Analytical and Numerical Study of Soil Disturbance Associated Wit

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University of Wollongong Thesis Collection University of Wollongong Thesis Collections

2010

Analytical and numerical study of soil disturbanceassociated with the installation of mandrel-driven

prefabricated vertical drainsAli GhandeharioonUniversity of Wollongong

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Recommended CitationGhandeharioon, Ali, Analytical and numerical study of soil disturbance associated with the installation of mandrel-drivenprefabricated vertical drains, Doctor of Philosophy thesis, Department of Civil Engineering, University of Wollongong, 2010.

http://ro.uow.edu.au/theses/3338
http://ro.uow.edu.au/http://ro.uow.edu.au/theseshttp://ro.uow.edu.au/thesesuowhttp://ro.uow.edu.au/http://ro.uow.edu.au/thesesuowhttp://ro.uow.edu.au/theseshttp://ro.uow.edu.au/http://ro.uow.edu.au/http://ro.uow.edu.au/


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ANALYTICAL AND NUMERICAL STUDY OF SOIL

DISTURBANCE ASSOCIATED WITH THE

INSTALLATION OF MANDREL-DRIVEN

PREFABRICATED VERTICAL DRAINS

A thesis submitted in fulfilment of the requirements

for the award of the degree of

Doctor of Philosophy

from

UNIVERSITY OF WOLLONGONG, AUSTRALIA

by

ALI GHANDEHARIOON, B.Sc., M.Sc.

Department of Civil Engineering

2010


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ii

CERTIFICATION

I, Ali Ghandeharioon, declare that this thesis, submitted in fulfilment of the

requirements for the award of Doctor of Philosophy in the Department of Civil

Engineering at the University of Wollongong, is wholly my own work unless

otherwise referenced or acknowledged. The document has not been submitted for

qualifications at any other academic institution.

Ali Ghandeharioon

October 2010


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iii

Abstract

Prefabricated vertical drains (PVDs) combined with preloading have gained in

popularity among the most effective ground improvement techniques available to

mitigate the unacceptable differential settlements caused by the heterogeneity and

high compressibility of soft soil deposits. In this thesis the installation of

mandrel-driven PVDs and associated disturbance in cohesive soils were studied by

conducting analytical investigations, laboratory experiments, and numerical

modelling. The pattern of disturbed regions surrounding the mandrels and the

distribution of stresses in soils obtained from the analytical and numerical predictions

agreed with the results of the laboratory tests. A number of case histories taken from

Malaysia, Australia and Thailand were also analysed to evaluate the associated soil

disturbance during installation of prefabricated vertical drains.

An analytical study of mandrel penetration and the resulting disturbance in soft

saturated clays was carried out with a new elliptical cavity expansion theory (CET).

This research postulated that installing PVDs in the field with commonly used

mandrels would create elliptical cavities with a concentric progression in the

horizontal plane. An elliptical CET was developed using modified Cam clay

parameters for undrained analysis with a formulation based on polar coordinates that

accounts for the rate of mandrel penetration and the time for predicting internal

pressure in the cavity, corresponding stresses and excess pore pressure in the soil


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while driving the mandrel. The pattern of distribution calculated for the excess pore

pressure was verified using data available in the literature. A more realistic elliptical

smear zone based on the elliptical CET was introduced while the disturbed soil

surrounding the mandrel was characterised by the plastic shear strain normalised by

the rigidity index.

A number of large-scale laboratory tests that incorporated the field conditions and

effects of confining pressures were performed. A consolidometer specifically

designed for the purpose, and a machine capable of driving mandrels at realistic rates

were used in these experiments. The variations of pore water pressure during

installation of a mandrel-driven PVD and withdrawal of the mandrel were monitored

by fast response pore pressure transducers connected to a digital data logger. The

extent of smear zone in the large-scale consolidometer was determined using the

results of moisture content tests on samples, which in relation to the installed PVD

were cored along different polar axes from various locations. The smear zone was

then analysed to establish a relationship between its size and the in-situ effective

stresses.

