DEVELOPMENT OF EVALUATION SYSTEM FOR GROUNDWATER LEVEL IN RELATION TO SLOPE STABILITY FORECASTING
NG SOON MIN
UNIVERSITI SAINS MALAYSIA 2016
DEVELOPMENT OF EVALUATION SYSTEM FOR GROUNDWATER LEVEL IN RELATION TO SLOPE STABILITY FORECASTING
by
NG SOON MIN
Thesis submitted in fulfillment of the requirements for the degree of
Doctor of Philosophy
PEMBANGUNAN SISTEM PENILAIAN PARAS AIR BUMI BAGI RAMALAN KESTABILAN CERUN
oleh
NG SOON MIN
Tesis yang diserahkan untuk memenuhi keperluan bagi ijazah
Doktor Falsafah
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ACKNOWLEDGEMENT
I would like to take this opportunity to express my utmost gratitude to all who gave me the possibility to complete this thesis successfully. First of all, I want to thank my main supervisor, Dr Mohd Ashraf Mohamad Ismail for his excellent guidance and advices throughout the completion of my PhD research. His valuable encouragement, time spent and patience dedicated to my study are greatly appreciated.
I would also like to thank my co-supervisor Professor Dr. Ismail bin Abustan who has given me a lot of invaluable comments and constructive suggestions to conduct this research successfully.
Special gratitude to Mr Ahmad Halmi, Mr Zabidi, Mr Dziauddin, Dr Tan Chee Ghuan, Mr Rais, Mr Firdaus and also all the staffs in school of Civil Engineering for their assistance in the laboratory, field and administrative works. My special thanks also go to my fellow peers for their moral support and interesting discourse sessions during my studies.
Aside from this, I would like to acknowledge Universiti Sains Malaysia for the Postgraduate Research Grant Scheme (USMRU-PRGS) (1001/PAWAM/8046010) and USM Research University Grant (1001/PAWAM/814192) that provide me the financial assistance to carry out this study smoothly.
To my family, I owe them a special debt for giving me such a huge encouragement and support throughout my studies. Finally, my deepest appreciation is also dedicated to my beloved soul mate Catherine Van for her continuous motivation and companionship to complete this thesis successfully.
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TABLE OF CONTENTS
Page
ACKNOWLEDGEMENT ii
TABLE OF CONTENTS iii
LIST OF TABLES viii
LIST OF FIGURES x
LIST OF ABBREVIATIONS xv
LIST OF SYMBOLS xvii
ABSTRAK xx ABSTRACT xxii CHAPTER 1 - INTRODUCTION 1.1 Background 1 1.2 Problem Statement 3 1.3 Objectives 6 1.4 Scope of work 8 1.5 Research Significance 9 1.6 Thesis outline 10
CHAPTER 2 - LITERATURE REVIEW
2.1 Introduction 13
2.2 Rainfall induced slope failure 16
2.2.1 Field Studies 18
2.2.2 Laboratory experiments 21
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2.3 Unsaturated Soil Mechanics 26
2.3.1 Soil Water Characteristic Curve (SWCC) 28
2.3.1.1 Laboratory tests for SWCC 30
2.3.1.2 Mathematical equations for SWCC 32
2.3.1.3 Prediction of SWCC 34
2.3.2 Hydraulic conductivity function 35
2.3.3 Unsaturated-saturated seepage flow 39
2.3.4 Shear strength 41
2.3.5 Slope stability analysis 43
2.3.5.1 Limit equilibrium method (LEM) 44
2.3.5.2 Finite element method (FEM) 46
2.4 Prediction of rainfall induced slope failure 49
2.4.1 Rainfall threshold 51
2.4.1.1 Empirical based model 52
2.4.1.2 Physical based model 54
2.4.2 In-situ instrumentation 54
2.4.2.1 The concept of tank model 57
2.5 Summary 59
CHAPTER 3 - RESEARCH METHODOLOGY
3.1 Introduction 60
3.2 Background of the study area 62
3.2.1 Subsurface investigation 67
3.2.1.1 Borehole drilling 69
3.2.1.2 Electrical Resistivity Survey 71
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3.2.1.4 Development of 3D subsurface model 80 3.2.2 Geotechnical properties of soil samples 82
3.2.2.1 Particle size distribution 84
3.2.2.2 Atterberg limits 84
3.2.2.3 Hydraulic conductivity test 85
3.2.2.4 Shear strength parameters 86
3.3 Multi tank model 87
3.4 Unsaturated-saturated seepage flow analysis 94
3.4.1 Volumetric water content function 96
3.4.2 Hydraulic conductivity function 97
3.5 Stability analysis 98
3.5.1 Slope stability assessment 100
3.6 One dimensional (1D) soil column study 101
