SIMULATION OF RIVER EMBANKMENT STABILITY: A CASE STUDY
ON FAILURE AND REMEDIAL METHOD AT MUAR RIVER,
PANCHOR, JOHOR
SITI NORAZELA BINTI HASAN
UNIVERSITI TEKNOLOGI MALAYSIA
PSZ 19:16 (Pind. 1/07)
UNIVERSITI TEKNOLOGI MALAYSIA
DECLARATION OF THESIS / UNDERGRADUATE PROJECT PAPER AND COPYRIGHT
Author’s full name :
SITI NORAZELA BINTI HASAN
Date of birth
:
6th AUGUST 1978
Title
: SIMULATION OF RIVER EMBANKMENT STABILITY; A CASE STUDY ON FAILURE
AND REMEDIAL METHOD AT MUAR RIVER, PANCHOR, JOHOR
Academic Session :
2009/2010 – SEM 2
I declare that this thesis is classified as:
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Date : 30TH APRIL 2010
NOTES
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PROF. DR. KHAIRUL ANUAR KASSIM
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Date :30th APRIL 2010
:*If the thesis is CONFIDENTAL or RESTRICTED, please attach with the letter from the
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this report is sufficient in terms of scope and quality for the
award of Master of Engineering (Civil-Geotechnics)”.
Signature
: ………………………………………………………
Name of Supervisor : PROF. DR. KHAIRUL ANUAR KASSIM
Date
: 30th APRIL 2010
SIMULATION OF RIVER EMBANKMENT STABILITY: A CASE STUDY ON
FAILURE AND REMEDIAL METHOD AT MUAR RIVER, PANCHOR, JOHOR
SITI NORAZELA BINTI HASAN
A report submitted in partial fulfillment of the
requirement for the award of the degree of Master of
Engineering (Civil-Geotechnics)
Faculty of Civil Engineering
Universiti Teknologi Malaysia
APRIL 2010
ii
“I declare that this project report is my own work except for the quotations and
summarizes which I have explained the source”
Tandatangan :
…………………………….
Nama Penulis :
SITI NORAZELA BINTI HASAN
Tarikh
30 April 2010
:
iii
To my beloved hubby Shermann and son Aiman Syahmi Shermann and those
who love me
iv
ACKNOWLEDGEMENTS
I would like to deeply praise the Almighty ALLAH SWT for allowing me passing all of
this moment and also I would like to take this opportunity to express my sincere
gratitude to all those who have contributed in completing this master project.
I wish to gratefully express my sincere gratitude to my supervisor of master project, PM.
Dr. Khairul Anuar Kassim for his gratitude, inspiration, support and friendship.
I would like to thank my boss, Mr. Ng Kim Hoy and to senior engineer, Mr. Ng Kok
Seng as for sharing the knowledge for me to accomplished these Master project. To my
hubby, Shermann Shamshudin and my son Aiman Syahmi Shermann for their
understanding and support and special thanks to my parents, friends - Zalina Mohamed,
Azhani Zukri, Vignesh Waran, Thanath and Vijay Kumar for the moral support.
v ABSTRACT
The soil movement on failed slope had caused substantial failure of soldier pile
wall at Muar River embankment, Panchor town, Johor. The existing retaining wall has
totally collapsed during low tide period is due to insufficient embedded length of
existing wall system and failure due to excessive deformation of the wall and slope
sliding under backfilling surcharge and human and traffic activities. To facilitate
investigating causes of the failure, a computer simulation of slope stability using
SLOPE/W is performed to simulate slope condition before and after construction of the
study area and to check the total displacement after the construction by using PLAXIS
V8.2. The river embankment collapsed during low tide period thus, the calculated back
analysis of factor of safety (FOS) is based on the different at every changes of water
level.The result of simulation analysis established the fact that global soil mass had a
lateral movement direction toward to installed soldier pile wall generates a combination
of mobilized shear force and lateral pressure larger than the capacity or strength of the
soldier pile wall. Furthermore, the simulation analysis deduces that the slope instability
become greater as moisture or pore-water pressure in the slope increase or decrease in
soil’s shear strength. FOS determined is 0.966 during low tide period where the existing
retaining wall has totally collapsed. Therefore there are 3 options of methods to be
introduced to overcome the failure which are all the options introduced show the FOS
ranging from 1.378 to 1.435. The anticipated settlement is in the order of 409mm over
25 years after construction.
vii ABSTRAK
Pergerakan bumi atas cerun telah menyebabkan kegagalan tembok cerucuk di Sungai
Muar, Pekan Panchor, Johor. Tembok cerucuk awalnya telah mengalami kegagalan
sewaktu air surut dan ianya berlaku disebabkan kedalaman tembok cerucuk yang tidak
mencukupi serta mengalami pergerakkan yang disebabkan oleh beban dan aktiviti
lalulintas di atasnya. Perisian SLOPE/W digunakan bagi menyiasat kegagalan sebelum
dan selepas pembinaan dan manakala perisian PLAXIS V8.2 juga digunakan untuk
menyemak pergerakan total selepas aktiviti pembinaan di kawasan tersebut. Oleh kerana
tambakan mengalami kegagalan sewaktu air surut, analisis bagi nilai faktor keselamatan
disemak berdasarkan pada setiap perubahan paras air. Keputusan analisis simulasi
menunjukkan bahawa keseluruhan tanah mengalami pergerakan sisi menuju ke arah
tembok cerucuk dan menghasilkan daya ricih dan tekanan sisi yang diaruh oleh
pergerakan ini adalah lebih besar daripada kekuatan tembok cerucuk. Keputusan analisis
ini juga mendapati bahawa kestabilan cerun akan terjejas dengan kenaikan tekanan air
atau dengan penurunan kekuatan ricih tanah. Oleh yang demikian, nilai faktor
keselamatan yang diperolehi sewaktu air surut adalah 0.966. bagi mengatasi masaalah
keruntuhan tembok cerucuk ini, 3 jenis kaedah kerja pembaikan dikenalpasti dan
dianalisa setiap satunya. Berdasarkan analisa yang dibuat, ketiga-tiga kaedah ini
memberi nilai faktor keselamatan di antara 1.378 - 1.435. Manakala tanah mengalami
pemendapan sebanyak 409mm bagi tempoh 25 tahun selepas pembinaan.
vii
TABLE OF CONTENT
CHAPTER
1
2
ITEM
PAGE
THESIS TITLE
i
DECLARATION
ii
DEDICATION
iii
ACKNOWLEDGEMENTS
iv
ABSTRACT
v
ABSTRAK
vi
TABLE OF CONTENT
vii
LIST OF TABLE
xii
LIST OF FIGURE
xiii
LIST OF SYMBOLS
xvii
INTRODUCTION
1
1.1
General
1
1.2
Problem statement
2
1.3
Objective and of Scope the Study
5
1.4
Limitation of the Study
5
1.5
Research Area
7
LITERATURE REVIEW
8
2.1
Introduction
8
2.2
Soft Soil
9
2.2.1 Soil Classification
9
2.3.2
Characteristic of Clay Soil
9
2.3.3
Problem of Clay Soil
10
2.3
Stability of Slope Embankment
12
viii
2.4
2.3.1
Type of Slope
12
2.3.2
Mode of Failure
13
2.3.3
Factor of Safety
14
2.3.4
Soft Soil behavior under the
Embankment
15
2.3.4.1 Settlement
15
2.3.4.2 Lateral Movement
18
2.3.5 Slope Stabilization Method
23
Review of Slope Stability Analysis
24
2.4.1 Type of Analysis
26
2.4.2
2.4.1.1 Method of Slice
27
Basic Requirements for Slope Stability
29
Analyses
2.4.3 Source of Uncertainty in Slope Stability
30
Analysis
2.4.4
2.4.3.1 Parameter Uncertainty
30
2.4.3.2 Model Uncertainty
31
2.4.3.3 Human Uncertainty
31
Selection of Parameter and Its
Variability
2.4.5
2.4.6
3
32
The Use of Finite Element Software
Package
33
Computer Modeling
35
2.4.6.1 PLAXIS
36
RESEARCH METHODOLOGY
38
3.1
Introduction
38
3.2
Regional Geology and Site Topography
40
3.3
Literature Review
43
3.4
Borehole with Standard Penetration
43
Test (SPT)
ix
3.5
Vane Shear Test
3.5.1
44
Undrained Shear Strength
of Cohesive Soils – general
evaluation basic
3.6
Laboratory Test
46
3.7
Material Properties
46
3.7.1
46
Soil classification Test
3.7.2 Particle Size Distribution
46
3.7.3
47
Atterberg Limits
3.7.4 Consolidation Test
3.7.5
3.7.6
3.8
4
45
47
Triaxial Test – Unconsolidated
Undrained
48
Sheet pile Wall
48
3.7.6.1 Limit State design
49
3.7.6.2 Fixed Earth Design
49
3.7.6.3 Softened Zone
50
Mathematical Modeling and Simulation
50
3.8.1
Finite Element Program
50
3.8.2
Features of PLAXIS
51
3.8.3
Type of Soil Model
52
ANALYSIS AND DISCUSSION
53
4.1
Introduction
53
4.2
Failure Occurred – Case Study
55
4.2.1 Ground Profile
55
4.2.2
Evaluation of Geotechnical Parameters
57
4.2.3
Factor of Safety
58
4.3
4.2.4 Back Analyses
58
Remedial Method
69
4.3.1
Option 1 - Continuous Sheet Pile Wall
with Tie Back System
70
x
4.3.2
Option 2 - Geogrid Wall with Pilling
System
4.3.3
4.3.4
4.4
70
Option 3 - Wellguard wall with Pilling
System
71
Summarize of analyses using SLOPE/W
71
Analyses by PLAXIS
81
4.4.1
81
Subsoil Profile and Ground
Characteristic
4.4.2 Shear Strengths
82
4.4.3 Laboratory Test Result
85
4.4.4
Deformation Characteristics
87
4.4.5
Soil Permeability
87
4.4.6 Geotechnical & Structural
Parameters
88
4.4.6.1 Sand fills for the
first 3m
88
4.4.6.2 Very Soft Clay for the
next 5m
88
4.4.6.3 Very Soft Clay
for the next 8m
88
4.4.6.4 Soft to Stiff Sandy
Clay for the next 8m
89
4.4.6.5 Very Stiff Clayey
4.5
89
4.4.6.4 Structural Members
91
Sequence of Construction
4.5.1
92
General Notes on Requirements
for the Reinstatement Works
96
Geometry of Model & Adopted Parameters
97
CONCLUSION AND RECOMMENDATIONS
99
5.1
99
4.6
5
Silt for the next 6m
General
xi
5.2
Conclusion
99
5.3
Recommendations
100
REFERENCESS
APPENDICES
Appendix A – Site Plan and Borehole Location
Appendix B – Field Testing: Vane Shear Test Result & Borelog Records
Appendix C – Laboratory Testing (Summary)
102
xii
LIST OF TABLES
TABLE
Table 1.1
ITEM
Summarizes the soil investigation works
PAGE
6
that was carried out at site
Table 2.1
Factor of safety
Table 2.2
Empirical correlation of lateral deformation on
21 embankments, (Tavenas et al., 1979)
Table 2.3
21
List of commonly used method of slice: assumption
concerning interslice force for different method of slice
Table 2.4
15
27
Characteristics of equilibrium methods of slope
stability analysis (Source: Duncan and Wright, 1980)
28
Table 2.5
Coefficient of Variation for Geotechnical Parameter
33
Table 3.1
Consistency of Clay versus N
44
Source: Terzaghi and Peck, R.B
Table 4.1
Interpreted Subsurface Profiles
Table 4.2
Geotechnical Parameters
Table 4.3
Recommended factor of safety for new slopes
(After Geotechnical Control Office, Hong Kong, 1984)
Table 4.4
55
57
58
Summarize of advantages and disadvantages of the
every option
69
Table 4.5
Summarize of analyses using SLOPE/W
71
Table 4.6
Drained Shear Strength Parameter
83
Table 4.7
Variation of Shear Strength and Deformation
Table 4.8
Parameters
90
Technical data for Sheet Pile Wall
92
xiii
LIST OF FIGURES
FIGURE
ITEM
Figure 2.1
Uncertainties in Soil Properties
Figure 2.2
PAGE
(Source: Christian, Ladd, Beacher, 1994)
16
Vertical displacement at the embankment
17
toe versus relative embankment height
(Hunter and Fell, 2003)
Figure 2.3
Vertical displacement beyond toe versus
17
relative embankment height
(Hunter and Fell, 2003)
Figure 2.4
Typical relation between maximum horizontal
19
displacement, ym and settlement, s under the
center of the embankment (Lerouiel et al.,1990)
Figure 2.5
Lateral surface displacements at embankment
20
toe versus relative embankment height
(Hunter and Fell, 2003)
Figure 2.6
Maximum lateral deformation for the 3m
22
control embankment at Muar Trial compared
with the selected empirical method
(Asrul Azam and Huat 2003)
Figure 2.7
Maximum lateral deformation for the 6m
23
control embankment at Muar Trial compared
with the selected empirical method
(Asrul Azam and Huat 2003)
Figure 2.8
Uncertainties in Soil Properties
31
(Source: Christian, Ladd, Beacher, 1994)
Figure 3.1
Flow Chart of Study Methodology
39
Figure 3.2
Regional Geology
41
Figure 3.3
Site Topography
42
Figure 3.4
Fixed Earth Support
50
xiv
Figure 4.1
(Before failure condition) Soldier Wall are
53
used and anchored to the pile size
150mm x 150mm
Figure 4.2
Site location
54
Figure 4.3
Borelog
56
Figure 4.4
Existing slope profile before failure
60
@ CH 250
Figure 4.5
Original profile (full water level)
61
@ CH 250
Figure 4.6
Original profile (full water level)
62
at chainage 250 with FOS = 1.546
Figure 4.7
Original profile (water level – 1m)
63
- CH 250
Figure 4.8
Original profile (water level – 1m)
64
@ CH 250 with FOS = 1.306
Figure 4.9
Original profile (water level – 2m)
65
@ CH 250
Figure 4.10
Original profile (water level – 2m)
66
@ CH 250 with FOS = 1.090
Figure 4.11
Original profile (water level – 3m)
67
@ CH 250
Figure 4.12
Original Profile (water level – 3m)
68
@ CH 250 with FOS = 0.966
Figure 4.13
Option 1- Continuous Sheet Pile Wall
72
with Tie Back System (Chainage 250)
Figure 4.14
Option 2 - Geogrid Wall with Pilling
73
System (Chainage 250)
Figure 4.15
Option 3 - Wellguard Wall with Pilling
74
System (Chainage 250)
Figure 4.16
Option 1- Continuous Sheet Pile Wall
75
Profile (Chainage 250)
Figure 4.17
Option 1- Continuous sheet pile wall profile
@ CH 250 with FOS =1.435
76
xv
Figure 4.18
Option 2 - Geogrid wall with pilling profile
77
@ CH 250
Figure 4.19
Option 2 - Geogrid wall with pilling profile
78
@ CH 250 with FOS = 1.437
Figure 4.20
Option 3 - Wellguard wall with pilling profile
79
@ CH 250
Figure 4.21
Option 3 - Wellguard wall with pilling profile
80
@ CH 250 with FOS = 1.378
Figure 4.22
Plot the undrained shear strength versus
82
depth from S.I works.