The installation of a mandrel was simulated numerically using a commercial finite

element software package, ABAQUS. The finite element models included coupled

analyses with a large-strain formulation. Coulombs law of friction and the penalty

method were incorporated into the numerical technique. It was shown that the soil


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surrounding the mandrel moved radially and downwards as the mandrel was installed.

The variations of pore water pressure at different locations during the installation of a

mandrel-driven PVD and withdrawal of the mandrel were illustrated. There was an

agreement between the pore pressures measured in the laboratory and the finite

element predictions. The extent of smear zone was studied according to a numerical

simulation of the mandrel installation.

The analytical formulation incorporating the elliptical CET presented in this thesis

was applied to case histories from the Muar clay region in Malaysia and the Sunshine

Motorway in Australia. The ratio of plastic shear strain to the rigidity index was

found useful for estimating the extent of the smear zone in the field because in

practical situations the basic soil parameters may be used without sophisticated

large-scale testing.

Moreover, the numerical model of mandrel installation was specifically developed to

study a case history from the Second Bangkok International Airport in Thailand. The

variations of pore pressure while installing a vertical drain and withdrawing the

mandrel were obtained. The plastic shear strain was evaluated to indentify different

aspects of disturbance in the soil elements. The results of this analysis indicated that

the model developed can be applied to field conditions.


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ACKNOWLEDGMENTS

vi

ACKNOWLEDGMENTS

The writer would like to express his profound gratitude to

Professor Buddhima Indraratna and Dr. Cholachat Rujikiatkamjorn for their

enthusiasm, invaluable help, and constructive criticism throughout the supervision

of this thesis. Their patience and suggestions regarding any inquiry were greatly

appreciated. Professor Buddhima Indraratna and Dr. Cholachat Rujikiatkamjorn

were the source of novel ideas during the twists and turns of this research, and

actively encouraged the writer in every aspect of his Ph.D. career.

Sincere appreciation is also extended to Mr. Alan Grant, senior technical officer at

the University of Wollongong for his valued help during the experimental phase

of this project. His advice, support and availability, even after hours, made the

complex laboratory work possible.

A special note of appreciation is offered to Dr. Hadi Khabbaz,

Professor Timothy McCarthy, Dr. Neaz Sheikh, and Dr. Jayan Sylaja Vinod for

their continuing support, friendly advice and useful comments throughout this

research.

The writer takes this opportunity to thank all past and present members of the

Department of Civil Engineering at the University of Wollongong for their

discussion and support. The writer would also like to thank the


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ACKNOWLEDGMENTS

vii

Cooperative Research Centre (CRC) for Railway Innovation for providing the

scholarship for this project.

The writer wholeheartedly and respectfully dedicates this piece of work as a

tribute to his beloved parents, Mr. Mohammad Ghandeharioon and

Mrs. Soodabeh Ataei, and his darling wife, Mrs. Mahgol Shekalzahi, for their

continuing love, prayers, encouragement and many sacrifices throughout this

research period, without which the writer could never have reached where he is

today. The writer is also grateful to his brother, Mr. Amir Ghandeharioon, for his

humour and support in times of stress.

Finally, and most importantly, the writer offers his heartfelt gratitude as a

compliment to his adored wife for her unfailing support during the highs and lows

of an academic career that started with a M.Sc. thesis at the Ferdowsi University

of Mashhad in Iran. Mahgols affections have been the source of the writers

strength and inspiration the whole time.

Ali Ghandeharioon

University of Wollongong, Australia


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PUBLICATIONS

viii

PUBLICATIONS

The following publications are related to this PhD thesis:

Ghandeharioon A., Indraratna B. and Rujikiatkamjorn C. (2010). Laboratory

and Finite Element Investigation of Soil Disturbance Associated with the

Installation of Mandrel-driven Prefabricated Vertical Drains. submitted to the

Journal of Geotechnical and Geoenvironmental Engineering.

Ghandeharioon A., Indraratna B. and Rujikiatkamjorn C. (2010). Analysis of

Soil Disturbance Associated with Mandrel-driven Prefabricated Vertical

Drains Using an Elliptical Cavity Expansion Theory.International Journal of

Geomechanics, 10(2), 53-64.