3.6.1 Laboratory physical modelling 102
3.6.1.1 Acrylic cylinder 103
3.6.1.2 Rainfall simulator system 105
3.6.1.3 Tensiometer-transducer system 105
3.6.1.4 TDR system 107
3.6.1.5 Pore air pressure sensor system 108
3.6.1.6 Data acquisition system 109
3.6.2 Experimental setup 111
3.7 Numerical modelling for 1D soil column 112
CHAPTER 4 - RESULTS AND DISCUSSION
4.1 Introduction 115
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4.2.1 Stratigraphy observation 116
4.2.2 Subsurface profiling 119
4.2.3 Delineation of water bearing zone 124
4.2.4 Potential failure zone 130
4.3 Input parameters for numerical modelling 131
4.3.1 Soil classification and physical properties 132
4.4 Application of multi tank model 133
4.4.1 Groundwater table calibration 134
4.4.2 Groundwater table prediction 137
4.5 Pore water pressure distribution in slope 139
4.6 Stability conditions 145
4.6.1 Stability prediction for the year 2012 145
4.6.2 Stability verification 148
4.7 1D soil column study 149
4.7.1 Properties for testing materials 150
4.7.2 Numerical simulation of 1D soil column experiment 153 4.7.2.1 Numerical assessment of 1D soil column experiment 155 4.7.3 Physical modelling of 1D soil column experiment 162 4.7.3.1 Response of pore air pressure 163
4.7.3.2 Response of matric suction 167
4.7.3.3 Response of volumetric water content 171 4.7.3.4 Behaviour of unsaturated soil during infiltration 175 4.7.4 Stability with consideration of pore air 177
CHAPTER 5 - CONCLUSIONS AND RECOMMENDATIONS
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5.2 Recommendations for future studies 182
REFERENCES 184
APPENDICES
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LIST OF TABLES
Page Table 2.1 Various causal factors of slope failure 14 Table 2.2 Summary of laboratory experiment techniques for matric
suction measurement
32
Table 2.3 Summary of mathematical equations for SWCC (Fredlund, 2000)
33
Table 2.4 Equations of static satisfied by limit equilibrium methods (Krahn, 2003)
46
Table 3.1 Relationship between in-situ rock quality and RQD (Deere, 1963)
70
Table 3.2 Resistivity of some common rocks and soil materials (Keller and Frischknecht 1966)
72
Table 3.3 Electrical resistivity of some types of water (Keller and Frischknecht 1966)
72
Table 3.4 Configuration for each resistivity lines 73 Table 3.5 Configuration for each seismic refraction lines 77 Table 3.6 Typical propagation velocities of seismic waves through
different types of geological media (Reynolds, 2011)
79
Table 3.7 Typical P-waves velocities, vp for various soils and rocks (Cornforth, 2005)
80
Table 3.8 Selected undisturbed sample for triaxial unconsolidated drained test
86
Table 3.9 Calculated weight for each rainfall station 92 Table 3.10 Data utilization based on date and purpose 94 Table 3.11 Experimental program for 1D soil column study 112 Table 3.12 Required input properties for numerical modelling 114
Table 4.1 RQD results from rock coring 117
Table 4.2 Summary for the detailed of three vp layers with the estimated ground condition
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Table 4.3 RMS results and number of iteration for each resistivity line 125 Table 4.4 Summary of resistivity distribution for the study area 125 Table 4.5 Geotechnical parameters for soil sample BH3 at 2 - 2.4m 133 Table 4.6 Result of parameter calibration after optimization 135 Table 4.7 Type of seepage analyses with different boundary conditions 140 Table 4.8 Basic soil properties for glass beads and soil sample Point 1 152 Table 4.9 Summary of findings for 1D soil column study 176
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LIST OF FIGURES
Page Figure 1.1 Groundwater behaviour in slope subjected to rainfall 3
Figure 1.2 Thesis outline 11
Figure 2.1 Stability variation for slope subjected to preparatory and triggering causal factors (Popescu, 1994)
15
Figure 2.2 Triggering factors of slope failure for worldwide literatures (Public Works Department of Malaysia, 2008)
15