Figure 4.23
Undrained shear strength of Port Klang
84
marine clay (after Dr. Ting Wen Hui)
Figure 4.24
Su determined from the field vane
84
shear test (VST) as a function
of the plasticity index (after Skempton)
Figure 4.25
Water content plot
85
Figure 4.26
Plasticity Chart
86
Figure 4.27
Cold Formed Sheet Pile Wall
91
Figure 4.28
Cold Formed Sheet Pile Wall – Z Section
91
Figure 4.29
Install 6 meter continuous Sheet Pile as
92
temporary protection
Figure 4.30
Excavation and Backfill crusher aggregate
93
as working platform
Figure 4.31
Install 250mm dia. Spun Pile at 2m C/C
94
and construct pile cap
Figure 4.32
Lay a layer of Geogrid (GX 600/50)
94
and backfill with sand
Figure 4.33
Install 20m length continuous Sheet Pile Wall
95
Figure 4.34
Tie back 20m continuous Sheet Pile Wall with
95
6m Sheet Pile Wall
Figure 4.35
Finite Element Model
97
Figure 4.36
Model Connectivities (Mesh)
97
Figure 4.37
Mode & Magnitude of total displacement
98
after construction
xvi
Figure 4.38
Rate and deformation magnitude of
settlement after construction
98
xvii
LIST OF SYMBOLS
c
:
Cohesion of Soils
Cc
:
Compression Index
Cv
:
Coefficient of Consolidation
D
:
Total deformed clay thickness
E
:
Modulus of elasticity
Gs
:
Specific gravity
Hnc
:
Threshold height
I
:
Moment of inertia
Ip
:
Plasticity Index
mv
:
Ceofficient of Volume Change
s
:
Settlement
Su
:
Undrained Shear Strength
wL
:
Liquid limit
wP
:
Plastic Limit
ym
:
Maximum Horizontal Displacement
σo
:
Initial total stress
σvo’
:
Initial effective stress
γd
:
Dry unit weight
γs
:
Saturated unit weight
CHAPTER I
INTRODUCTION
1.1
General
Evaluating the stability of the slope in soil is an important, interesting, and
challenging aspect of civil engineering. Concern with the slope stability has driven
some of the most important advance in our understanding of the complex behavior of
soil.
Experience with the behavior of slope and often with their failure, has led to
development of improved understanding of the changes in soil properties that can
occur over time, recognition of the requirements and the limitations of laboratory and
in situ testing for evaluating soil strength, developments new and more effective
types of instrumentation to observe the behavior of slope, improved understanding of
the principles of soil mechanics that connect soil behavior to slope stability, and
improved analytical procedures augmented by extensive examination of
the
mechanics of slope stability analyses, detailed comparisons with field behavior and
use of computers to perform thorough analyses.
This study will focus on riverbank slope failure and remedial method have
been done and to analyze the failure before and after the construction. Although
many mitigation works had been planned and designed prior to the construction of
the project, there still exist many uncertainties associated with the material, spanning
from it is complex origin.
2 The emergence of development in construction industry has minimized the
preferred site of geotechnical quality for construction although these sites are known
2 to reduce technical problems and thus the cost associated with their construction. By
that, socio-economic and political considerations have forced the use of sites of
lower quality and in particular, the sites covered by compressible soils. In developed
country such as Malaysia, the chances to have good quality construction sites
become rarer and it is necessary to choose sites that include compressible soils,
especially for industrial structures and transportation projects. Therefore, the tasks to
do constructions on these compressible soils have become a challenge for
geotechnical engineers all over the world.
Soils with characteristics of low strength and compressible exist all over the
world. One of the most significant problems arises because of its characteristics that
are difficulties in supporting loads on such foundation. The problem arises with low
strength is that it leads to difficulties in guaranteeing the stability of the structure on
this type of soil. On the other hand, this type of soil also associated with high
compressibility which leads to large settlements and deformations of the structure.
Clays, referring to the United Soil Classification System, are fine-grained
soils with more than 50% by weight passing No.200 US Standard sieve (0.075 mm).
Soft clay is defined as clay with shear strength below 25 kPa (Brand & Brenner,
1989). Soft soils have weak compressibility and known to engineers as very complex,
problematic, and treacherous materials. That is why many structures constructed on
soft clay experiencing failure.
Because of this, it is important to continue research effort on this problem in
order to resolve the problems posed by construction on soft clay.
1.2
Problem Statement
3 There are many circumstances in slope, where the civil engineer must
investigate the stability of slope by performing stability analysis. The construction on
soft cohesive soil is increasing lately because there are too many suitable sites for
construction or infrastructures or any other developments. The problems related to
this type of soil are stability and settlement. Due to that the understanding of
knowledge of engineering characteristic on soft clay are critical and should
understand by people related to this field. The selection of construction method on
this formation is restricted by cost, duration of completion and benefit.
The development in South East Asia had been so rapid that studying the soft
clay is very important. However the study have been done mostly concentrated on
major cities such as Kuala Lumpur, Singapore, Bangkok, Jakarta and
others.
Because of that the marine clay area in Muar, Johor are chosen in this study in order
to develop the simulation analysis of slope stability for riverbank of Sungai Muar,
Johor.
It is difficult to get samples from soft clay for laboratory testing, such as
shear strength. Some of the tests take a long time to complete and also need a careful
analysis. So correlations with basic properties play an important role to overcome
this problem. Besides that, the correlation of shear strength with depth could also
help the engineer to make prediction of the shear strength soil at certain depth below
ground level.
Existing method of slope stability analysis using slice (Bishop 1955, Janbu
1957) are based on the limit equilibrium theorem. An implicit assumption in
equilibrium analyses of slope stability is the stress-strain behavior of the soil is
ductile, i.e., the soil does not have a brittle stress-strain curve (where the shearing
resistance drops off after reaching a peak). This limitation result from the fact that
the method provide neither information regarding the magnitudes of the strain within
the slope, nor any indication about how they may vary along slip surface (Duncan,
4 1996). Besides it, the analysis only considered force and moment acting on the slice
with total disregard to the deformation developed in the slice. Thus, it is not possible
to obtain reliable result from the analyses of solely based on the method of slice
(Terado et al., 1999).
Thus, in order to obtain a unique solution it is necessary to introduce extra
conditions. Better analysis should therefore take into account the displacement and
deformation of the slices, and also the stresses in the soil mass in determining the
stability of slope.
In the other hand, the stability analyses are performed not only to provide a
factor of safety once the soil properties are know, but also to establish field shear
strength from the study of failures. It is rational to carry out the study determining
what actually happened after an unexpected instability has occurred. It is therefore
necessary to do some analyses in reverse, which is usually termed as ‘back analyses’.
The investigation is not mean to blame who or whom should be responsive to the
failure but it collects valuable information that could be used in designing the
remedial works as well as guidelines for further projects. The awareness of
importance of back analysis has resulted in development of various methods.
However the problem always arise in determining the suitable method of analysis
and the way back analysis can be carried out.
Failures of slopes will cause economic loss to the community. In addition to
the economic loss, sometimes there is loss of life too. There are many factors to
cause failure of a riverbank slope and very often these factors are interrelated. In the
design of a slope, stability analysis shall be carried out prior to the construction.
When the analysis results indicate undesired low factors of safety, strengthening
measures should be introduced. When a slope failed and remedial works are
required, it is essential to carry out failure investigation to find out the possible
causes. Suitable remedial design can only be carried out after knowing the causes of
failure.
5 Once the main causes of slope failure have been identified, the remedial
design can be carried out to correct the problems. Failure investigations were carried
out. The possible causes of slope failure were identified. Remedial measures adopted
were based on the investigation results, the site conditions, comparison of the
construction costs and technical knowledge of the remedial measure. . The
investigation is not mean to blame who or whom should be responsive to the failure
but it collects valuable information that could be used in designing the remedial
works as well as guidelines for further projects.
1.3
Objective and Scope of the Study
The objective of the study is:
1. To determine the stability of the slope before and after the construction of
Muar River at Panchor town, Johor.
2. To determine the total displacement after completion of construction on soft
soil
Stability analysis of slope is carried out based on the computer modeling using
SLOPE/W, limit equilibrium software and PLAXIS V8.2, a finite element package.
1.4
Limitation of the Study
The scope of the study includes several aspects as follows:
(i)
Literature review on previous embankment failure cases including the problems,
mode of failure, shear strength parameter obtain from the field test and
laboratory test and factor of safety.
6 (ii)
To simulate, verify and modify the various construction stages in term of
constructability,
stability
and
cost-effectiveness,
using
relevant
limit
equilibrium method software such as SLOPE/W.
(iii) The analysis also been conducted by using relevant finite element method such
as PLAXIS. Stresses of the soil mass along the critical slip surface as well as
the displacement and deformation are determined using the theory of finite
element.
Jabatan Pengairan dan Saliran (JPS) had commissioned Kumpulan IKRAM Sdn Bhd
to undertake soil investigation for detail study, while the JPS undertake the design of
the failure of the embankment of Sungai Muar, located at Pekan Panchor, Johor
based on the soil investigation. Field works for the investigation were carried out by
IKRAM Engineering Services Sdn. Bhd, IKRAM Selatan from 23 April 2008 to 26
April 2008, understanding supervision by Kumpulan IKRAM Sdn Bhd and JPS.
However the laboratory test was completed on 28 August 2008. Geotechnical
investigation to perform the Standard Penetration Test (SPT), carry out the provision
of disturbed, undisturbed sample and monitoring of ground water, in-situ Vane Shear
Test to determine the undrained shear strength of cohesive soil and to carry out
Laboratory Test on disturbed and undisturbed sample. The test locations are shown
in the site plan as shown in Appendix 1. Thus the result of field testing as attached in
Appendix 2.
The Table 1.1 summarizes the soil investigation works that were carried out at site.
The works were carried out in accordance with JKR specification.
Investigation works
Quantity
23 April 2008 - 26 April 2008
Boreholes
4 nos
Vane Shear Test (sampling in borehole)
4 nos
26 April 2008 - 28 August 2008
Laboratory testing
Refer Appendix 3
7 Table 1.1: Summarizes the soil investigation works that were carried out at site
Field explorations that are carried out using the Boring plant type ‘YWE’ which is
capable of boring and drilling to the depth required which was 30m deep. These
boring rings are also suitable for advancing the borehole, sampling, in-situ testing
such as vane shear test and rock drilling in accordance with the relevant specification
of each of these operations.
The methods for advancing the borehole were rotary boring, continuous sampling
rotary drilling or a combination of these methods. When undisturbed sample were
taken, a reasonably clean hole was provided and the portion of soil to be sampled
was not unduly disturbed. Disturbed sample were obtained by means of split spoon
samplers, which equipped with flap retainer or other attachments necessary for
cohesion less soil. The maximum amount of soil sample obtained was such that the
quantity is sufficient to carry out various classification tests. The vane shear test
results as attached in Appendix 4.
Soil sample were collected in the form of undisturbed. About 40mm of the soil were
removed from the top and bottom of the thin-wall sampling tube. Then the ends of
the tubes were filled with non-shrinking microcrystalline wax before sending to the
laboratory. Laboratory test that was carried out are as listed below:
(i)
Moisture Content
(ii)
Atterberg Limit
(iii)
Particle size Distribution
(iv)
Unconsolidated Undrained Triaxial Test
(v)
Consolidated test - 1D
(vi)
Visual and Manual examination
1.5
Research Area
8 This study presents the failure of embankment of Sungai Muar located at Pekan
Panchor Muar, Johor.
CHAPTER II
LITERATURE REVIEW
2.1
Introduction
Stability analyses are routinely performed in order to assess the safe and
functional design of an excavated slope (e.g. open pit mining, road cuts, etc.), fill
slope (e.g. embankment, earth dam), and/or the equilibrium conditions of a natural
slope. The analysis technique chosen depends on both site conditions and the
potential mode of failure, with careful consideration being given to the varying
strengths, weaknesses and limitations inherent in the methodology being used in the
analysis.
There are circumstances that need to be taken into consideration after the
construction of the embankment due to time such as excess pore pressure, settlement
and lateral movement. To obtain basic understanding of this study, literature reviews
focuses on three sections which is classification of soil, stability of slope and slope
stability analysis.
9 2.2
Soft Soil
2.2.1
Soil Classification
A soil consists of collection of separate particle of various shape and sizes. The
particle size analysis is to group these particles into separates ranges of sizes and so
determine the relative proportions by dry mass of each size range. Soil may be
separated into very broad categories: cohesion less, cohesive and organic soils. In the
case of cohesion less soils, the soil particles do not tend to stick together (Liu and
Evett, 2004). On the other hand, cohesive soil particle do tend to stick together and it
is categorized by very small particle size where the main element is due to effect of
surface chemical. Organic soil where the main element is due to effects of surface
chemical. Organic soils are typically spongy, crumbly and compressible. They are
undesirable for use in supporting structures.