Indraratna B., Rujikiatkamjorn C. and Ghandeharioon A.(2008). Modelling of

Soft Ground Consolidation via Combined Surcharge and Vacuum

Preloading. Proceedings of the 2nd International Workshop on Geotechnics

of Soft Soils: Focus on Ground Improvement, CRC Press, Taylor and Francis

Group, London, UK, 43-53.

Rujikiatkamjorn C., Indraratna B. and Ghandeharioon A. (2008). Finite

Element Simulation of Mandrel Penetration in a Normally Consolidated Soil.

Proceedings of the 2nd International Workshop on Geotechnics of Soft Soils:

Focus on Ground Improvement, CRC Press, Taylor and Francis Group,

London, UK, 287-292.


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TABLE OF CONTENTS

ix

TABLE OF CONTENTS

CERTIFICATION...ii

ABSTRACT....iii

ACKNOWLEDGMENTS..vi

PUBLICATIONS......viii

TABLE OF CONTENTS....ix

LIST OF FIGURES..xiv

LIST OF TABLES......xxxi

LIST OF SYMBOLS.xxxii

1 INTRODUCTION.........................................................................................1

1.1 General...1

1.2 Application of Vertical Drains ..6

1.3 Application of Cavity Expansion Theory....10

1.4 Objectives and Scope of the Study...15

1.5 Organisation of the Dissertation...17

2 LITERATURE REVIEW...20

2.1 Installation and Monitoring of Prefabricated Vertical Drains..20

2.2 Characteristics of Prefabricated Vertical Drains..24

2.2.1 Equivalent Drain Radius..24

2.2.2 Influence Zone of Drains.26

2.2.3 Smear Zone of Drains..27

2.2.4 Discharge Capacity of Drains..35


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TABLE OF CONTENTS

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2.2.5 Well Resistance of Drains38

2.2.6 Filtration Mechanism of PVDs and Apparent Opening Size of

Filters..39

2.3 Consolidation Theories43

2.3.1 Theory of Vertical Consolidation.43

2.3.1.1 Terzaghis Theory of 1-D Consolidation..45

2.3.1.2 Other Theories of 1-D Consolidation47

2.3.1.3 Evaluation of the Coefficients of Consolidation and

Permeability..48

2.3.2 Theory of Radial Consolidation...50

2.3.2.1 Barrons Theory of Radial Consolidation.50

2.3.2.2 Hansbos Theory of Radial Consolidation55

2.3.2.3

Method (Hansbo 1979, 1997, 2001).56

2.3.2.4 Evaluation of the Coefficient of Consolidation with

Radial Drainage.58

2.3.2.4.1 ( )rU1ln vs. t Approach...58

2.3.2.4.2 Plotting Settlement Data.59

2.3.3 Theory of Simultaneous Vertical and Radial Consolidation....60

2.4 Modelling Consolidation via Vertical drains Under Field

Conditions62

2.4.1 Permeability and Geometry Matching.63

2.4.2 Concept of Equal Discharge Rate64

2.4.3 Matching the Well Resistance .64


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TABLE OF CONTENTS

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2.4.4 The Concept of an Equivalent Parallel Drain Wall..65

