application of load, the primary and
secondary compressions are time dependent. The initial compression is due partly to
the compression of small pockets of gas within the pore spaces and the elastic
compression of soil grains. Primary consolidation is due to dissipation of excess
pore water pressure caused by an increase in effective stress whereas secondary
26
compression takes place under constant effective stress after the completion of
dissipation of excess pore water pressure.
The time required for the water to dissipate from the soil depends on the
permeability of the soil itself. In granular soil, the process is rapid and hardly
noticeable due to its high permeability. On the other hand, the consolidation process
may take years in clay soil. For peat, the primary consolidation occurs rapidly due to
high initial permeability and secondary compression takes a significant part of
compression.
2.2.1
Primary Consolidation
One-dimensional theory of consolidation developed by Terzaghi in 1925
carries an assumption that primary consolidation is due to dissipation of excess pore
water pressure caused by an increase in effective stress whereas secondary
compression takes place under constant effective stress after the completion of the
dissipation of excess pore water pressure. Other important assumptions attached to
the Terzaghi consolidation theory are that the flow is one-dimensional and the rate of
consolidation or permeability is constant throughout the consolidation process.
Consolidation characteristics of soil can be represented by consolidation
parameters such as coefficient of axial compressibility av, coefficient of volume
compressibility mv, compression index cc, and recompression index cr. Another
important characteristic of soil compressibility is the pre-consolidation pressure
(σc’). The soil that has been loaded and unloaded will be less compressible when it
is reloaded again, thus settlement will not usually be great when the applied load
remains below the pre-consolidation pressure. These parameters can be estimated
from a curve relating void ratio (e) at the end of each increment period against the
corresponding load increment in linear scale (Figure 2.3) or logarithmic scale (Figure
2.4).
27
0.75
0.70
void ratio (e)
0.65
0.60
0.55
av
0.50
0.45
0.40
0
200
400
600
800
1000
1200
1400
1600
1800
pressure (p)
Figure 2.3: Plot of void ratio versus pressure in linear scale (Nurly Gofar and
Khairul Anuar Kassim, 2005)
0.75
0.70
void ratio (e)
0.65
cc
0.60
0.55
cr
0.50
0.45
0.40
1
10
100
1000
10000
pressure (p)
Figure 2.4: Plot of void ratio versus pressure in logarithmic scale (Nurly Gofar and
Khairul Anuar Kassim, 2005)
As shown in Figure 2.3, the coefficient of axial compressibility av is the slope
of the e-p’ curve for a certain range of stress while the coefficient of volume
compressibility mv can be computed as:
28
mv =
av
1+ e o
(2.3)
where mv = coefficient of volume compressibility,
av = coefficient of volume compressibility, and
eo = initial void ratio.
The compression index cc and recompression index cr are the slope of the elog p’ curve (Figure 2.4) for loading and unloading stages.
Consolidation settlement is calculated based on the value of either the
coefficient of volume compressibility (mv) or the compression indices (cc and cr).
Due to construction, the total vertical stress on a soil element at depth z is increased
by ∆σ'. This increase of stress will results in the decrease of void ratio corresponds
to ∆e = eo-e1.
By knowing the ratio of the change in void ratio to the change in the effective stress
in e-p’ curve (Figure 2.3), then
⎛ e − e ⎞ ⎛ σ ' − σo ' ⎞
⎟⎟ H
Sc = ∆H = ⎜⎜ o 1 ⎟⎟ ⎜⎜ 1
⎝ σ1 ' − σ o ' ⎠ ⎝ 1 + e o ⎠
(2.4)
⎛ 1 ⎞
⎟⎟ (σ1 ' − σ o ') H = m v ∆σ' H
Sc = a v ⎜⎜
+
1
e
o ⎠
⎝
(2.5)
⎛ ∆e ⎞
⎟⎟ H
Sc = ⎜⎜
⎝ 1 + eo ⎠
(2.6)
By using the e-log p’ curve, the change in void ratio can be written as:
∆e = cc log
σ1
σo
(2.7)
29
and the settlement of a normally consolidated clay due to change of stress ∆σ’ is
given as:
Sc = c c
σ' + ∆σ
H
log o
1+ eo
σ' o
(2.8)
where Sc = ∆H = consolidation settlement,
H = thickness of consolidation soil layer,
∆σ’ = σ1’ - σ’o = the change in the effective in e-p’ curve,
∆e = eo – e1 = the change in void ratio, and
cc = compression index.
The soil that has been loaded and unloaded will be less compressible when it
is reloaded again.
Thus, it is also necessary to estimate the pre-consolidation
pressure i.e.: the stress carried by soil in the past (σc’) because consolidation
settlement will not usually be great when the applied load remains below the preconsolidation pressure. The pre-consolidation pressure can be obtained from the
consolidation curve by procedure suggested by Cassagrande.
If the pre-consolidation pressure obtained from laboratory test (σc’) is greater
then the existing overburden pressure (σo’) and the added stress increases the existing
pressure below the pre-consolidation pressure, then the compression index (cc)
should be replaced with the recompression index (cr) in Equation 2.8, which results
in Equation 2.9. If the additional stress increases the existing pressure beyond the
pre-consolidation pressure, then Equation 2.8 is modified as Equation 2.10.
Sc = c r
Sc = c r
σ' + ∆σ '
H
log o
σ' o
1+ e o
σ' + ∆σ
σ'
H
H
log o
log c + c c
σ' c
1+ e o
σ' o
1+ e o
where σ’c = pre-consolidation pressure, and
cr = recompression index.
(2.9)
(2.10)
30
The time rate of consolidation, and subsequently the time required for a
certain degree of consolidation to take place, can be obtained based on plot of
compression against time for each load increment. The Hydrodynamic equation
governing the Terzaghi one-dimensional consolidation is:
cv
∂ 2 u e ∂u e
=
∂z 2
∂t
(2.11)
where ue = excess pore water pressure,
t
= time,
z
= depth, and
cv = coefficient of rate of consolidation (m2/year or m2/sec) which
contains the material properties that govern the consolidation process.
cv =
k v 1+ eo
kv
=
γw av
m v γw
(2.12)
where kv = vertical coefficient of permeability, and
γw = unit weight of water (kN/m3).
General solution to Equation 2.11 is given by Taylor (1948) in terms of a Fourier
series expansion of the form:
n =∞
µ e = (σ 2 ' − σ1 ')∑ f1 (Z)f 2 (Tv )
n =0
where µe = excess pore water pressure,
σ2’ - σ’1 = the change in the effective stress,
Z = geometry factor = z/H, and
Tv = time factor.
(2.13)
31
The time factor is a dimensionless number which contain physical constants
of a soil layer influencing its time rate of consolidation. The time factor can be
written as:
Tv =
cv t
Hd
2
(2.14)
where Hd = length of drainage path for a particular pressure increment.
The relationship between the average degree of consolidation and time factor
are given in the form of curve (Figure 2.5) or equation 2.14.
Figure 2.5: Consolidation curve for two-way vertical drainage (Head, 1982)
For U < 60 %
T = (π/4) U2 = ((π/4) (U%/100)2
For U > 60 %
T = -0.933 log (1-U) - 0.085 = 1.781 – 0.933 log (100 – U %)
(2.15)
The coefficient of rate of consolidation for a particular pressure increment
from consolidation test can be determined by curve fitting methods. There are two
methods commonly used to determine the coefficient of rate of consolidation (cv)
i.e.: the logarithmic time (Cassagrande’s) method, and the square root time
32
(Taylor’s) method. These empirical procedures were developed to fit approximately
the observed laboratory test data to the Terzaghi’s theory of consolidation.
The Cassagrande methods use the plot of dial readings versus the logarithmic
of time (log t).
The idea is to find the reading at t50 or the time for 50 %
consolidation (Figure 2.6). The procedure is as follows:
1. Plot a graph relating dial reading (mm) versus logarithmic of time.
2. Produce a straight line for primary consolidation and secondary compression
part of the graph. The two lines will meet at point C.
3. The ordinate of point C is D100 = the deformation corresponds to U = 100 %.
4. Choose time t1 (point A), t2 = 4t1 (point B). The difference in the dial reading
is equal to x.
5. An equal distance x set off above point A fixes the point D0 = the deformation
corresponds to U = 0 %. Notes that Do is not essentially equal to the initial
reading may be due to small compression of air within the sample.
6. The compression between D0 and D100 is called the primary consolidation.
7. A point corresponding to U = 50 % can be located midway between D0 and
D100. The value of T corresponds to U = 50 % is 0.196.
8. Thus
cv =
0.196 H d
t 50
2
(2.16)
where Hd is half the thickness of specimen for a particular pressure increment.
33
6.8
6.6
6.4
6.2
6.0
5.8
5.6
5.4
5.2
5.0
4.8
4.6
4.4
4.2
4.0
3.8
D0
x
dial reading (D)
A
x
t1
B
t2
D50
Primary consolidation
C
D100
t50
tp
1
10
100
1000
10000
Time in minutes(log scale)
Figure 2.6: Determination of coefficient of rate of consolidation by Cassagrande’s
method (Nurly Gofar and Khairul Anuar Kassim, 2005)
The square root of time methods developed by Taylor is based on the similarity
of the shapes of experimental and theoretical curves when plotted versus the square
root of time (Figure 2.7). The following procedure was recommended:
1. Extent the straight line part of the curve to intersect the ordinate (t = 0) at
point D. The point shows the initial reading (Do). The intersection of this
line with the abscissa is P.
2. Take point Q such that OQ = 1.15 OP.
3. The intersection of line DQ and the curve is called point G.
4. Draw horizontal line from G to the ordinate (D90). The point shows the value
of √t90. The value of T corresponds to U = 90 % is 0.848.
5. Thus
cv =
0.848 H d
t 90
2
(2.17)
34
6.8
D0
6.6
6.4
6.2
6.0
dial reading (D)
5.8
5.6
5.4
5.2
G
5.0
D90
4.8
4.6
1.15d
d
4.4
4.2
t90
4.0
0
5
P 10 Q
time (minutes
15
1/2
20
)
Figure 2.7: Determination of coefficient of rate of consolidation by Taylor method
(Nurly Gofar and Khairul Anuar Kassim, 2005)
2.2.2 Secondary Compression
For some soils, especially those containing organic material, the compression
does not cease when the excess pore water pressure has completely dissipated but
continues at a gradually decreasing rate under constant effective stress. Thus, it is
common to differentiate the two processes as primary consolidation and secondary
compression. Secondary compression, also referred as creep, is thought to be due to
the gradual readjustment of the clay particles into a more stable configuration
following the structural disturbance caused by the decrease in void ratio.
35
Previous researchers (Leonards and Girault, 1961; Berry and Vickers, 1975;
Lefebvre et al., 1984; Hobbs, 1986; Kogure et al., 1986) have shown that both
primary consolidation and secondary compressions can take place simultaneously.
However, it is assumed that the secondary compression is negligible during primary
consolidation, and is identified after primary consolidation is completed. Secondary
compression of soil is conveniently assumed to occur at a slower rate after the end of
primary consolidation.
The rate of secondary compression in the standard
consolidation test can be defined by the slope (cα) of the final part of the void ratio
versus logarithmic of time curve (Figure 2.8).
3.0
Void ratio (e)
2.5
Primary
consolidation
2.0
1.5
1.0
cα
0.5
tp
0.0
1
10
100
1000
Secondary
compression
10000
100000
Time, t in minutes (log scale)
Figure 2.8: Determination of the coefficient of rate of secondary compression from
consolidation curve (Cassagrande’s method) (Nurly Gofar and Khairul Anuar
Kassim, 2005)
The axial rate of consolidation can be obtained from Figure 2.8 as the ratio of
change on the void ratio to the change on the logarithmic of time.
cα =
∆e
∆e
=
∆ log t log t f
tp
(2.18)
36
where cα = coefficient of secondary compression,
∆e = the change of void ratio from tp to tf,
The void ratio at time tp is denoted as eop.
This estimate is based on
assumptions that cα is independent of time, thickness of compressible layer, and
applied pressure. The settlement due to the secondary compression (Ss) is therefore:
Ss =
cα
t
H log f
1+ e o
tp
(2.19)
where Ss = settlement due to secondary compression,
Η = initial thickness,
tp = time of the completion of primary consolidation, and
tf = time for which the secondary compression settlement is required (design
life of a structure).
Research showed that the ratio of cα/cc is almost constant and varies from
0.025 to 0.06 for inorganic soil, while a slightly high range was obtained for organic
soils and peat (Holtz and Kovacs, 1981). A higher ratio was obtained for highly
compressible clay and organic soils, thus the amount of secondary compression
settlement may be quite significant.
2.3
Compressibility of Fibrous Peat
Fibrous peat undergoes large settlements in comparison to clays when
subjected to loading. The compression behavior of fibrous peat varies from the
compression behavior of other types of soils in two ways. First, the compression of
peat is much larger than of other soils. Second, the creep portion of settlement plays
a more significant role in determining the total settlement of peat than of other soil
types.
37
Researches (Mesri and Rokhsar, 1974; Mesri and Choi, 1985b; Mesri and
Lo, 1991; Lan, 1992) showed that Terzaghi’s theory of consolidation is not
applicable for the prediction of the compression of fibrous peat. Subsequently,
many theories of consolidation have been developed mainly as modifications to
Terzaghi’s theory. Such modifications, mostly intended for soft clays and silts,
include decrease in permeability with the progress of consolidation, the changes in
compressibility during consolidation, time related compressibility during and after
primary consolidation phase, the finite value of strains, and effect of self-weight. Of
all methods, few theories were developed solely to model compressibility of fibrous
peat (Gibson and Lo, 1961; Barden, 1968; Berry and Poskitt, 1972; den Haan, 1996).
Evaluation of the secondary compression of peat based on cα/cc has been used
widely. The evaluation of cα was done from the time-compression curve derived
from consolidation test.
Cassagrande (1936) method has been used for the
evaluation of the cα if the test results display an ideal “S” curve, which is the typical
of inorganic soil. The time-compression curve for organic soil, especially the fibrous
peat often deviates from the ideal curve. Therefore, an extension of Casagrande’s
method was developed by Dhowian and Edil (1980) was used for the evaluation of
time-compression curve derived from consolidation tests on organic soil.
This
method assumed that the secondary compression occurs following the completion of
primary consolidation, which is not true for fibrous peat. The method also neglects
the fact that secondary compression may have started before the completion of
excess pore water pressure dissipation.
Further development on the analysis of time-compression curve for the
evaluation of the compression of soil exhibiting non-linear relationship of secondary
compression with time was contributed by Sridharan and Prakash (1998). This
method separated the secondary compression from the primary consolidation by
assuming that the secondary compression follows the primary consolidation.
Robinson (1997) focused his research on the beginning of secondary compression
and found that the secondary compression may have started as early as 60 % degree
of primary consolidation. In this research, the completion of excess pore water
pressure dissipation was actually measured during the test (Robinson, 1999). The
38
complete procedure for the evaluation of primary and secondary compression of
fibrous peat was presented in Robinson (2003).
Mesri and Rokhsar (1974) developed a theory of consolidation based on
assumptions for soil properties that were more realistic than those in the original
Terzaghi theory of one-dimensional consolidation. The assumptions were that:
1. The soil undergoes a finite strain.
2. The compressibility and the permeability of the soil are variable during
consolidation.
3. The soil may display recompression and compression behavior.
4. A unique relationship between compressibility and effective stress and time.
The time related compressibility during the primary consolidation stage was
assumed to be equal to the degree of compression β multiplied by the secondary
compression index cα, measured during secondary compression stage.
Mesri and Choi (1985b) modified the theory of consolidation introduced by
Mesri and Rokhsar (1974) to include a nonlinear relationship between void ratio and
the logarithmic of effective vertical stress. Another modification was that the time
related compressibility was related to both the degree of compression β and the
compression index cc. Mesri and Lo (1991) further refined the Mesri and Choi
(1985a) formulation and also applied to consolidation with vertical drains. The
theory was incorporated in a computer program ILLICON, which was used
successfully to predict time-rate of settlement and excess pore water pressure
dissipation during primary consolidation (Ajlouni, 2000).
Lan (1992) claimed that the cα/cc concept is not applicable to peat
compression.
Therefore, based on the uniqueness of σ’v-e-e’ concept and the
relationship between e and σ’v, he proposed a constitutive equation for modeling the
primary consolidation and secondary compression of peat in the normally
consolidated range.
39
den Haan (1996) derived a consolidation equation for the deformation of nonbrittle soft clay and peat and solve the equation by finite difference technique for a
specified boundary and initial condition, and the nonlinear permeability-void ratio
relationship. This model is known as “abc” model and the solution was incorporated
in a computer program, CONSEF.
Another approach to modeling the consolidation process of peat soils is by
assuming that the structure of soils exhibiting secondary compression can be
evaluated based on Rheological model consisting of mass-spring dashpot as shown in
Figure 2.9. (Gibson and Lo, 1961; Barden, 1965; Barden, 1968; Berry and Poskitt,
1972). In this approach, the structural viscosity was assumed to be linier.
spring
spring
dashpot
Figure 2.9: Rheological model used for soil undergoing secondary compression
Berry and Poskitt (1972) proposed two different Rheological models to
symbolize the consolidation of amorphous and fibrous peat. The models consider
peat properties such as:
1. Finite strain.
2. Linear relationship between void ratio and the logarithmic of effective stress.
3. Linear relationship between void ratio and logarithmic of coefficient of
permeability.
4. Presence of time-related compressibility.
40
The consolidation equation was solved for a single homogenous layer
subjected to an increment of pressure and the solution was presented in the form of a
non-dimensional graphical solution.
Theoretical results that were obtained and
compared with experimental data on amorphous and fibrous peat samples showed a
general agreement, however the procedure for obtaining the theoretical results
includes curve fitting and arbitrary assumptions. In order to obtain the necessary
parameters, the secondary part of deformation-logarithmic of time relationship had to
be of a constant slope. The Rheological parameters involved in this model should be
obtained by non conventional engineering means make it very difficult to apply this
theory to data on peat.
2.4
Consolidation Test
The compressibility characteristics of a soil are usually determined from
consolidation tests. General laboratory tests for measurement of compression and
consolidation characteristics of a soil are: Oedometer test, Constant Rate of Strain
(CRS) test, and Rowe Cell test. The procedures for these tests are fully described in
BS 1377-6 and Head (1982, 1986).
2.4.1 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.
Oedometer cell can accommodate 50 mm
diameter and 20 mm thick samples. The schematic diagram of consolidation test on
Oedometer cell is shown in Figure 2.10.
41
Figure 2.10: Schematic diagram of Oedometer cell (Bardet, 1997)
Advantages and disadvantages of Oedometer test are outlined by Head
(1986). Among the advantages 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. The test provides a reasonable estimate of the
amount of settlement of structure on inorganic clay deposits.
On the other hand, 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.
The drainage in Oedometer test is entirely vertical.
As some soils are
strongly isotropic, their properties, particularly drainage, are very different in
horizontal and vertical direction. Drainage starts as soon as the load is applied. A
uniform pore pressure may not be developing throughout a sample, and the initial
undrained compression cannot be measured directly.
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.
Furthermore, the boundary effect from the ring enhances the friction of the sample.
Friction reduces the compression during loading and reduces swelling during
unloading.
42
For standard test, the samples were subjected to consolidation pressures with
load increment ratio of 1. 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 p’ plot, gives low value of pre-consolidation 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 fibrous peat.
The other limitation of the standard Oedometer test is that there is no means
of measuring excess pore water pressures, the dissipation of which control the
consolidation process. Therefore the estimation of compressibility is based solely on
the change of height of the specimen.
The analysis of compression of such soils presents certain difficulties when
the conventional methods are applied because the curves obtained from the
conventional Oedometer tests and the behavior exhibits by them differ from that of
clay. Furthermore, such soils are more prone to decomposition during Oedometer
testing.
Gas content and additional gas generation also may complicate the
interpretation of Oedometer tests (Edil, 2003). Some researchers (Berry and Poskitt,
1972; Ajlouni, 2000; Colleselli et al., 2000; Robinson, 2003) had presented the
behavior of fibrous peat and the recent advances in formulating their behavior.
2.4.2
Large Strain Consolidation Test (Rowe Cell)
Rowe consolidation cell (Figure 2.11) was introduced by Rowe and Barden in
1966 to overcome the disadvantages of the conventional Oedometer apparatus when
performing consolidation tests on non-uniform deposits such as fibrous peat. Rowe
cell has many advantages over the conventional Oedometer consolidation apparatus.
The main features responsible for these improvements are the hydraulic loading
system, the control facilities and ability to measure excess pore water pressure, and
the capability of testing samples of large diameter.
43
Figure 2.11: Schematic diagram of Rowe consolidation cell (Head, 1986)
Through hydraulic loading system, the sample is less susceptible to vibration
effects compared to the conventional Oedometer cell. Pressures of up to 1000 kPa
can be applied easily due to large sample size.
Corrections required for the
deformation of the loading system when subjected to pressure is negligible, except
perhaps for very stiff soils. Furthermore, the hydraulic loading system enables
samples of large diameter up to 254 mm diameter to be tested for practical purposes
and allows for large settlement deformations.
Three sizes of Rowe cell are commercially available i.e., 3 in (75 mm), 6 in
(151) mm, and 10 in (254 mm) diameters. The use of large samples enables the
effect of the soil fabric (laminations, fissures, bedding planes) to be taken into
account in the consolidation process, thereby enabling a realistic estimate of the rate
of consolidation to be made. Large samples (i.e. 150 mm diameter and 50 mm thick)
have been found to give higher and more reliable values of cv, especially under low
stresses, than conventional Oedometer test samples (Head, 1986). Better agreement
has been reported by McGown et al. (1974) and Rowe (1968 and 1972) between
44
predicted and observed rates of settlement, as well as their magnitude, may be partly
due to the relatively smaller effect of structural viscosity and fabric in larger samples.
Tests on high quality large diameter samples minimize the effect of sample
disturbance and therefore provide more reliable data for settlement analysis than
conventional one-dimensional Oedometer tests on small samples.
The most important feature of Rowe cell is the ability to control drainage and
to measure excess pore water pressure during the course of consolidation tests.
Drainage of the sample can be controlled, and several different drainage conditions
can be imposed on the sample. Control of drainage enables loading to be applied to
the sample in the undrained condition, allowing full development of pore pressure.
Consequently the initial immediate settlement can be measured separately from the
consolidation settlement, which starts when the drainage line is opened.
Excess pore water pressure can be measured accurately at any time and with
immediate response. Pore pressure readings enable the beginning and end of the
primary consolidation phase to be positively established. The volume of water
draining from the sample can be measured, as well as surface settlement.
The sample can be saturated by applying increments of back pressure until a
B value close to one is obtained, or by controlling the applied effective stress, before
starting consolidation. Tests can be carried out under an elevated back pressure,
which ensures fully saturated conditions, gives a rapid excess pore water pressure
response, and ensures reliable time relationships.
The sample can be loaded either by applying a uniform pressure over the
surface (free strain), or through a rigid plate which maintains the loaded surface
plane (equal strain). Fine control of loadings, including initial loads at low pressures,
can be accomplished easily.
45
Several drainage conditions (vertical or horizontal) are possible, and back
pressure can be applied to the sample. In this test, samples can be saturated and then
tested under the application of back pressure. Consolidation and permeability tests
can be successively conducted in Rowe cell providing data over a range of void
ratios or strain.
Figure 2.12 shows different types of consolidation tests using Rowe cell. A
different time factor is needed in every case related to drainage direction, boundary
conditions, and consolidation location (Table 2.6).
The drainage direction is
described as horizontal or vertical while the boundary condition is described as either
flexible or equal strain. The consolidation location is either described as ‘average’
for settlement or volume-change measurement, or is stated as the point at which pore
pressure is measured.
2.5
Evaluation of Compression Curves derived from Consolidation Test
The results of the consolidation test are presented by plotting height or
vertical strain against time for each load increment. This graph is required to observe
the time rate of consolidation, and subsequently the time required for a certain degree
of consolidation to take place. The second graph is the void ratio (e) at the end of
each increment period against the corresponding load increment. There are two
types of plot: the e-p’ curve and the e-log p’ curve. These graphs are needed to
obtain the coefficient of volume compressibility (mv), the compression index (cc),
and to calculate the magnitude of the consolidation settlement.
