GEOMETRICAL EFFECT ON THE BEHAVIOUR OF EMBANKMENT ON

SOFT GROUND

ALI SOBHANMANESH

A thesis submitted in fulfilment of the

requirements for the award of the degree of

Doctor of Philosophy (Civil Engineering)

Faculty of Civil Engineering

Universiti Teknologi Malaysia

JULY 2015

iii

To my lovely wife, your courage and compassion have taught me humility

To my beloved parents

iv

ACKNOWLEDGMENTS

I would like to take this opportunity to express my sincere gratitude and

appreciation to all those who have contributed in completing this project. I wish to

express profound gratitude to my supervisors Associate Prof. Ir. Dr. Ramli Bin Nazir

for his direct supervision, advice and guidance for bringing the idea into fruition,

especially during the writing of this thesis.

I also wishes my sincer thanks to Assoc. Prof. Dr. Wan Zuhairi Wan Yacoob

from Universiti Kebangsaan Malaysia (UKM) for allowing me to use the centrifuge

facilities and Assoc. Prof. Dr. Nurly Gofar from Universiti Teknologi Malaysia

(UTM) for her help in giving ideas and discussions. Thanks are also to all

researchers in the Department of Geotechnical and Transportation, Faculty of Civil

Engineering, UTM for invaluable discussion, assistance, and lasting friendship.

Acknowledgment is also conveyed to all Geotechnical laboratory technicians; in

particular Mr. Zulkifly Wahid who patiently provided assistance on laboratory test.

Finally, I wish to express special thanks to my beloved wife, dear Leila, for

her loves, sacrifices, patience, and continuous struggle towards the accomplishment

of this study. Especial thanks and appreciation to my dear father and mother for their

supports and love.

v

ABSTRACT

Many embankments constructed on soft ground are susceptible to failure and

large settlements due to its low strength soil condition. Geosynthetics are used

effectively as a reinforced material to increase the shear strength, and stiffness of the

reinforced embankment and consequently, to reduce the total and differential

settlements. In the first part of the study, four different cases of embankments with

and without reinforcement, constructed on soft and stiff grounds were studied

through small-scale physical modeling using centrifuge test and numerical modeling

using finite element simulation. Comparison between the results using both finite

element models and centrifuge tests was carried out to validate and identifies the

reliability of the finite element method. In centrifuge test, a model scale with various

sizes was simulated to a constant full-scale dimension using different acceleration

fields. The results show the different deformation behavior for these different

embankment cases and indicate the significant effect of the geosynthetics

reinforcement on increasing the stability of embankment. The comparison analysis

presents a good agreement between results of these two methods. It validated the

finite element technique in analysis of different embankment cases. The second part

of the study focus on the geometrical effects on the behavior and failure mechanism

of embankments. Two full-scale case history embankments in Malaysia and Canada,

the Muar trial embankment and Vernon highway embankment were verified. Three

dimensional effects on Muar trial embankment were evaluated by comparing the

results of two and three-dimensional analysis, in terms of predicted displacements,

lateral movements, excess pore pressure, factor of safety, and failure height of the

embankment fill. Moreover, this study attempt to evaluate the boundary limits for the

applicability of two and three-dimensional analyses by determining the suitable

geometry configuration of embankment in utilizing the geotechnical analysis. The

ratio of the calculated failure height of three to two dimensional Finite Element

analyses (Hf,3D/Hf,2D) has been determine for embankment cases with different base

aspect ratio of the length to width (L/B). Two shape-factor equations related to the

bearing capacity of spread footings and safety factor of embankments also utilized to

account for the geometrical behavior of the embankment regards to its geometrical

configuration. Results of three-dimensional analyses have better agreement with the

actual field measurements. It is concluded that neglecting the three dimensional

effects could mislead the design of the embankment in some condition. In

conclusion, it is recommended that for “long embankment” with the length to width

ratio more than two (L/B > 2), it may appropriate to use two-dimensional analysis as

the three-dimensional safety factor converges to two dimensional safety factor. For

“short embankment” with the length to width ratio less than two (L/B < 2), three

dimensional effects on the embankment behavior becomes considerably great and

should be considered as important factor in design and analysis of embankments.

vi

ABSTRAK

Kebanyakan tambakan yang di bina di atas tanah liat lembut terdedah kepada Kebanyakan benteng yang dibina di atas tanah lembut terdedah kepada kegagalan dan

enapan besar disebabkan keadaan tanah mempunyai nilai kekuatan yang rendah.

Geosintetik digunakan dengan berkesan sebagai bahan pemgukuh untuk meningkatkan kekuatan ricih, dan kekukuhan benteng bertetulang dan seterusnya, untuk mengurangkan

enapan jumlah dan perbezaan. Dalam bahagian pertama kajian ini, empat kes benteng

yang berbeza iaitu dengan dan tanpa menggunakan tetulang,yang dibina atas tanah dasar

lembut dan tegar telah dikaji menggunakan model fizikal berskala kecil melalui ujian centrifuge dan model berangka menggunakan simulasi unsur terhingga. Perbandingan

diantara keputusan menggunakan kedua-dua model unsur terhingga dan ujian centrifuge

telah dijalankan untuk mengesahkan dan mengenal pasti kebolehpercayaan kaedah unsur

terhingga. Dalam ujian centrifuge, skala model dengan pelbagai saiz telah disimulasikan kepada dimensi sebenar yang tetap menggunakan medan pecutan yang berbeza.