2.5 Analysis of Soil Disturbance due to Installation of the PVDs.68

2.5.1 Bearing Capacity Theory.68

2.5.1.1 Limit Equilibrium Method69

2.5.1.2 Slip-line Method70

2.5.2 Strain Path Method...72

2.5.3 Cavity Expansion Theory.77

2.5.4 Incremental Displacement Finite Element Method..81

2.6 Summary..88

3 THEORITICAL CONSIDERATIONS.90

3.1 General.90

3.2 Assumptions and Definition of the Problem91

3.3 Development of the Elliptical Cavity Expansion Theory93

3.3.1 Elastic Analysis...93

3.3.2 Plastic Analysis.102

3.4 Analysis of Soil Behaviour Around Mandrels Using the Elliptical

CET ...107

3.5 Validating the Elliptical CET with the Data Available in the

Literature....110

3.6 Summary118

4 LABORATORY STUDIES.120

4.1 General...120

4.2 Large-scale Laboratory Tests.121

4.2.1 Test Apparatus...121


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TABLE OF CONTENTS

xii

4.2.2 Test Materials.126

4.2.3 Test Procedure132

4.3 Results of the Large-scale Laboratory Tests..137

4.4 Summary150

5 FINITE ELEMENT MODELLING152

5.1 General...152

5.2 Development of the Finite Element Model154

5.3 Simulating Installation of the Mandrel.163

5.4 Results of the Finite Element Simulation.......166

5.5 Summary183

6 CASE STUDIES185

6.1 General...185

6.2 Muar Clay Region, Malaysia.187

6.2.1 Soil Conditions...189

6.2.2 Analytical Investigation of Smear Zone and Validating the

Prediction...191

6.3 Sunshine Motorway, Australia ..194


6.3.2 Analysis of the Variation of Smear Zone with Depth and its

Verification............................................................................197

6.4 Second Bangkok International Airport, Thailand..201


6.4.2 Finite Element Studying of the Installation of PVDs.205


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TABLE OF CONTENTS

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6.4.3 Numerical Prediction of Disturbance in Soil and its

Verification208

6.5 Summary211

7 CONCLUSIONS AND RECOMMENDATIONS..213

7.1 General Summary...213

7.2 Specific Observations.214

7.2.1 Developing a New Elliptical Cavity Expansion Theory to

Analyse Soil Disturbance...215

7.2.2 Large-scale Laboratory Program to Study the Installation of

PVDs .217

7.2.3 Using Finite Element Modelling to Evaluate the Installation of

PVDs..218

7.2.4 Case Histories Validating the Developed Models.220

7.3 Recommendations for Future Research.....221

REFERENCES...224

APPENDIX A: CONSTITUTIVE MODELLING OF SOILS...247

A.1 Cam Clay Model..252

A.2 Modified Cam Clay Model..255

APPENDIX B: CODE FOR NUMERICAL MODELLING......257


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LIST OF FIGURES

xv

LIST OF FIGURES

Figure 1.1 Structure built on an unstable soft soil deposit (a) just after the

construction; and (b) after differential settlement......1

Figure 1.2 Effect of vertical drains on providing short drainage paths2

Figure 1.3 Potential benefit of vertical drains combined with pre-loading (after

Lau and Cowland (2000)....3

Figure 1.4 Prefabricated vertical drains (a) typical front view (after Global

Synthetics 2010); and (b) a cross section of different types (unit: mm, after

Chai et al. 2004)7

Figure 1.5 Typical geometries of mandrel and shoe (after Saye 2001)...8

Figure 1.6 Common installation rigs on site (after Menard 2010)...8

Figure 1.7 General scheme of a system of prefabricated vertical drains combined

with pre-loading (after Geo-Technics America Inc 2010).....9

Figure 2.1 Common instrumentation of an embankment (after Rixner et

al. 1986)....22

Figure 2.2 Conversion of a typical PVD to a circular drain (a) PVD with cross-

section of mn; and (b) an equivalent circular vertical drain with radius

wr .....24

Figure 2.3 Equivalent radius of a PVD based on various studies..........................25

Figure 2.4 Radii of influence zones as a function of common installation patterns

(a) square pattern; and (b) triangular pattern....................................................27

Figure 2.5 Variations of horizontal permeability in relation to the radial distance

from the centre of the drain (after Onoue et al. 1991)......................................29


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LIST OF FIGURES

xvi

Figure 2.6 A schematic illustration of the consolidation apparatus (after Indraratna

and Redana 1998).............................................................................................30

Figure 2.7 Variations of the normalised horizontal coefficient of permeability in

relation to the radial distance from the centre of the drain (after Indraratna and

Redana 1998)...................................................................................................31

Figure 2.8 Variations of the normalised differential pore pressure in relation to the

radial distance from the centre of the drain (after Hird and Moseley

2000)................................................................................................................32