46
Figure 2.12: Drainage and loading conditions for consolidations tests in Rowe cell:
(a), (c), (e), (g) with ‘free strain’ loading, (b), (d), (f), (h) with ‘equal strain’ loading
(Head, 1986)
47
Table 2.6: Curve fitting data for evaluation of coefficient of rate of consolidation
(Head, 1986)
Test
ref.
Drainage
direction
Boundary
strain
Consolidation
location
(a)
and
(b)
Vertical
Free
And
Equal
Average
(c)
and
(d)
(e)
one way
Vertical
two way
Radial,
outward
(g)
Radial,
† Drain
T90
0.197
(Tc)
1.031
Free
And
Equal
Average
0.197
0.848
Free
Average
Free
Equal
ratio 1/20
Tc, Tro,Tri is theoretical time factors
Time
function
Power
curve
slope
factor
t0.5
1.15
t0.5
1.15
t0.465
1.22
t0.5
1.17
t0.5
1.17
t0.5
1.17
0.848
0.379
inward†
(h)
T50
Centre of base
Equal
(f)
Theoretical
time factor
(Tc)
0.0632
0.335
(Tro)
Central
0.200
0.479
Average
0.0866
0.288
(Tro)
Central
0.173
0.374
Average
0.771
2.631
(Tri)
r = 0.55 R
0.765
2.625
Average
0.781
2.595
r = 0.55 R
0.778
(Tri)
2.592
48
2.5.1
Time-Compression Curve
Figure 2.13 shows three types of time-compression curve derived from
laboratory test (Leonards and Girault, 1961). Type I curve is defined by Terzaghi’s
theory with S-shaped curve. 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 rate after the dissipation of excess pore water
pressure. Identification of the beginning of secondary compression (tp) and the rate
of secondary compression (cα) for Type I curve can be estimated based on
Cassagrande’s method by taking two straight lines from compression versus
logarithmic of time curve and the point of intersection is identified as the end of
primary consolidation (tp = t100). The procedures have been presented in section
2.2.2.
Compression (mm)
Type I curve
Type II curve
Type III curve
Time, t in minutes (Log scale)
Figure 2.13: Types of compression versus logaritmic of time curve derived from
consolidation test (Leonards and Girault, 1961)
49
The time-compression curves derived from results of one-dimensional
consolidation test on fibrous peat do not follow the type I curve. They resemble the
type II curve in which the primary consolidation is very rapid and secondary
compression does not vary linearly with logarithmic of time and tertiary compression
is actually observed after secondary compression. Therefore the quantification of
secondary compression based on conventional (Cassagrande) method frequently
under-estimate the settlement. Dhowian and Edil (1980) extended the Cassagrande
method to include the nonlinearity of secondary compression of fibrous peat by a
coefficient of secondary compression, cα1, and coefficient of tertiary compression,
cα2 (Figure 2.14).
In this case, time of secondary compression (ts) should be
identified in addition to the time for primary consolidation (tp). The term ‘tertiary
strain’ is introduced as a soil strain to designate the increasing coefficient of
secondary compression with time.
εi
εp
cα1
ts
εs
tp
cα2
εt
Figure 2.14: Vertical strain versus logaritmic of time curve of fibrous peat for one-
dimensional consolidation (Dhowian and Edil, 1980)
50
Identification of the beginning and rate of secondary compression from Type
I and Type II curves can also be made based on logarithmic of compressionlogarithmic of time (log δ-log t) as proposed by Sridharan and Prakash (1998). This
relationship yields two linear portions in which the point of intersection between the
two linear portions is regarded as the end of primary consolidation (tp) or the
beginning of secondary compression (Figure 2.15). An advantage of this method is
that the logarithmic of the secondary compression is found to be linear over a wider
extend of time.
The slope of the log δ-log t plot is defined as the secondary
compression factor, m.
m=
where m =
log (e1 /e 2 )
log (t 2 /t 1 )
(2.20)
secondary compression factor,
e1 =
void ratios of the compressible soil layer corresponding to
compression δ1 at time t1, and
e2 =
void ratios of the compressible soil layer corresponding to
compression δ2 at time t2 respectively.
1
Compression (mm) (log scale)
tp
A (δ1, t1)
B (δ2, t2)
m
10
0.1
1
10
100
1000
10000
100000
Time, t in minutes (Log scale)
Figure 2.15: Sridharan and Prakash log δ log t curve (Sridharan and Prakash, 1998)
51
It is evident that both Cassagrande and Sridharan & Prakash methods
assumed that the secondary compression begins at the completion of excess pore
water pressure (tp = t100). The methods also assumed that the secondary compression
occurs at a slower rate then the primary consolidation, thus tp is obtained at the
inflexion point in the curve. Therefore, the methods cannot evaluate secondary
compression of soils exhibiting Type III curve (Figure 2.13) because the curve does
not show an inflection point.
Previous researcher (Robinson, 1997) has pointed out that the full dissipation
of excess pore water pressure cannot be predicted based on settlement curve. Based
on his findings on consolidation test with measurement of excess pore water pressure
(Robinson, 1999), the excess pore water pressure dissipation is completed earlier
than the time predicted from the inflection point of the settlement curve. Further
analysis by the same researcher (Robinson, 2003) revealed that the secondary
compression actually starts during the dissipation of excess pore water pressure from
the soil. This observation was based on Terzaghi’s one dimensional consolidation
theory, whereby the relationship between dissipation of excess pore water pressure
and compression during primary consolidation can be represented by a straight line.
On the other hand, the actual curve derived from laboratory consolidation test on
peat soil was not actually follows a straight line. If the relationship does not form a
straight line, the settlement was actually due to the combination of excess pore water
pressure dissipation on primary consolidation and the secondary compression.
Robinson (2003) suggested a method for separating the primary consolidation
and secondary compression that occur during the consolidation process. The method
was developed based on time-compression and the time-excess pore water pressure
curves (Figure 2.16).
It can be observed that the dissipation of excess pore water pressure (Figure
2.16(b)) is actually completed earlier than predicted by the settlement curve (Figure
2.16(a)) (Robinson, 2003). It can be seen from Figure 2.13 that some settlement
curves do not exhibit the inflection point, thus the end of primary consolidation
cannot be predicted based on Cassagrande’s method. According to Robinson (2003),
the data from Figure 2.16(a) and 2.16(b) can be plotted as degree of consolidation
52
measured from the dissipation of excess pore water pressure versus total compression
of the soil in Figure 2.17(a)-(f).
Figure 2.16: (a) Compression-time curves, and (b) Degree of consolidation-time
from the measured excess pore water pressure dissipation curves for peat (Robinson,
2003)
53
Figure 2.17: Degree of consolidation from the excess pore water pressure dissipation
curves plotted against compression for several consolidation data for peat (Robinson,
2003)
54
Figure 2.17 (a) to (f) show similar trend in which the curve deviate from a
straight line at a certain degree of consolidation. The point where the curve diverges
from linearity is identified as the beginning of secondary compression.
The
compression corresponding to the point where the straight line meets the U = 100 %
axis is the total primary consolidation settlement (δp), while the compression below
the extrapolated line is the secondary compression (δs). Thus, using this procedure, it
is possible to separate the primary consolidation settlement and secondary
compression from time-compression data obtained from the laboratory onedimensional consolidation test. Figure 2.18 (a) and (b) show the total and primary
consolidation settlement after the removal of secondary compression respectively. A
clear S or Type I curve is obtained which is the shape expected if only the primary
consolidation is considered (Figure 2.18 (b)).
The secondary compression-time relationship is commonly represented by a
logarithmic function. Instead of using the consolidation curve derived directly from
the test results, the evaluation of the coefficient of consolidation of peat soil should
be based on the primary consolidation versus logarithmic of time curve (Figure
2.18(b)).
For Robinson’s method, as long as the secondary compression varies linearly
with logarithmic of time, the time-secondary compression relationship is
satisfactorily represented by the coefficient of secondary compression. The plot can
be obtained by subtracting the primary consolidation from total settlement. Note that
zero secondary settlement was obtained at t equal to to, where to is the beginning of
secondary compression. Figure 2.19 shows the plot of the secondary compression
(δs) against their corresponding time (t-to). The coefficient of secondary compression
of soil (cα) is the slope of the line shown in Figure 2.19.
55
Figure 2.18: (a) Total settlement-time curves for peat and (b) Primary settlement-
time curve after removing the secondary compression (Robinson, 2003)
0.02
Secondary compression, δ
s
(mm)
56
δs = 0.105 (t - t o )
R2 = 0.805
0.015
0.01
0.005
0
0
1
2
3
Log time (t - t o ) (t and t o are in minutes)
Figure 2.19: Secondary compression versus logarithmic of time curve for evaluation
of coefficient of secondary compression (Robinson, 2003)
2.5.2
The e-log p’ Curve
As mentioned previously, the void ratio (e) at the end of each increment
period is plotted against the corresponding load increment (e-log p’ curve) to obtain
the pre-consolidation pressure (σ’c), the compression index (cc), and to calculate the
magnitude of the settlement. The parameters are required for the evaluation of the
primary consolidation and to obtain the cα/cc values for evaluation of secondary
compression. The procedures for obtaining these parameters are described in Section
2.2.2.
Fox (2003) stated that the standard procedure for consolidation test specified
the load increment ratio (LIR) of one and each load is maintained for 24 hour. For
some soils, especially peat, the end of primary consolidation can be reached at time
much less than 24 hour. Thus, the estimation of the compression index (cc) based on
consolidation test conducted on fibrous peat in which the primary consolidation
occurs rapidly may not be accurate (Figure 2.20).
Some creep or secondary
compression took place before the application of the subsequent pressure.
57
Figure 2.20: Typical laboratory consolidation curve (Fox, 2003)
For this reason, measurement of excess pore water pressure during the
consolidation test is very critical in the observation of the end of primary
consolidation. Some corrections on the e-log p’ plot should be made if the load
increment is not added at the completion of excess pore water dissipation.
The void ratio obtained from each load after 24 hour is plotted as open points
in Figure 2.20. The end of consolidation can be determined from the 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 end of primary
consolidation (eop) and the void ratio at 24 hour was used as correction factor applied
to the original e-log p’ curve. The modified curve is plotted as solid line in Figure
2.20
The separation of secondary compression from the primary consolidation is
also suggested by Robinson (2003) for the evaluation of the time-compression curve
and time- excess pore water pressure curves.
CHAPTER 3
METHODOLOGY
3.1
Introduction
This chapter describes the research methodology adopted in this
investigation.
Section 3.2 explains the sampling procedure conducted in this
research, while Section 3.3 describes the preliminary test carried out in this study to
obtain the index properties and soil classification. Section 3.4 and 3.5 give details on
the equipment and the procedure of large strain consolidation, and data analysis. The
overall process of the study is presented in flow chart given in Figure 3.1.
Critical literature review was done in this study to provide rationale of the
research and to gather sufficient information on consolidation behavior of fibrous
peat. The background of the study was used to develop the hypothesis adopted for
this research i.e. the compressibility characteristics of fibrous peat can be analyzed
based on time-compression curve derived from the test results.
The sampling of the peat was carried out at Kampung Bahru, Pontian, West
Johore. Physical and chemical properties such as natural moisture content, specific
gravity, initial void ratio, unit weight, and acidity were determined to establish the
basic characteristics of the soil. The soil was classified based on von Post or degree
of humification, fiber content, organic content, and ash content.
The Scanning
Electron Micrograph (SEM) was performed to evaluate the structural arrangement of
59
Literature
Review
Problem
Identification
Index
Properties
Sampling of Peat,
Preparation of Material
and Equipment
Fabric by SEM
Classification
Preliminary Test and
Identification of
Peat Type
Shear
Strength
Consolidation Test
Standard Consolidation
Test (Oedometer)
Results & Analysis by
Cassagrande’s method
Published data
Comparisons
Comparisons
-
Effect of Fabric and
Structural Arrangement
-
Settlement Estimation
Compressibility
Characteristics
Conclusion
Figure 3.1: Flow chart of the study
Permeability
(Constant
Head)
Large Strain
Consolidation Test
(Rowe Cell)
Results
Data Analysis
by
Robinson’s (2003)
method
60
the fiber in soil mass. Engineering characteristics evaluated in this research include
permeability, strength, and compressibility. Permeability test was also conducted to
study the effect of fiber content and structural arrangement peat in the consolidation
behavior of fibrous peat.
The focus of the research is to evaluate the compressibility characteristics of
fibrous peat analyzed based on data obtained from large strain consolidometer test on
Rowe cell. Analysis of data includes the time-compression curve based on Robinson
(2003) method, analysis of consolidation curve, and settlement analysis. Comparison
was made between the results of consolidation test using Oedometer and Rowe cell.
The test results were also compared with published data.
All the laboratory test procedures are based on the manual of soil laboratory
testing (Head, 1981, 1982, 1986) in accordance with the British (BS) and U.S.
(ASTM) Standards.
3.2
Sampling of Peat
Block sampling method was used in this study to obtain the samples of the
fibrous peat from Kampung Bahru, Pontian, West Johore. The method was selected
because it is the best method for obtaining the most representative sample of peat at
shallow depth. At the time of sampling, the groundwater table was found at depth of
less then 1 m. Thus, the block sampling method was used to acquire the sample at a
depth below ground water surface or between 1 to 2 m.
The soil was excavated to a depth of 1 m and then a tube of 300 mm-diameter
and 300 mm high was pushed slowly into the soil. The surroundings of the sampler
was excavated so that samples could be then cut at the base and a thin wooden plate
was inserted at the bottom of the sample to cover the bottom of the sample before
taking it to the surface. The quality of samples was maintained by ensuring the
sharpness of the edge of the tube and knife used to cut the sample (Figure 3.2a). The
top and bottom of the sample were covered by wax and wooden plate before they
61
were transported to the laboratory.
The detailed procedures for obtaining the
samples are described in Appendix A.
Eighteen block samples were obtained from six different points, at least 2
meter apart, in one location. Each sample was transported in a well-cushioned
wooden box and was kept in the laboratory under constant temperature (air
conditioned room). All tests involved in this study were done within six months after
the sampling process in order to minimize the effect of biodegradation.
In order to estimate the initial permeability of the soil and to obtain more
accurate estimation of water content, six samples were retrieved using piston sampler
of diameter 105 mm and length 450 mm (Figure 3.2b). In this case, three samples
were obtained by pushing the piston in vertical direction and the other three were
obtained by pushing the piston in horizontal direction. The samples were used for
the determination of the natural water content and the initial permeability of the peat
using the constant head permeameter.
(a)
(b)
Figure 3.2: Sampling methods (a) block sample, (b) piston sample
62
3.3
Preliminary Tests
Preliminary laboratory test was conducted to identify the soil and to compare
the results to published data especially on Malaysia’s peat. The tests included the
determination physical and chemical properties of the soil and soil classification.
The Scanning Electron Microscope (SEM) was used to observe the fiber orientation
of fibrous peat.
Other tests include the shear strengths, permeability, and the
standard consolidation test on Oedometer cell.
3.3.1 Physical and Chemical Properties
Several fundamental tests were carried out to obtain physical and chemical
properties of peat. The natural moisture content was done following BS 1377-2
while the determination of the specific gravity (Gs) of peat soil was made using
kerosene following BS 1377-2. The initial void ratio (eo) can be calculated based on
the results from natural moisture content and specific gravity. As for unit weight (γ,
kN/m3), the value is calculated based on the natural moisture content, specific
gravity, and initial void ratio. Furthermore, the acidity of the peat was determined by
pH meter following BS 1377-3. Each test was conducted for at least six samples.
3.3.2
Classification
The peat soils were classified based on von Post degree of decomposition,
sieve analysis (BS 1377-2), fiber content (ASTM D1997-91), organic content, and
ash content (BS 1377-3). The classification based on the degree of decomposition
was proposed in which the degree of decomposition was grouped into H1 to H10
(Table 2.3 in Chapter 2). The sieve analysis was done to determine the fine contents
of the soil. The fiber content was determined from dry weight of fibers retained on
sieve no.100 sieve (more than 0.15 mm opening size) as a percentage of oven-dried
mass while the organic content and ash content were determined from the loss of
63
ignition test whereby the oven dried mass of soil is further heated in muffle furnace
at 440oC for 4 hours. Six samples were used for each test for soil classification.
3.3.3
Fiber Content and Fiber Orientation
The Scanning Electron Microscope (SEM) was used to observe the fiber
orientation in of the fibrous peat. The test follows the standard procedure outlined in
ASTM F 1392-93 and the standard procedures of G34-SUPRA 35 VP en 01 Carl
Zeisss SMT-Nano Technology System Division.
The Scanning Electron Microscope (SEM) is an instrument that is routinely
used for the production of strongly enlarged images of a specimen. The maximum
achievable magnification of the SEM is 500.000 x which use a combination of XRay and micro analysis.
The SEM is a simple tool with minimal specimen
preparation. Figure 3.3 shows the equipment used for SEM of the fibrous peat. The
equipment, test procedures, and the results are described in Appendix C.
Figure 3.3: The equipment for the Scanning Electron Microscope (SEM)
64
In order to study the fiber of the fibrous peat samples, the Scanning Electron
Microphotographs analysis were performed before and after the consolidation test.
The samples were cut in vertical and horizontal directions to enable the observation
of the rearrangement of the fiber forming peat at initial state and under consolidation
pressure, at three magnifications of 50, 200, and 400. The samples were prepared by
the drying technique.
3.3.4
Shear Strength
The assessment of in-situ shear strength of peat in this research was made by
65 mm diameter and 130 mm height field vane at depths of 1 and 2 m following
standard procedure BS1377-9 (Figure 3.4a). The test was done in each points of
sampling, thus six tests were done in each depth. The smallest size vane available in
the laboratory was selected in order to minimize the effect of fiber to the measured
shear strength. Rotational speed of 0.1deg/sec is used in the test.
Shear box test following standard procedure BS 1377-7 (Figure 3.4b) using
normal stress of 8, 16, and 22 kPa were done on twelve samples of the fibrous peat to
obtained the drained shear strength. Determination of normal stresses used for the
test is based on the estimation of overburden pressure on the soil at depth of 1 and 2
m.
(a)
(b)
Figure 3.4: Shear strength tests (a) Vane shear test carried out at site (b) Shear box
apparatus
65
3.3.5
Permeability
Since the peat soil can be as porous as sand, the constant head permeability
test was chosen to evaluate the initial permeability of the soil (Figure 3.5). The
constant head permeability test was done on sample obtained vertically and
horizontally using piston sampler (Figure 3.6a and 3.6b). The tests were performed
on three undisturbed vertical soil samples and three undisturbed horizontal soil
samples. The tests are done following standard procedures of ASTM D2434 using a
mould with 105.4 mm internal diameter and a height of 121.2 mm. The initial
permeability of the soil was computed on the basis of the amount water that passes
through the soil sample. The time for the water volume collected in a beaker from an
immersion tank with overflow was required for the computation of the rate of the
permeability of the soil.
Figure 3.5: Constant Head permeability test
66
(a)
(b)
Figure 3.6: Piston sampler (a) pushed in vertical direction (b) pushed in horizontal
direction
3.3.6
Standard Consolidation Tests
The standard consolidation test on Oedometer cell was conducted as
preliminary tests to estimate the consolidation behavior of the fibrous peat samples.
The tests are carried out based on the standard procedure outlined in BS 1377-5. The
Oedometer cell is 50 mm in diameter and 20 mm in height (Figure3.7). Since the
sample was taken from shallow depth (1 to 2 m), and subsequently the in-situ stress
is very low, then the consolidation test started at a very low pressure. The test is
conducted with load increment ratio (LIR) of one, and applied loads were 25 kPa, 50
kPa, 100 kPa, 200 kPa, and 400 kPa. Each load was maintained for two weeks or
20,000 minutes for loading stages during the first tests, but was modified to one
week or 10,000 minutes upon determination of the end of primary consolidation (tp)
and secondary compression (ts) of the soil. The standard consolidation test was
conducted on twelve samples.
67
(a)
(b)
Figure 3.7: Standard consolidation test (a) Oedometer cell (b) Assembly of all
components of Oedometer test
3.4
Large Strain Consolidation Test (Rowe Cell)
Large strain consolidation tests were performed using Rowe consolidation
cell (Figure 3.8) with internal diameter of 151.4 mm and height of 50 mm. The test
was done on six samples of fibrous peat with load increment (LIR) of one. Each
sample was subjected to large strain consolidation pressures of 25, 50, 100, and 200
kPa. This range of pressure was determined based on the results of the standard
consolidation test.
Figure 3.8: Rowe consolidation cell
68
The test was performed with two-way vertical drainage. The designation of
the large strain consolidation test with vertical drainage (two-way) is shown in
Figure 2.12d. A porous drainage disc is placed under the sample, and is connected to
the same back pressure system as the top drainage line for the consolidation stages.
In this type of test, drainage takes place vertically upwards and downwards while
pore pressure is measured at the center of the base.
Two types of measuring devices were used in the Rowe Consolidation test for
data measurement.
These measuring devices were linear variable displacement
transducer (LVDT) and pressure transducer. A 50 mm LVDT with an accuracy of
0.001 mm was used to measure vertical displacement of the soil sample in the Rowe
consolidation test (Figure 3.9). Four 1500 kPa pressure transducer with accuracy of
0.1kPa (Figure 3.10) were used to measure back pressure, diaphragm pressure, and
pore pressure from the top and the bottom of the specimen. All tubing connected to
back pressure, diaphragm pressure, and pore pressure must be saturated prior to
testing to ensure accurate pressure readings.
Figure 3.9: 50 mm Linear Variable Displacement Transducer (LVDT)
Figure 3.10: 1500 kPa pressure transducer
69
A serial pad 1 (advanced data logger system) was used to systematically read
and store the measurement data for a certain time interval. The serial pad 1 used for
the Rowe consolidation test has eight channels i.e. back pressure, diaphragm
pressure, volume change, pore pressure 1, pore pressure 2, and the displacement
which should be attached to the serial pad 1 when the Rowe consolidation test was
running.
All data were read and stored by a personal computer which use GDSLAB v
2.0.6 program to control the testing data saving progress from the serial pad 1.
Figure 3.11 and Figure 3.12 show the main page for the GDSLAB v 2.0.6 program
and the serial pad 1, while Figure 3.13 shows the schematic arrangement of control
system for the Rowe consolidation test.
The procedure for measurement using GDSLAB v. 2.0.6 is provided in
GDSLAB v2 Handbook (GDS Instruments Ltd, 2003).
Figure 3.11: Main page of the GDSLAB v 2.0.6 program for collecting data system
70
Figure 3.12: Serial pad 1
Volume
change
Back
pressure
Pore
pressure 1
CH 1
Diaphragm
pressure
Pore
pressure 2
Rowe
consolidation test
CH 2
CH 3
CH 5
CH 6
CH 7
Serial Pad 1
Program /
data recorder
Computer
timer
Printer /
plotter
Input channels to serial pad 1 unit:
CH 1 from pore pressure 1
CH 2 from axial displacement
CH 3 from pore pressure 2
CH 5 from volume change
CH 6 from diaphragm pressure
CH 7 from back pressure
Figure 3.13: Schematic arrangement of control system for the Rowe consolidation
tests
71
3.4.1
Calibration
Calibration is a vital factor in the use of instruments, and need to be carried
out from time to time in order to maintain a high standard of accuracy of test results.
Many instruments are issued with a manufacturer’s calibration certificate which
states the reading obtained at each of series of intervals of the characteristic being
measured. These values can be used as the basis of a calibration curve. However,
the performance of many instruments changes over a period of time, and can vary
with changes of temperature and other environmental conditions. Therefore it is
good laboratory practice to calibrate instruments regularly, and to ensure that the
latest calibration data are readily available for references when test are being carried
out and results analyzed.
Measurement instrument also need to calibrate with the GDSLAB v 2.0.6
program to get the accurate data when using serial pad 1.
The calibration of
instruments includes the linear displacement transducer (LVDT) and pressure
transducer.
For the linear displacement transducer (LVDT) calibration, total displacement
was measured by a caliper. After the maximum displacement were inserted, “zero”
button is clicked at the control part when releasing the LVDT to normal. When the
LVDT was pushed to the maximum value, the “gain” button was clicked to let the
GDSLAB v 2.0.6 program read the maximum displacement for the LVDT. The
whole LVDT calibration processes were show in the Figure 3.14.
