coupling effect of sction variation on riverbank...
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COUPLING EFFECT OF SCTION VARIATION ON RIVERBANK
STABILITY
MAZIAR FOROUTAN
UNIVERSITI TEKNOLOGI MALAYSIA
COUPLING EFFECT OF SUCTION VARIATION ON RIVERBANK STABILITY
MAZIAR FOROUTAN
A project report submitted in partial fulfilment of the
requirements for the award of the degree of
Master of Engineering (Civil - Geotechnics)
Faculty of Civil Engineering
Universiti Teknologi Malaysia
JUNE 2013
iii
In the name of God the most beneficent and merciful
To my beloved family
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ACKNOWLEDGEMENT
First and foremost, my sincere gratitude goes to my supervisor, Associate
Professor Dr. Ir. Azman bin Kassim. I thank him for the depth and breadth of his
guidance, for his constant willingness to help, for his kind and supportive words at
times, and for his careful reading of and constructive criticisms and suggestions on
this master project.
Lastly, I warmly thank my family, for their love, patience and support during
my study period.
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ABSTRACT
Failure of riverbank slope is a common problem in tropical country like
Malaysia. Problems are often encountered when calculating the real factor safety of
riverbank slope due to varying rainfall intensity and fluctuating of surface water table
applied on the slope. In this study, the coupling effect of rainfall infiltration and tidal
cycle on riverbank stability was investigated by measuring suction development in
the riverbank slope. The coupling effect of suction variation on riverbank stability
becomes more complicated as the up going tide of water on the riverbank saturates
the soil mass resulting in negative effect to the Factor Safety, and on the contrary, the
weight of surface water applied on the face of riverbank contributes to positive
Factor of Safety. Furthermore, the riverbank slope failure induced by rainfall,
involves infiltration through unsaturated zone above ground water table. Therefore,
to achieve stability analysis in term of rainfall effect, the slope should be considered
as an integral system of saturated- unsaturated soils. The critical factor of safety on
the riverbank slope was calculated by integrating simulated pore water distribution in
the slope by using SEEP/W into SLOPE/W software. From monitoring factor of
safety it was found that the coupling effect of extraordinary rainfall, which makes the
soil over-saturated and ponds on the slope, and tidal effect induces failure on the
riverbank slope.
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ABSTRAK
Kegagalan cerun tebing sungai adalah masalah biasa di negara tropika seperti
Malaysia. Masalah sering dihadapi apabila mengira faktor keselamatan sebenar cerun
tebing sungai disebabkan oleh perbezaan keamatan hujan dan naik turun paras air.
Untuk mencapai kestabilan analisis dari segi kesan hujan, cerun boleh dianggap
sebagai satu sistem keseluruhan tanah tepu-tak tepu. Kesan gandingan perubahan
sedutan terhadap kestabilan tebing sungai menjadi lebih rumit apabila air pasang
menaik di tebing sungai menjadi tepu dan melemahkan tanah cerun tetapi berat air
permukaan yang dikenakan ke atas muka tebing sungai menyumbangkan kesan
positif. Dalam disertasi ini, kesan gandingan penyusupan air hujan dan kitaran
pasang surut terhadap kestabilan tebing sungai telah disiasat dengan mengukur
perubahan sedutan dalam cerun tebing sungai.Faktor keselamatan kritikal di cerun
tebing sungai itu dikira dengan mengintegrasikan pengagihan liang air dalam cerun
yang disimulasi dengan perisian SEEP/W ke dalam SLOPE/W.Ia didapati bahawa
kitaran pasang surut yang menyebabkan paras air naik tidak menyebabkan kegagalan
cerun tebing sungai, kerana berat air menahan kegagalan jisim dan mengimbangi
pengurangan dalam nilai-nilai sedutan.Kesan gandingan hujan dan pasang surut air
juga didapati bukan sebab kegagalan. Fenomena yang menyebabkan kegagalan cerun
tebing sungai ialah takungan dan limpahan air di permukaan cerun yang berlaku
akibat hujan yang luar biasa atau kitaran pasang surut yang luar biasa.