Keputusan menunjukkan berlaku kelakuan ubah bentuk yang berlainan bagi kes-kes

tambak yang berbeza dan menunjukkan kesan yang ketara terhadap tetulang geosyntheic

di dalam peningkatan kestabilan benteng. Analisis perbandingan menunjukkan hubungan yang baik di antara keputusan kedua-dua kaedah. Ini mengesahkan penggunaan teknik

unsur terhingga dalam analisis untuk kes benteng yang berbeza. Bahagian kedua kajian

ini memberi tumpuan kepada kesan geometri terhadap tingkah laku dan kegagalan mekanisme benteng. Dua kes benteng berskala penuh di Malaysia dan Kanada, Benteng

Percubaan Muar dan Benteng Lebuh Raya Vernon telah disahkan. Kesan tiga dimensi di

Benteng Percubaan Muar dinilai dengan membandingkan hasil analisis dua dan tiga

dimensi, dari segi anjakan, ramalan pergerakan sisi, tekanan liang berlebihan, faktor keselamatan, dan ketinggian kegagalan benteng. Selain itu, kajian ini telah menilai had

sempadan yang sesuai untuk analisis dua dan tiga dimensi dengan menentukan

konfigurasi geometri benteng yang sesuai dalam menggunakan analisis geoteknikal.

Nisbah ketinggian kegagalan yang dikira menggunakan dua dan tiga dimensi analisis Unsur Terhingga (Hf,3D / Hf, 2D) telah ditentukan melalui kes-kes banteng yang

mempunyai nisbah yang berbeza untuk aspek asas panjang dan lebar (L / B). Dua

persamaan faktor bentuk yang berkaitan dengan keupayaan galas asas dan faktor

keselamatan benteng digunakan untuk mengambil kira kelakuan geometri benteng terhadap konfigurasi geometri itu. Keputusan analisis tiga dimensi mempunyai kesamaan

yang lebih baik dengan ukuran sebenar di tapak. Ia menyimpulkan bahawa dengan

mengabaikan kesan tiga dimensi, boleh mengelirukan reka bentuk benteng dalam beberapa keadaan. Kesimpulannya, adalah disyorkan bahawa untuk "benteng panjang"

dengan nisbah panjang ke lebar lebih daripada dua (L / B> 2), ia boleh memperuntukkan

untuk menggunakan dua analisis dimensi kerana faktor keselamatan tiga dimensi

menumpu kepada faktor keselamatan dua dimensi. Untuk "benteng pendek" dengan panjang ke lebar nisbah kurang daripada dua (L / B <2), kesan tiga dimensi ke atas

tingkah laku benteng menjadi agak besar dan boleh dianggap sebagai faktor penting

dalam reka bentuk dan analisis benteng.