1. Use the
caliper to
measure
the maximum
displacement
for the LVDT
2. LVDT
release to the
normal position
and click the
“zero” button in
the channel
configuration
3. LVDT is
push to the
maximum
value and
click the
”gain”
button on
the channel
configuration
Figure 3.14: Linear Displacement Transducer (LVDT) calibration process
72
Pressure transducer calibration was done on the back pressure, the diaphragm
pressure, volume change, and the pore pressure transducer. The 1500 kPa pressure
transducer used for testing was calibrated by the pressuring panel in the laboratory.
Maximum pressure can be measured by the pressure transducer was inserted in the
channel calibration program of GDSLAB v 2.0.6 “zero” button was clicked when no
pressure applied to the pressure transducer. When the maximum pressure of 1500
kPa was applied to the pressure transducer, ”gain” button was clicked to let the
GDSLAB v 2.0.6 read the maximum pressure value.
Beside the calibration of each measuring device, it is essential to do system
calibration of the equipment because the accuracy of the Rowe consolidation test was
based on the compression and the load pressure measurement.
Frictional error
between the specimen ring and the load platen could be generated when running the
Rowe consolidation test although the silicon grease was applied to the internal
surface of the specimen ring.
The setting up for the compression calibration was similar to the setting up
for Rowe consolidation test except the soil specimen inside the ring was changed to
the uncompressible solid steel within the range up to 10 kPa. The loading frame was
then started and the load and displacement were recorded by transducers with serial
pad 1. The load calibration was continued until the maximum load of the load cell
was achieved. All data was analyzed by the Microsoft Excel and loading calibration
curve was generated.
Correction on the testing data should be done to the
displacement for Rowe consolidation test. The sample procedures for calibration of
the pore pressure transducer are shown as follows:
1. The properties of transducer objects may be accessed by the clicking on the
relevant icon (Figure 3.15). From here the user may perform a read of the
transducer. The transducer channel and hardware connectivity are set in the
hardware configuration file and cannot be altered at this level of the program.
Physical connections may only be adjusted within the visual planner.
73
Figure 3.15: The transducer object
2. To ensure the transducer reading is correct, a number of compulsory values must
be entered in the ”Advanced” tab for the transducer as below (Figure 3.16):
Figure 3.16: The advanced tab for the transducer
74
3. Transducer calibrations must be entered for each transducer from the calibration
details tab (Figure 3.17 (a) and (b)). The minimum compulsory value is to enter
the sensitivity value for the transducer. This will be in units of engineering
units/returned units i.e. for a load cell the sensitivity is commonly in units of
kN/mV. The transducer may also be calibrated from scratch by pressing the ReCalibrate button and following the on screen wizard instructions.
When a
transducer has been configured the transducer name, serial number, and the last
calibrated data should be entered.
(a)
(b)
Figure 3.17: The transducer calibrations (a) The calibration detail tab (b) The results
of transducer calibrations
The complete calibration data of instrument and equipment such as pore
pressure, axial displacement, volume change, diaphragm pressure, and back pressure
done in this study is given in Appendix G.
75
3.4.2
Cell Assembly and Connections
Equipment & accessories needed for the large strain consolidation test are as
follows:
1. Rowe cell (diameter 150 mm).
2. Sintered bronze porous disc 3 mm thick with typical permeability 4 x 10-4 m/s
(the porous metal disc should be boiled after every test and carefully inspected in
order to prevent a gradual build-up of fine particles).
3. Dial gauge for measuring vertical settlement.
4. Spare porous insert for measuring excess pore water pressure.
5. Spare O Ring base seal.
6. Spare diaphragm.
7. Flange sealing ring.
8. Data acquisition system for measurement of
a. Diaphragm pressure.
b. Back pressure.
c. Excess pore water pressure.
d. Vertical settlement.
e. Volume of water draining out.
f. Time.
9. Consumables: Silicone grease
The arrangement of the Rowe cell and connections are described in the
following steps:
1. After covering the base with a film of water, place a saturated porous disc of
sintered bronze on the cell base without entrapping any air.
2. Fit the cutting rings containing soil sample on top of the Rowe cell body (Figure
3.18). Place the sample into the Rowe cell body by slowly and steadily pushing
the soil sample vertically downwards using a porous disc (Figure 3.19).
76
Figure 3.18: Cutting rings containing soil sample are fitted on top of the Rowe cell
Figure 3.19: A porous disc is used to slowly and steadily push the soil sample
vertically downward into the Rowe cell body
3. Flood the space at the top of the cell above the sample with de-aired water.
4. Place a saturated drainage disc through the water onto the sample by lowering
into position using the lifting handle. Avoid trapping air under the plate. Ensure
that there is a uniform clearance all round between the disc or discs and the cell
wall.
5. Connect a tube to valve F and immerse the other end in a beaker containing deaired water. The tube should be completely filled with de-aired water making
sure that there are no entrapped air bubbles.
77
6. Support the cell top at three points so that it is level, and with more than enough
clearance underneath for the settlement spindle attached to the diaphragm to be
fully extended downwards. The cell top should be supported near its edge so that
the flange of the diaphragm is not restrained. Fill the diaphragm with water using
rubber tubing about one-third the volume. The way de-aired water is filled into
the diaphragm can be diagrammatically observed in Figure 3.20 and realistically
observed in Figure 3.21. Open valve C.
Figure 3.20: Schematic diagram of filling of de-aired water into the diaphragm
(Head, 1986)
Figure 3.21: Realistic view of filling of de-aired water into the diaphragm
7. Place three or four spacer blocks, about 30 mm high, on the periphery of the cell
body flange. Lift the cell top, keeping it level, and lower it onto the spacers,
allowing the diaphragm to enter the cell body. Bring the bolt holes in the cell top
into alignment with those in the body flange.
78
8. Use rubber tube to add more water to the inside of the diaphragm so that the
weight of water brings the diaphragm down and its periphery is supported by the
cell body. Check that the cell body is completely filled with water. The whole of
the extending portion of the diaphragm should be inside the cell body, and the
diaphragm flange should lie perfectly flat on the cell body flange.
9. Hold the cell top while the supporting blocks are removed, then carefully lower it
to seat onto the diaphragm flange without entrapping air or causing ruckling or
pinching (Figure 3.22). Align the bolt holes. When correctly seated, the gap
between top and body should be uniform all round and equal to a diaphragm
thickness.
Open valve F to permit escape of excess water from under the
diaphragm.
Figure 3.22: Diaphragm inserted into Rowe cell body (Head, 1986)
10. Tighten the bolts systematically (Figure 3.23).
Ensure that the diaphragm
remains properly seated, and that the gap between the metal ranges remains
constant all round the perimeter.
79
Figure 3.23: Diaphragm is correctly seated (Head, 1986)
11. Open valve D, and press the settlement stem steadily downwards until the
diaphragm is firmly bedded on top of the plate covering the sample. Close valve
D when no more water emerges.
12. Connect valve C to a header tank of distilled water having a free surface about
1.5 m above the sample.
13. Completely fill the space above the diaphragm with water through valve C with
bleed screw E opened. Tilt the cell so that the last pocket of air can be displaced
through E. Maintain the supply of water at C when subsequently replacing the
bleed screw.
14. Maintain pressure at C, and as the diaphragm expands allow the remaining
surplus water from above the sample to emerge through valve F. Open valve D
for a moment to allow the escape of any further water from immediately beneath
the diaphragm. Escape of water from F due to diaphragm expansion may take
some considerable time because of the barrier formed by the folds of the
diaphragm pressing against the cell wall.
15. Close valve F when it is evident that the diaphragm has fully extended. Observe
the excess pore water pressure at the base of the sample, and when it has reached
a constant value record it as the initial excess pore water pressure, uo. This
corresponds to the initial pressure po under the head of water connected to C. If
80
the height from the top of the sample to the level of water in the header tank is h
mm, then:
po =
h x 9.81 h
=
kPa
1000 102
(3.1)
16. Maintain the pressure at C.
17. Connect the lead from the back pressure system to valve D without entrapping
any air. Open valve F for a while to let out the bubble from back pressure line.
3.4.3
Consolidation Test
The final arrangement of Rowe cell for two-way vertical drainage is
diagrammatically shown in Figure 3.24. The test is described under the following
stages: Preliminaries; Saturation; Loading; Consolidation; Further load increments;
Unloading; Conclusion; and Measurements and Removal of the sample.
Figure 3.24: Arrangement of Rowe cell for consolidation test with two-way vertical
drainage (Head, 1986)
81
3.4.3.1 Preliminaries
1. Close valve B to isolate the pore pressure transducer from the flushing system
throughout the test.
2. Set the vertical movement dial gauge at a convenient initial reading near the
upper limit of its travel, but allow for some upward movement if saturation is to
be applied.
3. Record the reading as the zero (datum) value under the seating pressure po.
4. Set the back pressure to the required initial value, with valve D closed. The back
pressure should be greater than the initial pore pressure (uo) but it should be 10
kPa less than the first increment of cell pressure (Head, 1986).
5. Record the initial reading of the volume gauge when steady.
3.4.3.2 Saturation
Saturation by the application of increments of back pressure is desirable for
undisturbed samples taken from above water table. For this type of test, application
of 10 kPa back pressure is used. Saturation is generally accepted completely when
the value of the pore pressure parameter B reaches about 0.96 (Head, 1986).
3.4.3.3 Loading Stage
1. With the drainage lines valve A and valve D closed and valve C open, increase
the diaphragm pressure steadily to the first increment. Open valve A valve D
when set. First increment of diaphragm pressure is taken as 25 kPa for this type
of test.
2. Open valve F to allow excess water to escape from behind the diaphragm for a
short time just to allow excess water from the top of the sample.
3. Wait until the pore pressure reaches a steady value equal to diaphragm pressure.
If the sample is virtually saturated the increase in pore pressure should almost
equal the pressure increment applied to the sample.
4. Record any settlement indicated by the dial gauge before starting consolidation.
82
3.4.3.4 Consolidation Stage
Consolidation is started by opening the drainage outlets (valve A and valve D
in Figure 3.25) and at the same instant starting the clock. Read the following data:
a. Vertical settlement.
b. Excess pore water pressure.
c. Volume change on back pressure line.
d. Diaphragm pressure (check)
The primary consolidation phase is completed when the pore pressure has
fallen to the value of the back pressure. Wait for secondary compression to take
place.
3.4.3.5 Further Load Increments
1. Increase the diaphragm pressure to give the next value of effective stress. Allow
excess water to drain from behind the diaphragm (valve F) if necessary.
2. The pore pressure should then be allowed to reach equilibrium before
proceedings to the next consolidation stage.
3. Repeat the above steps for 50 kPa, 100 kPa, and 200 kPa consolidation pressures.
3.4.3.6 Unloading
Unloading is needed to evaluate the effect of surcharge on the compressibility
characteristics of peat. In this case, the sample was loaded to the pre-consolidation
pressure (estimated based on standard consolidation test data, 30 kPa) and loaded to
100 kPa. At the end of consolidation test under 100 kPa, the soil was unloaded back
to 30 kPa. For unloading stage, diaphragm pressure is reduced with valve D closed.
It should be followed by swelling stage with valve D open, during which upward
movement, volume increase, and pore-pressure readings are taken in the same way as
consolidation process. The pore-pressure should be allowed to reach equilibrium at
83
the end of each stage before proceeding to the next stage of loading. The following
stage of loading in this case is 100 kPa and 150 kPa.
3.4.3.7 Conclusion of Test
1. Reduce the pressure to the initial seating pressure, po.
2. When equilibrium has been achieved, record the final settlement, volume change
and pore pressure readings.
3. Close valve A and open valves C, D and F, allowing surplus water to escape.
Unbolt and remove the cell top and place it on the bench supports.
3.4.3.8 Measurement and Removal of Sample
1. Remove the porous disc to expose the sample surface. Measure the diameter and
height of the sample.
2. Remove the cell body from the base and remove the sample intact from the cell.
Split the sample in two along a diameter.
3. Take two or more representative sample from one half of the sample for moisture
content measurements.
4. Allow the other half to air-dry to reveal the fabric and any preferential drainage
paths, which may have affected the test behavior.
5. Allow at least 4 hour before taking picture of the sample.
The cell components should be cleaned and dried before putting away, giving
careful attention to the sealing ring at the base. Porous bronze and ceramic discs and
inserts should be boiled and brushed; used porous plastic should be discarded.
Connecting ports and valves should be washed out to remove any soil particles. Any
corrosion growth on exposed metal surfaces should be scraped off, and the surface
made smooth and lightly oiled.
84
3.4.4
Consolidation Test with Horizontal Drainage
In addition to consolidation test with two-way vertical drainage, the tests
were also carried out with horizontal drainage to periphery in order to evaluate the
effect of fabric arrangement on the consolidation behavior of the peat.
Three
samples were tested using this arrangement. The arrangement of Rowe Cell for
consolidation test with horizontal outward drainage is shown in Figure 3.25 with
equal strain loading. The designation of the large strain consolidation test with
horizontal drainage to periphery is shown in Figure 2.12(f).
Linear displacement transducer
Rigid steel disc
Figure 3.25: Arrangement of Rowe cell for consolidation test with horizontal
drainage to periphery; pore pressure measurement from centre of base of sample
(Head, 1986)
85
The test is described under the following stages: (A) General Preparation, (B)
Fitting Peripheral Drain, and (C) Preparation of Sample.
A.
General Preparation
The cell base is made ready and the ceramic insert, which is situated at the
centre, is prepared for measuring excess pore water pressure. The transducer block,
with valve B and the connection to the pore pressure panel, is fitted on to valve A.
Since only one back pressure system is available, the back pressure system with
volume change gauge is connected to valve F for periphery. The port connecting to
ceramic inserts at the centre should be de-aired. The connection to valve D is not
used. The undisturbed sample is prepared and set up in the Rowe cell.
B.
Fitting Peripheral Drain
1. Cut a strip of the plastic material of width equal to the depth of the cell body, and
about 20 mm longer than its internal circumference. Cut the ends square using a
sharp blade and metal straight-edge.
2. Fit the plastic tightly against the wall of the cell body. Mark the end of the
overlap with a sharp pencil (Figure 3.26).
Figure 3.26: Fitting porous plastic liner in Rowe cell: (a) Initial fitting and marking,
(b) Locating line of cut, (c) Final fitting (Head, 1986)
86
3. Lay the plastic material on a flat surface and mark another line exactly parallel to
the first (i.e. square to the edges) at the following distance outside it (denoted by
x in Figure 3.26): for the 151.4 mm diameter Rowe cell: 3 mm.
4. Make a clean square cut on this line.
5. Fit the plastic in the cell body again, smooth face inwards, and trimmed ends
butting. Allow the additional length to be taken up in the form of a loop opposite
the joint (Figure 3.26).
6. Push the loop outwards and the plastic material will spring against the wall of the
cell. Check that it fits tightly, with no gaps.
7. Immediately before inserting the sample, remove the porous plastic for saturating
and de-airing in boiling water, then replace it in the cell. The inside face of
porous plastic must not be greased, because grease will prevent drainage.
Peripheral drain fitted into the Rowe cell body is shown in Figure 3.27.
Figure 3.27: Peripheral drain fitted into the Rowe cell body
C. Preparation of Sample
With exception of periphery drain and central drain installations, the procedure of
preparing, and setting up the sample in the cell for radial drainage to periphery and to
centre is the same as that of vertical drainage (two-way).
87
1. For ‘equal strain’ test, an impermeable steel disc is placed through the water on
to the soil sample, without entrapping air.
2. Fit and assemble the cell top to the body as described by the procedure for
vertical drainage (two-way).
Details differ from the arrangement for two-way vertical consolidation test in
the following ways:
1. The sample is surrounded by a drainage layer of porous plastic material.
2. The top surface of the sample is covered by an impermeable steel disc.
3. A back pressure system with volume gauge is connected to the rim drain at the
top of the cell.
4. Excess pore water pressure is measured at the base of the sample from the centre.
The pore pressure transducer housing block is connected to valve A which
replaces the blanking plug at that cell outlet (Figure 3.25).
5. The top drainage line is not used.
In this case, the thickness of horizontal consolidating layer is taken as half of
the diameter of the soil sample that is 74.2 mm. With equal strain loading and
sample saturation by applying back pressure, the diaphragm pressure line is the same
as used for the one-way vertical consolidation test. With exception of periphery
vyon porous plastic drain and installation, sample preparation is the same as that of
one-way vertical consolidation test.
3.4.5
Permeability Test
Permeability measurements were carried out on a sample in a Rowe cell with
laminar flow of water in the vertical direction (downwards). The arrangement of the
Rowe cell for the permeability test with vertical drainage is shown in Figure 3.28
while the designation of the permeability test with vertical flow of water downwards
is shown in Figure 3.29.
88
for downward flow (shown)…p1 > p2
flow to open burette
Figure 3.28: Arrangement of Rowe cell for permeability test with downwards
vertical flow (Head, 1986)
Diaphragm
pressure
flow to open burette
Figure 3.29: Downward vertical flow condition for permeability test in Rowe cell
(Head, 1986)
Two independently controlled constant-pressure systems are required for the
permeability test. One system is connected to valve C (Figure 3.26) to provide
pressure on the diaphragm. One back pressure system is connected to valve D, and
valve A is connected to an open burette.
Valve F remains closed during permeability test. The difference between the
inlet and outlet pressures should be appropriate to the vertical permeability of the
soil, and should be determined by trial and error until a reasonable rate of flow is
obtained. The pressures are adjusted to give downward flow.
89
Permeability tests are carried out in Rowe consolidation cell under ‘equal
strain’ conditions of known effective stress, with downward flow of water.
The arrangement of the cell and ancillary equipment is shown in Figure 3.28.
Three independent constant pressure systems are required, one for applying the
vertical stress, the other two on inlet and outlet flow lines but since, only two
independent constant pressure systems are available, valve A at the base of the Rowe
cell is connected to an open burette.
Since saturation by incremental back pressure is to be carried out initially, the
pore pressure transducer housing should be connected to valve A. During the
saturation stage, valve A should remain closed and water admitted to the sample
through valve D as usual. Since only two constant pressure systems are available,
the outlet from the sample is connected to an open burette via valve A whereas; the
inlet to the sample is connected to a back pressure system via valve D. That means
the direction of flow of water in the sample upon consolidation is downwards.
The arrangement shown in Figure 3.29 allows water to flow vertically
through the sample under the application of a differential pressure between the base
and top, while the sample is subjected to a vertical stress from the diaphragm
pressure as in a consolidation test. Since the flow is to an open burette, the outlet
pressure is zero if the free water surface in the burette is maintained at the same level
as the sample face from which the water emerges.
The sample is first consolidated to the required effective stress by the
application of diaphragm loading. Consolidation should be virtually completed, i.e.
the excess pore pressure should be at least 95 % dissipated before starting a
permeability test.
The procedure for permeability test using Rowe cell is as follows:
1. The test is first carried out by adjusting the pressure difference across the sample
to provide a reasonable rate of flow through it. The hydraulic gradient required
to induce flow should be ascertained by trial, starting with equal pressures on the
inlet and outlet lines and progressively increasing the inlet pressure, which must
90
never exceed the diaphragm pressure. Since only one back pressure system is
used, the outlet drainage is connected to an open burette as shown in Figure 3.30.
Figure 3.30: Arrangement for vertical permeability test using one back pressure
system for downward flow (Head, 1986)
2. When a steady rate of flow has been established, measure the time required for a
given volume to pass through. The volume of water is measured from an open
burette incorporated in the outlet of the soil sample via valve A.
3. Calculate the cumulative flow, Q (ml) up to the time of each reading, and plot a
graph of Q against time, t (minutes), as the test proceeds. Continue the test until
it can be seen that a steady rate of flow is reached, i.e. the graph is linear.
4. From the linear part of the graph, measure the slope to calculate the rate of flow,
q (ml/minute); i.e. q = δQ / δt (ml/minute).
5. Since the rate of flow is relatively small, the effect of head losses in the pipelines
and connections can be neglected and the pressure difference across the soil
sample is equal to p1-p2 = ∆p where, p2 = 0 since the free water surface in the
burette is maintained at the same level as the sample face from which the water
emerges.
The vertical coefficient of permeability is calculated from the following
equation:
kv =
qv
qv H
qv H
=
=
60 A i 60 A x102 ∆p 6120 A ∆p
(3.2)
91
where qv = rate of vertical flow (ml/minute),
t = time in minutes,
A = area of sample = 2πrH (mm2),
i = hydraulic gradient = (102 p1-h)/H,
∆p = pressure difference (kPa) = p1-p2,
H = height of sample (mm),
p1 = inlet pressure (kPa),
p2 = outlet pressure (kPa) = (9.81h)/1000,
h = head loss due to the height of water in the burette, and
kv = vertical coefficient of permeability (m/s).
3.4.6
Permeability Test for Horizontal Drainage
The arrangement of Rowe cell for permeability test with horizontal outward
drainage is shown in Figure 3.31. The designation of large strain permeability test
with horizontal outward drainage is shown in is shown in Figure 2.12(f).
Linear displacement
flow to open
Outflow,
Inflow, p1
Rigid steel
Back pressure
for horizontal flow (shown)…p1
Figure 3.31: Arrangement of Rowe cell for permeability test with horizontal
outward drainage (Head, 1986)
92
Permeability test on Rowe cell with horizontal outward drainage was carried
out according to the following steps:
1. The pressure difference across the sample is adjusted to give a reasonable rate of
flow by progressively increasing the inlet pressure without allowing it to reach
the diaphragm pressure.
2. Measure the rate of flow, when a steady state has been achieved.
3. Calculate the horizontal permeability from the equation below:
kh =
qh
qh r
qh r
=
=
60 A i 60 A x102 ∆p 6120 A ∆p
(3.3)
where qh = rate of horizontal flow (ml/minute),
r = radius of sample (mm), and
kh = horizontal coefficient of permeability (m/s).
3.5
Data Analysis
Analysis of the test data was carried out to determine the compressibility
parameters of fibrous peat such as pre-consolidation pressure (σ’c), compression
index (cc), coefficient of compressibility (av), coefficient of volume compressibility
(mv), the beginning of secondary compression (tp), the time of secondary
compression (ts), the rate of secondary compression (cα), representative coefficient of
rate of consolidation (cv), the coefficient of compressibility (av), the coefficient of
volume compressibility (mv), coefficient of permeability (kv). These parameters are
obtained or calculated from time compression curve and consolidation curve
generated from the test data.
93
3.5.1
Time-Compression Curve
The time-compression curves derived from the standard consolidation test on
Oedometer cell were analyzed using Cassagrande (1936) method, while the results of
large strain consolidation test were analyzed using method by Robinson (2003).
Robinson’s method requires the excess pore water pressure-logarithmic of time plot
together with the compression-logarithmic of time plot to develop the compressiondegree of consolidation curve. The coefficient of rate of consolidation (cv) as well as
the beginning and end of secondary compression are among the parameters required
for the analysis of secondary compression. Furthermore, the secondary compression
index (cα) was obtained from the curve.
3.5.2
The e-log p’ Curve
Besides the time-compression curve, a graph relating the void ratio at the end
of each loading stage with the effective pressure on a linear or logarithmic scale was
plotted for a complete set of consolidation test data.
Following the standard
procedure, the void ratio obtained from the standard consolidation test on Oedometer
cell was not corrected with the secondary compression which may occurred after the
completion of excess pore water pressure dissipation. For the results of large strain
consolidation test, the e-log p’ curve was plotted for primary consolidation only. In
this case, the void ratio is corrected by elimination of the creep occurred after the
completion of primary consolidation. The data was obtained from the construction
of the time-compression curve using method by Robinson (2003).
The e-p’ curve is used to obtain coefficient of axial compressibility av and
thus the coefficient of volume compressibility mv, while the e-log p’ is used to obtain
compression index, cc and pre-consolidation pressure (σ’c). These data are required
for evaluation of the magnitude of primary settlement and to obtain the ratio of cα/cc
for calculation of secondary compression.
94
3.5.3
Settlement Analysis
A hypothetical problem of an embankment of 2.5 m high constructed over a 5
m thick deposit of fibrous peat (Figure 3.32) was used for the settlement analysis.
The properties of fibrous peat deposit are based on the data obtained from the test
results. The groundwater table is assumed to coincide with the ground surface. The
embankment is constructed of sand fill over a geotextile layer so that uniform
settlement can be expected. For the ease of calculation, the unit weight of the sand
fill is taken as 20 kN/m3, and the unit weight of water is 10 kN/m3.