Based on simple definition, soil can be divided into component with particle size is
usually given in term of the equivalent particle diameter (Head, 1992):
(i)
Gravel – particle from 60 mm to 2 mm
(ii)
Sand - particle from 2 mm to 0.06 mm
(iii)
Silt – particle from 0.06 mm to 0.002mm
(iv)
Clay – particle (clay mineral) smaller than 0.002 mm
(v)
Fines are particles which pass a 63 µm sieve
(vi)
Clay Fraction is the percentage of particle smaller than 2 µm, as determined
by standard sedimentation procedure.
2.2.2
Characteristic of Clay Soil
Particle forming clay consists of complex minerals which are mostly flat and platelike or elongated and of a size less than 0.002 mm. the most significant properties of
clays are its plasticity and cohesion. Clay soils able to take and retain a new shape
when compressed or moulded (Whitlow, 1995). The size and nature of the clay
10 mineral particles, together with the nature of the adsorbed may be extremely high
and the soil extremely compressible.
Cohesive soils generally exhibit undesirable engineering properties compared
with those of granular soils. They tend to have lower shear strength further upon
wetting or other physical disturbances. They can be plastic and compressible and
they expand when wetted and shrink when dry.
Clay soil can creep (deform plastically) over time under constant load,
especially when the shear stress is approaching it shear strength, making them prone
land slide. They develop large lateral pressure and have low permeability. For these
reason, clay soil are generally poor material for retaining wall backfills. Being
impervious, however they make better core material for earthen dams and dikes.
With low permeability, cohesive soils compress much more slowly because
of the expulsion of water from the small soil pores is so slow. Hence, the ultimate
volume decrease of the cohesive soil and associated settlement of a structure built on
this soil may not occur until sometime after the structure is loaded.
2.2.3
Problem of Clay Soil
Saturated cohesive soil can be used susceptible to a large amount of settlement from
structural loads. It is usually the direct weight of the structure that causes settlement
of the cohesive soil. However secondary influences such as the lowering of the
groundwater table can also lead to settlement of cohesive soils. The soil parameter
normally employed and characterized in soft soil problem are:
(i)
Classification and Index Properties, and Natural Moisture Content
(ii)
Undrained Shear Strength (Su)
(iii)
Pre-Consolidation Pressure (σp)
(iv)
Compression Index (Cc) and the Coefficient of Volume Change (mv)
(v)
Coefficient of Consolidation (Cv)
11 The parameters are very important in analyzing the behavior of this soil so
that it can carry extra loads subjected to the soils. These nature creatures are widely
found in Malaysia along the coastal plains area and with the increasing economic
development over the soil; studies were carried out to determine the typical values of
the soil that can contribute to the failure of the soil structure.
For the soft marine clay in Malaysia, Broms (1990) has reported that typical
moisture contents range from 60% to 80%. This is different to what has Ting et al.
(1988) and Chen et al. (2003) reported; where the moisture content is typically about
80% to 130% in Penang area and 50% to 100% in Klang area respectively. Brand et
al. (1989) reported that the Muar clay has the water content as high as 100% and
generally exceeds the liquid limit. It is also very common that the moisture content of
the soil clay especially near to the ground level to be higher that the liquid limit.
The in-situ undrained shear strength, Su of soft clay can be directly measured
using field vane shear test. The Su is generally increasing with depth. Typically the
vane shear test result for clay at Klang areas are about 5kPa at depth 2m to 50kPa at
depth 18m (Chen et al., 2003). This is quite similar to Muar Clay and Juru Clay
where the Su range between 10kPa at depth 2m to 35kPa at depth 18m and 10kPa to
30kPa at depth 12m respectively.
12 2.3
Stability of Slope Embankment
2.3.1
Type of Slope
The stability of the slope is the main thing that should be focused in dealing with any
project involving slope. Slopes can be either:
a) Fill slope
Fill slopes means that soils from other places are brought to the field site and
properly placed and compacted on top of the existing soils. This slope is more critical
compared to the cut slope because the properties of the transported soils might not be
the same as the natural soil. Therefore, the difference in certain soils parameters such
as cohesion and friction angle might cause the instability of the slopes. Besides that,
due to improper soil compaction also might increase the chances of the slope to fail.
b) Cut slope
Different from fill slope, cut slope is a condition where the origin soil profile were
cut with certain degree and the top soil were transported to another places. Therefore,
no fill, foreign materials or soil needed to be placed on top of the existing soil. Due
to no extra materials added, the stability of the slope can be trusted because the effect
of the different soil properties can be minimized. However, the stability can also be
questionable due to other factors such as the gradient of the slope, appearance of
discontinuities in the soil profile, soil pressure, pore water pressure and many more.
c) Rock slope
Rock slope is another type of slope that could be found in Malaysia. This kind of
slope consists majority of rock. Soils that can be found on this type of slope are the
product of weathering of the original mass rock. The solid rocks have been subjected
to different level and grade of weathering. It might take years to see the end product
13 of weathering which is soil. As we go deeper inside the soil, the rocks have less
weathering effect compared to those upper one.
d) Slopes next to water
For slopes next to water courses such as river bank slopes, beaches, pond side slopes,
etc, the slope should be robustly designed by considering the probable critical
conditions such as saturated slope with rapid drawn-down conditions, scouring of
slope toe due to flow and wave actions, etc. Properly designed riprap or other
protection measures are needed over the tidal range.
2.3.2
Mode of Failure
According to Das (1997), finite slope failure occurs in one of the following modes:
a) When the failure occurs in such way that the surface of sliding intersects the slope
at or above its toe, it is called a slope failure. The failure circle is referred to as a toe
circle if it passes through the toe of the slope. If it is passes above the toe of the slope
it is called slope circle. Under certain circumstances, a shallow slope failure also can
occur.
b) When the failure occurs in way that the surface of sliding passes at some distance
below the toe of the slope, it is called a base failure. The failure circle in the case of
base failure is called a midpoint circle.
In general, the causes of slope failure can be simplified into three broad categories as
follows (Bromhead, 1992):
a) Strengths of subsoil – When the slope is high or steep, stronger strength of subsoil
will be needed to sustain the slope. Failures are often if the subsoil is weak.
Progressive deterioration of the strength of the subsoil may also cause an existing
slope becomes unstable.
14 b) Pore water pressure – Increase in pore water pressure in the subsoil will reduce the
effective stress thus reduce the shearing resistance at the slip surface. This is why
they are many slope failures after heavy downpours.
c) External influences such as seismic forces, scouring and undercutting at the toe of
the slope.
Once the main causes of slope failure have been identified, the remedial design can
be carried out to correct the problems. For example, if the slope failure was caused
by high pore water pressure, lower down the pore water pressure in the subsoil will
be the priority in the remedial design. If the slope failure was triggered by the
external influence such as undercutting at the toe, the remedial design shall focus on
the toe protection.
2.3.3
Factor of Safety
The definition of the Factor of Safety (FS) is expressed as:
a) Resisting Force, Fr / Driving Force, Fd
b) Resisting Moment, Mr / Driving Moment, Md
c) Critical Height, Hc / Slope Height, H
d) Available shear stress, s /shear stress at equilibrium, t
The last definition is the most widely used. The required FS depends on the
consequences of losses in terms of property, lives and cost of repair in the event of
slope failure. FS is also dependent on the reliability of design parameters. The
minimum FS recommended by various codes or guidelines vary from 1.2 to 1.5 as
Table 2.1.
15 Table 2.1: Factor of safety
REFERENCE
BS6031: 1981
NAVFAC DM 7.1
JKR Road Work
Geo-guide, Hong
Kong
2.3.4
FACTOR OF SAFETY
FS = 1.3 to 1.4 for first time slide.
FS = 1.2 for a slide with pre-existing slip surface.
FS = 1.5 for permanent loading condition.
FS = 1.15 to 1.2 for transient load such as earthquake
FS = 1.2 for unreinforced slope and embankment on soft
ground.
FS = 1.5 for reinforced slope.
FS = 1.0 to 1.4 for new slope depending on risk.
FS = 1.0 to 1.2 for existing slope depending on risk.
Soft Soil behavior Under the Embankment
2.3.4.1 Settlement
The settlement of foundation soil (clay soil) that occurred during and after
construction of embankment is due to applied loads with time, as shown in Figure
2.1. in the first phase, the foundation soil is in overconsolidated condition and has
high rate of consolidation. Therefore, the settlement is small and increase linearly
with increasing of embankment load (OP). the clay soils becomes normally
consolidated stage when height of embankment is greater than critical height
(H>Hnc)and start to respond in undrained condition. As illustrated in Figure 2.1,
settlement of P’A’ occurred when the rate of consolidation is decreasing. Finally,
after the end of construction the consolidation settlement is decreased at lower rate
with time (A’D’). However, settlement under the center of the embankment does not
provide an indication for an impending failure condition. The reason for this is
considered to be that in most cases the settlement monitoring point under the center
of the embankment is not located within the failure zone (Hunter and Fell, 2003).
16 Figure 2.1: Typical variation in embankment load and settlement with time
(Lerouiel et al., 1990)
Research conducted by Hunter and Fell (2003) show that the vertical deformation at
the toe of the embankment versus the relative embankment height, as shown in
Figure 2.2. Meanwhile, Figure 2.3 presents the vertical deformation behavior
beyond the embankment toe (approximately 5m distance beyond the toe). In all cases
the monitoring point beyond the toe was located within the eventual failure zone.
The vertical displacement at and particularly beyond the toe of the embankment is a
good indicator of an impending failure condition. For measurement points beyond
the toe, negligible vertical deformations were usually observed during the initial
period of embankment construction, and the impending failure condition was
identifiable by heave movements or large increase in the rate of heave movement
with increasing embankment height. These observations apply to a wide variety of
soil types from low sensitivity, ductile high plasticity clays to highly sensitive, low
plasticity clays and silts. Works by Hunter and Fell (2003) show that for the highly
sensitive, low plasticity clay foundation (St. Alban and James bay) the amount of
vertical deformations relatively small ( up to 10-15 mm) up to approximately 90% of
the eventual embankments failure height.
17 Figure 2.2: Vertical displacement at the embankment toe versus relative
embankment height (Hunter and Fell, 2003)
Figure 2.3: Vertical displacement beyond toe versus relative embankment height
(Hunter and Fell, 2003)
18 2.3.4.2 Lateral Movement
Asrul Azam and Huat (2003) defined lateral movement as horizontal outward flow of
soil when subjected to shear stresses which increase at the different rate with vertical
settlement due to anisotropic behavior of the soil. Embankment of limited width
induces vertical settlement and lateral deformation of the foundation soil whereas
large fill areas will experience significant vertical settlement. According to Asrul
Azam and Huat (2003), Duncan and Poulos (1981) categorized lateral deformation
for embankment as follows:
i- Deformation within the foundation soil which is relatively softer than the
embankment itself (for example deformation of road embankment on soft clays as
foundation) and
ii- Deformation within the embankment when the foundation soil is relatively stiffer
(more significant when dealing with dam’s foundation where stiffer foundation is
expected).
Lateral movement of foundation soil at the edge of the embankment shows the same
behavior as the settlement (Figure 2.4). During construction phase, overconsolidated
and drained condition causes the horizontal displacement lower than settlement
(OP’). This is happen when the effective stress path approach to Ko condition (zero
lateral strain). The lateral movement arises at the same rate as vertical movement
when the construction undrained condition. In the end, the lateral movement is less
than settlement when long term consolidation of the foundation soil takes place
(A’D”).
The observation lateral displacements at the toe of the embankments are
shown in Figure 2.5. From the observation, Hunter and Fell (2003) summarized that:
i. In most cases in increase in the rate of lateral displacement relative to the
embankment height occurs at about the threshold, Hnc, determined from the pore
19 pressure respond, which is in agreement with the interpretation of the behavior of
embankment on soft ground to limit state theory.
ii. A further increase in the rate of lateral surface displacement occure in most of the
case studies analyzed at relative embankment height of 70-90%. It is considered that
this behavior is an indication of an impending failure condition.
iii. For 11 of the 12 cases analyzed, the lateral surface displacement is a good
pending failure. These cases cover a broad range of soil types from brittle and low to
high plasticity.
Figure 2.4: Typical relation between maximum horizontal displacement, ym and
settlement, s under the center of the embankment (Lerouiel et al.,1990)
According to Hunter and Fell (2003), for embankment constructed at
relatively slow rates (Muar Embankment constructed at rates of 0.02-0.055 m/day)
the lateral surface displacement was a good indicator of the impending failure, but
for the embankments constructed at more rapid rates (Rio de Janiero at 0.1 m/day
and King’s at 0.67 m/day) the lateral surface displacement was not such a good
indicator.
20 Figure 2.5: Lateral surface displacements at embankment toe versus relative
embankment height (Hunter and Fell, 2003)
Asrul Azam and Huat (2003) presented the empirical correlation of lateral
deformation analyses from previous researchers (Mesri et al., 1994), tavenas et al.,
1979 and Bourges and Mieussens (1979). Empirical method normally correlates
maximum lateral deformation, ym to the maximum settlement, sm. However, Mesri et
al., 1994 correlates undrained shear strength of soil maximum lateral deformation as
follows:
(2.1)
21 Where Su is undrained shear strength of foundation and ym is maximum lateral
deformation within the foundation depth.
Tavenas et al., 1979 suggested that to separate lateral deformation into two major
stages which are during construction phase and after construction phase. They
concluded the empirical correlation on observation on 21 embankments as shown in
Table 2.2.