Figure 2.3 Triggering factors of slope failure in Malaysia (Public Works Department of Malaysia, 2008)
16
Figure 2.4 An overview of rainfall induced slope failure mechanism 17 Figure 2.5 Matric suction profile in horizontally layered unsaturated soil
under various surface flux boundary conditions (Hamdhan and Schweiger, 2011)
28
Figure 2.6 A typical soil water characteristic curve (SWCC) of soil (Fredlund and Xing, 1994)
29
Figure 2.7 Typical plots of the soil-water characteristic curve and the coefficient of hydraulic conductivity function for a sand and a clayey silt (Fredlund, 2000)
37
Figure 2.8 Extended Mohr Coulomb failure surface for unsaturated soil (Lu and Likos, 2004)
42
Figure 2.9 Slice discretization and slice forces (Krahn, 2003) 44
Figure 2.10 Definitions and terms used in FEM 47
Figure 2.11 The early warning system activities flowchart (Intrieri et al., 2012)
50
Figure 2.12 Conceptual model of tank model 58
Figure 2.13 Application of tank model in slope stability study 58
Figure 3.1 Methodology flowchart for this study 61
Figure 3.2 Rainfall intensity recorded by the rainfall station in Precinct 2, Putrajaya for March 2007 (Ahmed et al., 2011)
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Figure 3.3 Location of slope failure in Precinct 9, Putrajaya (extracted from Google Earth® and (Perbadanan Putrajaya, 2008))
63
Figure 3.4 The occurrence of slope failure in Precinct 9, Putrajaya on 22nd March 2007 (Lankapo, 2007)
64
Figure 3.5 Geological map of study area extracted from digital map of Peninsular Malaysia
65
Figure 3.6 Crib wall to stabilize the slope failure mass at Precinct 9, Putrajaya
66
Figure 3.7 Location of piezometers and inclinometers installed at the study area
66
Figure 3.8 Flowchart for subsurface characterization of the study area 68 Figure 3.9 Location of boreholes, resistivity lines and seismic lines 68 Figure 3.10 The arrangement of electrodes for a 2D electrical survey and
the sequence of measurement used to build up a resistivity section
73
Figure 3.11 The equipments for electrical resistivity survey 74 Figure 3.12 Flowchart of resistivity inversion process (Society of
Exploration of Geophysicist Japan, 2004)
74
Figure 3.13 Typical example of measured and calculated apparent resistivity pseudosections together with an inversion model (Loke, 2000)
75
Figure 3.14 Instruments for seismic refraction survey (a) Geode seismograph (b) 14Hz geophones (c) laptop (d) 200m geophone spread cable (e) 8kg sledgehammer (f) striker plate
77
Figure 3.15 Seismic refraction survey on-site configuration and the refracted wave
78
Figure 3.16 Flowchart of seismic refraction iterative data analysis (Takahashi et al., 2006)
78
Figure 3.17 Example of P-wave seismic refraction first signal arrivals 79 Figure 3.18 3D cells of voxels (extracted from RockWorks 15, 2010) 82 Figure 3.19 Additional sampling point for laboratory physical modelling 83 Figure 3.20 Flowchart of soil sampling laboratory tests 83
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Figure 3.21 Undisturbed sample for falling head test using core cutter method
85
Figure 3.22 Parameters in a tank model system 87
Figure 3.23 The multi tank model system on slope and the unknown parameters
88
Figure 3.24 General routine of multi tank model for groundwater tables evaluation
91
Figure 3.25 Four rainfall stations nearest to the slope at Precinct 9, Putrajaya
92
Figure 3.26 Daily rainfall intensity for year 2011 92 Figure 3.27 Daily rainfall intensity for year 2012 93 Figure 3.28 Flowrate measurements of horizontal drain for year 2011 93 Figure 3.29 Flowrate measurements of horizontal drain for year 2012 93
Figure 3.30 Routine for seepage flow analysis 95
Figure 3.31 Geometry of the finite element mesh used for the seepage flow analysis
96
Figure 3.32 Routine of stability analysis 100
Figure 3.33 Schematic diagram of 1D soil column apparatus 102 Figure 3.34 Actual experimental setup for 1D soil column experiment 103 Figure 3.35 Design of soil column cylinder in 3D view 104 Figure 3.36 Shower head with 2mm opening for rainfall simulator system 105 Figure 3.37 Tensiometer-transducer for 1D soil column experiment 107 Figure 3.38 2100F Soilmoisture probe (from Soilmoisture Equipment