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TABLE OF CONTENTS
CHAPTER TITLE PAGE
TITLE OF PROJECT
DECLARATION
DEDICATION
ACKNOWLEDGEMENT
ABSTRACT
ABSTRAK
TABLE OF CONTENTS
LIST OF TABLES
LIST OF FIGURES
LIST OF ABBREVIATIONS
LIST OF SYMBOLS
i
ii
iii
iv
v
vi
vii
x
xi
xiv
xv
1 INTRODUCTION 1
1.1 Background of the Study
1.2 Problem Description
1.3 Objectives and Scope of the study
1.4 Significance of the Study
1
2
3
5
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2 LITERATURE REVIEW 6
2.1 Introduction
2.2 Philosophy of suction measurement in unsaturated soil
2.2.1 Indirect Matric Suction Measurement
2.2.2 Direct Matric Suction Measurement
2.2.2.1 Principle of Tensiometer
2.3 Soil-Water Characteristic Curve (SWCC)
2.4 Hydraulic Conductivity Function in Unsaturated Soil
2.5 Rainfall Effect on Slope Stability Analysis
2.5.1 The Mechanism of Rainfall Induced Landslide
2.5.2 Infiltration and Steep drop in Apparent Shear
Strength
2.6 Slope Stability and Limit Equilibrium Method
2.6.1 The Ordinary Method (Fellenius, 1936)
2.6.2 Bishop’s Simplified Method
2.6.3 General Limit Equilibrium Method
2.6.3 Fredlund et al. (1991) Method
2.7 Selecting Suitable Analysis Method
2.8 Theory of Geostudio Software
2.8.1 Theory of Seep/W
2.8.2 Theory of Slope/W
6
6
7
8
8
9
11
14
15
16
17
18
20
21
23
24
25
25
28
3 RESEARCH METHODOLOGY 33
3.1 Introduction 33
3.2 Collection of Literature Review 34
3.3 Problem Identification and Site Equipment
3.3.1 Site Location
35
35
3.4 Problem Identification and Site Equipment
3.4.1 Suction Data Collection
36
37
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3.4.2 Preparing and Maintenance of Tensiometer
3.4.3 Properties of Soil
3.4.3.1 Creating SWCC
3.4.3.1.1 Sieve Analysis
3.4.3.1.2 Determination of
saturated volumetric
water content
3.4.3.1.3 Determination of SWCC
3.4.3.2 Hydraulic Conductivity Curve
3.4.3.2.1 Constant Head
Permeability Test
3.4.3.2.1 Compaction Test
3.4.4 Rainfall Data
3.5 Determination of FOS in Different Schemes
3.5.1 Simulation in Seep/W
3.5.1.1 Creation of Initial Condition
3.5.1.2 Water Table Fluctuation
3.5.1.3 Simulation of Rainfall Intensities
3.5.2 Simulation in Slope/W
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43
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53
54
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4 RESULTS AND DISCUSSIONS
4.1 Introduction
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4.2 Suction Measurement
4.3 Creation of SWCC of Material
4.3.1 Grain Size Distribution Data
4.3.2 Generating SWCC
4.4 Estimation of Hydraulic Conductivity
4.5 Determination of FOS in Different Schemes
4.5.1 Simulation in Seep/W
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63
64
67
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75
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x
4.5.2 Evaluation of Changes in Factor of Safety due to
Tidal Effect
4.5.3 Evaluation of Coupling Effect of Rainfall and
Tidal Effect on Riverbank Stability
4.5.4 Evaluation of water Ponding on the Riverbank
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78
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5 SUMMARY,CONCLUSIONS AND
RECOMMENDATIONS
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5.1 Summary 83
5.2 Summary of Results
5.2.1 Laboratory Results
5.2.2 Suction Distribution Result
5.2.3 Summary of Factor of Safeties
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85
5.3 Conclusion
5.4 Recommendation for Further Researches
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89
REFERENCES
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x
LIST OF TABLES
TABLE NO. TITLE PAGE
2.1
3.1
3.2
4.1
4.2
4.3
4.4
4.5
4.6
4.7
4.8
5.1
5.2
5.3
5.4
Empirical equation to determine K value
Particle size distribution data
Calculation table for permeability test
Water table at lowest level, 0.1m
Water table at middle level, 0.95m
Water table at highest level, 2m
Sieve analysis result
Mass of soil sample before and after drying process
The result of reading for different measures of flow
Mass of soil sample before and after drying process
Mass of soil sample before and after drying process
Basic properties of Gravelly sand material
Water table at lowest level. 0.1m
Water table at middle level.0.95m
Water table at highest level 2m
14
46
52
63
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63
64
67
70
73
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LIST OF FIGURES
FIGURE NO. TITLE PAGE
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
Main elements of tensiometer
SWCC for sandy soil, silty soil and Clayey soil
Typical soil-water characteristic for a silty soil
The mechanism of rainfall induced failure
Free body diagram of forces acting on a slice
Moment and Force Factors of Safety as a Function of the
Interslice Shear Force
Effect of different interslice functions
Moment equilibrium is sensitive to shear force
Research Frame work
Location of research area
Riverbank slope at Paya Mengkuang
Sets of tensiometers along riverbank
Top view of three sets of tensiometers
Water table at middle level
Water table at highest level
Water table at lowest level