vii

TABLE OF CONTENTS

CHAPTER TITLE PAGE

THESIS TITLE i

DECLARATION ii

DEDICATION iii

ACKNOWLEDGMENTS iv

ABSTRACT v

ABSTRAK vi

TABLE OF CONTENTS vii

LIST OF TABLES xiii

LIST OF FIGURES xv

LIST OF SYMBOLS xxiii

LIST OF ABBREVIATIONS xxvii

LIST OF APPENDICES xxix

1 INTRODUCTION 1

1.2.1 Problems related to reinforcement mechanism 3

1.2.2 Problems concerning the modeling of

embankment 4

1.2.3 Problems concerning the geometrical behavior

of embankment 5

viii

2 LITERATURE REVIEW 11

2.3.1 Soil-Reinforcement Interaction 17

2.4.1 Geotechnical Analytical Modeling 19

2.5.1 Geotechnical Centrifugal Modeling 33

2.6.1 Finite Element Simulation by PLAXIS 66

ix

2.7.1 Muar Trial Embankment Case 72

2.7.2 The Vernon Embankment Case 80

3 RESEARCH METHODOLOGY 95

3.3.1 Direct Shear Test on Soil-Reinforcement

Interface 99

3.3.2 Mini- Vane Shear equipment 102

3.3.3 Tensile strength test 103

3.5.1 Finite element analysis of full-scale prototype 106

3.6.1 Mini-Centrifuge Apparatus Utilized in this Study 110

3.6.2 Centrifuge Test Methodology 119

x

3.8.1 Study of Muar Trial Embankment

122

3.8.2 Study of the Vernon Highway Embankment 128

4 NUMERICAL AND PHYSICAL MODELING RESULTS

AND DISCUSSION 136

4.2.1 Full-Scale Embankment on Soft Ground (Case I) 137

4.2.2 Full-Scale Reinforced Embankment on Soft

Ground (case II) 146

4.2.3 Embankments on Stiff Ground (Case III and IV) 149

4.3.1 Embankment models constructed on soft ground 153

4.3.2 Embankment models constructed on stiff ground 156

4.4.1 Embankment constructed on soft ground (case I) 162

4.4.2 Reinforced embankment on soft ground (Case II) 163

4.4.3 Embankment on Stiff Ground (Case III) 165

4.4.4 Reinforced Embankment on Stiff Ground

(Case IV) 167

xi

5 RESULTS AND DISCUSSIONS OF CASE-HISTORY

EMBANKMENTS 174

5.2.1 Settlement and Heave 179

5.2.2 Lateral Movements 180

5.2.3 Excess Pore Pressure 183

5.2.4 Failure Height of the Embankment 185

5.2.5 Stability Analysis 187

5.3.1 Results of Waterline test fill 195

5.4.1 Shape Factor Equation of Bearing Capacity 200

5.4.2 Correction Equation of Factor of Safety 201

5.5.1 Effect of weathered crust layer 203

6 CONCLUSIONS AND RECOMMENDATIONS 212

xii

REFERENCES 217

Appendices A-D 226-236

xiii

LIST OF TABLES

TABLE NO. TITLE PAGE

2.1 Scale relation in small-scale physical modeling 32

2.2 Similarity requirement for the prototype in conventional and

centrifugal model (Ovesen, 1979) 43

2.3 Typical statistics for beam centrifuges (Wood, 2003) 55

2.4 Typical statistics for drum centrifuges (After (Springman

et al., 2001)) 63

2.5 Studies of construction of embankments on soft ground by

different geotechnical methods 87

3.1 Laboratory Tests for soil and reinforcement material 99

3.2 Details of four case models utilized in this research study 104

3.3 Properties of soil model for foundation and embankment fill 109

3.4 Specifications of small geotechnical centrifuge 114

3.5 Acceleration levels and their related times used in centrifuge

test 119

3.6 Properties of fill material for Mohr-coulomb model 127

3.7 Properties of foundation layers for Mohr-coulomb model 127

3.8 Strength profile of different layers of subsoil for Vernon

highway embankment 130

3.9 Parameters of fill material in Vernon highway embankment 130

3.10 Parameters of subsoil layers in Vernon highway embankment 131

4.1 Relation of finite element prototype and centrifuge small-scale

model 152

xiv

4.2 Vertical displacement for different stage of construction of

small-scale and full-scale models corresponded to Case I 155

4.3 Vertical displacement for different stage of construction of

small-scale and full-scale models corresponded to Case II 155

4.4 Vertical displacement for different stage of construction of

small-scale and full-scale models corresponded to Case III 158

4.5 Vertical displacement for different stage of construction of

small-scale and full-scale models corresponded to Case IV 158

4.6 Maximum measured settlement of the centrifuge model cases

at top of the fill slope for different embankment cases 159

4.7 Maximum vertical displacements resulted from the centrifuge

tests and finite element analyses of different cases 173

5.1 Differences between calculated failure height of embankment

by 2-D and 3-D analyses 187

5.2 Results of stability analysis for Waterline test fill calculated by

2-D and 3- D analysis 195

5.3 Results of Vernon highway embankment and Waterline test fill

by 2-D and 3-D analyses 198

5.4 Failure height ratio of embankments with different geometry

configuration 199

5.5 Failure height of embankment for different depth of crust

layers 209

5.6 2-D and 3-D safety factors obtained from analytical and

numerical methods 211

xv

LIST OF FIGURES

FIGURE NO. TITLE PAGE

1.1 Assumption shape of the failure surface in 2-D and 3-D

analysis 5

2.1 Schematically illustration of the reinforced embankment over

weak foundation soils in: (a) plain strain and (b) Three-

dimensional view 15

2.2 Example of time-dependent reinforcement application 17

2.3 Stress transfer mechanism at the soil-reinforcement interface 18

2.4 Details of circular arc slope stability analysis for (c, ϕ) shear

strength soils 21

2.5 Details of circular arc slope stability analysis for soil strength

represented by undrained conditions. 23

2.6 Geotextile embankment design based on bearing capacity

(After (Koerner et al., 1987)) 25

2.7 Geotextile embankment design based on global stability

(After (Koerner et al., 1987) 25

2.8 Required geosynthetic strength based on F.S=1.3. Chart

reflects surcharge height of 4 m. 26

2.9 Geotextile embankment design based on elastic deformation

(After (Koerner et al., 1987)) 27

2.10 Geotextile embankment design based on pullout or

anchorage (After (Koerner et al., 1987)) 28

2.11 Geotextile embankment design based on lateral spreading

(After Koerner et al., 1987) 30

2.12 (a) Angular velocity of an element in a circular path and (b)

element in a circular motion moving in different direction 34

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2.13 Object moving in steady circular orbit 36