Table 2.2: Empirical correlation of lateral deformation on 21 embankments,
(Tavenas et al., 1979)
Bourges and Mieussens (1979) expessed normalized deformation with depth in cubic
empirical correlations as follows:
Y = 1.78Z3 – 4.72Z2 + 2.21Z + 0.71
(2.2)
Where,
Y= y/ym = (lateral deformation at any depth, z) / ( maximum lateral deformation)
Z = z/D = (depth of any point,z) / (total deformation clay thickness)
Studies on lateral deformation analysis using empirical method for two selected
embankments (3m and 6m Muar trail) and compared with the available inclinometer
reading by Asrul Azam and Huat (2003) have shown that for 3m Muar Trial, the
deformation shape does not meet any agreement and it thought to be due to the
consolidation stage of upper and lower 10m of the clay layer, as illustrated in Figure
2.6. From the figure, the calculated maximum lateral deformation at the end of
22 construction is 143.6mm at depth 5.5m but the actual maximum lateral deformation
is 147.4mm at depth 5m. The ratio between maximum lateral deformation and
maximum settlement using empirical method proposed by Tavenas is 0.2, while for
actual behavior is range 0.13 to 0.2. Asrul Azam and Huat (2003) have presented the
calculated maximum lateral movement for 6m Muar Trial is much larger that he
observed. As shown in Figure 2.7, the maximum lateral deformation calculated is
191.98mm at depth against the actual 129.84mm at 3.5 depths. For this embankment
the ratio from 0.10 to 0.13 which very low as compared to 0.2 used in the empirical
method. Asrul Azam and Huat 2003 had concluded that empirical method does not
gives considerably good indication to predict lateral deformation because of the
complexity of clay behavior during the deformation process.
Figure 2.6: Maximum lateral deformation for the 3m control embankment at Muar
Trial compared with the selected empirical method (Asrul Azam and Huat 2003)
23 Figure 2.7: Maximum lateral deformation for the 6m control embankment at Muar
Trial compared with the selected empirical method (Asrul Azam and Huat 2003)
2.3.5
Slope Stabilization Method
Strengthening works on the collapsed riverbank slope is required to ensure the
stability of the slope. Slope stabilization usually involves some principles which are
altering the slope geometry, improvements to surface and subsurface drainage,
replacement of less desired slope soils with more competent material, providing
lateral or vertical support such as retaining structures i.e reinforced soil wall,
reinforced concrete wall, contiguous bored pile (CBP) or inserting inclusions such as
vertical piles, ground anchor, soil nails, rock bolts or geosynthetic reinforcement to
strengthen the slope. In selection of the best way of remedial works, some aspects
should be considered. The choice of method of stabilization is governed largely by
suitability, cost effectiveness, access, and availability of plant equipments, skills and
materials
There are many methods to increase the stability of a slope and to stabilize a
failed slope. These methods may be adopted singly or in combination. In general,
common adopted remedial measures can be grouped into three main categories
(Broms and Wong, 1985):
24 a) Geometrical method
This method is usually simple and cost effective. By changing the slope geometry
from a steep slope to a gentler slope, the stability of the slope can be increased. This
method can be done by cutting the slope and removal of any external load on top of
the slope or to backfill the toe of the slope. However, this method can only be
adopted if there is sufficient space.
b) Drainage method
One of the slope failure factors is saturation and pore water pressure building up in
the subsoil. If drainage system had been provided, the chances of building up pore
water pressure and saturation of subsoil can be minimized. This method can be very
effective.
However, the drainage system must be maintained in order to perform effectively. It
is easy to maintain the surface drains, but it is difficult to maintain the subsoil drains.
In general, this method is used in combination with other methods.
c) Retaining structures method
This method is generally more costly. However, due to its flexibility in a constrained
site, it is always the most commonly adopted method. The principle of this method is
to use a retaining structure to resist the downward forces of the soil mass. The
retaining structures include gravity types of retaining wall, cantilever wall,
contiguous bored piles, caisson, steel sheet piles and etc. Ground anchors or other tie
back system may be used together with the retaining structures if the driving forces
are too large to resist.
2.4
Review of Slope Stability Analysis
Slope stability analyses are usually conducted for one of the following reasons:
assessment of an existing natural slope, design of a proposed embankment or cut, or
back-analysis of a failed or failing slope (Lane and Griffiths, 1997).
25 Generally, there are two types of slope stability analyses methods, namely
conventional slope stability analysis and finite element numerical analysis.
Conventional slope stability analyses, based on the slip circle approach, are governed
by the imposed circle (or modified arc) and the detail of the analysis method (Lane
and Griffiths, 1997). Method of slices is one of the most common conventional slope
stability analyses methods in practice. When the slip surface under study for a soil
slope passes through soil materials whose shear strength is based upon internal
friction and effective stress, the method of slices is recognized as a practical means to
account for the expected variation in shearing resistance that develops along the
different portions of the assumed slippage arc (McCarthy, 2002).
On the other hand, finite element numerical analysis method requires good
assumptions and modeling technique on the initial condition of the slope failure
problems. Finite element numerical analysis method become increasingly important
due to the recognition on the shear strength in unsaturated soils contributed by
suction and other unsaturated soils properties which involves large amount of
variables and uncertainties. Several finite element codes in slope stability analysis
have been developed in the commercial market such as SLOPE/W, PLAXIS and etc.
Wesley (1994) stated that engineers are concerned with 2 (two) aspects of slope
stability, which are:
a) Evaluation of the existing stability of slope and its stability after certain
engineering works has been carried.
b) Devising measures to increase the stability of slope, in particular how to make
unstable slope stable again.
From it, analysis of slope stability may be implemented at 2 (two) distinct stages,
namely,
a) Pre-failure analysis, applied to assess stability in a global sense to ensure that the
slope will perform as intended; and
26 b) Post-failure analysis, also termed back analysis, responsive to the totality of
processes which led to failure (Eberhardt, 2002)
2.4.1
Type of Analysis
For slope relatively homogeneous soil, the failure surface is approximately by a
circular arc, along which the resisting rupturing forces can be analyzed. Various
techniques of slope stability analysis may be classified into three broad categories
(NAVFAC DM-7.1):
a) Limit Equilibrium Method
Most limit equilibrium method used in geotechnical practice assume the validity of
Coulomb’s failure criterion along an assumed failure surface. A free body of slope is
considered to act upon by known or assumeed forces. Shear stress induced on the
assume failure surface by the body and external forces are compared with the
availabe shear strength of the material. This method does not account for the load
deformation characteristics of the materials in question. Most of the methods of
stability analysis currently in use fall in this category.
b) Limit Analysis
This method considers yield criteria and the stress-strain relationship. It is based on
lower and upper bound theorems for bodies of elastic-perfectly plastic materials.
c) Finite Element Method
This method extensively used in more complex problem and where earthquake and
vibrations are part of total loading system accounts for deformation and is useful
where significantly different material properties are encountered. In recent years, FE
method is widely used in slope stability analysis.
27 2.4.1.1 Method of Slice
Bromhead (1992) conclude that simple sliding models fall into the category of limit
equilibrium method. In the method of slice, soil mass above the slip surface is
devided into wedges or slice. The method of slice is not an exact method because
there are more knowns than equilibrium equations. This requires that an assumption
be made concerning the inetrslice forces (Day 2000). Table 2.2 presnts a summary of
the assumption for the varios methods. Some characterictic of the commonly used
method of slice are given by Duncan & Wright (1980) and shown in Table 2.3
Table 2.3: List of commonly used method of slice: assumptions concerning inters
slice force for different method of slice (Source: Day, 2000)
Assumption concerning
inter slice forces
Resultant of the inter slice
forces is parallel to the
Ordinary Method of Slices
average inclination of the
slice
Resultant of the inter slice
Bishop simplified Method forces is horizontal (no
interslice shear forces)
Resultant of the inter slice
forces is horizontal (a
Janbu simplified Method correction factor is used to
account for interslice shear
force)
Location of interslice
Janbu generalized method normal force is defined by
an assumed line of thrust
Resultant of the inter slice
forces is of constant slope
Spencer method
throughout the sliding
mass
Direction of the resultant
Morgenstern – Price
of interslice force is
Method
determined by using a
selected function
Type of Method of Slice
Reference
Fellenius (1936)
Bishop (1955)
Janbu (1968)
Janbu (1957)
Spencer (1967, 1968)
Morgenstern and Price
(1965)
28 Table 2.4: Characteristics of equilibrium methods of slope stability analysis (Source:
Duncan and Wright, 1980)
Method
Slope Stability Charts (Janbu 1968;
Duncan et al., 1987)
•
•
Ordinary Method of Slice
•
•
•
Bishop’s Modified Method
•
•
•
•
Force Equilibrium Methods (e.g. Lowe
and Karafah 1960; U.S Army Corps of
Engineers 1970)
•
•
•
•
Janbu’s generalized Procedure of Slice
•
•
•
•
Morgenstern and Price’s Method
(Morgenstern and Price 1965)
•
•
•
Spencer’s Method (Spencer 1967)
•
•
•
Characteristic
Accurate enough for many
purposes
Faster than detailed computer
analysis
Only for circular slip surfaces
Satisfies moment equilibrium
Does not satisfy horizontal or
vertical force equilibrium
Only for circular slip surfaces
Satisfies moment equilibrium
Satisfies vertical force
equilibrium
Does not satisfy horizontal force
equilibrium
Any shape of slip surface
Satisfies all conditions of
equilibrium
Permits side force locations to be
varied
More frequent numerical
problems than some other method
Any shape of slip surface
Satisfies all conditions of
equilibrium
Permits side force locations to be
varied
More frequent numerical
problems than some other method
Any shape of slip surface
Satisfies all conditions of
equilibrium
Permits side force locations to be
varied
Any shape of slip surface
Satisfies all conditions of
equilibrium
Side force are assumed to be
varied
29 2.4.2 Basic Requirements for Slope Stability Analyses
Whether slope stability analyses are performed for drained or undrained conditions,
the most basic requirements is that equilibrium must be satisfied in term of total
stresses. All body force (weights), and all external loads, including those due to
water pressure acting on external boundaries, must be included in the analysis. These
analyses provide 2 (two) result:
1)
Total normal stress on the shear surface, and
2)
The shear stress required for equilibrium.
Equilibrium must be satisfied in term of total stress for all slope stability
analyses. In effective stress analyses, pore pressure are subtracted fromntotsl stresses
to evaluate the effective stresses on the shear surface. In total stress analyses, pore
pressures are not subtracted. Shear strength are related to total stresses. The basic
premise of total stress analyses is that there is a unique relationship between total
stress and efective strss. This is true only for undrained condition. Total stress
analyses are not applicable to drained condition. The time requred for drainage of
soil layers varies from minutes for sands and gravels to tens or hundred of years for
clays. In short term conditions, soils that drain slowly may best be characterized as
undrained, whilesoils that drain more quickly are best characterized as drained.
Analyses of such conditions can be performed by using effective stress
strength parameters for the drained soils and total stress strenght parameters for the
undrained soils. When effective stress strength parameter are used, pore pressures
determined from hydraulic boundary condition are specified. When total stress
strength parameter are used, no pore pressures are specified. An implicit assumption
of limit equilibrium analyses is that the soils exhibit ductile stress-strain behavior.
Peak strength should not be used for material such as stiff fissured claysand shales
which have brittle stress-strain characteristics, because progressive failure can occur
in these materials. Using peak strength can result in inaccurate and unconservative
evaluations of stability.
30 2.4.3
Source of Uncertainty in Slope Stability Analysis
Uncertainties in soil properties, environmental conditions, and theoretical models are
the most important source for a lack of confidence in deterministic analysis (Alonso,
1976). Morgenstern (1995) divided geotechnical uncertainty into three distinctive
categories: parameter uncertainty, model uncertainty and human uncertainty.
Parameter uncertainty is the uncertainty in the inputs of analysis; model uncertainty
is due to the limitation of the theories and models used in performance prediction
while human uncertainty is due to human errors and mistakes.
2.4.3.1 Parameter Uncertainty
The sources of variability in soil parameters are illustrated in Figure 2.8. The
parameter uncertainty is divided into data scatter and systematic error. Data scatter is
the dispersion of measurement around the mean. It is emanated from the spatial
variation of soil properties and random testing error. Spatial variability is the true
variation of soil properties from one point to another. It is inherent to the soil and
cannot be reduced. It must be considered in any analysis of uncertainty. Random
testing errors arise from factors related to the measuring process such as operator
error or a faulty device. Random errors should be removed from measurement prior
to analysis. Systematic error results in the mean of the measured property varying
from the mean of the desired property. Statistical error is the uncertainty in the
estimated mean due to limited sample size. While measurement bias occurs when
measured property is consistently overestimated or underestimated at all locations.
31 Figure 2.8: Uncertainties in Soil Properties (Source: Christian, Ladd, Beacher, 1994)
2.4.3.2 Model Uncertainty
The gap between the theory adopted in prediction models and reality is called model
uncertainty. Analytical models, particular in engineering are usually characterized by
simplifying assumptions and approximations. Model uncertainty is probably the
major source of uncertainty in geotechnical engineering (Wu et al., 1987;
Morgenstern, 1995; Whitman, 1996). Comparing model predictions with observed
performance or predictions of more rigorous and comprehensive model is probably
the most direct and reliable approach to quantify model uncertainty. However the
data needed for direct comparison with observed performance are seldom available
in practice.
2.4.3.3 Human Uncertainty
Human error caused uncertainty. This results when incorrect mechanism is modeled
or failure occurs due to mistake in correctly determining the chosen model or it may
due to carelessness and ignorance, misleading information, poor construction,
inappropriate contractual relationships and lack of communication between parties
involved in the project. Peck (1973) and Sowers (1991) provided examples of the
role of human uncertainty into geotechnical failure. The wide variability and
32 uniqueness of the human contribution from structure to another create difficulties in
identifying potential human errors, not to mention assessing their probabilities. (EIRamly, 2001)
2.4.4
Selection of Parameter and Its Variability
The first important step in modeling input variables is identifying which parameter to
treat as random variables. The decision relies on the observed variability in the
measured values of each parameter and the sensitivity of the output (e.g. factor of
safety) to variations in the magnitude of that parameter. Typically, strength
parameter (e.g cohesion, c, and angle of friction, φ.) and pore pressure are prime
concern. Other parameter (e.g. unit weight, geometry of unit boundaries and
peizometric line) could be treat as variables if found necessary. Table 2.4 provides a
summary of typical reported values for the coefficients of variation of commonly
encountered geotechnical parameters.