9
10
11
16
19
29
31
31
34
35
36
38
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40
40
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3.9
3.10
3.11
3.12
3.13
3.14
3.15
3.16
3.17
3.18
3.19
3.20
3.21
4.1
4.2
4.3
4.4
4.5
4.6
4.7
4.8
4.9
Equipment needed for sieve analysis
Particle size distribution curve
Soil vision window for volume mass prediction
SWCC estimation method in Geo-Studio
Details of constant head permeability test
General arrangement for constant head permeability test
Estimation of Hydraulic conductivity function
Initial water table condition
Unit flux applied to slope as an infiltration
Water table Fluctuation function
Rainfall hydraulic boundary function
Rainfall boundary condition
Material properties of Gravelly sand
Particle size distribution curve
Soil classification based grain size data
Calculation of Volume mass properties of soil
SWCC of Gravelly sand material
Graphical plot of rate flow against 1/t
Compaction curve of the material
Hydraulic conductivity curve of gravelly sand material
Suction variation at initial condition due to Seep/W
analysis
Negative pore water pressure distribution
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4.10
4.11
4.12
5.1
5.2
5.3
5.4
Variation of FOS due to tidal cycle
Coupling effect of rainfall and tidal cycle
Slope failure due ponding effect on the riverbank
Evaluation of Factor of safety due to tidal cycle
Evaluation of FOS due to coupling effect of rainfall and
tidal cycle
Slope failure due to effect of ponding on the riverbank
Comparison among factor of safeties in different cases
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80
85
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88
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LIST OF ABBREVIATIONS
LEM - Limit Equilibrium Method
FOS - Factor of Safety
SWCC - Soil-Water Characteristic Curve
AEV - Air Entry Value
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LIST OF SYMBOLS
c’ - Effective cohesion
k - Hydraulic conductivity
i - Hydraulic gradient
u - Pore pressure
ua - Pore-air pressure
uw - Pore-water pressure
(ua – uw) - Matric suction
v - Darcian velocity
V - Volume of typical slice
W - Total weight of soil
x - Perpendicular distance of the line of the slice weight from the
centre of rotation
- Inclination of slip surface at the middle of slice
- Slice base length
' - Effective friction angle
b - Unsaturated friction angle
- Minimum effective friction angle at failure
γ - Unit weight
γd - Unit weight of dry soil
γw - Unit weight of water = 9.81kN/m3
- Volumetric water content
wet - Wetted volumetric water content
field - Field volumetric water content
r - Residual volumetric water content
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- Total normal stress
( – ua) - Net stress
(a – uw)r - Residual suction
- Coefficient of friction
- Average shear stress developed along the potential failure
surface
1
CHAPTER 1
INTRODUCTION
1.1 Background of the study
One of the basic calculations in geotechnical engineering field is analysis of
slope stability in various conditions. The method of reaching stability analysis in
slope is the most important part of this field. Whereas, this method involves the
identification of the critical slip surface and the associated minimum factor of safety.
Available analytical methods include limit equilibrium methods, limit
analysis methods, rigid element methods, and finite element methods. The most
popular methods used for slope stability analysis is based on the limit equilibrium
theorem. These methods make use of the limit equilibrium theory to derive the factor
of safety against sliding for the slope. The factor of safety is defined as the ratio
between resisting and disturbing forces involved in the slope stability problem.
Therefore, when the factor of safety is below unity, e.g. 0.8, it’s assumed that the
failure happens. So, it’s very important to know about the parameters that cause loss
of factor of safety in slope. Furthermore, sometimes ignoring some parameters in
purpose of simplifying calculation will cause inaccuracy in obtaining factor of safety.
For example when effect of negative pore water pressure (suction), is not considered
in slope stability, maybe the relative FOS is below unity, for the standing slopes.
Besides, selecting the proper shear strength model which reflects the actual behavior
2
of the soil is another important aspect to achieve correct analysis of stability of the
slope. Although there are many different shear strength models for defining soil
behavior, none of them reflect the exact behavior of the soil, but some are closer to
reality. Furthermore, it is proved that using inappropriate shear strength model will
cause over estimation or sometimes under estimation in slope stability calculation.
Therefore, more research should be done on studying of the factors like
rainfall, suction, etc, to understand the real mechanism of slope failure which is very
important so that the same mechanism with the right soil properties can be coupled in
the slope stability analysis to achieve a reliable stability factor.