2.14 Element of soil (a) at surface of the earth and (b) on

centrifuge 37

2.15 Finite dimensions of two-dimensional centrifuge model of

embankment. 38

2.16 Parameters of circular footing resting on a dry sand surface

and the dimension of a sand grain (Ovesen, 1979). 42

2.17 Dimensionless load-settlement curves for test corresponding

to a 1m-diameter prototype footing (Ovesen, 1979). 44

2.18 Summary of a peak values obtained from centrifugal test and a

conventional test (Ovesen, 1979) 45

2.19 (a) Radial acceleration field on centrifuge and (b) flat surface

‘feels’ curved: soft soil may suffer ‘slope’ instability 46

2.20 Stress distribution in centrifuge modeling 47

2.21 Schematic diagram of beam centrifuge: model on swinging

platform 53

2.22 Diagram of Acutronic 680 beam centrifuge

(After (Wood, 2003)) 53

2.23 Beam centrifuge at Hong Kong University of Science and

Technology (Wood, 2003) 54

2.24 Beam centrifuge performance envelope (after De Souza,

2002) 55

2.25 Schematic section through a drum centrifuge: continuous

model of embankment 62

2.26 Diagrammatic section through drum centrifuge at Tokyo

Institute of Technology with actuator arranged for pull-out test

of enlarged base model footing (from (Gurung et al., 1998)) 63

2.27 Extent of the North-South Expressway, from Bukit Kayu

Hitam at the Malaysia-Thai border to Johor Bahru (MHA,

1989) 74

2.28 Location of Muar trial embankments (MHA, 1989) 74

2.29 Properties of the Malaysia Marine Clays (MHA, 1989) 75

2.30 Vane shear test results for Muar clay 76

xvii

2.31 13 full-scale trial embankments constructed on Malaysia

marine clay (MHA, 1989) 77

2.32 Statistics data of predicted failure heights of Muar

embankments 78

2.33 Failure and collapse of the Muar trial embankments (MHA,

1989) 79

2.34 Results of water content and shear strength profile of the

subsoil (After Crawford et al., 1995) 81

2.35 Plan view of Vernon highway embankment

(Crawford et al., 1995) 82

2.36 Longitudinal section of embankment

(after Crawford et al., 1995) 83

2.37 Construction stages, fill height and settlements at centerline

of station 27+80 during construction

(after Crawford et al., 1995) 83

3.1 Flowchart of thesis research 96

3.2 Big square shear box apparatus 101

3.3 Clamping of geotextile specimen at shear box 101

3.4 Mini-vane shear equipment 102

3.5 Mini-vane shear test on kaolin sample 102

3.6 Tensile strength test apparatus 103

3.7 Geometry dimension, generated FE mesh and boundary

fixities of considered cases of embankments (a) 2-D model

and (b) 3-D model 107

3.8 Closed consolidation boundaries at side of the model 108

3.9 Layer by layer staged-constriction of embankment fill 108

3.10 Overall view of mini geotechnical centrifuge apparatus 111

3.11 Inside view of mini geotechnical centrifuge apparatus 111

3.12 Section view of centrifuge system (acquired from UKM

Centrifuge lab) 112

3.13 Plan view and different parts of centrifuge system

(acquired from UKM Centrifuge lab.) 113

xviii

3.14 (a) Plan-view and (b) cross-section of considered model

cases 115

3.15 The state of placed kaolin, before performing the centrifuge

test 116

3.16 The state of placed kaolin, after performing the centrifuge

test 116

3.17 Mold used to construct the embankment fill 117

3.18 Constructed model of case I before running the test 118

3.19 Constructed model of case III before running the test 118

3.20 Generated mesh of FE model using centrifugal dimension 120

3.21 Acceleration levels with relative time of rotation 121

3.22 Geometry of Muar trial embankment (a) 2-D geometry (b)

3-D geometry 124

3.23 Generated mesh of Muar trial embankment (a) 2-D model and

(b) 3-D model 125

3.24 Soil strata and strength profile of the Muar trial embankment

(data from MHA 1989) 126

3.25 Undrained shear strength profile of Vernon highway

embankment (Lo and Hinchberger, 2006) 129

3.26 Plan view and cross-section of Vernon highway embankment 132

3.27 Three-dimensional mesh of Vernon highway embankment 133

3.28 Geometry of Waterline test embankment 133

3.29 Generated 3-D FE mesh of Waterline test embankment 134

4.1 Deformed mesh of the model after construction of the

embankment correspond to case I: (a) 2-D analysis and

(b) 3-D analysis 138

4.2 Displacement increments due to construction of the

embankment correspond to case I: (a) 2-D analysis and

(b) 3-D analysis 139

4.3 Induced vertical displacement at center of the embankment

due to the construction of the fill 140

xix

4.4 Excess pore pressure just before the failure: (a) 2-D and

(b) 3-D analysis 141

4.5 Development of excess pore pressure with time at point C

beneath the embankment by 2-D and 3-D analysis 142

4.6 Safety factor versus total displacements for point A (0,0) 143

4.7 Safety factor for different height of the embankment 143

4.8 Displacement increment in normal and update mesh analysis 145

4.9 Displacement increments at last phase of construction: (a)

2-D and (b) 3-D analysis 146

4.10 Development of vertical displacement for Cases I and II 147

4.11 vertical displacements induced by construction of

embankment layers reinforced by geotextile: (a) 2-D,

(b) 3-D model 148

4.12 Deformed mesh of case III: (a) 2-D and (b) 3-D analyses 149

4.13 Shading of total displacement of case III by: (a) 2D and (b)

3D analyses 150

4.14 Vertical displacement of cases III and IV due to construction

of the embankment 151

4.15 Deformation behavior of unreinforced embankment on soft

ground (case I) 154

4.16 Deformation behavior of reinforced embankment on soft

ground (case II) 154

4.17 Deformation behavior of unreinforced embankment on stiff

ground (case III) 156

4.18 Deformation behavior of reinforced embankment on stiff

ground (case IV) 157

4.19 Maximum measured settlement for different cases by

centrifuge test 159