33 Table 2.5: Coefficient of Variation for Geotechnical Parameter
2.4.5
The Use of Finite Element Software Package
In the past 25 years, great strides have been in the area of static stability and
deformation analyses. The widespread availability of microcomputer has brought
about considerable change in the computational aspect of slope stability analysis.
34 Analysis can be done much more thoroughly and from the point of view of
mechanics, and more accurately than was possible without computer. Still engineers
performing slope stability analyses must have than a computer program. They must
have a through mastery of soil mechanics and soil strength, a solid understanding of
the computer programs they use, and the ability and patience to test and judge the
result of their analyses to avoid mistake and misuse.
Realistic analyses of deformations of slope and embankments were not
possible until about 25years ago. They are possible now mainly because the finite
element method has been developed and adapted to theses applications (Duncan,
1996). In the past 25 years, the finite element method has been used to analyze a
large number of dam, as well as other embankments and slopes. The experience
gained over this period of time-element method for use in practical engineering
problems.
As information technology develops, a problem facing is to decide to what
extent details of the computing processes need to be visible to the user. In
electronics, for example; integrated circuits have greatly reduced the need for circuit
designers to deal with components. Now treat a finite element package in the same
way as an integrated circuit. Need to know only the general principle of how inputs
is transformed into out. Within the information explosion there seems little point in
gaining knowledge that will not be used (Macleod, 1991).
The purpose of finite element analyses is to predict the behavior of physical
systems. The physical system to be modeled is described here as the ‘prototype’ and
the person responsible for creating the model is the ‘user’. Iain A. Macleod in his
paper title ‘Required Knowledge for Finite Element Software Use; summarized 5
(five) steps in the finite element method, namely:
a.
Define the constitutive relationship
b.
Define the element relationship
c.
Form the system model
d.
Solve
e.
Back substitute
35 Constitutive relationship represents the primary definition of material behavior.
Knowledge of the basic assumption used in deriving them and how they relate to the
behavior of the prototype are essential user requirements.
Element relationship can be further divided into 2 (two) classes:
(i)
Type A
These types are those which require no further assumption beyond the constitutive
relationship to establish the element stiffness matrix. It is most important to realize
that these elements do not need mesh refinement shape of these elements but the
functions used satisfy the constitutive relationship and therefore no approximations
are involved.
(ii)
Type B
For type B elements, further approximations are required and mesh refinement is
needed to improved accuracy. An important question is: ‘What is the desirable extent
of user knowledge for these assumptions?’ the assumptions for more complex
elements (e.g hybrid elements) do not help the user to understand how the element
will behave. Knowledge of the performance of Type B elements comes mainly from
observing their behavior in use. User should develop ‘favorite elements’ and get to
know them well. The assumptions for the system model that for the element include:
1.
Definition of the boundary conditions
2.
Definition of loading
2.4.6
Computer Modeling
Computer model are numerical calculation procedures implemented in a computer
program and meant for the solution of a specific physical process or a particular
problem (Brinkgreve, 2002)
36 Nowadays analyses can be done much more thoroughly and from the point of
view of mechanics more accurately than was possible without computers. One must
have a mastery of soil mechanics and soil strength, a solid understanding of the
computer program they use and the ability and patience to test and just the results of
their analyses to avoid mistake and misuse.
In this study, SLOPE/W and PLAXIS V8.2 (Professional Edition) is used.
2.4.6.1 PLAXIS
Slope stability of an idealized homogenous slope is modeled by using Version 8.2 of
the PLAXIS finite element program. This program designed for geotechnical
analysis features automatic mesh generation, pore pressure generation, a non-linear
elastic-plastic Mohr-Coulomb iterative solution algorithm and a phi-C reduction
procedure for calculation of safety factors.
The factor of safety FS in PLAXIS is defining as:
(Eqn. 2.1)
Where σ’n is effective normal stress acting on a plane C and Ф are the input
cohesion and angle of internal friction and CR and ФR are a reduced cohesion and
reduced angle of internal friction calculated in the program as products of the input
values and a multiplier. The parameter CR and ФR are then iteratively changed until
they are just large enough to maintain equilibrium and then used in the above
equation to give the factor of safety on an element-by element basis.
When the factor of safety approaches 1, failure is imminent in critically stress
elements. The factor of safety for a slope is the mean of the factors of safety for all
elements approaching failure in the finite-elements model. Savage and others (200a)
found that the safety factor procedure in PLAXIS gives results similar to
conventional slip-circle analyses where the safety factor ratio of the true strength to
37 the minimum strength required for equilibrium of a postulated slip surface (Debray
& Savage, 2001).
CHAPTER III
RESEARCH METHODOLOGY
3.1
Introduction
This chapter discusses the research methodology that includes the analyses the soil
investigation data and to analyze the failure of river embankment by using
SLOPE/W and analyze the stability of river embankment using PLAXIS V8.2. As
well as, preparation on literature search were carried out to determined basic
understanding of behavior of soft soil, the review of the stability of slope and the
stability analyses of the river embankment.
This chapter also discussed about material used in this study which are soft
clay, Geogrid and sheet pile wall. Analysis of the test result especially the
engineering properties of these materials are discussed in this chapter.
The analyses data based on the soil investigation report to determine the
engineering properties. Thus the analysis using the SLOPE/W software is to identify
the failure occurred based on the factor of safety value. Remedial works has been
done, therefore the stability on the river embankment need to verify using the
PLAXIS V8.2 finite element. The research methodology exercise to carry out this
study is presented in the form of flow chart as illustrated in Figure 3.1.
39 Figure 3.1: Flow Chart of Study Methodolog
3.2
Regional Geology and Site Topography
Based on the geological map of the site as shown in Figure 3.2 which is extracted
from the ‘Geological Map of Peninsular Malaysia’ (8th print, 1985) published by the
Director General of the Mineral and Geosciences Department (JMG) of Malaysia,
the proposed site is Quaternary deposit mainly comprised of marine and continental
deposit. Also within the bedrock formation of sedimentary and metamorphic rock
type consisting of shale, mudstone, siltstone, phylite and slate.
Based on the topographical map of peninsular Malaysia as shown in Figure 3.3
which is published by the Director of National Mapping, Malaysia (1996), the site is
located adjacent to the straits of Malacca and there are many tributaries of Sungai
Batu Pahat and Sungai Kesang surrounding the site. In addition there is also
watercourse – Bukit Pengkalan next to the site.
41 Figure 3.2: Regional Geology
42 Figure 3.3: Site Topography
43 3.3
Literature Review
Literature review was conducted to obtain such as material properties, geological
information, behavior of embankment, settlement and information of common
problem that happen to embankment underlying soft soil for more understanding
about this study. All information’s are based on journal, books and internet sources.
3.4
Borehole with Standard Penetration Test (SPT)
Borehole sometime called deep boring. The details of boring, sampling and testing
are decried in BS5930: 1981. Rotary open hole drilling by circulating fluid (water,
bentonite or air foam) is the most common method. The other commonly used
method is wash boring which utilize the percussive action of a chisel bit to break up
material and flush to the surface by water pumping down the hollow drill rods.
Detail S.I is usually carried out after optimum layout has been selected and
confirmed and aims to achieve the following objectives (Ir. Dr. Gue See Sew & Ir.
Tan Yean Chin, 2000):
(i) Plan for critical areas of concern
(ii) Refine subsoil profile
(iii)Obtained necessary soil parameters for detailed design of foundations
(iv) At area with difficult ground conditions (e.g. very soft soils, etc.)
(v) Major fill or cut areas that are more critical
(vi) Location with structures (e.g. retaining walls, areas with large loading)
Soil Investigation (SI) is an essential phase in engineering foundation works.
It is aimed to identify the project development site’s subsurface condition, profile
44 and relevant characteristics. Field test that were carried out consists of Standard
Penetration Test (SPT-N), Vane Shear Test (VST) and Piezometer Test.
The consistency of cohesive soils has been correlated with the N values, as
shown in Table 3.1. In general, these values are to be considered only approximate
guidelines, since clay sensitivity can greatly affect the N value (Schmertmann, J.H
1975). Although the correlation with N value in clay commonly are considered to be
less reliable than those in sand, increasing N values do in general, reflect increasing
stiffness and therefore decreasing liquidity index. To express this general correlation,
the consistency index (CI) has been defined as follows:
(Eqn. 3.1)
N Value
(blow/ft or 305mm)
0 to 2
2 to 4
4 to 8
8 to 15
15 to 30
Consistency
Very Soft
Soft
Medium
Stiff
Very stiff
Hard
¾ 30
Table 3.1: Consistency of Clay versus N
Source: Terzaghi and Peck, R.B
3.5
Vane Shear Test
Field vane shear test is suitable to test very soft to firm clay. However the results will
be misleading if tested in peats, sands, gravels, or in clays containing laminations of
silt, sand, gravel or roots. The field vane shear test is used only to obtained
undisturbed peak undrained shear strength, and remolded undrained shear strength
thus sensitivity of the soil.
45 The field vane shear tests are widely used to obtain the representative Su
profile of cohesive soils. The sensitivity, St of the material can also be obtained. The
most common error that occurred are wrong computation of spring factor and if the
clay contains organic material (e.g. seashells, decayed woods, peat, etc)
3.5.1
Undrained Shear Strength of Cohesive Soils – general evaluation basic
The undrained shear strength (Su) may very well be the most widely used parameter
for describing the consistency of cohesive soils. However, Su is not a fundamental
material property. Instead, it is measured respond of soil during undrained loading
which assume zero volume change. As such Su is affected by the mode of testing,
boundary condition, rate of loading, confining stress level, initial stress state, and
other variables. Consequently, although not fully appreciated by many users, Su is
and should be different for different test type.
Many detailed studies have shown that the unconsolidated undrained (UU)
and triaxial and unconfined (U) compression tests often in gross error because of
sampling disturbance effect and omission of a reconsolidation phase. Based on
studies such as these, the CIUC test also is considered to be the minimum quality
laboratory test for evaluating the undrained shear strength of cohesive soil. Therefore,
these tests should only be considered general indicator of relative behavior. They
should never be used directly for design.
Since Su is stress-dependent behavior, its value commonly is normalized by
the vertical effective overburden stress (σ’vo) at the depth where Su is measured. This
undrained strength ratio, Su/ σ’vo, has been expressed in many alternate forms in the
literature, including Su/ σ’o Cu/ σ’v,, c/p etc. all are equal to Su/ σ’vo.
46 3.6
Laboratory Test
Soil sample collected from the borehole are as follows:
(i)
Wash sample: from soil washed out from the borehole for oil strata.
(ii)
Disturbed Soil Sample: from split spoon samplers after SPT
(iii)
Undisturbed Soil Sample: using piston sampler, thin wall sampler, continuous
sampler.
Laboratory works focused on the test of foundation. After the soil sampling works,
several tests were conducted such as soil classification, one dimensional
consolidation test and triaxial test. Soil classification test consist of particle size
distribution, Atterberg limit, natural moisture content liquid limit, plastic limit and
calculation of plastic index.
3.7
Material Properties
3.7.1
Soil classification Test
Purpose of the classification test is to separates group of soil with different behavior.
Classification tests describe the basic properties of soil such as plasticity, specific
density, textures and grain size of a soil. These basic properties influence the
behavior of a soil and it was an important variable in subsequent analysis. All
classification test are based on the British Standard (BS 1377: Part 2:1990)
3.7.2 Particle Size Distribution
Particle size distribution also referred also referred as sizing testing. Dry sieve was
used as it is suitable for both type of soil. The sample of soil was place on a tray and
47 is allowed to dry overnight in an oven maintained at 105°C – 110°C. after drying the
sample were 5mm, 3.35mm, 2mm, 1.18mm, 600µm, 425µm, 300µm, 212µm,
150µm and 63µm. sieve are nested together with the largest aperture sieve at the top
and the receiving pan under the smallest aperture sieve at the bottom. Mechanical
sieve shaker were used and is shaken for 10 minutes for all particle smaller than each
sieve was measured. As for clay, sedimentation tests (using hydrometer) were carried
out on particles passing 63µm. the procedure of sedimentation test is explained in
detail in BS 1377: Part 2: 1990:9.5.
3.7.3
Atterberg Limits
The Atterberg Limit is used to investigate in detail the variations of soil properties
which occurred within a limit zero. There were 2 (two) test carried out for Atterberg
Limit test which is Liquid limit wL, and Plastic Limit, wP in accordance to BS 1377:
Part 2: 4.3, 5.3.
The plasticity Index, Ip is the numerical difference between Liquid Limit and Plastic
Limit, Ip = wL –wP where value of Ip will categorized the plasticity of the soils using
plasticity chart.
Clay sample were prepared for testing from natural state. While sample of sand are
sieved passing 425µm then it was mixed with distilled water and were leave for 24
hours to mature before proceeding with the test.
3.7.4
Consolidation Test
The consolidation test carried out in this study was one-dimensional consolidation or
odometer test. This test was conducted to identify the compression index Cc, the
vertical coefficient of consolidation Cv and effective stress Pc. The test was carried
out by applying a sequence of some four to eight vertical loads to a laterally confined
48 specimen having a height of about one quarter of its diameter. The vertical
compression under each load is observed over a period of time, usually up to 24hour.
Since no lateral deformation is allowed, the deformation only occure in one direction
which is vertical direction. The procedure of consolidation test can be obtained in BS
1377: Part 5: 1990.
3.7.5
Triaxial Test – Unconsolidated Undrained
This test method covers determination of the strength and stress-strain relationship of
a cylindrical specimen of either undisturbed or remolded soil. Specimens are
subjected to a confining fluid pressure in a triaxial chamber. No drainage of the
specimen is permitted during the test. The specimen is sheared in compression
without drainage at a constant rate of axial deformation.
The unconsolidated undrained triaxial strength is applicable to situation
where the loads are assumed to take place so rapidly that there is insufficient time for
the induced pore-water pressure to dissipate and pre consolidation to occur during
the loading period.
3.7.6
Sheet pile Wall
A sheet pile retaining wall has a significant portion of its structure embedded in the
soil and a very complex soil/structure interaction exists as the soil not only loads the
upper parts of the wall but also provides support to the embedded portion. Current
design method for retaining wall does not provide a rigorous theoretical analysis due
to the complexity of the problem. The method that have been developed to overcome
this, with the exception of finite element modeling techniques, introduce empirical or
empirically based factors that enable an acceptable solution to the problem to be
found. As s result, no theoretically correct solution can be achieved and large number
of different approaches to this problem devised.