Calculation of settlement was made based on the data and the timecompression curve derived from Rowe consolidation test. Robinson (2003) method
was used to interpret the data obtained from the test and is extended for the
calculation of settlement. Calculation of settlement based on cα/cc concept proposed
by Cassagrande was made for comparison purposes.
For this calculation,
Cassagrande’s method was used to analyze the time-compression curve derived from
Oedometer test and Rowe consolidation test.
5m
Ground Level
2m
3.65 m
3.65 m
Proposed Embankment
Fibrous Peat
2m
5m
2,5 m
5m
Figure 3.32: Hypothetical problem for analysis of settlement
CHAPTER 4
GENERAL CHARACTERISTICS
This chapter reports the results of standard laboratory tests carried out on peat
obtained from Kampung Bahru, Pontian, West Johore. The tests were done to
identify the general characteristics of the soil including water content, specific
gravity, and initial void ratio.
Organic content and fiber content are used to
determine the classification of the peat.
The other properties disscused in this
chapter are the fiber orientation, shear strength, initial permeability, and
compressibility obtained from the standard consolidation test on Oedometer cell.
4.1
Physical and Chemical Properties
The preliminary identification of the soil was made based on the index
properties and classification tests conducted on six samples. Index properties include
the determination of water content, specific gravity, bulk unit weight, and the initial
void ratio. The summary of index properties is presented in Table 4.1 while the
results of each index test are presented in Appendix B.
96
Observation made in the location showed that the peat is categorized as deep
peat with thickness of more than 5 m. Groundwater table exists at depth less than 1
m at the time of sampling. Visual identification showed that the peat is dark brown,
very soft, and contains a large amount of fiber. Plant structures such as roots are
easily recognizable from the soil. Long, slender roots, and rootlets are identified as
the remaining of forest vegetation.
The texture is coarse and results in large
permeability.
In-situ measurement of water content was not possible. Thus, sufficient care
was taken during the sampling of the peat in order to maintain the natural water
content.
Three samples were acquired by piston sampler for water content
determination in laboratory.
The average natural water content obtained from
laboratory tests is 608 % which indicates that the peat has a high water-holding
capacity. This value is within the range obtained by previous researchers for peat
soil in West Malaysia (Table 4.1).
Table 4.1: The summary of index properties of peat soil in West Malaysia
Parameters
Index
properties
Natural moisture
content (%)
Specific Gravity
(Gs)
Bulk unit weight
(kN/m3)
Dry unit weight
(kN/m3)
Initial void ratio
(eo)
Acidity
(pH)
Results
from
this study
Published data
(ranges)
608
200 – 700
Huat (2004)
1.47
1.30 – 1.90
Huat (2004)
10.02
8.30 – 11.50 Huat (2004)
1.40
1.00-1.65
8.92
3 – 15
3.24
3.0 – 4.5
Al-Raziqi et al. (2003)
Huat (2004)
Muttalib et al. (1991)
The average specific gravity obtained using kerosene on pycnometer test is
1.47 and it is within the range for fibrous peat (Table 4.1). The samples were taken
below water table; thus it is expected to be in fully saturated condition. As shown in
Figure 4.1, for water content of 608 %, specific gravity of about 1.47. The initial
void ratio is 8.92 which is within the range given by Huat (2004). The average bulk
97
density obtained from this study is a little bit lower than predicted by Huat (2004).
The average unit weight of the peat is 10.02 kN/m3 which give a bulk density of
1.002 Mg/m3 (Figure 4.1). The value is also within the range given in Table 4.1.
The dry unit weight of the peat is 1.40 kN/m3 and it is slightly less than predicted by
Al-Raziqi et al. (2003) based on the natural water content (Figure 4.2).
Present study
Figure 4.1: Correlation of bulk density, water content, specific gravity, and degree
of saturation of fibrous peat (Hobbs, 1986)
1.80
1.60
3
Dry density (Mg/m)
1.40
Peat in West Malaysian
1.20
1.00
Present study
0.80
0.60
0.40
0.20
0.00
0
100
200
300
400
500
600
700
800
900
1000
Natural water content (%)
Figure 4.2: Correlation of dry density and natural water content for West Malaysian
peat (Al-Raziqi et al., 2003)
98
From the information indicated above, the unit weight of the peat in this
study is close to the unit weight of water; hence in-situ effective stress is very small
and the void ratio of the peat is very large. The void ratio also includes the volume
of gas generated during decomposition process. The average void ratio for the
fibrous peat obtained in Pontian is 8.92 and this is within the range given for West
Malaysian peat (3-15) predicted by Huat (2004).
Peat in Malaysian Peninsular is known to have low pH value and the acidity
tends to decrease with depths. The test results showed that the average pH value of
the fibrous peat used in this study is 3.24 which is in the lower side of the range
published for Malaysia peat (3.0-4.5) predicted by Muttalib et al. (1991).
4.2
Classification
The peat in this study was classified based on the degree of humification (von
Post scale) and the organic and the fiber content. The von Post scale is based on the
appearance of soil water that is extruded when a sample of the soil is squeezed in the
hand. When brown water comes out from the soil and the soil left on the hand has a
large amount of fiber, then the peat is classified as fibrous peat with H4 degree of
decomposition according to von Post scale.
The organic content of the peat is found as 97 % which is quite high but still
correlate well with its specific gravity and water content (Figure 4.3 and Figure 4.4).
The loss of ignition or ash content is 3 %. The fiber content of 90 % is considered
very high as compared to published data around the world (Table 4.2) but this is a
typical fibrous peat obtained in West coast of Peninsular Malaysia (Muttalib et al.,
1991 and Huat, 2004). The average percentage of particle passing from 0.063 mm
sieve is 2.37 % which show that the soil contain a large amount of fiber.
99
Present
study
Figure 4.3: The range of organic content of fibrous peat based on specific gravity
(Lechowicz et al., 1996)
120
Organic content (%)
100
Present
study
80
60
Peat in
West Malaysian
40
20
0
0
100
200
300
400
500
600
700
800
Natural water content (%)
Figure 4.4: The range of organic content of fibrous peat based on water content (AlRaziqi et al., 2003)
100
The summary of the classification tests results are presented in Table 4.2
while the results of each test are presented in Appendix B.
Table 4.2: The summary classification test results in West Malaysia peat
Parameters
Classification
4.3
von Post
humification
of peat
Organic
content (%)
Ash
content (%)
Fiber
content (%)
Results from
this study
Published data
(ranges)
von Post
(1922)
H4
H1- H4
97
more than 90
3
less than 10
90
more than 20
Huat
(2004)
Huat
(2004)
Molenkamp
(1994)
Fiber Orientation
Fiber orientation is identified as a dominant factor in the structure of fibrous
peat.
The presence of the fiber induces the natural soil imperfections or
discontinuities such as, fissures, cracks, rootlets, and pockets of organic material
which may results in the high permeability of the soil.
The application of
consolidation pressure may induce a rearrangement of fiber orientation and
drastically reduces the void, causing a significant reduction in the vertical
permeability.
Even though most of the features of anisotropy of the fibrous peat are visible
to the naked eye, a more detailed analysis on the microstructure of the fiber and the
fiber content can be examined under a Scanning Electron Microscope (SEM). The
examination is important because previous researcher have shown that the fiber
content appears to be a major compositional factor in determining the way in which
peaty soils behave (Dhowian and Edil, 1980).
The samples were cut in vertical and horizontal sections to enable the
observation of the rearrangement of the fiber due to consolidation pressure. Figure
101
4.5 and 4.6 show the typical fiber orientation obtained by Scanning Electron
Microscope for the fibrous peat obtained from Kampung Bahru, Pontian, West
Johore, at initial state and under consolidation pressure of 200 kPa. Comparison of
the two sets of microphotographs shows obvious structural anisotropy for the fibrous
peat in which the fiber is more oriented in the vertical direction.
Individual
microstructures may have been destroyed by breaking and squeezing during
compression under high-stress conditions. This implies that for the fibrous peat, the
initial vertical rates of permeability is larger than its respective horizontal rates of
permeability but the situation changes remarkably under appreciation of
compression. The results of Scanning Electron Microphotograph of fibrous peat
samples under various consolidation pressure and sections are given in Appendix C.
4.4
Shear Strength
The in-situ shear strength was obtained by field vane on six locations at depth
of 1 and 2 m. A small size vane of diameter 65 mm and slow torque (0.1 mm/sec)
were selected in order to minimize the effect of fiber in the measured undrained
shear strength of peat (cu). The initial undrained shear strength of peat obtained by
the field vane shear test is 10.10 kPa which is comparable to the undrained shear
strength of peat obtained in Sarawak (Huat, 2004). The peat is identified as very
sensitive to disturbance with sensitivity of 5.64, which is also with in the range give
by Huat 2004 (2-11).
The laboratory evaluation of shear strength was made by shear box test on
twelve samples. The shear box test is chosen because it is suitable for evaluating the
drained shear strength of fibrous peat, even-though the high friction angle obtained
from the test might not be an indication of the real strength of the soil. The results
showed an average effective cohesion (c’) of 3.10 kPa, and average effective angle of
internal friction (φ’) equal to 25.4o. The cohesion value is slightly lower compared to
the published data on peat in West Malaysia (Huat, 2004). The result of the shear
box test is shown in Figure 4.7. The detailed results of field vane shear as well as
shear box test for each sample are given in Appendix D.
102
(a)
(b)
Figure 4.5: The Scanning Electron Microphotographs (SEM) of fibrous peat samples
at initial state (a) horizontal section x 400, (b) vertical section x 400
(a)
(b)
Figure 4.6: The Scanning Electron Microphotographs (SEM) of fibrous peat samples
under consolidation pressure of 200 kPa (a) horizontal section x 400 (b) vertical
section x 400
103
30
Test 1
Test 3
Test 5
Test 7
Test 9
Test 11
Average
Shear strength (kPa)
25
20
Test 2
Test 4
Test 6
Test 8
Test 10
Test 12
15
φ’ = 25.42o ± 1.97
10
5
c = 3.10 kPa ± 1.06
0
0
5
10
15
20
25
30
Normal stress (kPa)
Figure 4.7: Results of the shear box test
4.5
Initial Permeability
The initial permeability of the soil is observed through constant head
permeability test. The samples for the test were obtained by piston sampler. The
sample was transferred directly to the permeameter for the test to ensure minimum
disturbance. The purpose of the tests was to determine the initial rate of permeability
of the soil.
The test results revealed that at initial state, the average vertical
coefficient of permeability of the soil at standard temperature of 20°C, kv (20°) is
1.20 x 10-4 m/s, hence the soil can be classified as medium permeability or the soil
has a good drainage characteristic.
The relationship between the initial coefficient of permeability of the soil at
standard temperature (kv 20°C) and it’s initial void ratio (eo) is plotted with typical
range of data obtained by previous research in Figure 4.8. It can be observed from
Figure 4.8 that the fibrous peat samples have high initial void ratios with the void
104
ratios range from 4 to 12 and permeability range from 5.07 x 10-10 m/s to 2.19 x 10-4
m/s. This shows that the fibrous peat is as porous as clean sand. Figure 4.8 shows
that the initial vertical permeability of the fibrous peat is within the range observed
by other researchers (Hanrahan, 1954; Lea and Browner, 1963; Mesri and Olson,
1971). The detailed results of initial permeability test for each sample are given in
Appendix E.
Figure 4.8: Effect of initial void ratio (eo) on the initial permeability of soil (Hobbs,
1986)
4.6
Compressibility
Twelve sets of the standard consolidation test were conducted on Oedometer
cell according to the standard procedure outlined in BS 1377 Part 5. The test was
carried out to establish the range of stress to use in large strain consolidation test and
to establish the preliminary estimation of the possible response of the peat to loading.
Data acquired from Oedometer test is also used for comparison with the results of
large strain consolidation test on Rowe cell. Each sample has a thickness of 20 mm,
105
a diameter of 50 mm, and was subjected to consolidation pressures with load
increment ratio (LIR) of 1. The pressures applied to the soil sample are 25 kPa, 50
kPa, 100 kPa, 200 kPa, and 400 kPa. Each pressure is maintained for one week or
10,000 minutes to enable observation of secondary compression behavior. During
this time, deformation of specimen was observed in specified time (e.g. ¼, ½, 1, 2, 4,
8, 15, 30, 60, 120, 240, 480, 1440, 2880, 4320, 5760, 7200, 8640, 10080 minutes).
The results were presented in term of time-compression curve and the e-log p’ curve
and discussed in the following sections accordingly.
4.6.1
Analysis of Time-Compression Curve
Typical logarithmic of time-compression curve derived from the standard
consolidation test is shown in Figure 4.9. The figure shows that the plot of timecompression data resembles the Type II curve (Figure 2.10) in which the secondary
compression varies non-linearly with time and tertiary compression was observed for
all ranges of consolidation pressure. It can be observed from the figure that the
primary consolidation is still dominant in the compression of the peat, but the
consolidation occur in a relatively shorter time as compared to clay. Secondary
compression, even though less significant than the primary consolidation in term of
magnitude, could be very important in term of the design life of a structure. Tertiary
compression was observed from the test results, but may not be very significant in
term of the design life of a structure because as shown in Figure 4.9, the secondary
compression takes a significant amount of time.
106
0
25 kPa
1
50 kPa
100kPa
2
200 kPa
400 kPa
3
4
Compression (mm)
5
6
7
8
9
10
11
12
13
14
0.1
1
10
100
1000
10000
100000
Time, t in minutes (log scale)
Figure 4.9: Typical compression versus logarithmic of time curves from Oedometer
test
Cassagrade’s method was used to evaluate the time-compression curves to
obtain the end of primary consolidation (t100 = tp), the coefficient of rate of
consolidation (cv), the coefficient of secondary compression (cα), and end of
secondary compression (ts). Based on Cassagrande’s method, described in Section
2.2.1 the time of the completion of primary consolidation was identified from the
time compression curve as the time where the curve shows a maximum curvature
(Figure 2.7). Extension of Cassagrande method proposed by Edil and Dhowian
(1980) was used to determine the completion of the secondary compression as the
107
time where the curve shows a sharp change in the slope. The coefficient of rate of
consolidation (cv) and the coefficient of secondary compression (cα) were determined
based on Cassagrande’s method.
Analysis on typical compression-time curve is shown in Figure 4.10 while the
results for all data are summarized in Table 4.3.
0.0
Compression (mm)
0.5
1.0
ts
cα
1.5
2.0
tp
2.5
3.0
0.1
1
10
100
1000
10000
Time, t in minutes (log scale)
Figure 4.10: Analysis of the compression-time curves from Oedometer test
Table 4.3: Compressibility parameters obtained from consolidation curves
Consolidation
End of
Coefficient of
Coefficient of
End of
pressure
primary
rate of
secondary
secondary
consolidation
consolidation
compression
compression
(p’, kPa)
(t100 = tp, minutes)
(cv, m2/year)
(cα)
(ts, minutes)
25
38.50
2.074
0.147
3633
50
32.75
1.646
0.166
3350
100
28.83
1.355
0.173
3117
200
25.58
1.085
0.114
2958
400
22.17
0.850
0.146
2717
108
As indicated in Table 4.3, the end of primary and secondary compression and
the coefficient of rate of consolidation vary with the consolidation pressure. On the
other hand, no trend can be observed for the relationship between the coefficients of
secondary compression with consolidation pressure. The variation of the end of
primary and secondary compression and the coefficient of rate of consolidation
varies with the consolidation pressure are presented in Figures 4.11, 4.12, and 4.13
respectively. The figures show a clear indication that these parameters decrease nonlinearly with increasing consolidation pressure.
70
60
T est 1
T est 2
50
T est 3
tp (minutes)
T est 4
T est 5
40
T est 6
T est 7
T est 8
30
T est 9
T est 10
T est 11
20
T est 12
Average
10
0
0
100
200
300
400
500
600
Consolidation Pressure (p',kPa)
Figure 4.11: Variation of the time of completion of primary consolidation with
consolidation pressure
109
6000
Test 1
5000
Test 2
Test 3
ts (minutes)
Test 4
Test 5
4000
Test 6
Test 7
Test 8
Test 9
3000
Test 10
Test 11
Test 12
Average
2000
1000
0
100
200
300
400
500
600
Consolidation Pressure (p',kPa)
Figure 4.12: Variation of the time of completion of secondary compression versus
consolidation pressure
4.0
3.5
Test 1
Test 2
Test 3
Test 4
Test 5
Test 6
Test 7
Test 8
Test 9
Test 10
Test 11
Test 12
average
2
cv (m /year)
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0
100
200
300
400
500
600
Consolidation Pressure (p',kPa)
Figure 4.13: Variation of the coefficient of rate of consolidation with consolidation
pressure
110
Figure 4.14 shows the variation of the coefficient of secondary compression
with consolidation pressure and the figure indicates that there is no trend of the
coefficient with consolidation pressure. Since there is no trend observed for the
coefficient of secondary compression with the consolidation pressure, then the values
are averaged over the whole range of consolidation pressure. The analysis yields the
range coefficient of secondary compression from 0.017 to 0.368 with an average of
0.149.
0.40
Test 1
Test 2
Test 3
Test 4
Test 5
Test 6
Test 7
Test 8
Test 9
Test 10
Test 11
Test 12
Average
0.35
0.30
Cα
0.25
0.20
0.15
0.10
0.05
0.00
0
100
200
300
400
500
600
Consolidation Pressure (P', kPa)
Figure 4.14: Variation of the coefficient of secondary compression with
consolidation pressure
4.6.2
Analysis of the e-log p’ Curve
As stated in Chapter 2, primary consolidation settlement occurs when the
applied stress and subsequently the excess pore pressure had caused the water to
dissipate from the voids in saturated soils. Parameter such as pre-consolidation
pressure (σ’c) and compression index (cc) can be determined from the e-log p’ curve,
while the coefficient of compressibility (av) and the coefficient of volume
compressibility (mv) can be obtained from e-p’ curve.
111
The e-log p’ curve derived from the standard consolidation test on Oedometer
cell was constructed for raw data in which the total compression was considered.
Figure 4.15 shows the e-log p’ curve plotted for all sets of data obtained from the
standard consolidation test on Oedometer cell.
12
11
Slope = cc
10
Void Ratio (e)
9
8
7
6
5
4
Test 1
Test 5
Test 9
3
Test 2
Test 6
Test 10
Test 3
Test 7
Test 11
Test 4
Test 8
Test 12
2
10
100
1000
Consolidation Pressure (p', kPa)
Figure 4.15: The e-log p curves obtained from the standard consolidation test on
Oedometer cell
It can be seen from the figure that all curve show the same shape but different
initial void ratio. The difference in the initial void ratio may be due to the time of
test execution, the earlier the test was conducted the higher the initial void ratio was.
The curves do not show a clear indication of pre-consolidation pressure.
112
Even though not very clear from the curves in Figure 4.15, the analysis of the
e-log p’ curve has shown that the pre-consolidation pressure obtained from
Oedometer test is about 45 kPa. This value is higher than estimated by equation σ’p
= 162 eo
-0.988
kPa proposed by Kogure and Ohira (1977) which yields in a pre-
consolidation pressure of less than 20 (Figure 4.16). This indicated that the peat
deposit may have been overconsolidated.
The compression index cc obtained from the test is 3.253 ± 1.121. This
compression index is much lower than estimated by the simple correlation cc =
ωo/100 and c c =
ω o 1.07
128
proposed by Kogure and Ohira (1977) which results in a
compression index of about 6 and 7 respectively (Figure 4.17).
Present
Study
Figure 4.16: Relationship between pre-consolidation pressure and in-situ void ratio
(Kogure and Ohira, 1977)
113
Present study
Figure 4.17: Relationship between compression index and natural water content
(Kogure and Ohira, 1977)
The initial void ratio obtained from the standard consolidation test on
Oedometer cell is 9.934 ± 1.426 which is slightly higher than the natural void ratio
calculated from the water content and the specific gravity of the soil (8.925).
Table 4.4 shows the variation of the average coefficient of volume
compressibility from the standard consolidation test on Oedometer cell with
consolidation pressure.
Table 4.4 indicates that the coefficient of volume
compressibility decreases significantly at the lower range of pressure but the effect is
decreasing for large pressure. This trend is in agreement with the consolidation
theory.
114
Table 4.4: The average coefficient of volume compressibility
4.6.3
Consolidation pressure
Coefficient of volume compressibility
(p’, kPa)
(mv, 1/kPa)
25
0.00555
50
0.00274
100
0.00171
200
0.00035
400
0.00026
Coefficient of Permeability based on the Standard Consolidation Test
Coefficient of permeability can be evaluated based on the coefficient of
consolidation obtained from consolidation test through Equation 2.4. The coefficient
of permeability for the range of consolidation pressure used in the test is in Table 4.5.
The data shown in Table 4.5 indicated that the application of consolidation pressure
has the effect of decreasing the coefficient of permeability of fibrous peat. All test
suggested a significant decrease in permeability as consolidation pressure increases.
Table 4.5: Average coefficient of permeability for each consolidation pressure
Consolidation pressure
Coefficient of permeability
(p’, kPa)
(kv , m/s)
25
3.69092x10-10
50
1.43629 x10-10
100
4.87719 x10-11
200
1.14917 x10-11
400
6.53644 x10-12
115
4.6.4 Summary
The summary of the consolidation parameters obtained from twelve
Oedometer test results including the coefficient of volume compressibility (mv), end
of primary consolidation (tp), end of secondary compression (ts), rate of consolidation
(cv), coefficient of secondary compression (cα), and coefficient of permeability (kv)
as a function of consolidation pressure is presented in Table 4.6.
Table 4.6: The summary of data obtained from Oedometer test
Consolidation
pressure
Consolidation Parameters
mv
tp
cv
cα
cc
ts
kv
(x10-10)
(p’, kPa)
(1/kPa)
(minutes)
(m2/year)
25
0.00555
38.50
2.074
50
0.00274
32.75
100
0.00171
200
400
(minutes)
(m/s)
0.147
3633
3.69092
1.646
0.166
3350
1.43629
28.83
1.355
0.173
3117
0.48772
0.00035
25.58
1.085
0.114
2958
0.11492
0.00026
22.17
0.850
0.146
2717
0.06537
3.253
CHAPTER 5
COMPRESSIBILITY CHARACTERISTICS
5.1
Introduction
This chapter presents the data obtained from analysis of the results of large
strain consolidation test on Rowe cell, comparison of the data with data obtained
from the standard consolidation test on Oedometer cell, and the data obtained from
researches carried out in the past. The presentation is divided into three sections.
Section 5.2 presents the results of large strain consolidation test on Rowe cell and
analysis of compressibility characteristics data obtained from the test. Section 5.3
discusses the comparison of the data with that obtained from the standard
consolidation test. Comparisons of the data from large strain consolidation test
(Rowe cell) with published data are deliberated in Section 5.4. The results and
comparisons are presented and discussed in terms of time-compression curve and
consolidation curves.
The chapter also discusses the results in terms of the effect of the fiber
orientation on the compressibility characteristics of fibrous peat (Section 5.5) and
illustrates the application of the results of the calculation of settlement of peat deposit
under embankment loads based on a hypothetical problem (Section 5.6). Section 5.7
presents the discussion of the findings.
117
5.2
Test Results and Analysis
Large strain consolidation tests (Rowe cell) were conducted on six soil
samples obtained from Kampung Bahru, Pontian, West Johore. The sample was
obtained using block sampling method and maximum care was taken during the
sampling process, transportation of the sample, and the storage. The test was carried
out following the standard procedure outlined by Head (1986). For this test, the
sample was placed on a Rowe consolidation cell with diameter of 151.4 mm and
height of 50 mm and subjected to a hydraulic pressure with load increment ratio
(LIR) of one. Pressure increments of 25, 50, 100, and 200 kPa were applied during
the test. Drainages are allowed from top and bottom boundaries through porous
stones. Deformation and excess pore water pressure were observed by GDSLAB v
2.0.6 program (Section 3.4) and the subsequent increment was applied after the
completion of excess pore water pressure dissipation indicated by the pore pressure
reading.
As for the standard consolidation test, the results of large strain
consolidation test are presented in terms of time-compression curve and the e-log p’
curve and discussed in the following sections accordingly. Complete data on the
results of large strain consolidation test is given in Appendix H.
5.2.1 Analysis of Time-Compression Curve
The logarithmic of time versus compression curve obtained from large strain
consolidation test (Rowe cell) on six samples are shown in Figure 5.1 a to f
representing test 1 to test 6 respectively. Tests 1 to 3 show almost identical curves,
while Test 4, 5, and 6 shows some deviation from the previous tests in terms of
compression. This may be caused by the change in the natural moisture content.