Corporation)
107
Figure 3.39 Trime-Pico 32 for water content measurement 108 Figure 3.40 PTFE membrane for pore air pressure sensor 109
Figure 3.41 Kyowa low pressure transducer 109
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Figure 3.43 Flowchart for numerical modelling routine 113 Figure 3.44 Finite element model for 1D soil column 114
Figure 4.1 3D multi borehole logs 118
Figure 4.2 3D stratigraphy model 118
Figure 4.3 Stratigraphy fence diagram for the study area 119 Figure 4.4 Seismic velocity profiles for (a) S1 (b) S2 (c) S3 (d) S4 (e) S5
(f) S6
121
Figure 4.5 3D vp model of the study area 123
Figure 4.6 Correlation coefficient for 3D vp model interpolation 123 Figure 4.7 Seismic velocity sections for (a) A-A’ (b) B-B’ 124 Figure 4.8 2D resistivity model for survey line (a) R1 (b) R2 (c) R3 (d) R4
(e) R5 (f) R6 (g) R7 (h) R8 (i) R9
126
Figure 4.9 3D resistivity model of the study area 129 Figure 4.10 Correlation coefficient for 3D electrical resistivity model
interpolation
129
Figure 4.11 Resistivity cross-sections for (a) A-A’ (b) B-B’ 130
Figure 4.12 Conceptual model of the study area 131
Figure 4.13 Microsoft Excel worksheet for parameter calibration 135 Figure 4.14 Parameter calibration result for the calculated groundwater
levels at the crest, middle and toe of the slope
136
Figure 4.15 Predicted groundwater levels at the crest, middle and toe of the slope for the year 2012
138
Figure 4.16 Hydraulic properties function for silt (a) volumetric water content (b) hydraulic conductivity
140
Figure 4.17 Application of boundary conditions in seepage analysis using (a) fluctuating groundwater table (b) fixed groundwater table
141
Figure 4.18 Initial pore water pressure distribution for the slope model with (a) fluctuating groundwater table (b) fixed boundary (c) observed groundwater table
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Figure 4.19 Pore water distribution during highest rainfall day analyzed with three types of boundaries condition at (a) crest (b) middle slope (c) toe
144
Figure 4.20 Variation of FOS in 2012 computed using LEM and FEM for slope model with (a) groundwater table fluctuations (b) fixed groundwater table
146
Figure 4.21 Slip surface on 21st September 2012 for slope analyzed by (a) LEM (b) FEM
147
Figure 4.22 LEM stability to validate the multi tank model approach with observed groundwater table data
149
Figure 4.23 Particle size distribution for glass beads 151 Figure 4.24 Particle size distribution at different depth for soil sample Point
1
151
Figure 4.25 Hydraulic properties function for glass beads and soil sample from site (a) volumetric water content (b) hydraulic conductivity
152
Figure 4.26 Infiltration depth with pore water pressure distribution at t=5min for numerical simulation (a) E1 (b) E3 (c) E5 (d) E7
154
Figure 4.27 The response of pore air pressure by numerical analysis for (a) E1 (b) E3 (c) E5 (d) E7
156
Figure 4.28 The response of matric suction by numerical analysis for (a) E1 (b) E3 (c) E5 (d) E7
158
Figure 4.29 The response of volumetric water content by numerical analysis for (a) E1 (b) E3 (c) E5 (d) E7
160
Figure 4.30 Position of monitoring instruments with elevation indicator used for analysis
163
Figure 4.31 Results of pore air pressure due to rainfall infiltration for (a) E1 (b) E2 (c) E3 (d) E4 (e) E5 (f) E6 (g) E7 (h) E8
165
Figure 4.32 Results of matric suction due to rainfall infiltration for (a) E1 (b) E2 (c) E3 (d) E4 (e) E5 (f) E6 (g) E7 (h) E8
169
Figure 4.33 Results of volumetric water content due to rainfall infiltration for (a) E1 (b) E2 (c) E3 (d) E4 (e) E5 (f) E6 (g) E7 (h) E8
173
Figure 4.34 Comparison of FOS analyzed with and without consideration of pore air
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LIST OF ABBREVIATIONS
1D One dimensional
2D Two dimensional
3D Three dimensional
AEV Air entry value