49 3.7.6.1 Limit State Design
The design calculation prepared to demonstrated the ability of a retaining wall to
perform adequately under the design conditions must be carried out with full
knowledge of the purpose to which the structures is to be put. In all cases, it is
essential to design for the collapse condition or Ultimate Limit State (ULS) and some
situation it may also be appropriate to assess the performance of the wall under
normal operating conditions, the Serviceability Limit State (SLS). SLS calculation
should be carried out where wall deflections and associated ground movement are
important
3.7.6.2 Fixed earth design
A wall designed on fixed earth principle acts as a proposed vertical cantilever.
Increased embedment at the foot of the wall prevents both translation and rotation
and fixity is assumed as presented in Figure 3.4. Once again a tie or prop provides
the upper support reaction. The effect of toe fixity is to create a fixed end moment in
the wall, reducing the maximum bending moment for a given set of conditions but at
the expense of increased pile length. The assumption of fixed earth condition is
fundamental to the design of a cantilever wall where all the support is provided by
fixity in the soil.
When a retaining wall is designed using the assumption of fixed earth support,
provided that the wall is adequately propped and capable of resisting the applied
bending moments and shear force, no failure mechanism relevant to an overall
stability check exists.
50 Figure 3.4: Fixed Earth Support
3.7.6.3 Softened zone
Where soft cohesive soils are exposed at dredge or excavation level it is advisable
when calculating passive to assume that the cohesion increase linearly from zero to
the design cohesion value over a finite depth of passive soil.
3.8
Mathematical Modeling and Simulation
3.8.1
Finite Element Program
Finite element method has been widely used in most geotechnical problems due to its
ability to accommodate difficulties such as non-homogeneity, non-linear stress-strain
behavior and complicated boundary conditions.
Accuracy of the finite element
analysis depends on the accuracy of model used and the correctness of the input
parameters.
51 For the purpose of the mathematical modeling and simulation, PLAXIS finite
element codes version 8.6 is employed. This is a two-dimensional program for
deformation and stability analyses which is considered adequate for a linear plainstrain problem such as a embankment over soft ground.
3.8.2 Features of PLAXIS
PLAXIS is equipped with features to deal with various aspects of complex
geotechnical structures. Some of these are elaborated as follows:
(i)
Geometry of mathematical model is created with graphical input. The input
of soil layers, structures, construction stages, loads and boundary conditions
is based on convenient CAD drawing procedures. A 2D finite element mesh
is then generated
(ii)
Automatic mesh generation is allowed. Triangular mesh is used which is
more accurate. In addition 15-node triangular elements are available
(iii)
Plate element allows the modeling of sheet pile wall, lining, etc
(iv)
Geogrid element is used to represent internal element such as geotextile
(v)
Soil models like Mohr-Coulomb model is available which is robust and
simple non-linear model using well-known soil parameters. It incorporates
special features to allow gain in strength and stiffness with depth to simulate
natural ground behavior
(vi)
PLAXIS uses factor of safety defined by ratio of available shear strength to
the minimum shear strength needed for equilibrium which is suitable and
more appropriate for sheet pile wall and embankment
(vii)
PLAXIS has enhanced graphical output
52 3.8.3
Type of Soil Model
The elastic-plastic Mohr-Coulomb model is used in the finite element analysis. It
involves five input parameters: E and v for soil elasticity; ø and c for soil plasticity
and Ψ as angle of dilatancy which are commonly available.
Advanced modeling feature allows increase of stiffness and cohesion with
depth and the use of tension cut-off. This relates to the reality of the ground where
soil investigations have demonstrated the undrained shear strength increases with
depth.
Since the problem deals with soft ground condition, the critical loading case
is under undrained condition. There are several methods of modeling undrained
behavior in PLAXIS, Methods A, B and C. Method B is used where undrained
parameters C=Cu, ø=0 and Ψ=0 are adopted. The rest are in the form of effective
parameters i.e. E’, v’
53 CHAPTER IV
ANALYSIS AND DISCUSSION
4.1 Introduction
The failure is due to excessive deformation of the wall and slope sliding under
backfilling surcharge and human/traffic activities. The Existing Retaining Wall has
totally collapsed during low tide period probably due to insufficient embedded length
of existing wall system as shown in Figure 4.1.
Figure 4.1: (Before failure condition) Soldier Wall are used and
anchored to the pile size 150mm x 150mm
54 The rectification works need to be carried out immediately to avoid road
from the retrogressive failure hazard. Normally, immediately after failure, the slope
is still in a very unstable and critical condition. Any disturbance or vibration would
cause retrogressive movement and more slide. However, some immediate measures
had been carried to ensure the safety of live and properties.
Figure 4.2: Site location
The survey drawing show, the length of river embankment along the Sungai Muar
approximately 3m above the water level and the approximately length of the failure
occurred is 100 meter. The retaining wall failed during the low water tide and the
causes of the failure is due to length of the soldier wall is not sufficient enough to
resist the slip failure zone.
Meanwhile, site visit also conducted at Sungai Muar, Pekan Panchor, Muar
Johor to have general view of the site. Figure 4.2 shown the location of the failure
occurred. The failure of the river embankment due to the lateral displacement at the
existing retaining structure type soldier pile concrete panel lagging and there is
tension crack line along the riverbank of Sungai Muar.
55 4.2
Failure Occurred – Case Study
4.2.1
Ground Profile
From the soil investigation data obtained, the following sequence of subsoil
stratum has been interpreted:
LAYER
GENERAL SOIL
DESCRIPTION
DEPTH
RANGE
(m)
1
Very Soft Marine Clay
15 ∼ 18
2
Medium Dense SAND
23 ∼ 26
3
Hard Clayey SILT
30
Table 4.1: Interpreted Subsurface Profile
For all 4 nos of boreholes indicates the thickness of the overlying soft marine
clay is about 18 m with SPT-N value equal to zero. Refer Figure 4.3.
56 4.6
Construction Methodology
Figure 4.3: Borelog of Muar River
57 4.2.2
Evaluation of Geotechnical Parameters
For primary design the parameter using based on empirical correlation SPT-N
value. Based on the field tests, the following Table 4.2 geotechnical parameters
are estimated for the subsoil stratum:
LAYER
TYPE
UNIT
WEIGHT, γB
(KN/M3)
COHESION, C’
(KPA)
PHI,
φ‘
(DEG
REE)
1
Very
Soft
Marine
CLAY
15.0
7
10o
2
Medium
Dense
SAND
17.0
10
27o
3
Hard
Clayey
SILT
19
12
29o
Table 4.2: Geotechnical Parameters
58 4.2.3
Factor of Safety
The factor of safety (FOS) is defined with respect to shear strength and the FOS
is computed for an assumed slip surface.
For the case at Pekan Panchor, Muar, Johor, adopting a factor of safety of 1.4
against a rotational failure mode for the repaired slopes would be appropriate.
ECONOMIC RISK RISK TO LIFE Negligible Low High Negligible >1.0 1.2 1.4 Low 1.2 1.2 1.4 High 1.4 1.4 1.4 Table 4.3: Recommended factor of safety for new slopes
(After Geotechnical Control Office, Hong Kong, 1984)
4.2.4
Back Analyses
Refer to the slope profile before failure as shown in Figure 4.4, I-beam with
conjunction of soldier wall had been installed up to depth of 9 meter. The soldier
wall had an anchored to pile size 150mm x 150mm. Based on the sketches shown
that the primary system is only floating at a very soft clay layer.
Stability analyses are performed not only to provide a factor of safety once
the soil properties are known, but also to established field shear strength from the
study of failure. Back analyses of the failure by using the SLOPE/W software with
consideration of changes of water level from high tide to low tide. The analyses
calculated with the water level at the ground level. As shown in Figure 4.4 the
existing slopes profile before failure.
59 The analyses that have been done using SLOPE/W as show in Figure 4.5 to
Figure 4.12 the original profile from the condition of full water level until the water
level decrease 3 meter below the ground surface and given the value of factor of
safety based on the different conditions.
60 1 5 0m m S q u a r e p il e
E x is t in g S o ld i er W a ll
9m
1 8m
Su n g a i M u a r
V e r y S o ft
m ar i n e C L A Y
S AN D
C la y ey S IL T
Figure 4.4: Existing slope profile before failure @ CH 250
61 Total Activating Force:
Total Activating Moment: 7512.8
Total Resisting Force:
Total Resisting Moment: 11643
Total weight: 3703.8
Total Volume: 278.03
36
34
36
34
32
32
30
TRAFFIC LOAD = 10kN/m2
30
29
28
12
31
27
431
28 30
26
Elevation, m
24
9
11
10
Description: Marrine CL
Wt: 15
24
Phi: 10
22
C: 7
26
8
22
5
SUNGAI MUAR
20
7
20
32
18
16
18
Description: CLAY
Wt: 17
14
Phi: 27
12
C: 10
16
5
12
14
6
2
12
16 15
10
14
13
10
17
8
21
18
6
3
23
24
4
2 20
19
4
4
25
10
20
30
40
50
60
70
80
Distance, m
Figure 4.5: Original profile (full water level) @ CH 250
8
6
22
26
0
0
28
90
2
0
100
Description: Hard CLAY
Wt: 19
Phi: 29
C: 12
62 Total Activating Force: 235.6
Total Activating Moment: 5242.8
Total Resisting Force: 363.32
Total Resisting Moment: 8107.1
Total weight: 3166
Total Volume: 236.09
36
34
36
34
1.546
32
30
28 30
26
24
Elevation, m
32
28
31
12
27
431
TRAFFIC LOAD = 10kN/m2
29
9
11
10
5
SUNGAI MUAR
20
28
Description: Marrine CLAY
Wt: 15
24
Phi: 10
22
C: 7
26
8
22
30
7
20
32
18
18 Description:
16
12
14
6
2
12
16 15
10
14
13
21
18
6
3
24
4
20
19
23
4
4
25
10
20
30
40
50
60
70
80
8
6
22
26
0
0
Wt: 17
14
Phi: 27
12
C: 10
10
17
8
2
CLAY
16
5
90
Distance, m
Figure 4.6: Original profile (full water level) at chainage 250 with FOS = 1.546
2
0
100
Description: Hard CLAY
Wt: 19
Phi: 29
C: 12
63 Total Activating Force: 0
Total Activating Moment: 0
Total Resisting Force: 0
Total Resisting Moment: 0
Total weight: 0
Total Volume: 0
37
37
28
33
12
30
27
431
32
31
Elevation, m
27
TRAFFIC LOAD = 10kN/m
9
10
8
5
34
12
6
2
16 15
12
2
14
13
17
21
18
3
24
20
19
Marrine CLAY
22Description:
5
17
32Description:
Wt: 15
27Cohesion: 7
Phi: 10
7
22
7
29
11
32
25
23
22
4
26
CLAY
Wt: 17
17Cohesion: 10
Phi: 27
12
Description: Hard CLAY
Wt: 19
7 Cohesion: 12
Phi: 29
2
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94 96 98
Distance, m
Figure 4.7: Original profile (water level – 1m) - CH 250 64 Total Activating Force: 306.28
Total Activating Moment: 7685.1
Total Resisting Force: 397.55
Total Resisting Moment: 10038
Total weight: 2918.4
Total Volume: 211.5
37
28
33
12
30
27
4 31
32
31
27
Elevation, m
37
1.306
TRAFFIC LOAD = 10kN/m
9
10
8
34
22
12
6
2
16 15
12
14
13
17
21
18
3
24
20
19
23
22
25
4
26
2
CLAY
Wt: 17
17Cohesion: 10
Phi: 27
12
Description: Hard CLAY
Wt: 19
7 Cohesion: 12
Phi: 29
2
0
2
4
6
Marrine CLAY
22Description:
5
17
32Description:
Wt: 15
27Cohesion: 7
Phi: 10
7
5
7
29
11
32
8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94 96 98
Distance, m
Figure 4.8: Original profile (water level – 1m) @ CH 250 with FOS = 1.306
65 Total Activating Force: 0
Total Activating Moment: 0
Total Resisting Force: 0
Total Resisting Moment: 0
Total weight: 0
Total Volume: 0
35
35
30
28
34
12
33
3127 1
43
Elevation, m
30
25
TRAFFIC LOAD = 10 kN/m229
9
11
32
10
8
7
5
20
12
2
16 15
10
14
13
21
18
3
5
24
0
19
Description: CLAY
Wt: 17
15 Cohesion: 10
Phi: 27
10
17
20
Description: Marrine CLAY
Wt: 15
25 Cohesion: 7
Phi: 10
20
35
5
15
6
30
23
25
22
5
4
26
0
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94 96
Distance, m
Figure 4.9: Original profile (water level – 2m) @ CH 250
Description: Hard CLAY
Wt: 19
Cohesion: 12
Phi: 29
66 Total Activating Force: 331.99
Total Activating Moment: 6855.9
Total Resisting Force: 360.92
Total Resisting Moment: 7473
Total weight: 2503.7
Total Volume: 174.79
35
1.090
30
28
34
12
33
3127 1
43
30
25
Elevation, m
35
TRAFFIC LOAD = 10 kN/m2
29
9
11
32
10
8
7
5
20
12
2
16 15
10
14
13
21
18
3
5
24
0
19
Description: CLAY
Wt: 17
15
Cohesion: 10
Phi: 27
10
17
20
Description: Marrine CL
Wt: 15
25
Cohesion: 7
Phi: 10
20
35
5
15
6
30
25
23
22
5
4
26
0
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94 96
Distance, m
Figure 4.10: Original profile (water level – 2m) @ CH 250 with FOS = 1.090
Description: Hard CLAY
Wt: 19
Cohesion: 12
Phi: 29
67 Total Activating Force: 0
Total Activating Moment: 0
Total Resisting Force: 0
Total Resisting Moment: 0
Total weight: 0
Total Volume: 0
30
31
2530
28
34
12
33
27
431
TRAFFIC LOAD = 10 kN/m2
29 30
9
11
32
10
8
Elevation, m
7
20
15
5
20
35
5
12
6
2
16 15
10
14
13
17
3
5
24
0
19
Description: CLAY
15 Wt: 17
Cohesion: 10
10 Phi: 27
21
18
20
Description: Marrine CL
Wt: 15
25
Cohesion: 7
Phi: 10
23
25
22
5
4
26
0
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94 96 98
Distance, m
Figure 4.11: Original profile (water level – 3m) @ CH 250
Description: Hard CLAY
Wt: 19
Cohesion: 12
Phi: 29
68 Total Activating Force: 381.96
Total Activating Moment: 8245.5
Total Resisting Force: 370.19
Total Resisting Moment: 7962
Total weight: 2519.8
Total Volume: 174.41
30
0.966
31
2530
28
34
12
33
27
431
TRAFFIC LOAD = 10 kN/m2
29
9
30
11
32
10
25
8
Elevation, m
7
20
15
5
20
35
5
12
6
2
16 15
10
14
13
17
3
5
24
20
0
19
Description: CLAY
Wt: 17
Cohesion: 10
10 Phi: 27
15
21
18
25
23
22
5
4
26
0
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94 96 98
Distance, m
Figure 4.12: Original Profile (water level – 3m) @ CH 250 with FOS = 0.966
Description: Marrine C
Wt: 15
Cohesion: 7
Phi: 10
Description: Hard CLAY
Wt: 19
Cohesion: 12
Phi: 29
69 4.3
Remedial Method
There are 3 method option are proposed for remedial works at Sungai Muar, Panchor
Johor. The analyses for these 3 (three) method are using SLOPE/W and only option
1 are selected to do the analyses by using the PLAXIS V8.2 since the option 1 is the
method selected to construct as remedial works due to reason as shown in the Table
4.4 below.