Despite of these deviations, the curve shows a similar shape indicating the
compressibility of the fibrous peat consists of primary consolidation and secondary
compression. The primary consolidation is still dominant in the compression of the
peat, but the consolidation occurs rapidly. The secondary compression is non linear
with time and this condition was observed from the curve at all ranges of
consolidation pressure. The secondary compression, even though less significant
118
than the primary consolidation in term of magnitude, could be very important in term
of the design life of a structure.
The shape of the compression curve resembles the Type II curve (Leonards
and Girault, 1961), which is typical of compression of peat soil. The shape of the
time-compression curve indicates that deformation process of fibrous peat deviates
from the simple model used in Terzaghi’s consolidation equation, which is the basis
for the Cassagrande and Taylor’s evaluations of primary consolidation and the
estimation of the coefficient of rate of consolidation. The time-compression curves
did not give a clear indication of an inflection point where the primary consolidation
is assumed to end and the secondary compression is assumed to start. As shown in
Figure 5.1, the secondary compression may have started during the process of excess
pore water pressure dissipation.
The figure also suggested that the secondary
0
0
10
10
Compression (mm)
Compression (mm)
compression does not occur at a constant rate.
20
30
20
30
40
40
0.1
1
10
100
1000
10000
Time, t in minutes (log scale)
(a. Test 1)
0.1
1
10
100
1000
10000
Time, t in minutes (log scale)
(b. Test 2)
Figure 5.1: The compression versus logarithmic of time curve obtained from large
strain consolidation tests on Rowe cell
0
0
10
10
Compression (mm)
Compression (mm)
119
20
20
30
30
40
40
0.1
1
10
100
1000
0.1
10000
1
100
1000
10000
Time, t in minutes (log scale)
Time, t in minutes (log scale)
(c. Test 3)
(d. Test 4)
0
0
10
10
Compression (mm)
Compression (mm)
10
20
30
20
30
40
40
0.1
1
10
100
1000
10000
0.1
Time, t in minutes (log scale)
(e. Test 5)
1
10
100
1000
10000
Time, t in minutes (log scale)
(f. Test 6)
Figure 5.1 (Cont’): The compression versus logarithmic of time curve obtained
from large strain consolidation tests on Rowe cell
120
The large strain consolidation test on Rowe cell allows the continuous
measurement of the excess pore water pressure. Thus, the time-excess pore pressure
dissipation curve could be used to indicate the end of primary consolidation. A
typical time compression curve obtained based on the result of Test 4 under the range
of consolidation pressure is presented in Figure 5.2, while the corresponding excess
pore water pressure measurement for the test is given in Figure 5.3.
Robinson (1999) identified that the time of the completion of primary
consolidation can be easily identified from the excess pore water pressure curve.
Figure 5.3 shows that the completion of excess pore water pressure dissipation (t100)
is about 30 minutes after the start of the test while observation on Figure 5.2 shows
unclear inflection at two points before and after the completion of excess pore water
pressure dissipation. Robinson (1997) suggested on his research on fibrous peat that
the secondary compression of the peat is actually started before the completion of the
dissipation of excess pore water pressure; hence the earlier point may indicate the
start of the secondary compression. He suggested that the beginning of secondary
compression can be identified at the time when the compression-degree of
consolidation curve deviates from a straight line. As explained in Section 2.2, the
primary consolidation is linearly correlated with the degree of consolidation, thus the
degree of primary consolidation where the curve deviates from a straight line is
identified as the beginning of secondary compression. The primary consolidation
and secondary compression occurred beyond this point should be separated to form a
primary consolidation and secondary compression curves.
The primary
consolidation curve was used for the evaluation of the coefficient of rate of
consolidation (cv), while the secondary compression part was used for evaluation of
the coefficient of secondary compression (cα). Figure 5.4 shows the relationship
between the degree of consolidation where the secondary compression is actually
started. This figure is plotted based on Figure 5.2 and Figure 5.3, and is useful for
separating the primary consolidation from the secondary compression.
121
0
Compression (mm)
2
4
6
25 kPa
50 kPa
100 kPa
8
200 kPa
10
0.1
1
10
100
1000
Time, t in minutes (log scale)
Figure 5.2: Compression versus logarithmic of time curves for Test 4
0
Dissipation of excess pore water pressure, Uv (%)
10
20
30
40
50
60
25 kPa
50 kPa
70
100kPa
80
200 kPa
90
100
0.1
1
10
100
1000
Time, t in minutes (log scale)
Figure 5.3: Excess pore water pressure versus logarithmic of time curves for Test 4
122
0.00
Primary
consolidation
0.05
Compression (mm)
0.10
Uv
0.15
0.20
0.25
δs
0.30
Secondary
compression
0.35
cα
0.40
0
10
20
30
40
50
60
70
80
90
100
Dissipation of excess pore water pressure, Uv (%)
Figure 5.4: Typical compression versus degree of consolidation curve from large
strain consolidation test with two-way vertical drainage
Robinson (2003) method was used for the more accurate analysis of timecompression curve. The method was used for the evaluation of the time-compression
curves to obtain the completion of excess pore water pressure dissipation (t100), the
beginning of secondary compression (tp), the coefficient of rate of consolidation (cv),
and the coefficient of secondary compression (cα). The Robinson (2003) method is
described in Section 2.3, while the analysis for a typical set of data is given in
Appendix H.
Table 5.1 summarizes the results of the analysis in terms of the time needed
for the dissipation of excess pore water pressure (t100) and the beginning of secondary
compression (tp) over the range of consolidation pressure used in this research.
123
Table 5.1: Average time for end of primary consolidation (t100) and the beginning of
secondary compression (tp) obtained from Rowe test results
Consolidation
End of
The beginning
pressure
primary
of secondary
consolidation
compression
(p’, kPa)
(t100, minutes)
(tp, minute)
(U, %)
25
27.67
19.83
61.67
50
25.83
17.67
65.00
100
23.50
15.83
69.00
200
23.00
14.33
70.50
Degree of
consolidation
Figure 5.5 and 5.6 shows the variation of the end of primary consolidation
and the beginning of secondary compression with consolidation pressure
respectively.
The curves show a clear indication that the end of primary
consolidation decreases non-linearly with increasing consolidation pressure. The
higher the consolidation pressure, the faster the dissipation of excess pore water
pressure, and the shorter the time needed for primary consolidation. The beginning
of the secondary compression also decreases with increasing consolidation pressure
but the degree of consolidation where the secondary compression started increases
with consolidation pressure.
The average coefficient of rate of consolidation for each pressure obtained
from large strain consolidation tests is evaluated using the first part of Figure 5.4 and
the results are shown in Table 5.2. Figure 5.7 shows the variation of the coefficient
of consolidation obtained from the tests with consolidation pressure.
It is clear from Figure 5.7 that the coefficient of rate of consolidation
decreases almost linearly with increasing consolidation pressure. This finding is in
agreement with the theory of consolidation, which stated that the coefficient of rate
of consolidation decreases with increasing consolidation pressure (Holtz and Kovacs,
1981).
124
40
30
T est 1
t100 (minutes)
T est 2
T est 3
T est 4
20
T est 5
T est 6
Average
10
0
0
100
200
300
400
Consolidation Pressure (p', kPa)
Figure 5.5: Average time of completion of primary consolidation versus
consolidation pressure
40
30
T est 1
tp (minutes)
T est 2
T est 3
T est 4
20
T est 5
T est 6
Average
10
0
0
100
200
300
400
Consolidation Pressure (p', kPa)
Figure 5.6: Variation of the beginning of secondary compression with consolidation
pressure for sample tested under vertical consolidation
125
Table 5.2: Average coefficient of rate of consolidation for each pressure
Consolidation pressure
Coefficient of rate of consolidation
(p’, kPa)
(cv , m2/year)
25
5.689
50
4.947
100
4.179
200
3.259
10
8
T est 1
T est 3
T est 4
T est 5
2
cv (m /year)
T est 2
6
T est 6
4
Average
2
0
10
100
1000
Consolidation Pressure (p',kPa)
Figure 5.7: Variation coefficient of rate of consolidation with consolidation pressure
The coefficient of secondary compression is evaluated from the second part
of Figure 5.4. The average coefficient of secondary compression corresponding to
each pressure is given in Table 5.3 while Figure 5.8 shows the variation of the
coefficient of secondary compression versus consolidation pressure. Table 5.3 and
Figure 5.8 indicate that the coefficient of secondary compression increases with
126
increasing consolidation pressure. This is not in agreement with the popular theory
which suggested that the coefficient of secondary compression is constant (Holtz and
Kovacs, 1981; and Mesri and Godlewski, 1977). However some researchers such as
Lea and Browner (1963) and Fox et al. (1992) suggested that the coefficient of
secondary compression have some correlations with the consolidation pressure.
Table 5.3: Average coefficient of secondary compression
Consolidation pressure
Coefficient of secondary compression
(p’, kPa)
(cα)
25
0.109
50
0.124
100
0.157
200
0.211
0.40
0.30
T est 1
T est 2
cα
T est 3
0.20
T est 4
T est 5
T est 6
Average
0.10
0.00
0
100
200
300
400
Consolidation Pressure (p', kPa)
Figure 5.8: Variation coefficient of secondary compression versus consolidation
pressure
127
Figure 5.4 also shows a sharp change in the slope of time-compression curve
at some time after the beginning of secondary compression. The compression of soil
after this point is often referred as the tertiary compression. The time where this
point exists is known as the time of secondary compression (ts) and the value
calculated for the results of consolidation test on Rowe cell is presented in Table 5.4.
It can be seen from Table 5.4 that the time of secondary compression decreases with
increasing consolidation pressure, which mean that the consolidation pressure have
an effect of reducing the time of secondary compression stage.
Table 5.4: Average time of secondary compression
5.2.2
Consolidation pressure
The time of secondary compression
(p’, kPa)
(ts,minutes)
25
1216.67
50
1066.67
100
875.00
200
750.00
Analysis of the e-log p’ Curve
Peat is sensitive to the effect of disturbance, which can influence the
relationship between void ratio and pressure (e-p’ and e-log p’ curve) derived from
large strain consolidation test on Rowe cell. Hence the need for extreme care in the
preparation of test specimen. Analysis of the e-p’ and e-log p’ curve were performed
to obtain the pre-consolidation pressure, compression index, coefficient of
compressibility, and the coefficient of volume compressibility.
The e-log p’ curve derived from large strain consolidation test on Rowe cell
was constructed for raw data in which the compression is due to primary and
secondary settlement and for the case where secondary settlement was excluded from
analysis (Figure 5.9). It can be seen from the figure that there is a difference in
128
settlement, and the slope of the line for primary consolidation is slightly milder than
that obtained from total compression, resulting in the lower cc value.
9
Void ratio (e)
8
Primary only
Total
7
6
5
4
0
50
100
150
200
250
Consolidation pressure (p', kPa)
(a)
9
Void ratio (e)
8
Primary only
Total
7
6
5
4
10
100
1000
Consolidation pressure (p', kPa)
(b)
Figure 5.9: The consolidation curve from large strain consolidation test on Rowe
cell based on primary and total settlement (a) typical e-p’ curve, (b) typical e-log p’
curve
129
Figure 5.10 shows the relationship between void ratios and the logarithmic of
the consolidation pressure curves based on the primary settlement only for the data
obtained from the results of large strain consolidation test (Rowe cell). The curves
give similar slope of compression index except Test 2 and Test 3 which is slightly
deviate from the common trend.
Figure 5.10 suggests that the curve shows a
different initial void ratio for each test in which the initial void ratio evaluated from
test 1 is higher than test 6. The reduction in the initial void ratio from the tests may
be due to the change in natural moisture content which relates to time. The large
strain consolidation test was done on one cell only, so the test should be carried out
subsequently. Even though the samples was kept in a constant temperature, the
waiting time for testing may results in the change of natural moisture content and
biodegradation of the fiber (Mesri et al., 1997), therefore the change in the initial
void ratio.
10
9
Slope = cc
Void Ratio (e)
8
7
6
Test 1
Test 2
Test 3
5
Test 4
Test 5
Test 6
4
10
100
1000
Consolidation Pressure (p', kPa)
Figure 5.10: The void ratio versus logarithmic of consolidation pressure curve of
large strain consolidation test on Rowe cell based on primary settlement
130
The void ratio obtained from the results of large strain consolidation test
(Rowe cell) is about 8.854 ± 0.291 which is within the ranges of 3 to 15 suggested by
Huat (2004) for fibrous peat in West Coast of Malaysian Peninsular. The average
compression index (cc) obtained from the set of data is 3.128 ± 0.037 which is much
lower than predicted based on natural moisture content (Figure 4.17).
It is necessary to estimate the pre-consolidation pressure because
consolidation settlement will not usually be great when the applied load remains
below the pre-consolidation pressure. Even though not very clear from the curves,
the pre-consolidation pressure obtained from the results of large strain consolidation
test is about 41 kPa. As mentioned in section 4, this value is higher than the preconsolidation pressure predicted from formula proposed by Kogure and Ohira (1977)
which gives a pre-consolidation pressure of less than 20 (Figure 4.16).
The coefficient of volume compressibility (mv) can be determined based on
consolidation (e-p’) curve (Figure 5.11). This parameter is very useful to estimate
the primary consolidation settlement. Table 5.5 shows the average coefficient of
volume compressibility obtained from large strain consolidation tests. A curve of
coefficient of volume compressibility versus consolidation pressure was plotted as
shown in Figure 5.11. Table 5.5 and Figure 5.11 indicate that the coefficient of
volume compressibility decreases as the consolidation pressure increases.
Table 5.5: The average coefficient of volume compressibility
Consolidation
Coefficient of volume
pressure
compressibility
(p’, kPa)
(mv, 1/kPa)
25
0.00049
50
0.00171
100
0.00121
200
0.00073
131
3.5E-03
mv(1/kPa)
3.0E-03
2.5E-03
Test 1
2.0E-03
Test 2
Test 3
Test 4
1.5E-03
Test 5
1.0E-03
Test 6
Average
5.0E-04
0.0E+00
0
100
200
300
400
Consolidation Pressure (p',kPa)
Figure 5.11: Variation of coefficient of volume compressibility versus consolidation
pressure
It should be noted that lower value was obtained for the coefficient of volume
compressibility (mv) under consolidation pressure of 25 kPa due to swelling of the
soil sample because this consolidation pressure is actually lower than the preconsolidation pressure (41kPa).
5.2.3
Evaluation of Permeability
It has been mentioned in the previous part that the rate of consolidation is a
function of permeability of the soil. Thus, the coefficient of permeability can be
indirectly evaluated based on the coefficient of rate of consolidation, the coefficient
of volume compressibility and the unit weight of water. Equation 2.11 was used for
the calculation of the coefficient of permeability.
The vertical coefficient of
permeability for the range of consolidation pressure used in the study is summarized
in Table 5.6.
The data shown in Table 5.6 indicated that the application of
consolidation pressure has the effect of decreasing the coefficient of permeability of
fibrous peat.
Table 5.6 suggests a significant decrease in the coefficient of
permeability with increasing consolidation pressure. Note that the calculation of
132
coefficient permeability for consolidation pressure of 25 kPa is not included here due
to the variation in the value of the coefficient of volume compressibility (mv) as
mentioned in Section 5.2.2.
Table 5.6: Vertical coefficient of permeability based on large strain consolidation
test
5.2.4
Consolidation pressure
Vertical coefficient of permeability
(p’, kPa)
(kv, m/s)
50
3.09893x10-10
100
1.76332x10-10
200
7.51869x10-11
Summary of Test Results
The summary of the data obtained from the large strain consolidation tests
results are presented in Table 5.7. The results show that the time of the completion
of primary consolidation (t100), the time for the beginning of secondary compression
(tp), the time of secondary compression (ts), coefficient of rate of consolidation (cv),
the coefficient of volume compressibility (mv), the coefficient of permeability (kv),
and the coefficient of secondary compression (cα) decreases as the consolidation
pressure increases.
Table 5.7: The summary of large strain consolidation data
Consoli-
Consolidation Parameters
dation
pressure
(p’,
t100
tp
ts
cv
2
(minutes) (minutes) (minutes) (m /year)
cα
cc
kPa)
25
27.67
19.83
1216.67
5.689
0.109
50
25.83
17.67
1066.67
4.947
0.124
100
23.50
15.83
875.00
4.179
0.157
200
23.00
14.33
750.00
3.259
0.211
mv
(1/kPa)
kv
(x10-11)
(m/s)
0.00049 8.97302
3.128
0.00171 0.30989
0.00121 0.17633
0.00073 7.51869
133
5.3
Comparison with Oedometer Data
The analysis of consolidation curve from the large strain consolidation test
(Rowe cell) is compared with the data obtained from the standard consolidation test
on Oedometer cell. It should be noted here that the analysis of the consolidation
curve for the large strain consolidation test was analyzed with Robinson’s (2003)
method, while for the standard consolidation test on Oedometer cell data was
analyzed with Cassagrande’s (1963) method. Figure 5.12 shows the typical strain
versus logarithmic of time curve from consolidation test on Rowe cell and
Oedometer cell under consolidation pressure of 50 kPa. It can be seen from the
figure that the time-strain curves obtained from large strain consolidation test is
similar in shape with the result of the standard consolidation test on Oedometer cell,
except that the secondary compression appear to vary non linear with time.
0
2
Strain (%)
4
6
8
Rowe cell
Standard Oedometer
Oedometer (corrected)
10
12
0.1
1
10
100
1000
10000
Time, t in minutes (log scale)
Figure 5.12: The typical strain versus logarithmic of time curve from Rowe cell and
Oedometer test
134
It can be observed from Figure 5.12 that the curve shows different
displacement reading for the beginning of strain between Rowe cell and Oedometer
test. Head (1981) mentioned that initial reading of Oedometer test is mainly due to
the shock occurred when placing load on hanger, deformation of the apparatus
related to the elasticity of the frame, and bedding effect on contact surfaces. The
different strain between Rowe cell and Oedometer test may also be caused by the
effect of side friction between the specimen and the ring as well as the change in
initial moisture content. Besides, initially the soil from Oedometer test was not
completely saturated which may result in sudden decrease in volume on saturation
due to collapse of the grain structure and the small size of the specimen. Thus, the
initial reading for Oedometer data does not show a displacement equal to zero. This
deformation added to the settlement of the specimen, hence the results in unaccurate
prediction of the compression in index (cc) and the coefficient of volume
compressibility (mv).
Figure 5.12 also shows a corrected strain from the data
obtained from Oedometer test with respect to initial reading. This error is eliminated
when consolidation test is carried on Rowe cell because the load is applied as
hydraulic pressure.
The comparison of the corrected curve with the data obtained from large
strain consolidation test on Rowe cell still suggested that larger strain was obtained
from Oedometer test data. This suggests that the effect of the specimen size and
change in fiber texture is as significant as the effect of shock at the beginning of load
application.
The comparison between void ratio versus consolidation pressure (e-p’ and elog p’) curve from large strain consolidation test and standard consolidation test is
shown in Figure 5.13(a) and (b). It should be noted that the void ratio from Rowe
cell was measured after the completion of dissipation of excess pore water pressure
while the void ratio from Oedometer test measured at the end of each load increment
(one week).
135
10
9
Rowe
Standard Oedometer
Void ratio (e)
8
7
6
5
4
3
2
0
100
200
300
400
500
Consolidation pressure (p', kPa)
(a)
10
9
Rowe
Standard Oedometer
Void ratio (e)
8
7
6
5
4
3
2
10
100
1000
Consolidation pressure (p', kPa)
(b)
Figure 5.13: Void ratio versus consolidation pressure curve from Rowe cell and
Oedometer test (a) typical e-p’ curve, (b) typical e-log p’ curve
136
It can be seen in Figure 5.13 that the curve obtained from consolidation test
on Rowe cell is quite similar with the curve obtained from the standard consolidation
test on Oedometer cell even though the parameter evaluated based on this figure is
quite different. The different method used in the evaluation of consolidation curves
results in the different value of compressibility parameter obtained from Rowe cell
and the standard consolidation test.
Table 5.8 shows the comparisons of the compressibility parameters obtained
based on test results on Rowe cell and Oedometer test for the range of consolidation
pressure used in this research.
Table 5.8: Compressibility parameters obtained from Rowe cell and Oedometer tests
Consolidation
Parameters
The completion of primary
consolidation,
t100 (minute)
The beginning of
secondary compression,
tp (minute)
Coefficient of rate
of consolidation,
cv (m2/year)
Coefficient of secondary
compression,
cα
Coef. of vol.
compressibility,
mv (1/kPa)
Compression index,
cc
Pre-consolidation pressure,
σ’c (kPa)
Initial void ratio,
eo
Rowe Cell
Tests
Oedometer
Tests
50 kPa
100 kPa
200 kPa
50 kPa
100 kPa
200 kPa
25.83
23.50
23.00
32.75
28.83
25.58
17.67
15.83
14.33
32.75
28.83
25.58
4.947
4.179
3.259
1.646
1.355
1.085
0.124
0.157
0.211
0.166
0.175
0.114
0.00171
0.00121
0.00073
0.00274
0.00171
0.00035
3.128
3.253
41
45
8.854
9.934
The results of Rowe consolidation test shown in Table 5.8 indicate that the
completion of primary consolidation (t100) is relatively faster than that obtained from
Oedometer test under the same range of consolidation pressure. The beginning of
secondary compression (tp) obtained from Rowe consolidation test is even much
lower compare to that predicted from Oedometer test. The results suggested that
Cassagrande’s method does not give the actual time for the completion of excess
137
pore water pressure dissipation, and the time does not represent the beginning of
secondary compression.
The average value of the coefficient of rate of consolidation (cv) obtained
from the result of Rowe consolidation test is higher than that obtained from
Oedometer test. This may be due to the size of the sample and the relative size of the
fiber to the size of sample.
The coefficient of secondary compression (cα) obtained from the result of
Rowe consolidation test varies with the consolidation pressure, while from
Oedometer test, the coefficient of secondary compression does not vary with
consolidation pressure or constant at 0149.
The coefficient of secondary
compression taken from the results of Rowe consolidation tests are ranging from
0.124 to 0.211. It can be observed that the coefficient of secondary compression
obtained from the large strain consolidation (Rowe cell) test is generally higher than
the result obtained from Oedometer test. Referring to the discussion by Mokhtar
(1997) for secondary compressibility of peat, this difference suggests that the results
of large strain consolidation test (Rowe cell) are less susceptible to the effect
disturbance and thus more reliable. This also shows that the coefficient of secondary
compression can be better observed from the large strain consolidation.
Under consolidation pressure of 50 kPa, the coefficient of volume
compressibility (mv) obtained from Rowe cell and Oedometer test are 0.00171 and
0.00274 respectively, while under consolidation pressure of 200 kPa, the coefficient
of volume compressibility (mv) obtained from Rowe cell and Oedometer test are
0.00073 and 0.00035 respectively.
The coefficient of volume compressibility
determined from Rowe consolidation test data is slightly lower than that obtained
from Oedometer test under consolidation pressure of 50 kPa.
However, under
consolidation pressure of 200 kPa, the coefficient of volume compressibility
determined from Rowe consolidation test data is slightly higher than that obtained
from Oedometer test. This shows that the results of consolidation test on Rowe cell
is more stable and less affected by consolidation pressure. The compressibility of
sample in Oedometer cell is reduced due to the reduction of the thickness and
rearrangement of the fiber in the soil.
138
It can be observed from Table 5.8 that the results obtained from large strain
consolidation test on Rowe cell and the standard consolidation test on Oedometer
cell gave comparable values for consolidation parameter. The average value of
compression index (cc) obtained from the result of Rowe consolidation test is 3.128,
while the compression index (cc) obtained from the standard consolidation test on
Oedometer cell is 3.253. The higher value of compression index observed in the
standard consolidation test on Oedometer cell is caused by various factors mentioned
in the preceding paragraphs such as effect of initial reading, sample disturbance, the
small size of the specimen, and the effect of secondary compression occur during and
after the primary consolidation. The pre-consolidation pressure estimated from the
Rowe consolidation test data is 41 kPa, which is slightly lower than that obtained
from the standard consolidation test on Oedometer cell (45 kPa). The higher preconsolidation pressure observed in the standard consolidation test on Oedometer cell
may be due to the compression during sample preparation and the thickness of the
sample.