4.20 Settlements on top of the fill slope due to stress increments

on each velocity field 160

4.21 Deformed mesh of case I after FE simulation of one -hour

rotation in different acceleration field 162

xx

4.22 Vertical displacements of case I after FE simulation of one -

hour rotation in different acceleration field 162

4.23 Vertical displacement of points G (70 mm, 50 mm) for case I 163

4.24 Deformed mesh of case II after FE simulation of one -hour

rotation in different acceleration field 164

4.25 Total displacements of case II after FE simulation of one -

hour rotation in different acceleration field 164

4.26 Vertical displacement of points G (70 mm, 50 mm) for

case II 165

4.27 Deformed mesh of case III after FE simulation of one -hour

rotation in different acceleration field 166

4.28 Vertical displacement of case III after FE simulation of one

-hour rotation in different acceleration field 166

4.29 Vertical displacement of points G (70 mm, 50 mm) for

case III 167

4.30 Comparison of deformation pattern resulted in centrifuge

and FE models for case I 168

4.31 Comparison of deformation pattern resulted in centrifuge

and FE models for case II 168

4.32 Comparison of deformation pattern resulted in centrifuge

and FE models for case III 168

4.33 Comparison of deformation pattern resulted in centrifuge

and FE models for case IV 169

4.34 vertical displacements due to construction of embankment

layers corresponded to the centrifuge test and FE analysis 170

5.1 ( a) Two-dimensional and (b) three-dimensional deformed

mesh after construction of Muar trial embankment 176

5.2 Results of Muar embankment prior to failure (H = 4 m) and

at failure (H > 4 m): (a) Plastic points, (b) velocity field,

and (c) shading of incremental displacement 177

5.3 Three-dimensional displacement increments at failure: (a)

velocity field and (b) shading contours 178

5.4 Displacement profiles (settlement and heave) along ground

surface for different height of Muar embankment by 2-D

analysis 179

xxi

5.5 Displacement profile (settlement and heave) along ground

surface at failure by 3-D analysis 180

5.6 lateral movement of the Muar trial case after construction

of the embankment by: (a) 2-D and (b) 3-D analysis 181

5.7 (a) section A-A at embankment toe, (b) Lateral movements of

foundation along the depth for section A-A by 3-D analysis 182

5.8 Profiles of lateral movements of foundation ground along the

depth for different height of Muar trial embankment 183

5.9 Principal directions of the excess pore pressure distribution 184

5.10 Excess pore pressure with depth for different height of

embankment 184

5.11 Induced settlement of Muar trial embankment due to

increasing the fill height in 2-D and 3-D analyses 186

5.12 Net fill height of Muar trial embankment versus the fill

height in 2-D and 3-D analyses 186

5.13 Safety factor for different height of embankment 187

5.14 Vertical displacement versus fill height of Vernon highway

embankment in 2-D analysis for different soil strength

profiles 189

5.15 Net fill height versus fill height of Vernon highway

embankment in 2-D analysis for different soil strength

profiles 189

5.16 Vertical displacement of Vernon highway embankment in

2-D and 3-D analysis for M strength profile 190

5.17 Net fill height of Vernon highway embankment in 2-D and

3-D analysis for M strength profile 190

5.18 Surface settlement profile of Vernon highway embankment

in 3-D model 191

5.19 Deformed mesh of Vernon highway embankment at failure:

(a) by 2-D analysis, (b) by 3-D analysis 192

5.20 Velocity field of total displacements of Vernon highway

embankment at failure: (a) by 2-D analysis, (b) by 3-D

analysis 193

xxii

5.21 Shading contours of total displacements of Vernon highway

embankment at failure: (a) by 2-D analysis, (b) by 3-D

analysis 194

5.22 Vertical displacement versus fill height of Waterline

embankment in 2-D analysis for different soil strength

profiles 196

5.23 Net fill height versus fill height of Waterline embankment

in 2-D analysis for different soil strength profiles 196

5.24 Vertical displacement versus fill height of Vernon highway

embankment in 3-D analysis for L and M strength profiles 197

5.25 Net fill height versus fill height of Vernon highway

embankment in 3-D analysis for L and M strength profiles 197

5.26 Failure height ratio for different base aspect ratios 200

5.27 Safety factor ratio for different base aspect ratios 202

5.28 Deformation velocity field of Muar trial embankment (a)

with crust layer (b) without crust layer 203

5.29 Lateral displacement of ground for 2-D model: (a) with

crust layer and (b) without crust layer 204

5.30 Lateral displacement of 3-D model without crust layer:

(a) deformed mesh and (b) shading contour 205

5.31 Lateral displacement of 3-D model with crust layer: (a)

deformed mesh and (b) shading contour 206

5.32 Evaluation of safety factor for Muar trial embankment with

2 m crust layer 208

5.33 Effect of surface crust layer on the stability of Muar trial

embankment 209

xxiii

LIST OF SYMBOLS

Am - Local acceleration of model

Ar - Radial acceleration

B - Width of embankment

C - Cohesion of the soil

Ca - Adhesion of the soil to the geosynthetic

Cv - Coefficient of uniformity

, - Total and effective cohesions, respectively

D - Diameter of footing

DR - Thickness of failed region

dg - Average grain size

E - Shear, or frictional, efficiency of geosynthetic to soil

Eg - Coefficient of elasticity of grain material

e - Void ratio

FS - Factor of safety

Fr - Number of revolution per unit time

H - Embankment height

Hallow - Allowable height of embankment

c c

xxiv

Hf - Failure height of embankment

Hm - Height of model

Hp - Height of prototype

hi - Height of water above base of circle for each slice

Ka - Coefficient of active earth pressure = tan2 (45 – ϕ /2)