ADVANTAGES
DISADVANTAGES
OPTION 1
1. Easier to construct
2. Shorter construction time
1.
OPTION 2
1. Cheaper alternative
2. Messy construction
3. Longer construction time
OPTION 3
1. Relatively easy construction
2. Shorter construction time
3. Expensive
Steel expensive
Table 4.4: Summarize of advantages and disadvantages of the every options
The river embankment construction is to be investigated under the following
proposed methodologies:
(i)
With steel sheet pile as cut-off barrier against seepage and for construction
robustness.
(ii)
Similar to the above but including Geogrid reinforced foundation and
geotextile mattress together with common earth and rock fills
Modeling and simulations of each methodology were executed to ascertain
the most appropriate method and cost-effective construction.
construction alternatives were simulated:
The following
70 4.3.1
Option 1 - Continuous Sheet Pile Wall with Tie Back System
As referring in Figure 4.13, using continuous sheet pile wall with tie back system.
Continuous sheet pile wall with 20 meter length was installing up to 3 meter depth
from the original wall.
The wall than to be anchored with Galvanized Tie Rod to the sheet pile wall
and installed 6 meter depth or dead man.
The Spun Pile was installed in between sheet pile purposely to reduce the
load acted to the sheet pile.
Spun Pile will carry the overburden load and will increase the unit weight
like composite material which are increase in strength and decrease in
compressibility.
4.3.2
Option 2 - Geogrid Wall with Pilling System
The next method is Geogrid Wall and Spun Pile is used to carry the surcharge from
the top as show in Figure 4.14.
Facial wall will be installed at river embankment. There are many type of
facial wall available in the market but for this type condition of failure the
readymade precast concrete like key stone L-shape retaining wall are advisable to be
used. It’s to ensure the installation at site would be easy and time saving.
The concrete SPUN pile will take the load from the facial wall up to 20 meter
depth which reaching the hard layer. At the slope of embankment amour rock is use
to strengthen the slope and geotextile is use as a separator in between amour rock
and the soil.
71 4.3.3
Option 3 - Wellguard wall with Pilling System
For the third option is using the Wellguard wall and tie back system where the steel
I-beam will install up to hard layer and installation of the concrete panel in a
between.
This kind of method are similarly the existing method but enhance on
installation on Spun pile for strengthen the soil at the area. At the slope of
embankment amour rock is use to strengthen the slope and geotextile is use as a
separator in between amour rock and the soil.
4.3.4
Summarize of analyses using SLOPE/W
The results are tabulated below in terms of minimum factor of safety against basal
failure of the slope. From the Table 4.5 below, it is apparent that the bearing
capacity of the existing slope is not adequate. Hence, appropriate stabilization
measures are needed to prevent further distress and possible failure of the slope.
CASE
DESCRIPTION
MINIMUM
F.O.S.
Existing Profile (Back Analysis)
1
CH 250 - Existing Profile With Full Water Level
1.546
2
CH 250 - Existing Profile With -1m Water Level
1.306
3
CH 250 - Existing Profile With -2m Water Level
1.09
4
CH 250 - Existing Profile With -3m Water Level
0.966
72 6 m l e n g th C o n ti n u o u s
S h e e t P ile W a ll
G a l v a n i ze d T i e
R od
1 l a y er o f G e o g r i d
2 0 m Sh e e t P ile W a ll w it h
c a p p in g b e a m
F -F i l tr a ti o n G e o te x ti l e (F 6 8 )
A m o ur R oc k
14m
S un ga i Mu a r
V e ry S of t
m ar i n e C L AY
1 2 m @ 2 5 0 d ia .
S p u n P ile a t
2 m c /c
SAND
C la y e y SIL T
Figure 4.13: Option 1- Continuous Sheet Pile Wall with Tie Back System (Chainage 250)
73 1 2 m L e n g th S p u n Pi le @
2 5 0 m m d ia . a t 2 m c /c
R e in f o r c e d G e o g ri d
F ac ia l B lo c k (A n y T y p e )
F -F ilt r at io n G e o te x tile (F 68 )
16m
A m o ur R oc k
S u n g a i Mu a r
V e ry S of t
m ar in e C L AY
SAND
C la y e y SIL T
Figure 4.14: Option 2 - Geogrid Wall with Pilling System (Chainage 250)
74 6m le n g t h C o n tin u o u s
Sh e et P ile W a ll
12 m le n g th Sp u n P ile
(G ra d e 8 0 ) 2 5 0 m m d ia .
at 2m c /c
G alv a n ize d T ie R o d
R ein fo rc e d G e o g r id
W e llg u ar d W a ll P a n e l w ith
c a p p in g b e a m
F -F iltr a tio n G e o t ex t ile ( F 6 8 )
A m o ur R oc k
14m
S u n g a i Mu a r
St e el I-B e a m
V e ry S of t
m a rin e C L A Y
S AND
C la y e y SIL T
Figure 4.15: Option 3 - Wellguard Wall with Pilling System (Chainage 250)
75 Total Activating Force: 0
Total Activating Moment: 0
Total Resisting Force: 0
Total Resisting Moment: 0
Total weight: 0
Total Volume: 0
35
35
45
37
36
29
17
7 30
43
30
14
15
38
44
16
5
2
7
5
20
31
3
15
33
35
4
27
3
26
10
4
11
24
25
12
23
Description: CLAY
Wt: 17
15
Cohesion: 10
Phi: 27
8
22
5
0
47
39
20 19
21
Marrine CLA
20
28
18
21
10
30Description:
Wt: 15
25Cohesion: 7
Phi: 10
6
25
Elevation, m
34
6
32
46
41
40
42
10
9
5
1
13
0
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94 96 98100 104 108
Distance, m
Figure 4.16: Option 1- Continuous Sheet Pile Wall Profile (Chainage 250)
Description: Hard CLAY
Wt: 19
Cohesion: 12
Phi: 29
76 Total Activating Force: 994.58
Total Activating Moment: 45847
Total Resisting Force: 1355.1
Total Resisting Moment: 62651
Total weight: 13279
Total Volume: 939.55
1.435
35
35
30
14
15
45
37
36
29
17
7 30
43
32
34
6
46
41
40
42
38
44
16
30
Description: Marrine CLAY
Wt: 15
25
Cohesion: 7
Phi: 10
6
25
Elevation, m
5
2
7
5
20
31
3
15
33
35
4
27
3
20
25
26
28
18
21
10
10
20 19
8
21
4
22
5
11
24
Description: CLAY
Wt: 17
15
Cohesion: 10
Phi: 27
47
39
12
23
10
9
5
1
13
0
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94 96 98100
104
0
108
Distance, m
Figure 4.17: Option 1- Continuous sheet pile wall profile @ CH 250 with FOS =1.435
Description: Hard CLAY
Wt: 19
Cohesion: 12
Phi: 29
77 Total Activating Force:
Total Activating Moment: 20442
Total Resisting Force:
Total Resisting Moment: 29716
Total weight: 8887.6
Total Volume: 795.54
40
40
= 20kN/m2
58 TRAFFIC LOAD 59
39
28
30
42
43
60
44
45
46
47
27
29
3817 19 21 23 25
48
49
3
37
50
51
5
53
41 52
54
55
56
57
11 14 12
35
30 40
Elevation, m
36
35
25
6
18 20 22 24 26
20
Description: Marrine
Wt: 11
Cohesion: 5
25
Phi: 18
30
20 Description:
33
15 Cohesion: 5
4
Phi: 27
2
10
13 15 16
10
3
6
7
5
0
5
10
1
314
32
CLAY
Wt: 17
1
34
15
35
8
2
5
9
0
0 2 4 6 8 101214161820222426283032343638404244464850525456586062646668707274767880828486889092949698100 104 108 112 116 120
Distance, m
Figure 4.18: Option 2 - Geogrid wall with pilling profile @ CH 250 Description: Hard CL
Wt: 19
Cohesion: 8
Phi: 29
78 40
Total Activating Force: 221.92
Total Activating Moment: 2238
Total Resisting Force: 317.62
Total Resisting Moment: 3215.5
Total weight: 1339.8
Total Volume: 108.63
1.437
40
= 20kN/m2
58 TRAFFIC LOAD 59
39
28
30
42
43
60
44
45
46
47
27
29
3817 19 21 23 25
48
49
3
37
50
51
5
53
41 52
54
55
56
57
11 14 12
35
3040
Elevation, m
36
35
25
6
18 20 22 24 26
20
33
15
35
Description: Marrine
Wt: 11
Cohesion: 5
25
Phi: 18
30
20 Description:
Wt: 17
1
34
15 Cohesion:
4
10
13 15 16
3
6
7
5
32
0
5
10
1
314
8
5
Phi: 27
2
10
CLAY
2
5
9
0
0 2 4 6 8 101214161820222426283032343638404244464850525456586062646668707274767880828486889092949698100 104 108 112 116 120
Distance, m
Figure 4.19: Option 2 - Geogrid wall with pilling profile @ CH 250 with FOS = 1.437 Description: Hard CL
Wt: 19
Cohesion: 8
Phi: 29
79 Total Activating Force:
Total Activating Moment: 22140
Total Resisting Force:
Total Resisting Moment: 30316
Total weight: 7726.2
Total Volume: 686.1
30
TRAFFIC LOAD =4520 kN/m2
25
2524
44
26
12
46
23
5 4129
30133 35 37 39
43
28
30
31
43
25
8
Elevation, m
7
20
15
20
4
5
6
34 36 38 40 42
10
11
12
13
6
10
14
27
15
20
0
16
21
Description: CLAY
Wt: 17
Cohesion: 5
10 Phi: 27
15
17
2
5
32
9
19
18
5
3
22
0
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94 96 98
Distance, m
Figure 4.20: Option 3 - Wellguard wall with pilling profile@ CH 250 Description: Marrine C
Wt: 11
Cohesion: 5
Phi: 18
Description: Hard CLA
Wt: 19
Cohesion: 8
Phi: 29
80 Total Activating Force: 558.22
Total Activating Moment: 22140
Total Resisting Force: 767.93
Total Resisting Moment: 30504
Total weight: 7726.2
Total Volume: 686.1
30
1.378
TRAFFIC LOAD =4520 kN/m2
25
2524
44
26
12
46
23
5 4129
30133 35 37 39
43
28
31
43
8
Elevation, m
7
20
15
5
14
27
15
20
0
21
Description: CLAY
17
Cohesion: 5
10 Phi: 27
15 Wt:
17
2
5
16
9
34 36 38 40 42
10
11
12
13
6
10
32
Description: Marrine
25 Wt: 11
Cohesion: 5
Phi: 18
20
4
6
30
19
18
5
3
22
0
0 2 4 6 8 10 1214 16 1820 22 24 2628 30 3234 36 38 4042 44 4648 50 5254 56 58 6062 64 6668 70 72 7476 78 8082 84 8688 90 92 9496 98
Distance, m
Figure 4.21: Option 3 - Wellguard wall with pilling profile@ CH 250 with FOS = 1.378
Description: Hard CLA
Wt: 19
Cohesion: 8
Phi: 29
81 4.4
Analyses by PLAXIS
4.4.1
Subsoil Profile and Ground Characteristic
Generally the soil investigation indicates the following ground characteristics:
(i)
There are 4 (four) borehole have been done for investigate the soil foundation.
(ii)
Very week ground easily up to 15 meter to maximum 18 meters with SPT-N
value of zero comprising very soft marine clay.
(iii)
Vane shear test – undrained shear strength could be as low as 4 kPa at
shallow depth and only increase marginally up to 16 kPa at about 11 meter
depth. Refer Figure 4.22.
(iv)
Undrained strength values from tests are agreeable to the VST results and
close to the theoretical value of Cu=0.3σv’ to 0.4σv’
(v)
Plastic indexes range from 25% to 66% with an average of 56%
(vi)
The in-situ moisture content is higher than the liquid limit throughout the
entire depth of the soft clay
(vii)
The mean bulk density of the clay is about 16.0 to 17.8 kN/m3
(viii) The degree of disturbance caused by wash boring as well as undisturbed
sample abstraction on the strength and deformation properties of the soft clay
could be appreciable.