Furthermore, the initial void ratio (eo) obtained from both Rowe consolidation
test is lower than obtained from the standard consolidation test on Oedometer cell.
The initial void ratio obtained from large strain consolidation test on Rowe cell and
the standard consolidation tests on Oedometer cell are 8.854 and 9.934 respectively.
The higher value of initial void ratio determined in the standard consolidation test on
Oedometer cell may be caused by the different in the time of execution between the
Rowe cell and the standard consolidation test on Oedometer cell.
Due to the
availability of the cell in Geotechnics Laboratory, only one large strain consolidation
test can be carried out on Rowe cell at a time. On the other hand, several Oedometer
test can be done at the same time. The delay in the execution of the test allows the
redistribution of the moisture content and biodegradation of the fiber and therefore
changes the initial void ratio. Note that the duration of each test is 10 days hence the
last test was done about three months after sampling.
139
5.4
Comparison with Published Data
A number of researchers have been carried out on compressibility
characteristics of different types of soils including fibrous peat (Berry and Poskitt,
1972; Edil and Dhowian, 1979; Robinson, 2003; Sridharan and Prakash, 1998;
Ajlouni, 2000; Holtz and Kovacs, 1981). Consequently, analysis of consolidation
curve was done to compare the curve obtained in this study with the published data
in terms of time-compression curve and consolidation curve.
Figure 5.14 shows the comparison of the typical strain versus logarithmic of
time curves obtained from this study with published data of amorphous granular peat
(Berry and Poskitt, 1972; and Edil and Dhowian, 1979), fibrous peat (present study,
Robinson, 2003; and Berry and Poskitt, 1972), kaolinite (Robinson, 2003), and clay
(Sridharan and Prakash, 1998).
0
2
4
Strain (% )
6
8
Amorphous granular peat (Berry&Poskitt, 1972)
10
Amorphous granular peat (Edil&Dhowian, 1979)
Present study
12
Fibrous peat (Robinson, 2003)
14
Kaolinite (Robinson, 2003)
Clay (Sridharan&Prakash, 1998)
16
0.1
1
10
100
1000
Time, t in minutes (log scale)
Figure 5.14: Strain versus logarithmic of time curves
10000
140
As shown in Figure 5.14, there are variations in the typical shape of strain
versus logarithmic of time curves for different types of soil. The time-compression
curve for fibrous peat in the present study is relatively similar in shape with the
curves obtained for fibrous peat (Robinson, 2003; and Berry and Poskitt, 1972). The
data obtained for the present study is in accordance with the published data in which
the end of primary consolidation is relatively difficult to identify and the secondary
compression varies non-linearly with time. The time-compression curve for the
fibrous peat is slightly different from that obtained for amorphous granular peat (Edil
and Dhowian, 1979) in terms of the amount of secondary compression as compared
to the primary consolidation. The time-compression curves for kaolinite (Robinson,
2003) and clay (Sridharan and Prakash, 1998) show an idealized curve where all
parameters can be identified such as recompression phase, primary consolidation
phase and secondary compression phase.
A distinct point where the primary
consolidation finished and the secondary compression is assumed to start can be
easily identified. The typical curve from clay (Sridharan and Prakash, 1998) presents
a higher value of strain, but most of the strain was due to primary consolidation. The
present study showed that the secondary compression plays an important role in the
compression of fibrous peat.
Figure 5.15 shows the relationship between the dissipation of excess pore
water pressure and the logarithmic of time for fibrous peat obtained in the present
study. The curve is compared with data obtained from previous studies on fibrous
peat (Robinson, 2003), amorphous granular peat (Berry and Poskitt, 1972), and
kaolinite (Robinson, 2003). It can be seen that the dissipation of excess pore water
pressure from fibrous peat in the present study is relatively faster as compared to
other soils, but the end of primary consolidation is quite similar to that predicted for
different type of fibrous peat (Robinson, 2003). Slower rate of dissipation of excess
pore water pressure is identified for kaolinite (Robinson, 2003).
Figure 5.16 shows the relationship between the void ratio and the logarithmic
of consolidation pressure (e-log p’) obtained from the present study as compared to
published data on fibrous peat (Berry and Poskitt, 1972; Ajlouni, 2000), amorphous
granular peat (Edil and Dhowian, 1981; Berry and Poskitt, 1972), and soft clay
(Holtz and Kovacs, 1981). It can be seen that the curve obtained from the present
141
study is similar in shape with the curve obtained by Ajlouni (2000) on the same type
of soil. The curves show a change in the slope indicating that the soil is exhibiting a
pre-consolidation pressure. The other curves for fibrous peat (Berry and Poskitt,
1972) and amorphous peat (Edil and Dhowian, 1981; Berry and Poskitt, 1972) show
an almost straight line indicating no recompression phase. Consolidation curve for
soft clay clearly indicates the recompression and compression phase. Figure 5.16
also indicate that fibrous peat has relatively high initial void ratio compared with
amorphous granular peat and soft clay. The initial void ratio of the sample used in
the present study is comparable to the fibrous peat used by Ajlouni (2000).
The coefficient of compressibility and the coefficient of volume
compressibility can be obtained from void ratio versus consolidation pressure (e-p’)
curve (Figure 5.17). The curve is similar curve in shape with the published data
indicating that the coefficient of compressibility and the coefficient of volume
compressibility decrease as the consolidation increases. However, the present study
suggested that the change in the coefficient of volume compressibility is not as
significant as suggested by previous researchers.
Excess pore water pressure dissipation, U (%)
0
Present study
10
Fibrous Peat (Robinson, 2003)
20
Kaolinite (Robinson, 2003)
30
Amorphous granular peat
(Berry&Poskitt,1972)
40
50
60
70
80
90
100
0.1
1
10
100
1000
Time, t in minutes (log scale)
Figure 5.15: Excess pore water pressure versus logarithmic of time curves
142
12
Present Study
11
Fibrous Peat
(Berry&Poskitt, 1972)
Fibrous Peat (Ajlouni,
2000)
Amorphous Peat
(Edil&Dhowian, 1981)
Amorphous granular peat
(Berry&Poskitt, 1972)
Clay (Holtz&Kovacs,
1981)
10
9
Void ratio (e)
8
7
6
5
4
3
2
1
0
1
10
100
1000
Consolidation pressure (p', kPa)
Figure 5.16: Void ratio versus consolidation pressure (logarithmic scale)
12
11
Present Study
10
Fibrous Peat (Berry&Poskitt, 1972)
9
Fibrous Peat (Ajlouni, 2000)
8
Amorphous Peat (Edil&Dhowian, 1981)
Void ratio (e)
Amorphous granular peat (Berry&Poskitt, 1972)
7
Clay (Holtz&Kovacs, 1981)
6
5
4
3
2
1
0
0
100
200
300
400
500
600
700
800
Consolidation pressure (p', kPa)
Figure 5.17: Void ratio versus consolidation pressure
900
1000
143
Comparison of the data obtained from the present study with published data
is summarized in Table 5.9. It can be concluded that the present study yields in a
comparable values of initial void ratio. The compression index and the coefficient of
secondary compression are much lower than the published data, while the primary
consolidation take longer time to complete compared with the published data. The
coefficient of rate of consolidation obtained from present study is slightly higher than
that published data.
Table 5.9: Comparison of the data obtained from the analysis of data obtained in the
present study with published data
Consolidation
Parameters
5.5
Present Study
Published Data on Fibrous Peat
Compression
index,
cc
Initial void ratio,
eo
Coefficient of
rate of
consolidation,
cv (m2/year)
4.947
(under
consolidation
pressure of 50 kPa)
3.61 (Berry and Poskitt 1972 under
consolidation pressure of 28-56
kPa)
Coefficient of
secondary
compression,
cα
0.109-0.211
(in the pressure
range of 25-200
kPa)
0.54 (Ajlouni 2000 in the pressure
range of 80-200 kPa)
0.40 (Berry and Poskitt 1972 under
consolidation pressure of 14-28
kPa)
The end of
primary
consolidation,
tp (minute)
25 minutes
3.128
4.4 (Berry and Poskitt 1972)
6-9 (Ajlouni 2000)
8.854
11
9
(Berry and Poskitt 1972)
(Ajlouni 2000)
Less than 15 minutes
(Fibrous peat, Mesri et al., 1997)
Effect of Fiber
The effect of fiber arrangement of the fibrous peat obtained from Kampung
Bahru, Pontian, West Johore on compressibility characteristics are studied through
the Scanning Electron Micrograph (SEM), permeability test using samples obtained
in the vertical and horizontal directions, and large strain consolidation tests
conducted with horizontal drainage to perimeter.
144
As mentioned in Section 4.1, the fibrous peat used in the present study was
obtained below water table at depth of 1 to 2 meter. Thus, it is considered as a
shallow deposit. The Scanning Electron Micrograph (SEM) was taken on samples
cut in vertical and horizontal directions to enable the observation of the
rearrangement of the fiber of fibrous peat at initial state and under consolidation
pressure. Comparison of the results of SEM on samples cut in vertical and horizontal
direction demonstrates the difference in the fiber arrangement in both directions, and
the effect of application of consolidation pressure on the fiber arrangement. Figure
4.5.b indicated that long slender roots are identified and these results in high initial
permeability in vertical direction. Application of load induces a rearrangement of
solid particles and redistributes the fiber (Figure 4.6.b), thus reduce the flow in
vertical direction.
Permeability test is conducted in this research to evaluate the effect of fiber as
well as application of consolidation pressure on the reorientation of the fiber and
permeability of the soil.
The initial coefficient of permeability in vertical and
horizontal directions is determined based on the results obtained from constant head
permeability test as described in section 4.5. The results show that the average
coefficient of initial permeability for samples obtained in vertical direction is 1.20 x
10-4 m/s, while for sample obtained in horizontal direction is 9.48 x 10-5 m/s. The
higher the coefficient of permeability obtained for samples in vertical directions
shows the effect of rootlets in the soil.
The vertical and horizontal coefficient of permeability under consolidation
pressure can be evaluated based on the coefficient of consolidation obtained from
large strain consolidation test (Rowe cell) through Equation 2.3. Table 5.10 shows
the coefficient of volume compressibility and the coefficient of permeability
obtained for the range of consolidation pressure used in the test.
As shown in Table 5.10, the horizontal coefficient of permeability decrease in
the slower rate of with increasing consolidation pressure compared to the vertical
coefficient of permeability, thus the ratio of kh/kv increase significantly as
consolidation pressure increases.
145
Table 5.10: Coefficient of volume compressibility and coefficient of permeability
based on large strain consolidation test
No.
Consolidation
Pressure
Coefficient
of volume
compressibility
Coefficient
of volume
compressibility
(p’)
mv
mh
(1/kPa)
0.00171
(1/kPa)
(kPa)
1.
50
2.
100
3.
200
0.00121
0.00073
Coefficient
of vertical
permeability
kv
(x10-10)
(m/s)
3.09893
0.00243
0.00140
1.76332
0.75187
0.00091
Coefficient
of
horizontal
permeability
kh
(x10-10)
Ratio
kh/kv
(m/sec)
22.84050
13.93300
8.29176
7.370
7.482
11.028
Permeability test was also carried out at the end consolidation test on Rowe
cell i.e. under consolidation pressure of 200 kPa. The data shown in Table 5.11
indicated that the application of consolidation pressure has the effect of decreasing
the coefficient of permeability of fibrous peat. The data also shows that the effect of
consolidation pressure is more significant on the vertical coefficient of permeability
as compared to the horizontal one, thus the ratio of kh/kv is higher for higher
consolidation pressure. The ratio of kh/kv increases from 0.79 for initial condition to
about 5 under consolidation pressure of 200 kPa.
Table 5.11: Effect of consolidation pressure on coefficient of permeability
Type of
permeability test
Constant-head
Large strain permeability
permeability test
test (under 200 kPa
consolidation pressure)
Coefficient of permeability
kh (20°C) (m/s)
9.48 x 10-5
2.60 x 10-9
kv (20°C) (m/s)
1.20 x 10-4
5.07 x 10-10
kh/kv
0.79
5.13
146
The results indicate that the flow of water is initially larger in the vertical
direction, but changing as consolidation pressure is applied. Thus, the effect of
consolidation pressure can be reduced if the water is allowed to flow in horizontal
direction. All test suggested a significant decrease in permeability as consolidation
pressure increases. However, the effect is more dominant to the permeability in
vertical direction and the ratio of kh/kv is increasing as the consolidation pressure
increases.
The application of consolidation pressure also has the effect of decreasing the
void ratio in the soil. The relationship between the void ratio and the coefficient of
permeability in horizontal and vertical direction is presented in Figure 5.18, which
also suggest that the effect of decreasing in void ratio is more prominent in the flow
of water in vertical direction.
12
11
10
9
Void ratio (e)
8
7
6
5
4
3
2
1
kh from consolidation test
kv from consolidation test
kv from constant head
kh from constant head
kv from permeability test
on Rowe cell
kh from permeability test
on Rowe cell
0
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
Coefficient of permeability (k, m/s)
Figure 5.18: The relationship between the void ratio and the coefficient of
permeability in horizontal and vertical direction
147
5.6
Settlement Estimation
5.6.1 Introduction
The application of the time-compression curve on the analysis of settlement
for soil exhibiting secondary compression was introduced by Cassagrande (1936)
and was presented in many geotechnical engineering textbook such as Holtz and
Kovacs (1981).
This involves the determination of preliminary consolidation
(Equation 2.8) and secondary compression (Equation 2.19).
In this method the calculation of primary consolidation settlement is based on
e-log p’ curve for which the compression index (cc) can be obtained as the slope of
the curve, while the determination of secondary compression is based on e-log time
curve and coefficient of secondary compression (cα). The method assumes that the
ratio of cα/cc is constant. This assumption is actually an oversimplication of real
behavior.
According to the cα/cc concept of compressibility, the magnitude and
behavior of the coefficient of secondary compression (cα) with time directly related
to the magnitude and behavior of the compression index (cc) with the effective
vertical stress (σ’v). In general, cα remains constant, decreases, or increases with
time, for constant stress while cc remains constant, decreases, or increases with
effective stress. Another problem with this analysis is that the time of completion of
primary consolidation (tp) is difficult to determine based on the time-compression
curve. It is clear that for fibrous peat, the primary consolidation and the secondary
compression can occur at the same time. For peat, the primary consolidation occurs
rapidly due to high initial permeability and secondary compression takes a significant
part of compression. The end of primary consolidation (tp) can be determined from
excess pore water pressure measurements using pressure transducers or by using
graphical methods, such as the Cassagrande’s method.
148
As pointed out in Chapter 2 section 2.3, some researchers (Dhowian and Edil,
1980; Sridharan and Prakash, 1998; Mesri and Lo, 1991) have tried to solve the
complication related to the calculation of primary consolidation and secondary
compression based on time-compression curve. The most advanced research done
on this topic is made by Robinson (1997, 1999, and 2003). He started the research
on the definition of the beginning of secondary compression based on timecompression curve (1997). The modification of the method was made possible by
the measurement of excess pore water pressure dissipation in Oedometer test. Thus
the actual primary consolidation process can be monitored by the excess pore water
pressure reading (1999).
The evaluation of the beginning of the secondary
compression based on both excess pore water pressure reading and time-compression
curve was presented by Robinson (2003). The procedure is outlined in Appendix H.
Settlement analysis made on a hypothetical problem based on the timecompression curve obtained from Oedometer test and Rowe cell using Cassagrande
(1936) and Robinson (2003) methods.
5.6.2 Hypothetical Problem
A hypothetical problem shown in Figure 3.33 is used to illustrate the
settlement estimation of construction over fibrous peat deposit based on Cassagrande
analysis and the method proposed by Robinson (2003). An embankment of 2.5 m
high is constructed over a 5 m thick deposit of fibrous peat. The overburden pressure
at the middle of peat layer is 25 kPa. The embankment constructed directly on the
soil by five layers of 0.5 m will induce a stress increment of 50 kPa to the soil. The
groundwater table is assumed to coincide with the ground surface and permeable
layer is assumed to exist below the peat deposits. The embankment is constructed of
sand fill over a geotextile layer so that uniform settlement can be expected. The
design life of the structure is 20 years. The problem is redrawn in Figure 5.19, while
the properties of fibrous peat deposit derived from large strain consolidation test and
Oedometer test for stress increment of 50 kPa is given in Table 5.12.
149
5m
Ground Level
2m
3.65 m
3.65 m
2m
5m
Proposed Embankment
2,5 m
Fibrous Peat
5m
Permeable Layer
Figure 5.19: Geometry and soil properties for the hypothetical problem
Table 5.12: The properties of fibrous peat deposit obtained from large strain
consolidation test and Oedometer test for consolidation pressure 50 kPa
Parameters
Unit weight of the fill
material (kN/m3)
Unit weight of the fibrous
peat (kN/m3)
Initial void ratio (eo)
Initial void ratio at t100 (e100)
Initial void ratio at tp (ep)
Compression index (cc)
Coefficient of rate of
consolidation (cv , m2/year)
Coefficient of secondary
compression (cα)
End of primary consolidation
(t100, minutes)
The beginning of secondary
compression (tp, minute)
Degree of consolidation at
the beginning of secondary
compression (U, %)
Results from Rowe cell test
Results from
Oedometer
test
compression
compression+excess
pore water pressure
measurement
20
20
20
10
10
10
9.934
5.076
5.076
3.253
8.854
5.109
5.109
3.804
8.854
4.810
5.780
3.128
1.646
3.393
4.947
0.166
0.106
0.124
33
40
26
33
40
18
-
-
65
150
5.6.3 Settlement Analysis by Cassagrande (1936) Method
The settlement was calculated using cα concept proposed by Cassagrande
based on the data taken from Oedometer test and Rowe consolidation test under
application consolidation pressure of 50 kPa.
Based on Oedometer test data, the primary consolidation settlement is:
Sc = c c
σ' + ∆σ
25 + 50
H
5
log o
= 3.253
= 0.710 m = 710 mm
log
1+ e o
σ' o
1 + 9.934
25
Following the standard procedure, time to reach 90% consolidation (t90)
should be calculated to estimate the end of primary consolidation which is:
t 90 =
Tv H2 d
Cv
0.848 x (2.5) 2
=
= 3.22 years or about 3 years
1.646
Thus 90 % of settlement (639 mm) will occur in about 3 years, while the time
required to finish the primary consolidation for the hypothetical curve based on the
time of primary consolidation (tp) value obtained from laboratory test is about 21
years which is more than the design life of the structure of 20 years which means that
the method can not be used to predict the secondary compression because the timecompression curve obtained by Oedometer test does not give a clear indication of the
beginning of secondary compression.
Based on large strain consolidation test on Rowe cell, the primary
consolidation settlement is:
Sc = c c
H
σ' + ∆σ
5
25 + 50
log o
= 0.921 m = 921 mm
= 3.804
log
1+ eo
σ'o
1 + 8.854
25
Following the standard procedure, time to reach 90% consolidation (t90)
should be calculated to estimate the end of primary consolidation which is:
151
t 90 =
Tv H2d
Cv
=
0.848 x (2.5)2
= 1.56 years or about 19 months
3.393
The time required to finish the primary consolidation based on the time of
primary consolidation (tp) value obtained from laboratory test is about 10 years. The
secondary settlement of the peat layer after the end of primary consolidation until the
completion of design life of 20 years is:
Ss = cα
20
5
t
H
log = 0.02611 m = 26 mm
log = 0.106
10
1 + 5.109
tp
1 + e op
Tables 5.13 summarize the results of settlement calculation for the
hypothetical problem based on Rowe consolidation test.
Table 5.13: The results of settlement calculated based on Rowe consolidation test
Primary Consolidation
(mm)
Total
(mm)
0 - tp (0 – 10 years)
921
921
tp - 20 years (10 years – 20 years)
26
26
Total
947
947
Time
The relationship of the settlement with time based on Rowe consolidation test
is presented in Figure 5.20. The analysis shows that 97 % of settlement was actually
due to primary consolidation and secondary compression contributes only 3 % of the
total settlement which is negligible. This is due to the assumption that secondary
compression occur after the end of primary consolidation.
152
0
Settlement (cm)
20
40
60
80
100
0.001
0.01
0.1
1
10
100
Time, t in year (log scale)
Figure 5.20: The curve of settlement with time based on Rowe consolidation test
5.6.4
Settlement Analysis by Robinson (2003) Method
Based on the data taken from Rowe consolidation test, the primary
consolidation settlement is:
Sc = c c
σ' + ∆σ
25 + 50
H
5
log o
log
= 0.757 m = 757 mm
= 3.128
1+ e o
σ' o
1 + 8.854
25
Following the standard procedure, t90 should be calculated to estimate the end
of primary consolidation which is:
t 90 =
Tv H2 d
Cv
=
0.848 x (2.5) 2
= 1.07 year or about 12 months
4.947
The data suggested that the secondary compression started at degree of
primary consolidation of 65 % (Table 5.1). The water can flow in two directions and
153
the length of drainage path is equal to half of the thickness of the peat deposit or 2.5
m. The time factor for 65 % consolidation is:
For U > 60 %:
Tv = -0.933 log (1-U) - 0.085 = 1.781 – 0.933 log (100-U %)
Tv = 1.781 - 0.933 log (100-65) = 0.340
t 65 =
Tv H2d
Cv
=
0.340 x (2.5)2
= 0.43 year = 5 months
4.947
The settlement at this time is:
S65 = U x Sc = 65 % x 757 mm = 492.05 mm = 492 mm
Based on the test result, the time to reach the completion of the primary
consolidation (t100) is 26 minutes; therefore the time for this layer to reach 100 %
consolidation can be calculated using square rule and resulted in 6.3 years. The time
between 65 % consolidation to the completion of primary consolidation is equal to
5.9 years. The primary consolidation settlement during this time is:
(Sc – S65) = (1-U) x Sc = (100 - 65) % x 757 mm = 264.95 mm = 265 mm
The secondary compression (tp) occurred between 65 % and 100 % of
primary consolidation is:
Ss = cα
H
t
5
6 .3
log = 0.124
log
= 0.106614 m = 107 mm
1+ e p
tp
1 + 5.780
0.43
The time where the secondary compression curve shows a sharp change slope
(ts) for consolidation pressure of 50 kPa is 1067 minutes, thus the time required for
secondary compression in this case is 265.5 years, which is much longer than the
design life of a structure. Hence, the secondary compression can be evaluated based
on average cα over the time period between 6.3 and 20 years.
154
The secondary compression settlement after the completion of primary
consolidation to the end of the design life of the structure (20 years) is:
Ss = cα
20
5
t
H
= 0.124
= 0.053537 m = 54 mm
log
log
6 .3
1 + 4.810
t100
1 + e100
Tables 5.14 summarize the results of settlement calculation for the
hypothetical problem shown in Figure 3.29. It can be seen from Table 5.15, the total
settlement of the embankment during the design life of the structure is 918 mm or
about 1 m which consists of 757 mm of primary consolidation and 161 mm
secondary compression. The primary consolidation is completed in 6.3 years
however 90 % consolidation is achieved within 1 year.
Table 5.14: The results of settlement calculated based on Robinson’s method
Total
Primary
Consolidation
(mm)
Secondary
Compression
(mm)
(mm)
0 - tp (0 – 5 months)
492
0
492
tp - t100 (5 months – 6.3 years)
265
107
372
-
54
54
757
161
918
Time
t100 - 20 years (6.3 years – 20 years)
Total
The analysis shows that much of the settlement (83 %) was actually due to
primary consolidation, however the secondary compression of the peat deposit
should also be considered since it contributes to the total settlement occur during the
design life of a structure. The secondary compression started as early as 5 months
after construction. The relationship of the settlement with time is presented in Figure
5.21.
155
0.00
0.02
Settlement (mm)
0.04
U (65 %)
0.06
0.08
Primary
δ
0.10
Total
cα = 0.124
0.12
Primary +
Secondary
Primary
0.14
0.1
1
10
100
Secondary
1000
10000
Elapsed time (minutes)
Figure 5.21: Settlement versus logarithmic of time curve based on Robinson’s
method (2003)
5.6.5
Discussion
The settlement analysis made on the hypothetical problem of an embankment
on the fibrous peat deposit showed that the total settlement of the structure based on
Oedometer test by Cassagrande’s method (710 mm) is less than the total settlement
evaluated based on Rowe consolidation test data (947 mm). The method assumes
that the secondary compression only occur upon the completion of primary
consolidation. Analysis based on Oedometer data can not predict the secondary
compression during the design life of the structure because the end of primary
consolidation (eop) predicted based on this data is 21 years which is longer than the
design life.