kx - Horizontal Permeability

ky - Vertical Permeability

Larc - Length of the failure arc

Lm - Length dimensions in the model; Suffix m = model

Lp - Length dimensions in the prototype; p = prototype

Lreqd - Required anchorage length behind the slip plane

m - Number of geotextile layers

n - Number of slices

N - Scale factor or gravity level

Nc - Bearing capacity factor

Ni - Wi cos θi

- , in which

Pa - Rankine active pressure

- Allowable bearing capacity

qu - Unconfined compression strength of soil

R - Radius of the failure circle

N i Ni -uiDxi

qallow

xxv

Ra - Radius of rotating arm

S - Distance travel along circular path

Tact - Actual stress in the geosynthetic

Ti - Allowable tensile strength of various geotextile layers

Tv - Consolidation time factor

t - Time travel

tm - Model time

tp - Prototype time

V - Vertical external load

Vr - Radial velocity

Vs - Volume of sand

ui - hi γw = pore-water pressure

W - Weight of failure zone

, - Total and effective weight of each slice

X - Moment arm to center of gravity of failure zone

yi - Moment arm of geotextile layers

γ - Unit weight of embankment soil

γm - Model unit weight

γp - Prototype unit weight

γw - Unit weight of water

- Artificial gravity induced by centrifugal forces

Wi W i

xxvi

θ - Angle of pile inclination/about center of rotation

- Angle of intersection of horizontal to tangent at center of slice

- Arc length of each slice

- Width of slices

ϕ - Friction angle of the soil

, - Total and effective angles of shearing resistance, respectively

σc - Cohesive force between sand grain

σg - Crushing strength of grain materials

σv - Average vertical stress = γH

σvm - Model vertical stress

σvp - Prototype vertical stress

δ - Friction angle of the soil to the geosynthetic

δreq - Required friction angle of geosynthetic to soil

ε - Maximum error developed in centrifugal machine

ρ - Soil density

ω - Angular velocity

qi

Dli

Dxi

f f

xxvii

LIST OF ABBREVIATIONS

ASTM - American Standard Testing Method

BIS - Bureau of Indian Standards

BS - British Standard

BSI - British Standards Institution

CRE - Constant Rate of Extension

CSPE - Chlorosulfonated Polyethylene

CU - Consolidated Undrained

EPS - Expanded Polystyrene

EPWP - Excess Pore Water Pressure

FD - Finite-Difference

FE - Finite Element

HF - Failure Height

HDPE - High Density Polyethylene

ISO - International Organization for Standardization

LCD - Liquid Crystal Display

LDPE - Low Density Polyethylene

LL - Liquid Limit

LVDT - Linear Variable Different Transducer

MC - Mohr-Coloumb

MHA - Malaysian Highway Authority

PA - Polyamide

PET - Poly- Ester

xxviii

PI - Plasticity Index

PL - Plastic Limit

PP - Polypropylene

PVC - Polyvinyl Chloride

PRC - People Republic of China

UK - United Kingdom

UKM - Universiti Kebangsaan Malaysia

ULS - Ultimate Limit State

USA - United States of America

UTM - Universiti Teknologi Malaysia

SS - Soft Soil Constitutive Model

2-D - Two-dimensional

3-D - Three-dimensional

xxix

LIST OF APPENDICES

APPENDIX TITLE PAGE

A Atterberg Limit Tests 227

B Proctor Compaction Test 228

C Direct Shear Test 230

D Direct Shear Test on Reinforced Sand 234

1

CHAPTER 1

INTRODUCTION

Background of Study

Embankments are needed in construction of many industrial structures.

Today, a large number of industrial structures and embankments are constructed in

areas with low strength grounds such as harbor and river inlets zones. Many

embankments constructed on such soft grounds are susceptible to failure and large

settlements due to the incompatible weak condition of the ground soil.

Many conventional methods and ground improvement techniques have been

used in the past to increase the shear strength of the soft soils. In the conventional

method of construction, the soft soil is replaced by a suitable soil or it is improved by

preloading, dynamic consolidation, injected additives, lime/cement mixing or

grouting prior to the placement of the embankment. Other options such as staged

construction with sand drains, the use of stabilizing berms and piled foundations are

also available for application. All of these methods have a degree of applicability,

but it is clear all suffer from being either expensive, time-consuming, or both. Hence

an alternative method such as soil reinforcing by geosynthetics materials, which is a

fast and economical technique, could cope with this problem to some extent.

The utilizing of geosynthetics as ground reinforcement has enhanced the

concept of ground improvement and being used for a wide range of applications e.g.

slope stabilization, construction of retaining structures, bridge abutment walls and

embankments. As a deformable material, geosynthetics have the effect of not only

2

increasing the strength and ductility of soil, but also creating a more flexible

structure. In the construction of geosynthetic reinforced soil structures, successive

layers of free draining soil are compacted between sheets of reinforcement. This

procedure results in a stable composite structure that can extend to significant height.

Such structures can undergo fairly large deformation without catastrophic collapse

and often without their serviceability be affected. From a mechanical standpoint,

reinforcing soil provide the benefit of stiffening earthwork structures without

increasing their mass.

The other important issue in designing and analyzing of the embankment

construction on soft ground is to consider the correct behavior of embankment and

define all possible failure mechanisms. The behavior of embankments is originally

three-dimensional (3-D) but in many cases two-dimensional (2-D) analysis can give

an acceptable and reasonable results. In general, two-dimensional (2-D) analysis can

be categorized into two types: (1) 2-D plane stress which is usually applied for stress

analysis of thin plate structure by assuming the stress in the direction perpendicular

to the plate is equal to zero and (2) 2-D plane strain which is defined as the strain

state in the direction perpendicular to the plane is equal to zero. Most researches

assumed plane strain condition for numerical simulations of reinforced earth

structures.