BSCS British Soil Classification System CU Consolidated undrained triaxial test
EDM Electronic distance meter
FEM Finite element method
FEM-SSR Finite element shear strength reduction method
FOS Factor of safety
GLE General limit equilibrium
GP Genetic programming
GPS Global positioning system
GWT Groundwater table
HAE High air entry
ID Intensity-duration
IDW Inverse distance weighting
LEM Limit equilibrium method
LP Local polynomial
lpm Litre per minute
MG Gravelly silt
MH High plasticity silt
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MS Sandy silt
NN Nearest neighbour
NSMP National Slope Master Plan of Malaysia
PTFE Polytetrafluoroethylene
RMS Root mean square
RQD Rock quality designation
SP Poorly graded sand
SPT Standard penetration test
SSR Shear strength reduction
SWCC Soil Water Characteristic Curve
TCR Total recovery ratio
TDR Time domain reflectometry
TSA Trend surface analysis
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LIST OF SYMBOLS
σ’ Effective normal stress σ Total normal stress
ua Pore air pressure
ψ Matric suction
uw Pore water pressure θ Volumetric water content
θs Saturated volumetric water content a Air entry value
n Parameter that controls the slope of SWCC function
m Parameter related to residual water content
Cr Correction factor
kw coefficient of hydraulic conductivity
ks saturated coefficient of hydraulic conductivity
Θ θ/θs
p fitting parameter corresponding to the slope of hydraulic conductivity function
e natural number of 2.71828
C(ψ) Correction factor
θr Residual water content
b ln(1000000)
y dummy variable of integration representing the logarithm of integration
h total hydraulic head
kx Coefficient of hydraulic conductivity in x-direction
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Q Applied boundary flux such as evaporation, infiltration etc.
mw Slope of the SWCC
τ Shear stress on the failure plane
c’ Effective cohesion
’ Effective angle of internal friction
b Angle indicating the rate of increase in shear strength relative
to the change in matric suction, (ua-uw)f cf Factored cohesion
ϕf Factored angle of internal friction vp P-waves velocity
I Rainfall intensity
D Rainfall duration
Rc Computed data for inverse distance weighting method Ri Measured data at site
N Total points measured
wi Weighting for each point
di Distance from each points measured to the unknown points
α Exponent
R Coefficient correlation
c Total cohesion
Total angle of internal friction
ka Dielectric constant
cv Velocity of light in free space (3 x 108 m/s)
t Time
L Length
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βi Discharge coefficient for infiltration in tank model
Hi Height of lateral outlet in tank model Qi Lateral flow discharge in tank model
Zi Water level in tank model Ii Vertical infiltration discharge
E Evaporation rate
GWTref Reference groundwater table
GWTc Computed groundwater table
GWTo Observed groundwater table
M Total number of data measurements
ψi Suction pressure corresponding to the water content occurring at the inflection point of the curve
s Slope of the line tangent to the function that passes through the inflection point
Ns Slice base normal force
Ws Slice weight
ll Line load
β, R, x, f, d, w Geometric parameters of a circular slip surface
α Inclination of slice base
Fm FOS equations with respect to moment equilibrium
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PEMBANGUNAN SISTEM PENILAIAN PARAS AIR BUMI BAGI RAMALAN KESTABILAN CERUN
ABSTRAK