The settlement evaluation based on data obtained from Rowe
consolidation test predicts a very small amount of secondary compression settlement
(3 %). This shows that Cassagrande’s method based on time-compression curve can
not be used to predict settlement on fibrous peat deposit used in this study.
156
The evaluation of settlement based on Robinson (2003) method shows a total
settlement of 918 mm, 83 % of which is due to primary consolidation. The results
also showed that the secondary compression started as early as 5 months after
construction, at this time the primary consolidation has reached 492 mm or 65 %
degree of consolidation. As shown in Table 5.14 and Figure 5.21 much of the
secondary compression (107 mm) actually occur before the completion of primary
consolidation, only amount of settlement (54 mm) occurred after the end of primary
consolidation.
The previous discussion indicates that the use of large strain consolidation
test to evaluate consolidation characteristics of soil exhibiting secondary
consolidation is advantageous because it enable long term observation of
deformation of fibers and the resulting strain can be easily observed. The excess
pore water pressure measurement made on the large strain consolidation test enables
the elimination of the effect of secondary compression on the evaluation of primary
consolidation, thus the evaluation of secondary settlement can be made separately
from the primary settlement especially that occurs before the completion of excess
pore water pressure dissipation.
CHAPTER 6
SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS
6.1
Summary
An extensive laboratory testing program was conducted on undisturbed
specimens of fibrous peat deposits from Kampung Bahru, Pontian, West Johore for
the purpose of studying the compressibility characteristics of the peat. In addition,
literature study on the geotechnical properties of peat was conducted to provide
rationale of the research and to gather sufficient background information on the
consolidation behavior of fibrous peat. The literature study was used to develop the
hypothesis adopted for the study, i.e. the compressibility characteristics of fibrous
peat can be evaluated based on the results of large strain consolidation test on Rowe
cell, and analyzed using the method suggested by Robinson (2003).
The focus of this research was to investigate the compression behavior of
fibrous peat based on the results of consolidation test using Rowe Cell, and to
develop the suitable model for the analysis of settlement of construction on fibrous
peat deposit.
The peat samples were obtained by block sampling method.
In
addition, some samples were retrieved using piston sampler for evaluation of natural
water content and initial permeability of the soil. Field vane shear test was utilized
to obtain the preliminary estimates on the shear strength of the deposit.
158
Laboratory testing program included standard laboratory testing used for
identification and classification purposes, and determination of basic engineering
characteristics. Consolidation tests were performed on the standard Oedometer and
Rowe cell by incremental loading with load increment ratio of one.
Scanning
Electron Micrograph (SEM) of the sample was taken to enable the evaluation of the
fiber orientation and the effect of pressure on the fiber arrangement and their effect
on the permeability and the compression behavior of the fibrous peat. Data obtained
in this study were compared with published data.
The applicability of consolidation theories to evaluate the amount and rate of
primary consolidation and predict secondary compression based on the timecompression curve was studied in this research by a hypothetical case of an
embankment placed on fibrous peat deposit.
6.2
Conclusions
Conclusions are derived based on the results obtained from the current
research on fibrous peat obtained from Kampung Bahru, Pontian, West Johore, and
data from the literature. The generalization of the research data was not attempted in
this research since it is fully understood that the properties of peat soil are unique to
location. The conclusions of this study are indicated in the followings:
1.
The peat deposit is categorized as deep peat with thickness of more than 5 m.
The natural water content of the peat is 608 % which corresponds to initial void
ratio of about 9. The peat is classified as fibrous peat with low to medium
degree of decomposition (H4 in von Post scale) and very high organic and fiber
content. The literature study indicated that this is the typical peat found in West
Malaysia.
2. The undrained shear strength of peat is 10.10 kPa with sensitivity of 5.64. The
drained shear strength parameters are c’ = 3.10 kPa and φ’ = 25.4o. The shear
159
strength is slightly lower compared to the published data on peat in West
Malaysia.
3. The observation of the sample on Scanning Electron Micrograph at initial stage
showed the direction of the fiber was mostly in vertical direction which results in
a higher coefficient of permeability in vertical direction. The initial permeability
in the vertical and horizontal direction are 1.2x10-4 m/s and 9.48x10-5 m/s
respectively.
4. The comparison between the results of consolidation test on Oedometer and
Rowe cells showed that the use of Rowe cell for the evaluation of the
consolidation characteristics of soil exhibiting secondary compression is
advantageous because it enables long term observation of deformation of fibers.
The excess pore water pressure measurement made in the large strain
consolidation test enables the elimination of the effect of the secondary
compression on the evaluation of the primary consolidation.
5. The compression index (cc) obtained from the large strain consolidation test on
peat in the present study is 3.128 while the coefficient of secondary compression
from 0.102 to 0.304 for the range of consolidation pressure of 25 to 200 kPa.
6. The secondary compression started as early as 65 % degree of consolidation.
The average time of the beginning of secondary compression (tp) for the fibrous
peat used in the present study is 18 minutes while the completion of primary
consolidation is 26 minutes.
7. The coefficient of rate of consolidation (cv) obtained from large strain
consolidation test ranged from 5.689 to 3.259 for pressure range of 25 to 200 kPa
which is comparable to published data on fibrous peat.
8. The permeability and the end of primary consolidation are highly influenced by
the application of consolidation pressure. The ratio of kh/kv increases from 0.79
for initial condition to about 5 under consolidation pressure of 200 kPa. The ratio
of ch/cv is increasing from 6 to 9 for consolidation pressure of 25 to 200 kPa.
160
9. Settlement analysis on the hypothetical problem showed that the large part of
settlement occurs during primary consolidation stage. Secondary compression
can not be evaluated based on time-compression curve alone because accurate
prediction of the beginning of secondary compression can not be made based on
Cassagrande’s method.
10. Robinson method is the most suitable method for evaluation of the settlement of
fibrous peat deposit because it has the ability to predict secondary compression
occur before the end of primary consolidation.
6.3
Recommendations for Future Research
The primary objective of this research is to study the compressibility
characteristics of the fibrous peat based on the results of consolidation test using
large strain consolidometer (Rowe Cell).
Effect of factors such as fiber
orientation is considered as important in the compressibility characteristics, thus
evaluation on the consolidation and the permeability in horizontal direction is
also studied. Finally, the compressibility parameters obtained from consolidation
test on Rowe cell was used in the evaluation of settlement of embankment over
peat deposit.
Further experimental work is needed to overcome the limitation of the study
and to verify the application of the settlement analysis made on this study. Some
further research works are suggested:
1. The effect of time of execution of the test on the consolidation properties of peat
should be studied. Development of the relationship between the time and the
initial void ratio and the compression index is of great importance.
2. An evaluation on the increase in shear strength due to application of
consolidation pressure and draining process is recommended as an extension of
161
this research to study the application of stage loading as one improvement
method for fibrous peat.
3. Field embankment tests are needed to further examine the consolidation theory
for fibrous peat found in Kampung Bahru, Pontian, West Johore. The field test
programs should be accompanied with extensive laboratory tests to define the
peat profile to better evaluate the applicability of the consolidation theory on the
analysis of settlement of embankment over peat deposits.
162
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169
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170
APPENDIX A
SAMPLING PROCEDURE
The procedures for sampling of peat:
1.
Excavate soil to a depth below ground water level by using hoe. Clean the
base and throw away the twigs and roots.
surface
1m
2.
Push a 300 mm diameter and 300 mm height tube into the soil carefully to get
a sample (Figure A1). The sharpness of the tube has to be ensured to cut the
fiber from blocking the tube and to control the quality of the sample. Sharp
knife was used to help cutting any fiber from outside the tube when needed.
Surface
Push a tube into the soil
1m
300 mm
300 mm
171
3.
Excavate the surrounding of the tube then cut the base of sample using a
sharp knife.
surface
1m
the base of
an excavation
cut the base of sample
4.
Insert a piece of cylindrical wood plate below the sample and keep the sample
still in by blocking the top and bottom of the tube using a wood piece which
of the same diameter with the inside of the tube (Figure A2).
5.
Take the tube out carefully and put on a safe place. Cover the top and bottom
of the tube with wax to maintain the moisture of soil (Figure A2).
6.
Cover again the top and bottom of the using square wood plate (500 mm x
500 mm) and secure it with ropes to stabilize the tube during transportation
(Figure A3).
7.
Arrange two sample tubes in one wooden box (Figure A3). Cover the wall of
the wooden box and fill the voids with layers of sponge to minimize the
effect of vibration during transportation from site to Geotechnical Laboratory
at UTM.
8.
A thin wall fixed piston sampler (Figure A5 and A6) was also used to take
samples horizontally and vertically for checking the quality of samples, water
content determination, and constant head permeability test
172
Figure A1: A block sampler was Figure A2: A peat block was covered
manually pushed into the bottom of a test with 2 cylindrical pieces of wood, and
sealed with melted candles, which
pit
hardened at normal temperature to
preserve the natural moisture content
Figure A3: Each peat block was covered with 2 pieces of wood, tied with ropes,
and then put into a wooden box to prevent the soil sample from moving during
transportation and then transported and kept in the laboratory
173
Figure A4: The fibrous peat soil block sample
Figure A5: A thin wall fixed piston
samplers was manually pushed and
carved from the bottom of a test pit to
obtain vertical undisturbed fibrous
peat soil samples
Figure A6: A thin wall fixed piston
sampler was sealed with moistureresistant plastic covers to preserve the
moisture content of undisturbed fibrous
peat soil sample in the sampler
174
APPENDIX B
INDEX TESTS DATA
1.
Natural Moisture Content
Table B1: Typical test sheet for natural moisture content
Location : Geotechnical Laboratory
Soil Description : PEAT
Job ref.
Bore hole/
Pit no.
Sample no.
Depth
Date
Test method
ASTM D2216-92 / BS 1377 : Part 2:1990 : 3.2
Related test
Speciment ref.
1
2
Container no.
A1-1
A1-2
Mass of wet soil + container (m2)
g
543
471
Mass of dry soil + container (m3)
g
312
248
Mass of container (m1)
g
276
214
Mass of moisture (m2-m3)
g
231
223
Mass of dry soil (m3 –m1)
g
36
34
641.167
655.882
m
m
−
3
Moisture content ω = 2
x100 %
m 3 − m1
Average
ω = 632.349 %
Table B2: Results for natural moisture content
No
Test 1-1
Test 1-2
Test 1-3
Test 2-1
Test 2-2
Test 2-3
Test 3-1
Test 3-2
Test 3-3
AVERAGE
Natural Moisture Content (ω, %)
641.167
655.882
600.000
621.490
601.714
631.298
555.717
577.973
584.219
608
1
1-2 m
14/01/2005
3
A1-3
481
259
222
222
37
600.000
175
2.
Specific Gravity (Gs)
Table B3: Typical test sheet for specific gravity (Gs)
Location : Geotechnical Laboratory
Soil Description : PEAT
Test method ASTM D854-92 / BS 1377 : Part 2 : 1990 : 8.3 / 8.4
Method of preparation : pycnometer method
Small/Large pycnometer
Speciment references
Pycnometer number
Mass of bottle + soil +water (m3)
g
Mass of bottle + soil (m2)
g
Mass of bottle full of water (m4)
g
Mass of bottle (m1)
g
Mass of soil (m2 –m1)
g
Mass of water in full bottle (m4-m1)
g
Mass of water used (m3-m2)
g
Volume of soil particles (m4-m1)- (m3-m2)
mL
m
m
−
2
1
Specific gravity Gs =
Mg/m3
(m4 − m1 ) − (m3 − m2 )
Job ref.
Bore hole/
Pit no.
Sample no.
Depth
Date
1
1567
137.745
46.642
135.273
35.476
11.166
99.797
91.103
8.694
1.284
3.
Specific Gravity (Gs)
1.284
1.325
1.442
1.439
1.513
1.510
1.482
1.534
1.509
1.543
1.544
1.486
1.468
Initial Void Ratio
Based on average natural moisture content & average specific gravity:
e0 =
Gs × w
γw
= 8.925
1459
137.924
47.489
135.245
36.556
10.933
98.689
90.435
8.254
1.325
Average Gs = 1.305
Table B4: Results for specific gravity (Gs)
No
Test 1-1
Test 1-2
Test 2-1
Test 2-2
Test 3-1
Test 3-2
Test 4-1
Test 4-2
Test 5-1
Test 5-2
Test 6-1
Test 6-2
AVERAGE
1
1-2 m
08/02/2005
176
4.
Organic Content and Ash Content
Table B5: Typical test sheet for organic content and ash content
Job ref.
Bore hole/
Pit no.
Sample no.
Depth
Date
Location : Saint Laboratory
Soil Description : PEAT
1
1-2 m
22/02/2005
Test method
ASTM D1997-91 / BS 1377 : Part 3:1990 : 4.3
Related test
Speciment ref.
1
Crucible no.
C14
C40
Mass of crucible (m1)
g
32.3868
34.7537
Mass of crucible + soil (m2)
g
37.3868
39.7537
Mass of crucible + soil after ignition (m3)
g
32.8248
34.8945
91.240
97.184
m
m
−
2
3
Organic Content OC =
%
x100
m 2 − m1
average
OC = 94.212
Ash Content AC = 100 % - OC
5.
%
AC
= 5.788
Fiber Content
Table B6: Typical test sheet for fiber content
Location : Geotechnical Laboratory
Soil Description : PEAT
Test method
ASTM D1997-91
Related test
Speciment ref.
Cointainer no.
Mass cointainer
g
Mass of dry soil of fibers retained #100sieve (m1)
g
Mass of dry soil of fibers retained #100sieve after ignition (m2) g
Fiber Content FC = m 2 x100
%
m1
Job ref.
Bore hole/
Pit no.
Sample no.
Depth
Date
A-1
1-2 m
04/02/2005
1
1
9.641
45.201
40.814
90.294
2
9.873
44.162
39.880
90.304
Average FC = 90.299
Table B7: Results for organic content, ash content, fiber content and pH
No. of Test
Test 1
Test 2
Test 3
AVERAGE
Organic
content (%)
94.212
98.520
98.542
97.091
Ash
Content (%)
5.788
1.480
1.458
2.909
Fiber
Content (%)
90.299
90.435
89.621
90.118
pH
3.04
3.26
3.42
3.24
177
6.
Sieve Analysis
100
90
Cumulative (%) Passed
80
70
60
50
40
30
20
10
0
0.01
0.1
1
10
100
Test sieve aparture size (mm)
Figure B1: Cumulative (%) passed versus sieve aparature size (mm)
Table B8: Results for sieve analysis
Test No.
Sieve Size
(mm)
Mass Passing
Percentage Passing
Cumulative
(g)
(%)
1
0.063
9
2.26
2
0.063
11
2.74
3
0.063
AVERAGE
13
3.23
2.74
178
APPENDIX C
SOIL FABRIC
1.
Apparatus
Figure C1: Assembly Plan for SEM test, (21) Emergency shutdown button, (56)
Rotary pump, (50) Water solenoid valve, (57) Exhaust hose, (51) Water main
valve, (58) Discharge line, (52) Compressed air-Main valve, (59) Grounding, (53)
Nitrogen-Main valve, (60) Switchbox, (54) Dynamic vibration-damper, (61)
Computer with keyboard and mouse, (55) Static damper with adsorption trap,
(62) Miniature circuit breaker, Ground fault circuit interrupter-emergency
shutdown-switch
179
Figure C2: The equipment for SEM test
2.
Procedure Scanning Electron Microscope (SEM)
The procedure for Scanning Electron Microphotograph (SEM) follows the
standard procedure outlined in ASTM F 1392-93 and the standard procedures of
G34-SUPRA 35 VP en 01 Carl Zeisss SMT-Nano Technology System Division. The
procedures as follows:
1.
Switching the instrument on.
The emergency shutdown button must be unlocked, and the master’s switch
must be switched on. Then open the cooling water valve, the nitrogen valve,
the cover on the yellow STANDBY-button and press the button.
2.
Starting the Smart SEM program.
Double-click on the Smart SEM icon with the left mouse button. While the
program loads, the screen will also show you which systems.
3.
Loading the specimen chamber.
Take hold of the door handle and carefully open the chamber door. Next,
load specimen containers into specimen holder and tighten laterally with an
Allen wrench and load samples into specimen containers. Place the prepared
specimen holder on the table. The specimen table can be moved in three
180
directions, tipped, and rotated around the beam axis. After that close the
chamber door by pressing lightly on the front with the palm of your hand, or
use the door handle.
4.
Evacuating the specimen chamber.
When the specied vacuum has been reached, you will see the message “Vac
Status ready”, and the red X next to the “Vac” icon in the bottom toolbar will
change to a green check mark.
5.
Activating the electron beam.
Left-click on “GUN” and “EHT” (on the bottom toolbar). Subsequently the
cathode will heat up, electrons will be emitted, the acceleration voltage will
be on, and the image on the screen will turn lighter.
6.
Focusing the electron beam.
The objective focuses the electron beam on the surface of the specimen. The
specimen must be placed in the correct position under the electron beam
before you bring it into focus.
7.
Modifying the image.
The Smart SEM program has many functions to help you obtain the desired
results. Information of interest can be accessed via Windows help, program
help, or context-based help.
8.
VP-Mode.
When examining non-or only slightly conductive preparations, charges can
be induced on their surfaces, which are difficult or impossible to divert and
which result in an altered image. In VP-Mode, these surface charges are
avoided or reduced and high-quality images can be produced, even from such
preparations.
9.
Finishing examination of a specimen.
You can save or print out an image if it meets your quality requirements.
10.
Placing the SEM in standby mode.
Standby mode is the normal status for the SEM once you have finished
examining a specimen.
11.
Switching off the SEM.
The SEM must be shut down for maintenance, repairs, if the instrument will
not be used for an extended period of time or in case of an emergency.
12.
Shutting down the SEM completely.
181
3.
Results of Scanning Electron Microscope (SEM) in Vertical Section
A
C
E
B
D
F
Figure C3: Scanning Electron Microphotographs (SEM) of Kampung Bahru,
Pontian, West Johore Peat. (A) Vertical Section before Compression x50, (B)
Vertical Section after Compression under 200kPa x50, (C) Vertical Section before
Compression x200, (D) Vertical Section after Compression under 200kPa x200, (E)
Vertical Section before Compression x400, (F) Vertical Section after Compression
under 200kPa x400
182
4.
Results of Scanning Electron Microscope (SEM) in Horizontal Section
A
C
E
B
D
F
Figure C4: Scanning Electron Microphotographs (SEM) of Kampung Bahru,
Pontian, West Johore Peat. (A) Horizontal Section before Compression x50, (B)
Horizontal Section after Compression under 200kPa x50, (C) Horizontal Section
before Compression x200, (D) Horizontal Section after Compression under 200kPa
x200, (E) Horizontal Section before Compression x400, (F) Horizontal Section after
Compression under 200kPa x400
183
APPENDIX D
SHEAR STRENGTH
1.
Field Vane Shear Test
Table D1: Test results for field vane shear 1
Sample No.
Blad NR Page No.
Djup Depth
Vinge Vane
Vane Factor
Undisturbed M1
Remoulded Sample M3
Rod Friction M2A
Rod Friction M2B
Torque MU = M1-M2A
Torque Mr = M1-M2B
Shearing Strengths
τ fu =
Mu
1
2
(m)
1/2
1
2/2
2
(Vc)
(Nm)
(Nm)
(Nm)
(Nm)
(Nm)
(Nm)
14
4.9
2.1
3.8
11.9
1.1
(kPa)
3
1/2
1
2/2
2
1/2
1
2/2
2
11.1
4.8
5.5
3.4
5.6
1.4
13.9
4.0
2.0
1.7
11.9
2.3
1.01
15.5
9.6
11.0
5.3
4.5
4.3
8.6
2.0
1.8
1.4
6.8
0.6
12.3
5.1
2.5
2.5
9.8
2.6
11.78
5.54
11.78
4.46
6.73
9.70
1.09
1.39
2.28
4.26
0.59
2.57
10.81
3.99
5.17
1.05
11.41
3.77
φ 65 x H 130 mm
Vc
Shearing Strengths
(τ fu )r
Sensitivity
St =
=
Mr
Vc
τ fu
(τ fu )r
(kPa)
For 1 m depth:
For 2 m depth:
30.29
= 10.10
3
3.96
(τ fu )r =
= 1.32
3
27.93
St =
= 9.13
3
τ fu =
τ fu =
19.70
= 6.57
3
8.22
(τ fu )r =
= 2.74
3
St =
8.81
= 2.94
3
184
Table D2: Test results for field vane shear 2
Sample No.
Blad NR Page No.
Djup Depth
Vinge Vane
Vane Factor
Undisturbed M1
Remoulded Sample M3
Rod Friction M2A
Rod Friction M2B
Torque MU = M1-M2A
Torque Mr = M1-M2B
Shearing Strengths
Mu
τ fu =
1
2
(m)
1/2
1
2/2
2
(Vc)
(Nm)
(Nm)
(Nm)
(Nm)
(Nm)
(Nm)
16.7
3.8
1.3
2.7
15.4
1.1
(kPa)
3
1/2
1
2/2
2
1/2
1
2/2
2
7.7
6.4
3.9
3.8
3.8
2.6
18.8
4.8
1.7
1.5
17.1
3.3
1.01
16.4
9.6
3.5
4.3
12.9
5.3
15
5.1
2.2
3.6
12.8
1.5
14.1
5.6
4.2
3.9
9.9
1.7
15.25
3.76
16.93
12.77
12.67
9.80
1.89
2.57
3.27
5.25
1.48
1.68
8.07
1.46
5.18
2.43
8.56
5.83
φ 65 x H 130 mm
Vc
Shearing Strengths
(τ fu )r
Sensitivity
St =
=
Mr
Vc
τ fu
(τ fu )r
For 1 m depth:
τ fu =
44.85
= 14.95
3
(τ fu )r
=
St =
6.64
3
= 2.21
21.81
= 7.27
3
(kPa)
For 2 m depth:
τ fu =
26.33
= 8.78
3
(τ fu )r
=
St =
Initial shear strength (from 1m to 2 m depth)
Sensitivity:
St(ave) = 5.64
3
= 3.17
9.72
= 3.24
3
Average
Cu(ave) = 10.10 kPa
9.50
185
Shear Box Test
30
Test 1
Test 2
Test 3
Test 4
Test 5
Test 6
Test 7
Test 8
Test 9
Test 10
Test 11
Test 12
Average
25
Shear strength (kPa)
2.
20
φ’ = 25.4o
15
10
5
c = 3.10 kPa
0
0
5
10
15
20
25
30
Normal stress(kPa)
Figure D1: Shear stress at failure (σf) versus normal stress (σn)
Table D3: Typical test results for shear box
Number of Test
Parameters of Shear Strength
Cohesion, c’ (kPa)
Friction, φ (0)
Test 1
1.80
27
Test 2
4.90
24
Test 3
3.25
25
Test 4
2.80
24
Test 5
1.80
27
Test 6
1.90
27
Test 7
2.20
28
Test 8
3.70
26
Test 9
4.40
22
Test 10
3.00
23
Test 11
4.20
27
Test 12
3.30
25
Average
3.10
25.4
186
APPENDIX E
INITIAL PERMEABILITY TEST
1.
Apparatus for Constant Head Permeability
Figure E1: The piston sample
using for permeability test
Figure E2: The equipment for
permeability test
Figure E3: Constant Head permeability test
187
2.
Procedure of Constant Head Permeability
The apparatus for constant head permeability such a: (a) Permeameter cell,
fitted with loading piston, perforated plates, flow tube connections, piezometer
nipples and connections, air bleed valve, sealing rings, (b) Glass piezometer tubes,
(c) Rubber tubing, (d) Uniform fine gravel, or glass balls, for end filter layers, (e)
Two disc of wire gauze, of the same diameter as the internal cell diameter, (f) Two
porous stone or sintered bronze disc of the same diameter, (g) Measuring cylinders:
500 ml and 100 ml, (h) Constant head reservoir, (i) Outlet reservoir with overflow to
maintain a constant water level, (j) Supply of clean water, (k) Small tools: funnel,
tamping rod, scoop, etc., (l) Thermometer, (m) Stop-clock (minutes timer). The
general arrangement diagram of the test system is shown in figure E4.