Figure 2.9 A radial profile of the moisture content (after Sharma and Xiao

2000)..32

Figure 2.10 Variations of normalised horizontal coefficient of permeability in

relation to the radial distance from the centre of the drain (after Sathananthan

and Indraratna 2006)........................................................................................33

Figure 2.11 Variations of moisture content in relation to the radial distance (after

Sathananthan and Indraratna 2006)..................................................................34

Figure 2.12 Common discharge capacities of different types of PVD under unit

hydraulic gradient (after Rixner et al. 1986)....................................................36

Figure 2.13 Deformation modes of a PVD associated with ground compression

(a) uniform bending; (b) sinusoidal bending; (c) local bending; (d) local

kinking; and (e) multiple kinking (after Holtz et al.

1991)................................................................................................................36

Figure 2.14 One-dimensional consolidation (a) void ratio vs. effective stress; and

(b) void ratio vs. Permeability..........................................................................44


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LIST OF FIGURES

xvii

Figure 2.15 Variations of the vertical average degree of consolidation in relation

to the modified time factor (after Lekha et al.

2003)....48

Figure 2.16 A schematic view of the soil with a vertical drain modelled as a unit

cell51

Figure 2.17 Evaluating the horizontal coefficient of consolidation (after Aboshi

and Monden 1963)...58

Figure 2.18 Estimating the horizontal coefficient of consolidation (after Asaoka

1978)................................................................................................................59

Figure 2.19 Converting an axi-symmetric unit cell into a plane strain

condition...........................................................................................................62

Figure 2.20 Variation of excess pore pressure at the periphery of the drain in

relation to the depth (after Chai et al. 1995)....................................................65

Figure 2.21 Assumed failure mechanism for the deep penetration problem

(a) Terzaghi (1943); (b) De Beer (1948), Meyerhof (1951); (c) Berezantzev et

al. (1961), Vesic (1963); and (d) Biarez et al. (1961), Hu (1965) (after

Durgunoglu and Mitchell 1975)...70

Figure 2.22 Slip-line network for the wedge and cone penetration analysis

(after Yu and Mitchell 1996)71

Figure 2.23 Strain path method for deep penetration viewed as a problem

involving steady flow (after Baligh 1985)...73

Figure 2.24 Strain paths due to cone penetration for the soil elements located

initially at a radial distance of one cone radius from the axis of penetration

(a) 1E vs. 2E ; and (b) 3E vs. 2E (after Teh and Houlsby 1991)74


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LIST OF FIGURES

xviii

Figure 2.25 Location of the elastic-plastic boundary in the cone penetration

problem (after Teh and Houlsby 1991)....75

Figure 2.26 Expansion of a cylindrical cavity in clay (after Cao et al. 2001)........80

Figure 2.27 Variation of normalised elastic-plastic boundary with the isotropic

overconsolidation ratio (after Cao et al. 2001).................................................81

Figure 2.28 Eulerian method for cone penetration (after van den Berg et al.

1996)................................................................................................................83

Figure 2.29 Numerical simulation of piezocone penetration (after Abu-Farsakh et

al. 2003)............................................................................................................85

Figure 2.30 Contours of excess pore pressure in kPa at the end of (a) Stage 1; and

(b) Stage 2 (after Abu-Farsakh et al. 2003)......................................................86

Figure 3.1 Expansion of an elliptical cavity in an infinite soft saturated cohesive

soil, shown in polar coordinates...92

Figure 3.2 The strain components due to (a) radial displacements; and

(b) tangential displacements.....96

Figure 3.3 The Effective Stress Path (ESP) for constant volume deformation of

(a) normally consolidated soil; and (b) lightly overconsolidated soil

(after Wood 1990)..103

Figure 3.4 An undrained triaxial compression test on normally consolidated soil

(a) effective stress path; and (b) deviator stress/excess pore pressure versus

shear strain.108

Figure 3.5 The distribution patterns for stresses near a mandrel in the horizontal