Statement of Problem 1.2

There are many problems and issues concerning the modeling and analyzing

of reinforced embankment on soft ground as described in following:

3

Problems related to reinforcement mechanism 1.2.1

There are many factors that affect the mechanism and behavior of

geosynthetic reinforced soil / embankment, but the most important ones are:

Characteristics of soil

Characteristics of geosynthetic reinforcement

Interaction between soil and reinforcement

In construction of embankments, the characteristics of soil are very important

and have a significant influence on stability and failure height of embankment. If the

soil has weak geotechnical characteristics (soft soil), it causes many limitations and

problems, i.e. the weak shear strength of soil considerably reduces and limits the

height of embankment fill and the deformability, compressibility and low

permeability of soil induce excessive settlements because of developing of excess

pore water pressure due to construction of embankment on such a compressible soils.

The characteristics of geosynthetics also have a great influence on behavior of the

model. With regards to the characteristics of geosynthetic reinforcements, different

reinforcement mechanisms e.g. membrane type, shear type, and anchorage (pull-out)

type should be considered. Moreover, soil–geosynthetics interface plays an important

role in the reinforced structures.

Aforementioned factors have been studied by many researchers but despite

the large number of experiences related with using geosynthetics to enhance the

stability of embankments and other geotechnical projects, the reinforcement

mechanisms and its interaction with the adjacent soil are not completely well-

defined. Analytical analyses based on failure modes are simplified and do not

provide an integrated picture of stress-strain and deformation behavior of the

complete system. The mechanism of load transferring among different elements,

includes embankment fill, foundation soil, reinforcement and soil-reinforcement

interaction is complex and is influenced by the properties of the individual elements

4

as well as the relative magnitudes of the properties with respect to each other

(Varadarajan, 1999).

Problems concerning the modeling of embankment 1.2.2

As mentioned before, analytical methods cannot furnish a comprehensive

mechanism of reinforced embankment system on soft ground. Therefore other

methods of modeling such as physical modeling by means of full-scale or small-scale

(centrifuge test) modeling and numerical modeling by means of finite element (FE)

or finite difference simulations are needed to give a deeper insight of the behavior of

these structures. Due to economical and time concerns, centrifuge test is considered

as a preferable technique in physical modeling but there are many factors that affect

the behavior of embankment in a centrifuge test, which makes some errors and

differences compare to the results of the prototype. These factors are:

Radial gravity of centrifuge tests

Different geometry of embankment in each stage of construction due to the

different gravitational acceleration field

Interaction between the side wall of the model box and the model

Limitation payload capacity of centrifuge apparatus

In numerical simulation of centrifuge test, most of researchers have

considered FE simulations based on prototype full-scale dimensions without

considering the above factors. Therefore, numerical simulations utilizing small-scale

dimensions of centrifugal models with considering the above factors are essential for

a realistic comparison between the numerical results and centrifugal measurements

and to minimize the differences between these two modeling methods.

5

Problems concerning the geometrical behavior of embankment 1.2.3

The other issue that should be considered as the most important factor that

affects the analysis of embankments is geometrical effects (2-D and 3-D behavior) of

embankment. Generally, as a simple and quick approach, most researchers have

assumed two-dimensional (2-D) plane strain condition, while there can be a

difference between the assumption shape of the failure surface in 2-D and 3-D

analysis. As shown in Figure 1.1, for 2-D analysis an infinite cylindrical surface is

considered while for 3-D analysis a finite curved surface is assumed which is closer

to the actual failure surface in many cases. Consequently, direction of maximum

stress and sliding of soil can not be recognized by 2-D analysis in some cases, which

leads to inaccurate design of embankments on soft grounds.

Usually in the factor of safety approach, with a few exceptions, two-

dimensional analysis yields conservative results compared to three-dimensional

analysis (FS2D < FS3D), while with increasing width of the failing soil wedge

assumed in a 3-D analysis, FS3D converges to FS2D.

Figure ‎1.1 Assumption shape of the failure surface in 2-D and 3-D analysis

6

Based on above explanations, it can be conclude that, 2-D analysis can

give proper results in linear fill cases (long embankments) in which the length of the

fill is much larger than the width such as roadway embankments, while gives a

conservative and less accurate results compare to 3-D analysis in a area fill (short

embankment) in which the length and width of the site are approximately equal.

Therefore, define a proper behavior of embankment based on its geometrical aspects

is very important in analysis of such structures. Moreover, 3-D analysis has been

rarely considered in previous works and researches and the field is still open for

further studies of 3-D behavior and geometrical effects on behavior of embankments

on soft ground.

Objectives of Study 1.3

The major aim of this thesis is evaluation of geometrical effects on the

behavior and failure mechanism of embankment and to define that under what

geometry configuration, the failure mechanism is three-dimensional.

In order to attain aims of this thesis, following objectives had been fulfilled:

1. To determine the influence of important parameters on the deformation

behavior and failure mechanism of embankment.

2. To evaluate the geometrical (3-D) effects on deformation behavior and failure

mechanism of embankment on soft ground.