Figure E4: General arrangement for Constant Head permeability test (downward
flow)
188
The test procedure for constant head permeability:
1.
Preparation of ancillary apparatus.
2.
Preparation of permeameter cell.
3.
Selection of sample.
4.
Preparation of test sample.
5.
Placing sample in cell.
6.
Assembling cell.
7.
Connections to cell.
8.
Saturation of sample.
9.
Connections for test.
10.
Running the test.
11.
Repeat tests.
12.
Dismantling cell.
13.
Calculations.
For the calculations, a quantity of water Q ml flows through a sample in a
time of t min, the mean rate of flow q is equal Q/t ml/min or Q/60t ml/s. The
hydraulic gradient i between two adjacent manometers points a distance L mm apart,
giving manometer levels h1, h2 mm above a datum, is calculated from the equation:
i=
h1 - h 2
L
If the area of cross-section of the sample is equal to A mm2, the permeability KT
(m/s) of the sample at ToC is calculated from equation:
KT =
Q
60 Ait
189
3.
Results of Constant Head Permeability Test of Fibrous Peat Soil Samples
obtained from Kampung Bahru, Pontian, West Johore
A.
Results of Horizontal Samples
Table E3: Typical test results for horizontal samples no. 1
Hydraulic
gradient,
i
3.34
4.99
6.64
7.47
7.88
8.29
Horizontal rate Horizontal rate Horizontal flow
of flow,
velocity,
of flow,
v (m/s)
q (ml/min)
q (m3/s)
10.31
0.00000017
0.00001970
13.85
0.00000023
0.00002646
20.47
0.00000034
0.00003910
22.02
0.00000037
0.00004206
27.07
0.00000045
0.00005171
24.99
0.00000042
0.00004774
Horizontal flow velocity,
(v, m/s)
6.E-05
5.E-05
4.E-05
3.E-05
2.E-05
kh = 5.90 x 10-6 m/s
kh (20 ˚C) = 4.84 x 10-6
/
1.E-05
0.E+00
0
1
2
3
4
5
6
7
8
9
Hydraulic gradient, i
Figure E5: Horizontal flow velocity (v) versus hydraulic gradient (i) for sample 1
Table E1: Data of coefficient of permeability at 20°C for horizontal samples
Dry
Moisture
Horizontal Total mass Total volume
Bulk
sample
of initial
of initial
density, content, density,
no.
soil sample,
sample,
1
2
3
Average
MT
(kg)
0.960
1.028
0.960
0.995
VT
ρ
(m3)
(kg/m3)
0.0010574838 907.82
0.0010574838 972.12
0.0010574838 907.82
0.0010574838 940.915
ω
(%)
460.50
522.64
609.60
564.375
ρd
(kg/m3)
161.97
156.13
127.93
573.64
Initial Horizontal
void coefficient of
ratio, permeability
at 20°C,
eo
8.36
8.71
10.85
9.70
kh (20°C)
(m/s)
0.00000484
0.00000745
0.00027200
0.00009476
190
B. Results of Vertical Samples
Table E2: Typical test results for vertical samples no. 1
Hydraulic
gradient,
i
3.34
4.99
6.64
7.47
7.88
8.29
Horizontal rate Horizontal rate Horizontal flow
of flow,
velocity,
of flow,
v (m/s)
q (ml/min)
q (m3/s)
85.67
0.00000143
0.00016365
142.92
0.00000238
0.00027301
214.62
0.00000358
0.00040997
237.77
0.00000396
0.00045419
255.03
0.00000425
0.00048716
274.67
0.00000458
0.00052467
Vertical flow velocity,
(v, m/s)
6.E-04
5.E-04
4.E-04
3.E-04
2.E-04
kv = 6.08 x 10-5 m/s
1.E-04
kv (20 ˚C) = 4.99 x 10-5
0.E+00
0
2
4
6
8
10
Hydraulic gradient, i
Figure E6: Vertical flow velocity (v) versus hydraulic gradient (i) for sample 1
Table E3: Data of coefficient of permeability at 20°C for vertical samples
Dry
Moisture
Horizontal Total mass Total volume
Bulk
sample
of initial
density, content, density,
of initial
no.
sample,
soil sample,
1
2
3
Average
MT
(kg)
0.946
0.956
0.989
0.064
VT
ρ
(m3)
(kg/m3)
0.0010574838 894.58
0.0010574838 904.03
0.0010574838 935.24
0.0010574838 911.28
ω
(%)
526.68
578.02
679.16
594.62
ρd
(kg/m3)
142.75
133.33
120.03
132.04
Initial Horizontal
void coefficient of
ratio, permeability
at 20°C,
eo
kv (20°C)
(m/s)
9.62
0.00004990
10.37 0.00021900
11.63 0.00009100
10.54
0.0001199
191
APPENDIX F
STANDARD CONSOLIDATION TEST
1.
Analysis of Time-Compression Curve
0.0
Compression (mm)
0.5
1.0
ts
cα
1.5
2.0
tp
2.5
3.0
0.1
1
10
100
1000
10000
Time, t in minutes (log scale)
Figure F1: Typical compression versus logarithmic of time-compression from
Oedometer test
192
Table F1: The results of Oedometer test
Consolidation
Pressure
Test 1
Test 2
Test 3
Test 4
Test 5
Test 6
Test 7
Test 8
Test 9
Test 10
Test 11
Test 12
Average
tp (minutes)
ts (minutes)
cα
cv (m2/year)
tp (minutes)
ts (minutes)
cα
cv (m2/year)
tp (minutes)
ts (minutes)
cα
cv (m2/year)
tp (minutes)
ts (minutes)
cα
cv (m2/year)
tp (minutes)
ts (minutes)
cα
cv (m2/year)
tp (minutes)
ts (minutes)
cα
cv (m2/year)
tp (minutes)
ts (minutes)
cα
cv (m2/year)
tp (minutes)
ts (minutes)
cα
cv (m2/year)
tp (minutes)
ts (minutes)
cα
cv (m2/year)
tp (minutes)
ts (minutes)
cα
cv (m2/year)
tp (minutes)
ts (minutes)
cα
cv (m2/year)
tp (minutes)
ts (minutes)
cα
cv (m2/year)
tp (minutes)
ts (minutes)
cα
cv (m2/year)
25 kPa
50 kPa
100 kPa
200 kPa
400 kPa
60
4200
0.252
1.087
60
5000
0.174
0.999
28
3500
0.226
3.187
29
3000
0.130
1.558
20
3500
0.022
3.273
30
4500
0.028
1.429
60
3000
0.181
1.919
40
4000
0.074
1.361
30
4000
0.198
3.478
55
3000
0.109
1.013
20
2900
0.243
2.943
30
3000
0.123
2.636
38.50
3633
0.147
2.074
60
4000
0.287
1.044
50
5000
0.158
0.581
27
3400
0.124
2.026
25
2400
0.280
1.290
18
3500
0.048
2.147
30
4300
0.028
1.330
40
2800
0.160
1.810
30
3000
0.108
1.123
25
3500
0.172
3.134
35
3000
0.152
1.006
18
2500
0.336
2.138
35
2800
0.133
2.120
32.75
3350
0.166
1.646
50
4000
0.351
0.748
40
4500
0.147
0.414
26
3200
0.176
1.366
25
2800
0.183
1.154
20
3500
0.017
1.514
30
4000
0.028
1.258
30
2500
0.143
1.744
20
3000
0.067
1.076
20
3000
0.347
2.870
35
2500
0.309
0.829
20
2500
0.179
1.894
30
1900
0.123
1.387
28.83
3117
0.173
1.355
40
3900
0.227
0.714
20
3500
0.131
0.626
24
3100
0.190
0.844
30
2900
0.165
0.574
17
3200
0.021
1.289
32
3800
0.026
1.178
30
2500
0.042
1.646
18
3000
0.080
1.001
20
3000
0.096
1.811
33
2400
0.162
0.751
18
2400
0.125
1.311
25
1800
0.101
1.275
25.58
2958
0.114
1.085
25
3800
0.250
0.545
20
3000
0.165
0.574
22
3000
0.149
0.702
23
2800
0.193
0.369
12
3000
0.033
1.094
30
3500
0.035
1.088
30
2500
0.060
1.518
9
2000
0.165
0.886
20
3000
0.085
1.261
32
2200
0.132
0.552
18
2000
0.368
1.039
25
1800
0.113
0.570
22.17
2717
0.146
0.850
193
Analysis of e-p’ and e-log p’ Curve
9
Void ratio (e)
8
7
6
5
4
3
0
100
200
300
400
500
Consolidation pressure (p', kPa)
Figure F2: Typical e-p’ curves from Oedometer test
9
8
Void ratio (e)
2.
7
6
5
4
3
10
100
Consolidation pressure (p', kPa)
Figure F3: Typical e-log p’ curves from Oedometer test
1000
194
Table F2: The results from the analysis of e-p’ and e-log p’ curve
Initial Void
Compression
Pre-consolidation
Ratio
Index
Pressure
(eo)
(cc)
(σp’)
1
10.035
3.578
47
2
7.589
2.040
45
3
11.955
4.977
46
4
11.290
5.042
44
5
8.253
2.104
45
6
7.565
3.147
44
7
10.393
2.960
43
8
10.548
4.327
44
9
11.152
4.023
46
10
10.539
2.270
40
11
9.759
2.079
45
12
10.127
2.493
46
Average
9.934
3.253
45
Test
No.
195
3.
Calculation of Permeability
Table F3: The results from permeability test
Consolidation
Pressure
Test 1
Test 2
Test 3
Test 4
Test 5
Test 6
Test 7
Test 8
Test 9
Test 10
Test 11
Test 12
Average
mv (1/kPa)
cv (m2/year)
kv (m/s)
mv (1/kPa)
cv (m2/year)
kv (m/s)
mv (1/kPa)
cv (m2/year)
kv (m/s)
mv (1/kPa)
cv (m2/year)
kv (m/s)
mv (1/kPa)
cv (m2/year)
kv (m/s)
mv (1/kPa)
cv (m2/year)
kv (m/s)
mv (1/kPa)
cv (m2/year)
kv (m/s)
mv (1/kPa)
cv (m2/year)
kv (m/s)
mv (1/kPa)
cv (m2/year)
kv (m/s)
mv (1/kPa)
cv (m2/year)
kv (m/s)
mv (1/kPa)
cv (m2/year)
kv (m/s)
mv (1/kPa)
cv (m2/year)
kv (m/s)
mv (1/kPa)
cv (m2/year)
kv (m/s)
25 kPa
50 kPa
100 kPa
200 kPa
400 kPa
0.00645
1.087
0.00221
1.044
0.00124
0.748
0.00038
0.714
0.00025
0.545
2.2215x10-10
7.3261x10-11
2.9388 x10-11
8.6714 x10-12
4.3205 x10-12
0.00591
0.999
0.00373
0.581
0.00118
0.414
0.00041
0.626
0.00034
0.574
1.8722 x10-10
6.8627 x10-11
1.5452 x10-11
8.1585 x10-12
6.1885 x10-12
0.00393
3.187
0.00191
2.026
0.00111
1.366
0.00055
0.844
0.00026
0.702
3.9686 x10-10
1.2245 x10-10
4.8037 x10-11
1.4586 x10-11
5.6986 x10-12
0.00461
1.558
0.00231
1.290
0.00108
1.154
0.00040
0.574
0.00028
0.369
2.2780 x10-10
9.4492 x10-11
3.9374 x10-11
7.2260 x10-12
3.3114 x10-12
0.00751
3.273
0.00365
2.147
0.00227
1.514
0.00011
1.289
0.00008
1.094
7.7943 x10-10
2.4849 x10-10
1.0898 x10-11
4.4961 x10-12
2.7752 x10-12
0.00631
1.429
0.00347
1.330
0.00192
1.258
0.00014
1.178
0.00010
1.088
2.8593 x10-10
1.4634 x10-10
1.5708 x10-11
5.0428 x10-12
3.4500 x10-12
0.00391
1.919
0.00114
1.810
0.00248
1.744
0.00019
1.646
0.00012
1.518
2.3799 x10-10
6.5660 x10-11
1.3715 x10-10
9.9169 x10-12
5.7763 x10-12
0.00473
1.361
0.00321
1.123
0.00195
1.076
0.00047
1.001
0.00064
0.886
2.0405 x10-10
1.1431 x10-10
4.6994 x10-12
1.4855 x10-11
1.7981 x10-11
0.00546
3.478
0.00245
3.134
0.00135
2.870
0.00057
1.811
0.00045
1.261
6.0229 x10-10
2.4318 x10-10
1.2286 x10-10
3.2733 x10-11
1.7994 x10-11
0.00492
1.013
0.00137
1.006
0.00071
0.829
0.00061
0.751
0.00035
0.552
1.5801 x10-10
4.3703 x10-11
1.8690 x10-11
1.4408 x10-11
6.1263 x10-12
0.00577
2.943
0.00305
2.138
0.00216
1.894
0.00008
1.311
0.00002
1.039
5.3809 x10-10
2.0644 x10-10
1.2973 x10-10
3.4546 x10-12
6.5893 x10-13
0.00705
2.636
0.00441
2.120
0.00302
1.387
0.00036
1.275
0.00023
0.570
5.8929 x10-10
2.9660 x10-10
1.3282 x10-11
1.4353 x10-11
4.1572 x10-12
0.00555
2.074
0.00274
1.646
0.00171
1.355
0.00035
1.085
0.00026
0.850
3.6910- x10-10
1.4363 x10-10
4.8772 x10-11
1.1492 x10-11
6.5364 x10-12
196
APPENDIX G
SYSTEM CALIBRATION FOR CONSOLIDATION TEST ON ROWE CELL
1.
Pore Pressure 1
197
Figure G1: The calibration for pore pressure 1
198
2.
Axial Displacement
Figure G2: The calibration for axial displacement
199
3.
Pore Pressure 2
Figure G3: The calibration for pore pressure 2
200
4.
Volume Change
Figure G4: The calibration for volume change
201
5.
Diaphragm Pressure
Figure G5: The calibration for diaphragm pressure
202
6.
Back Pressure
Figure G6: The calibration for back pressure
203
APPENDIX H
LARGE STRAIN CONSOLIDATION AND PERMEABILITY TEST
(ROWE CELL)
1.
Large Strain Consolidation Test (Rowe cell)
a.
Procedures for Analysis of Time-Compression Curve based on
Robinson’s (2003) Method
Step 1: Plot the compression (mm) versus the logaritmic of time curves from large
strain consolidation test (Rowe cell).
0
Compression (mm)
1
2
3
4
cα
5
ts
tp
6
7
8
0.1
1
10
100
1000
10000
Time, t in minutes (log scale)
Figure H1: Typical compression versus the logaritmic of time curves from large
strain consolidation test (Rowe cell)
204
Step 2: Plot the dissipation of excess pore water pressure in percentage versus the
logarithmic of time. The starting and ending points of the excess pore
water pressure dissipation curve are defined as the beginning and ending of
primary consolidation of the soil (d0 and d100) and their corresponding times
are denoted by t0 and t100 respectively. Based on excess pore water pressure
measurement, vertical coefficient of rate of consolidation (cv) can be
determined.
0
Dissipation of excess pore water
pressure, Uv (%)
10
20
30
40
50
60
70
80
90
100
1
10
100
Time, t in minutes (log scale)
Figure H2: Typical the dissipation of excess pore water pressure versus the
logarithmic of time from large strain consolidation test (Rowe cell)
Step 3: Plot the compression versus the dissipation of excess pore water pressure (in
percentage). The point where the curve diverges from linearity is identified
as
the
beginning
of
secondary
compression.
The
compression
corresponding to the point where the straight line meets the U = 100% axis
is the total primary consolidation settlement (δp), while the compression
below the extrapolated line is the secondary compression (δs).
205
0.00
Primary
consolidation
0.05
Compression (mm)
0.10
Uv
0.15
0.20
0.25
0.30
Secondary
compression
0.35
δs
cα
0.40
0
10
20
30
40
50
60
70
80
90
100
Dissipation of excess pore water pressure, Uv (%)
Figure H3: Typical compression versus the dissipation of excess pore water pressure
curve from large strain consolidation test (Rowe cell)
Step 4: Plot the total settlement corresponding to the dissipation of excess pore
water pressure against the logarithmic of time. Subtract the secondary
compression (δs) during the dissipation of excess pore water pressure from
the total compression of soil to give primary consolidation of soil free from
the influence of secondary compression.
0
Total Settlement(mm)
0.4
0.8
1.2
1.6
2
0.1
1
10
100
Elapsed Time (minutes)
Figure H4: Typical total settlement versus the logarithmic of time curve from large
strain consolidation test (Rowe cell)
206
Step 5: Plot the primary consolidation versus the logarithmic of time. It can be
seen that this curve indicate the end of primary consolidation at time tp
similar with that obtained from the excess pore water pressure dissipation
curve (Figure H3).
Primary Settlement(mm)
0
0.2
0.4
0.6
0.8
1
0.1
1
10
100
Elapsed Time (minutes)
Figure H5: Typical primary consolidation versus the logarithmic of time curve from
large strain consolidation test (Rowe cell).
Step 6: Plot the secondary compression during the dissipation of excess pore water
pressure from soil (δs) against their corresponding time (t-to). The
coefficient of secondary compression of soil (cα) is determined by dividing
the slope of the linear relationship between the secondary compression
during the dissipation of excess pore water pressure from soil (δs) and their
corresponding time (t-to), by the thickness of the consolidating soil layer, H.
Secondary compression, δs (mm)
0.2
δs = 0.1273
0.15
0.1
0.05
0
0
0.2
0.4
0.6
0.8
1
Time (t - to) (t and to are in minutes)
Figure H6: Typical secondary compression during the dissipation of excess pore
water pressure curve from large strain consolidation test (Rowe cell).
207
b.
Analysis of Consolidation Parameters
Table H1: The results of Rowe consolidation test
Consolidation
Pressure
Test 1
Test 2
Test 3
Test 4
Test 5
Test 6
Average
t100 (minutes)
U(%)
tp (minutes)
ts (minutes)
cv (m2/year)
cα
t100 (minutes)
U(%)
tp (minutes)
ts (minutes)
cv (m2/year)
cα
t100 (minutes)
U(%)
tp (minutes)
ts (minutes)
cv (m2/year)
cα
t100 (minutes)
U(%)
tp (minutes)
ts (minutes)
cv (m2/year)
cα
t100 (minutes)
U(%)
tp (minutes)
ts (minutes)
cv (m2/year)
cα
t100 (minutes)
U(%)
tp (minutes)
ts (minutes)
cv (m2/year)
cα
t100 (minutes)
U(%)
tp (minutes)
ts (minutes)
cv (m2/year)
cα
25 kPa
50 kPa
100 kPa
200 kPa
25
60
20
1500
8.802
0.103
30
68
19
900
4.084
0.117
25
50
20
1700
7.748
0.118
26
60
18
1300
5.458
0.102
31
68
22
900
3.542
0.108
29
64
20
1000
4.500
0.105
27.67
61.67
19.83
1216.67
5.689
0.109
24
60
18
1300
8.031
0.166
28
85
19
900
4.076
0.127
21
50
14
1300
6.939
0.124
25
58
15
1200
3.878
0.103
30
72
22
850
3.214
0.115
27
65
18
850
3.546
0.109
25.83
65.00
17.67
1066.67
4.947
0.124
23
58
16
1200
7.695
0.226
20
70
16
600
4.015
0.148
20
64
12
700
5.130
0.193
24
80
15
1100
2.489
0.120
28
70
19
900
3.000
0.128
26
75
17
750
2.744
0.124
23.50
69.50
15.83
875.00
4.179
0.157
22
55
12
900
6.985
0.304
20
70
15
550
3.885
0.255
20
70
10
500
4.000
0.248
24
85
14
1000
1.427
0.175
27
67
19
850
1.695
0.132
25
76
16
700
1.561
0.154
23.00
70.50
14.33
750.00
3.259
0.211
208
Analysis of e-p’ and e-log p’ Curve
9
Void ratio (e)
8
7
6
5
4
0
50
100
150
200
250
Consolidation pressure (p', kPa)
Figure H7: Typical e-p’ curves from Rowe consolidation test
9
8
Void ratio (e)
c.
7
6
5
4
10
100
Consolidation pressure (p', kPa)
Figure H8: Typical e-log p’ curves from Rowe consolidation test
1000
209
Table H2: The results from analysis of e-p’ and e-log p’ curve
1
Initial Void
Ratio
(eo)
8.814
Compression
Index
(cc)
3.189
Preconsolidation
Pressure
(σp’)
43
2
9.048
3.122
42
3
9.321
3.085
40
4
8.513
3.108
40
5
8.781
3.116
43
6
8.6477
3.147
40
Average
8.854
3.128
41
Test No.
d.
Analysis of Permeability based on Rowe Consolidation Tests
Table H3: The results of permeability based on Rowe Consolidation tests
Consolidation
Pressure
Test 1
Test 2
Test 3
Test 4
Test 5
Test 6
Average
mv (1/kPa)
cv (m2/year)
kv (m/s)
mv (1/kPa)
cv (m2/year)
kv (m/s)
mv (1/kPa)
cv (m2/year)
kv (m/s)
mv (1/kPa)
cv (m2/year)
kv (m/s)
mv (1/kPa)
cv (m2/year)
kv (m/s)
mv (1/kPa)
cv (m2/year)
kv (m/s)
mv (1/kPa)
cv (m2/year)
kv (m/s)
25 kPa
50 kPa
100 kPa
200 kPa
0.00070
8.802
0.00142
8.031
0.00125
7.695
0.00078
6.985
1.95377x10-10
3.61619x10-10
3.05009x10-10
1.72764x10-10
0.00135
4.084
0.00278
4.076
0.00127
4.015
0.00032
3.885
1.74829x10-10
3.59313x10-10
1.61690x10-10
3.94216x10-11
0.00041
7.748
0.00074
6.939
0.00042
5.130
0.00021
4.000
1.62825x10-10
6.83219x10-11
2.66362x10-11
0.00013
5.458
0.00330
3.878
0.00273
2.489
0.00194
1.427
2.24994x10-11
4.05803x10-10
2.15467x10-10
8.77848x10-11
0.00020
0.464
2.24994x10-11
0.00024
0.421
0.00014
0.393
0.00010
0.222
2.44597x10-10
1.33181x10-10
5.37481x10-11
2.24632E-11
4.500
0.00177
3.546
0.00144
2.744
0.00102
1.561
2.24813x10-11
3.25200x10-10
1.74324x10-10
7.07664x10-11
0.00049
5.689
0.00171
4.947
0.00121
4.179
0.00073
3.259
1.00732x10-10
8.97302x10-11
3.09893x10-10
1.76332x10-10
7.51869x10-11
210
2.
Permeability Test
Table H4: The results of permeability tests under consolidation pressure of 200 kPa
Type of
permeability test
Two-way
Vertical
Drainage
Permeability
test
Test 1
Test 2
Test 3
Test 4
Test 5
Test 6
Average
Consolidation
Pressure
(kPa)
Vertical coefficient
of permeability
at 20o C (kv, m/s)
200
200
200
200
200
200
2.36 x 10-10
8.82 x 10-10
3.43 x 10-10
4.54 x 10-10
4.02 x 10-10
7.25 x 10-10
5.07 x 10-10
Table H5: The results of permeability tests under consolidation pressure of 100 kPa
3.
Type of
permeability test
Permeability
test
Two-way
Vertical
Drainage
Test 1
Test 2
Test 3
Average
Consolidation
Pressure
(kPa)
100
100
100
Vertical coefficient
of permeability
at 20o C (kv, m/s)
2.10 x 10-9
1.32 x 10-9
3.83 x 10-9
2.42 x 10-9
Apparatus
Figure H9: Two independently Figure H10: Power supply and readout
controlled water pressure systems, unit for the electric pore pressure
giving maximum pressure up to 1000 transducer
kPa used for large strain consolidation
and permeability tests in laboratory
211
Figure H11: Volume change
gauge
Figure H12: Sintered bronze disc of 4 mm
thickness
Figure H13: Rowe cell top attached to
diaphragm
Figure H14: Rowe cell body of 151.4
mm internal diameter
Figure H15: Rowe cell base
Figure H16: Bolt tightened Rowe cell
connected to linear transducer
212
Figure H17: A burette connected to Rowe consolidometer for large strain
permeability test