1.2 Problem Description
There are many functioning elements that ignoring each of them will cause
inaccuracy in calculating the factor safety of riverbank slope. One of the most
challenging factors in tropic conditions is rainfall effect that may induce shallow
failure due to lowering shear strength at infiltrated areas. Moreover, there is another
challenging aspect that is fluctuation of water table in the river due to tidal cycle and
rainfall. In the flood season that there is high volume of rainfall, the water in the river
will go up on the riverbank, and in the season that there is less rainfall the lowering
of water table could be seen in the river, this also could be occurred during a day
because of tidal effect. Furthermore, this phenomenon would cause variation of
suction in both cases. Therefore, when the water will go up on the riverbank it will
decrease shear strength of soil to the lowest value because it will eliminate suction on
the riverbank. On the other hand, the water on the riverbank, will act in the opposite
way to the sliding mass of soil on the slip failure that will help to keep stability of
slope. In addition, when the water goes down slope will face inverse situation, it
means that the suction will increase shear strength that will keep the factor of safety,
but there is lack of the effect of weight of water.
3
Another aspect of difficulties in slope analysis is rainfall effect. Furthermore,
when this problem is combined with suction variation, it is necessary to investigate
which one causes the loss of factor safety. Firstly people believed that evaluation of
suction could be ignored because the weight of the water could be a make up for lack
of shear strength caused by saturation, but in this case another problem may arise is
the rainfall intensity may causes shallow failure on the slope. This shallow failure is
because of infiltration of the rainfall on the riverbank that creates unsaturated-
saturated zone, so that the saturated area would face reducing shear strength and loss
of factor safety. In this case, the depth of the failure is up to the depth of infiltration
of rainfall. Shallow mode of rainfall induced landslide is complex soil mechanics
behaviors. Hence it needs a more realistic state-of-the art theoretical concept for the
shallow rainfall infiltration induced failure.
All in all is that, a kind of integrated analysis based on real data which will be
accompanied by precise result should be done. Since, this result could be used to
predict the time that any failure is near to happen or do remedial works to prevent
failure from happening or reducing damages of failure, also achieving to factors that
govern the stability of slope among many different factors.
1.3 Objectives and Scope of the Study
The aim of this study is to investigate the coupling effect of rainfall
infiltration and surface water pressure on riverbank stability by observing suction
variation in the riverbank slope. This study is undertaken in order to achieve the
following objectives:
i. To measure suction distribution on riverbank slope under different
surface water level due to tidal cycle and rainfall.
ii. To determine the factor safety of riverbank slope at different surface
water table on the riverbank slope.
4
iii. To determine the factor of safety of the riverbank slope under
different levels of rainfall intensity coupling with fluctuation of
surface water table due to tidal cycle.
iv. To determine the factor of safety of the riverbank slope under
different levels of rainfall intensity coupling with fluctuation of
surface water table due to tidal cycle which resulted in ponding on the
riverbank.
In order to measure suction on the site, tensiometers were installed on the
riverbank slope located at Paya Mengkuang, Gelang Patah, Johor which is about 45
km from UTM. The measurement was done based on the suction development due to
rainfall and tidal cycle. Consequently, the pore water pressure distribution was
simulated and analyzed with SEEP/W, and factor safety of riverbank slope was
calculated with SLOPE/W by an integrated analysis between these two softwares. In
this study tidal cycle were applied by defining different surface water table along the
riverbank slope. Then, low, medium and high rainfall intensity was applied to
analyze riverbank stability against coupling effect of rainfall and tidal cycle which
can induce failure. Furthermore, this coupling effect was evaluated in case of
ponding and non-ponding on slope.
1.4 Significance of the Study
Slope failure involves a very complicated mechanism that could be effected
by different parameters. Therefore, studying and evaluating these various parameters
can be useful to find out the dominant factors which will cause loss of factor of
safety.
In this study the coupling effect of suction due to fluctuation of water level
and rainfall effect is evaluated. Moreover, by getting details of analysis result, the
5
periods of time which are more susceptible to failure could be known. Since, the
results which are achieved from this study are based on real and exact in-situ suction
measurement by tensiometer and could be used for interpretation of same cases with
similar soil type and similar rainfall condition.
90
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Hamdhan, I. N., & Schweiger, H. F. Slope Stability Analysis of Unsaturated Soil
with Fully Coupled Flow-Deformation Analysis.
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Md. Noor, M. J. (2007). Slope Stability Method Incorporating the Curved Surface
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Press
Natural Resources Conservation Service (2009). Tensiometer – Job Sheet.
Washington: USDA
Pan, H., Qing, Y., & Pei-yong, L. (2010). Direct and indirect measurement of soil
suction in the laboratory. Electronic Journal of Geotechnical
Engineering,15(3), 128.
Rahardjo, H., & Leong, E. C. (2006, April). Suction measurements. ASCE.
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Nevada.
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Sukanta Chakraborty (2009). Numerical Modelling for Long Term performance of
soil-Bentonite Cut-off Walls in Unsaturated Soil Zone. Master of Engineering.
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Sreedeep, S., & Singh, D. N. (2011). Critical review of the methodologies employed
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