82 Figure 4.22: Plot the undrained shear strength versus depth from S.I works.
4.4.2
Shear Strengths
Plot of the undrained shear strength with depth from the present in-situ vane shear
test is presented in Figure 4.22. The results demonstrate consistent increment of
undrained strength with depth, typically of normally consolidated clay behavior. The
lower bound values may be confined by the line Cu = 0.21σv’. It is noted that results
of a vane shear test slightly lower value due to the degree of disturbance caused by
wash boring as well as undisturbed sample abstraction on the strength and
deformation properties of the soft clay could be appreciable. A classical case on
undrained shear strength for Port Klang marine clay has been published by Dr. W.H.
Ting with similar relationship of Cu=0.4σv’ and
is shown in Figure 4.23 for
reference.
Early work by Skempton suggested the general correlation in Figure 4.24 for
Su determined from the field vane shear test (VST) as a function of the plasticity
index. All of the data are for normally consolidated (NC) clay. A linear fit of these
data result in:
83 (Eqn. 4.1)
However, in a surprisingly large number of case histories, direct use of Su
from the field VST in stability analyses of numerous embankments, excavations and
footing in clay has led to failure. A back analysis of these failures has led to
empirical correction factor for the field VST (Skempton).
However for design purpose the shear strength defined by the lower gradient
line Cu = 0.34σv’ shall be adopted. Undrained strength values from tests are
agreeable to the VST results and close to the theoretical value of Cu=0.3σv’ to 0.4σv’
The drained shear strength parameters from laboratory CIU tests are summarized in
the Table 4.6 below.
Borehole
Sample
BH1
UD1
BH2
Depth (m)
Laboratory Test Result
C’ (kPa)
Ф’ (deg)
4.0 – 4.5
5
26
UD1
3.0 – 3.5
5
14
BH3
UD1
5.0 – 5.5
5
16
BH4
UD2
12.0 – 12.5
6
18
Table 4.6: Drained Shear Strength Parameter
84 Figure 4.23: Undrained shear strength of Port Klang marine clay
(after Dr. Ting Wen Hui)
Su = 0.34 σ’vo PI = 5 6% Figure 4.24: Su determined from the field vane shear test (VST) as a function of the
plasticity index (after Skempton)
85 4.4.3
Laboratory Test Result
The natural moisture content generally decreases with depth and close to the liquid
limits ranging from 95% to 29% as shown in Figure 4.25. Liquid Limit (LL) and
Plastic Index (PI) are in the range of 25% to 66% with an average of 56% as
tabulated and shown in Figure 4.26.
Figure 4.25: Water content plot
86 Figure 4.26: Plasticity Chart
87 4.4.4
Deformation Characteristics
Laboratory 1-D consolidation results appear to give very low soil modulus values
compared to the corresponding values from vane shear test results. This could be due
to high degree of sample disturbance that could have occurred along the line from
sample abstraction, transportation to testing in the laboratory. As such deformation
parameters shall be derived from correlation with the undrained shear strength
(Duncan & Buchignani – 1976): i.e. Eu = 300Cu. Effective modulus, E’ = 2Eu (1+v)/3
= 0.86Eu for common value of Poisson’s ratio, v=0.3. The field measurement were
used in the numerical analyses and the undrained modulus (Eu) and effective strength
parameter (C’, Ф’) were also determine from the appropriate triaxial tests, and
summarized in Table 4.7 for the various clay layers.
4.4.5
Soil Permeability
The consolidation tests reveal the soil coefficient of permeability is in the order of
1x10-10 m/sec. This conforms to the results of the previous soil investigations.
88 4.4.6
Geotechnical & Structural Parameters
4.4.6.1 Sand fills for the first 3m
The medium dense sand layer from about 16m depth has an average SPT ‘N’ value
of 15. The correlated modulus E’= 1500xN = 20,000 kPa.
4.4.6.2 Very Soft Clay for the next 5m
The soft clay undrained shear strength and stiffness profiles for the 3 meter below
ground level to 8m depth are presented below.
3m
Depth
Cu = 7kPa; E’ = 7x300x.86 = 1806 say 1,800kPa
Cinc = (8-7)/5 = 0.2kPa/m
Einc = (2000-1800)/5 = 40kPa
8m
Cu = 8kPa; E’ = 8x300x.86
= 2,064 say 2,000kPa
4.4.6.3 Very Soft Clay for the next 8m
From 8m to 16m (Cu=30kPa) depth the very soft clay layer, exhibits the following
stiffness profile:
8m
Depth
Cu =8kPa; E’= 2,000kPa
Cinr = (10-8)/8 = 0.25kPa/m
Einc = (2580 - 2000)/8 = 72.5kPa/m
16m
Cu = 10kPa; E’= 10 x 300 x 0.86 = 2580 kPa
89 4.4.6.4 Soft to Stiff Sandy Clay for the next 8m
The soft clay undrained shear strength and stiffness profiles for the 3 meter below
ground level to 8m depth are presented below.
16m
Cu = 10kPa; E’= 10 x 300 x 0.86 = 2580 kPa
Depth
Cinc = (11-10)/8 = 0.13kPa/m
Einc = (2800-2580)/8 = 27.5kPa
24m
Cu = 11kPa; E’ = 11x300x.86
= 2838say 2800kPa
4.4.6.5 Very Stiff Clayey Silt for the next 6m
From 8m to 16m (Cu=30kPa) depth the very soft clay layer, exhibits the following
stiffness profile:
24m
Depth
Cu =11kPa; E’= 2800kPa
Cinr = (12-11)/6 = 0.17kPa/m
Einc = (3096-2800)/6 = 49.kPa/m
30m
Cu = 12kPa; E’= 12 x 300 x 0.86 = 3096 kPa
90 Table 4.7: Variation of Shear Strength and Deformation Parameters
Depth
(m)
Soil Type
Type
Eu
(kPa)
C’
(kPa)
Ф’
(degrees)
v
γunsat
γsat
Einc
Cinc
0-3
Sand fill
Drained
20,000
0.5
30
0.30
20
22
0
0
3-8
Very Soft
Clay
Undrained
1900
7
0
0.35
11
15
94
0.4
8-16
Very Soft
Clay
Undrained
2500
8
0
0.35
11
16.5
387
1.5
16-24
Soft to Stiff
Sandy Clay
Undrained
3000
10
27
0.30
20
22
94
0.4
24-30
Very Stiff
Clayey Silt
Undrained
4000
12
29
0.30
22
24
88.3
3.7
91 4.4.6.4 Structural Members
Design and modeling parameters for structural members such as steel sheet pile and
tie back anchor rod shall be assigned with high enough capacity to accommodate the
anticipated induced loads and stresses. The type of sheet pile are used is cold formed
sheet pile (OZ Section) as show in Figure 4.27 below and the technical data of the
sheet pile has been used are as summarized in Table 4.8.
Figure 4.27: Cold Formed Sheet Pile Wall
Figure 4.28: Cold Formed Sheet Pile – Z section
92 Type
Thickness,
(t) mm
Cross
Section Area
(Ac) cm2
Moment of
Inertia, (I)
cm4
Section
Modulus, (Z)
cm3
OZ 16A
8.00
173.8
33350
1670
OZ 28A
11.5
266
62667
2620
Table 4.8: Technical data for Sheet Pile Wall
4.5
Sequence of Construction
Construction sequence and timing will very much depend on the type of ground
improvement and effects from the tides.
The following aspects of construction sequence and technical strategy form
the key to the success of the construction methodology:
1)
Stage 1
Figure 4.29: Install 6 meter continuous Sheet Pile as temporary protection
93 2)
Stage 2
Figure 4.30: Excavation and Backfill crusher aggregate as working platform
3)
Stage 3
Figure 4.29: Remove Existing Retaining Wall
94 4)
Stage 4
Figure 4.31: Install 250mm dia. Spun Pile at 2m C/C and construct pile cap
5)
Stage 5
Figure 4.32: Lay a layer of Geogrid (GX 600/50) and backfill with sand
95 6)
Stage 6
Figure 4.33: Install 20m length continuous Sheet Pile Wall
7)
Stage 7
Figure 4.34: Tie back 20m continuous Sheet Pile Wall with 6m Sheet Pile Wall
96 4.5.1
General Notes on Requirements for the Reinstatement Works
1)
The proposed steel sheet piles shall be supplied new, clean, minimum Grade
355 with section modulus 2450cm3 and of 20m length.
2)
The proposed steel sheet piles for temporary protection and dead man shall be
supplied new clean, minimum grade 355 with section modulus 1650cm3 and
of 6m length.
3)
The steel sheet piles shall be painted with a layer of approved paint for
corrosion protection prior to installation.
4)
The steel sheet piles shall be installed in an interlocking manner so as to
achieve its maximum capacity. The steel sheet piles shall be installed to the
required level as per the relevant details.
5)
All reinforcement bars shall be deformed high tensile steel bars of the
specified size with yield strength, fy = 460 N/mm2.
6)
Concrete grade shall be minimum 35 N/mm2 unless otherwise specified.
7)
Spun pile for road embankment shall be 250mm dia. and 12m length.
Working load capacity of pile shall be taken as 40kN unless otherwise
specified.
8)
Minimum clear concrete cover to main reinforcement bars shall be 40mm
unless otherwise specified.
9)
Actual alignment may be subject to some site adjustments and shall be
confirmed prior to installation/construction.
97 4.6
Geometry of Model & Adopted Parameters
The geometry, construction components and parameters discussed above are
depicted in Figures 4.35 – Figure 4.37 below.
Figure 4.35: Finite Element Model
Figure 4.36: Model Connectivities (Mesh)
98 Crst settlement = 409mm Figure 4.37: Mode & Magnitude of total displacement after construction
C B Figure 4.38: Rate and deformation magnitude of settlement after construction
CHAPTER V
CONCLUSION AND RECOMMENDATIONS
5.1
General
This research has been successfully carried out in fulfilling the objective outlined at
the very beginning of the research works. The performance of sheet pile wall
5.2
Conclusion
A well planned and full-time supervised subsurface investigation (S.I) is necessary
to obtain reliable subsoil information and parameter for safe and economical design.
Although there may be an increase in awareness of the need for subsurface
investigation, however this does not necessary means is an increase in understanding
of what subsurface investigation can achieve. Hence clients need to be made to
understand that insufficient and unplanned subsurface investigation will lead to poor
design and subsequently means higher cost and sometimes unsafe design for project.
Generally, the simulation analysis shows that the water pressure greatly
influences the stability of the slope, in which the increasing water level up to the
ground level (crest) will lead to the increasing of the stability as show in the
computer simulation using SLOPE/W. As such deformation parameters shall be
derived from correlation with the undrained shear strength (Duncan & Buchignani –
1976): i.e. Eu = 300Cu. Effective modulus, E’ = 2Eu (1+v)/3 = 0.86Eu for common
value of Poisson’s ratio, v=0.3.
100 The deformation of sheet pile wall and Spun pile are actually not resulted
from the localized failure but actually is caused by the movement of the soil mass
globally resulted from the displacement at the crest of the slope. Based on the finite
element modeling, the sheet pile wall and spun pile have been used; they can restrain
the movement of the soil mass.
I-beam with conjunction of soldier wall had been installed up to depth of 9
meter. The soldier wall had an anchored to pile size 150mm x 150mm. Based on the
sketches shown that the primary system is only floating at a very soft clay layer.
Stability analyses are performed not only to provide a factor of safety once
the soil properties are known, but also to established field shear strength from the
study of failure. Back analyses of the failure by using the SLOPE/W software with
consideration of changes of water level from high tide to low tide. The analyses
calculated with the water level at the ground level.
Based on the result of analysis, it show that the application of Option #1
which is Continuous Sheet Pile Wall with Tie Back System in construction was
worthwhile. The anticipated settlement is in the order of 409mm over 25 years after
construction.
5.3 Recommendations
Some recommendations for future research are given below:
1) Displacement and deflection of the retention system need to be observed over
period of time. (e.g: Inclinometer and settlement marker)
101 2) Determine the displacement and lateral pressure acting on the installed sheet pile
and spun pile by using PLAXIS for case back analysis of failure occurred.
3) The applied of seismic loading also can be include in future study in order to
identify the effect of earthquake.
102 102 REFERENCES
Liew, S.S., Gue, S.S. & Liong, C.H. Geotechnical investigation and monitoring
results of a landslide failure at southern peninsular Malaysia (Part 2 : Back
analyses of shear strength and remedial works).
Bromhead, E. N. (1992). The stability of slope, second edition, Blackie Academic &
Professional.
C.S. Chen & C.S. Lim. Some Case Histories of Slope Remedial Works
Roy Whitlow 2001. Basic Soil Mechanics
J.Michael Duncan and Stephen G. Wright, 2005, Soil Strength and Slope Stability.
Gue See-Sew & Cheah Siew-Wai. Geotechnical Challenges In Slope Engineering of
Infrastructures
Braja M. Das. Fundamental of Geotechnical Engineering 1999
Braja M. Das. Priciple of Foundation engineering Fifth edition 2004
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Kulhawy, F.H, Duncan, J.M and Seed H.B. Finite Element Analysis of Stress and
Movement in Embankment During Construction.
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on Performance of Earth and Earth Support Structure, Vol. 2, Lafayette 1972,
pp1-54
B. Indraratna. Performance of Test Embankment Constructed to failure on Soft
Marine Clay (1992)
Dr. W.H. Ting and Dr. C.T. Toh. Property of Some Soft Soil Deposit in Malaysia
(1986), Proceeding Seminar on Geotechnical Development in Malaysia.
103 Das, B.M. 1997. Principles of Geotechnical Engineering: pp. 583.
Marto.A (2007). Current condition and simulation modeling of muar trial
embankment. Proceeding of 2nd International Conference on Advances in Soft
Soil Engineering and Technology. July 2-4, Malaysia, pp. 221-235
Asrul Azam, A., and huat, B.B.K (2003). Lateral Deformation Analysis – A brief
Review on The Empirical Method. Proceeding of 2nd International Conference
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353-360.
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Engineering Purposes, Part 2: Classification Test. London: BS1377
APPENDIX A
Site Plan and Borehole Location Appendix B
Field Testing: Vane Shear Test Result & Borelog Records
Appendix C
Laboratory Testing (Summary)