3. To define the suitable geometry configuration of embankment, for utilizing in

geotechnical analysis (2-D or 3-D analysis).

7

4. To perform numerical modeling, utilizing small-scale centrifugal model

dimensions and considering important factors of centrifuge test in FE

simulation.

Scopes of Study 1.4

This thesis is divided in two parts: The first part deals with the evaluation of

important factors on the behavior of reinforced embankments by physical (small-

scale centrifuge tests) and numerical modeling (finite element simulation) of

assumed cases. The second part describes the geometrical behavior and 3-D effects

on behavior of embankments by FE simulation of case-history embankments. The

scope of this research comprised of different types of geotechnical modeling and

analysis with considering different materials in order to achieve the objectives of this

study. Following scopes and limitations had been covered:

1. Hypothetical analysis of initial embankment model on soft ground was

performed based on limit equilibrium analysis of different failure elements

(e.g., bearing capacity analysis, global stability analysis, elastic deformation

analysis, pull-out or anchorage analysis, lateral spreading analysis).

2. Four cases of embankment models based on different type of foundation soil

and reinforcement condition were considered in centrifuge test and finite

element analysis. Moreover, two case-history embankments namely ‘Muar trial

embankment and Vernon highway embankment’ were considered in

parametric and geometric analyses.

3. In modeling of four embankment cases, Kaolinite and compacted sand were

used as soft and stiff foundations respectively. Clayey-sand was utilize as

embankment fill material and a proper textile was considered as a

reinforcement material. These materials were considered based on the available

compatible materials regards to the models of this study.

8

4. Geotechnical laboratory tests were performed to define the properties of

materials of the study. These tests include direct shear test, compaction

(proctor) test, mini-vane shear test and tensile strength test. The characteristics

and properties of case-history embankments considered based on the previous

works of other researchers on these embankments.

5. Small-scale physical modeling by means of centrifuge test was performed in a

mini-centrifuge apparatus of Universiti Kebangsaan Malaysia (UKM). This

apparatus did not enable a comprehensive quantitative study of the models due

to its small capacity and payload limitation, which affected the results of this

study. The small size of the strongbox makes it possible to study a small

embankment model with a fill slope of 1:1 only and limited boundary

conditions. Furthermore, It did not equipped with necessary measurement

sensors, transducers, cells and gauges. Finally, the effect of step loading cannot

be studied completely, because in-fight loading was not possible with this

apparatus.

6. Numerical modeling by means of two-dimensional (2-D) and three-

dimensional (3-D) finite element simulation were carried out. “PLAXIS 2-D”

and “PLAXIS 3-D FOUNDATION” programs were used for finite element

simulation and analysis.

7. 2-D and 3-D parametric and geometric analyses were performed on considered

cases and two full-scale case-history embankments.

Research Significances 1.5

The weak and compressible condition of soft ground leads to embankment

failure and collapse, which cause wasting of budget, time and consequences in

stopping or postponing the project. Therefore, the study of the construction of

embankments on compressible soft soils has been a frequent task for geotechnical

9

engineers all over the world and considering a proper and developed method of

designing and analyzing of embankments on soft ground is very important and

necessary.

Totally, utilizing the 2-D plane strain analysis seems to be conservative in

some cases, which result in inaccurate strength of subsoil foundations. This can lead

to an inappropriate designs of embankment over soft ground and cause catastrophic

failure and collapse. To deal with this issue, three-dimensional analysis is essential

and significant to evaluate the influence of geometric conditions and investigate the

3-D effect on deformation behavior and failure mechanism of embankments on soft

grounds. Considering 3-D effect especially in analyzing the short embankments can

contribute in increasing the stability of work by giving more accurate and realistic

results.

Moreover, the parametric study of this research can give a better insight to

the researchers and engineers about the influence of important variables on the

deformations and displacements of embankment in two and three-dimensional (2-D

and 3-D) analyses.

Finally, The results of this research study can be a useful guidance for

engineers in actual and industrial field of embankment construction. It shows the

proper method of deign and analysis (2-D or 3-D analysis) based on the basal aspect

ratio of length to width (L/B) of embankment.

Thesis Organization 1.6

Chapter 1 presents an introduction of thesis research about construction of

embankments on soft grounds, including background of the research, statement of

problems, aim and objectives of study, scopes of study and significance of this

research.

10

Chapter 2 gives a review of construction of embankment on soft ground,

reinforcing the embankments by geosynthetics, 2-D and 3-D failure mechanism,

geotechnical modeling and their application in analyzing the embankments e.g

analytical, physical and numerical modeling and finally an overview of some case-

history embankments built to failure in Malaysia and Canada.

Chapter 3 explains the methods and technics that used in this research to

fulfill the objectives of study include geotechnical laboratory test methods, small-

scale physical centrifuge test and numerical finite element simulation and analyses

for different embankment case models.

Chapter 4 present and discuses the results obtained from physical and

numerical modeling and analysis for various case embankments with different shear

strength of foundation and reinforcement condition and to compare these results to

validate the finite element analysis.

Chapter 5 describes the results obtained from 2-D and 3-D geometric and

parametric analyses of two full-scale case-study embankments to investigate the 3-D

effect and compare the 2-D and 3-D results.

Chapter 6 depicts useful conclusions based on results of this research study

especially on utilizing the three-dimensional analysis in construction of

embankments on soft grounds. Moreover, this chapter provides recommendations for

further research works.

217

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