Penyelidikan ini bertumpu kepada pembangunan sistem penilaian paras air bumi bagi ramalan kestabilan cerun berdasarkan kajian kes bertempat di cerun Presint 9, Putrajaya, Malaysia. Sistem ini melibatkan pembangunan model konseptual subpermukaan, pembangunan model untuk penilaian turun naik paras air bumi dan penilaian kestabilan dan penyiasatan perilaku tanah tak tepu yang merangkumi tekanan liang udara. Pencirian subpermukaan untuk kawasan kajian telah dijalankan dengan menggunakan integrasi lubang gerudi, kajian pembiasan seismik dan kaedah keberintangan elektrik. Model subpermukaan tiga dimensi (3D) yang terdiri daripada stratigrafi, halaju seismik dan keberintangan elektrik berjaya mengenal pasti zon kegagalan cerun berpotensi tinggi yang juga merangkumi kawasan kegagalan cerun lampau. Zon kegagalan yang terdiri daripada tanah kelodak dikenal pasti mempunyai ciri-ciri kandungan air yang tinggi dengan julat keberintangan elektrik dari 10Ωm ke 300Ωm dan julat halaju gelombang P (vp) yang sederhana dari 500m/s ke 1000m/s. Ini diikuti dengan pembangunan model tangki pelbagai dan analisis berangka untuk menentukan perubahan air bumi dan taburan tekanan liang air di dalam cerun. Model tangki pelbagai berjaya menghasilkan ralat punca kuasa dua (RMSE) bernilai 0.156 dan 0.169 untuk model kalibrasi yang menggunakan data tahun 2011 dan model ramalan yang menggunakan data tahun 2012. Faktor keselamatan (FOS) cerun sepanjang tahun 2012 menunjukkan perubahan berdasarkan turun naik paras air bumi yang dianalisis menggunakan kaedah keseimbangan had (LEM) dan kaedah unsur
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terhingga (FEM). Disamping itu, FOS untuk model cerun yang menggunakan paras air bumi model tangki pelbagai memperoleh ralat sebanyak 1% berbanding dengan model cerun yang menggunakan paras air bumi ukuran di tapak. Keputusan kajian membuktikan bahawa sistem penilaian yang dibangunkan ini boleh diterapkan sebagai satu sistem penilaian untuk ramalan kestabilan cerun. Selain itu, komponen tekanan liang udara yang sering diabaikan dalam perilaku tanah tak tepu semasa penyusupan air hujan juga disiasat menggunakan simulasi berangka dan permodelan fizikal kolum tanah satu dimensi (1D). Keputusan kajian menunjukkan bahawa pembangunan tekanan liang udara adalah dipengaruhi oleh jenis tanah, keamatan hujan dan kandungan air permulaan. Keputusan simulasi berangka dan eksperimen makmal menunjukkan bahawa sampel tanah dari tapak dengan keamatan hujan sebanyak 26751.6mm/hr dan kandungan air permulaan sebanyak 21% menghasilkan tekanan liang udara yang maksimum. Saiz zarah yang halus serta intensiti hujan dan kandungan air permulaan yang tinggi akan menyebabkan penyusupan air hujan tidak dapat menggantikan liang udara dengan lancar maka mengakibatkan tekanan liang udara meningkat.
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DEVELOPMENT OF EVALUATION SYSTEM FOR GROUNDWATER LEVEL IN RELATION TO SLOPE STABILITY FORECASTING
ABSTRACT
The aim of this study focused on developing a new evaluation system of groundwater level for slope stability forecasting by conducting a case study at a cut slope in Precinct 9, Putrajaya, Malaysia. This system involved the development of subsurface conceptual model, development of groundwater table fluctuation model, stability assessment and investigation of unsaturated soil behaviour which incorporates the response of pore air pressure. Subsurface characterization of the study area was carried out by using the integration of borehole drilling, electrical resistivity survey and seismic refraction survey. Three dimensional (3D) models of stratigraphy, electrical resistivity and seismic velocity were proven to be effective in identifying the potential failure zone that coincides with the past slope failure zone. The potential failure zone that consist of silt soil was identified to have high water content with electrical resistivity ranges from 10Ωm to 300Ωm and intermediate P-waves velocity (vp) of 500m/s to 1000m/s. Subsequently, the multi tank model and numerical analysis was developed to determine the groundwater table fluctuations and pore water pressure distributions of the slope. The multi tank model was able to produce a root mean square error (RMSE) of 0.156 and 0.169 for the calibration model using year 2011 data and prediction model using year 2012 data respectively. The factor of safety (FOS) of the slope throughout the year 2012 varies according to the fluctuations of groundwater table analyzed by using both limit equilibrium method (LEM) and finite element method (FEM). Furthermore, the FOS for the