plane, 0.5m below the surface just after the mandrel installation

(preconsolidation pressure = 20 kPa).110


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LIST OF FIGURES

xix

Figure 3.6 The distribution patterns predicted for excess pore pressure with the

radial distance using elliptical CET and cylindrical CET along the major axis

of the mandrel 0.5m below the soil surface, and measured when tip of the

drain shoe passed the horizontal plane under consideration, with a (a)

preconsolidation pressure = 30 kPa; and (b) preconsolidation pressure =

50 kPa.....111

Figure 3.7 The distribution pattern of excess pore pressure 0.5m below the surface

predicted using the developed elliptical CET and conventional cylindrical

CET (preconsolidation pressure= 30 kPa) along the (a) o45 polar axis; (b) o90

polar axis113

Figure 3.8 The distribution pattern of excess pore pressure 0.5m below the surface

predicted using the current elliptical CET and conventional cylindrical CET

(preconsolidation pressure = 30 kPa) along the mandrel quadrant114

Figure 3.9 Variations in the ratio of the horizontal coefficient of permeability to

the vertical coefficient of permeability and the plastic shear strain in relation

to the radial distance normalised by the equivalent elliptical radius of the

mandrel...114

Figure 3.10 Variations in (a) effective stress in the qp : plane; and (b) strain in

the pqp

V : plane associated with the mandrel installation highlighting the

locations described in Figure 3.9...115


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LIST OF FIGURES

xx

Figure 3.11 The distribution pattern for the ratio of the plastic shear strain to the

rigidity index in relation to the radial distance normalised by the equivalent

elliptical radius of the mandrel characterising the disturbed soil surrounding a

PVD...117

Figure 4.1 The schematic design of the assembled consolidometer cell

(unit: mm)...122

Figure 4.2 Doughnut pressure chamber mounted on top of the consolidometer

cell..123

Figure 4.3 Fast response pore pressure transducer used in the tests....123

Figure 4.4 The dead weight testing machine used for calibrating the

transducers......124

Figure 4.5 Radial positions (planar view) of the fast response pore pressure

transducers (Ts) in relation to the centre of the cell, at levels identified in the

Figure 4.1 (unit: mm).....125

Figure 4.6 The digital data logger used for monitoring and recording the signals

from the transducers...126

Figure 4.7 The mechanical mixing bowl used for mixing clay and water...127

Figure 4.8 The clay deposit held under water level to ensure full saturation..128

Figure 4.9 The prefabricated vertical drain used in the tests (a) roll sourcing the

vertical drain; and (b) section of vertical drain..129

Figure 4.10 The mandrel used for installing the prefabricated vertical drains

(a) the front view; and (b) the section view.......130

Figure 4.11 The conical shoe attached to the PVD for anchoring purposes (a) the

front view; and (b) the side view...131


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LIST OF FIGURES

xxi

Figure 4.12 Schematic illustration of the mandrel attached to the driving module

(a) the front view; and (b) the side view (unit: mm)..133

Figure 4.13 Positioning the mandrel-driving machine on top of the consolidometer

(a) transferring process; and (b) alignment process...134

Figure 4.14 Final setup of the specially designed machine for driving mandrels

mounted on the large-scale consolidometer...135

Figure 4.15 Planar view of the locations of samples cored in order to evaluate the

extent of smear zone (each mark represents three adjacent samples)....136

Figure 4.16 Variations of excess pore pressure measured in the laboratory during

installation of a PVD and withdrawal of the mandrel at (a) T3 and T6; (b) T2

and T5; and (c) T1 and T4, as identified in Figure 4.5 (surcharge

loading=20 kPa).139




loading=20 kPa).....140




loading=32.5 kPa)..141




loading=32.5 kPa)..142


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LIST OF FIGURES

xxii




loading=50 kPa).143




loading=50 kPa).144



of the mandrel 0.26 m below the soil surface, and measured when base of the

drain shoe passed the horizontal plane under consideration, with a surcharge

loading=20 kPa...145





loading=32.5 kPa....146





loading=50 kPa...146


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LIST OF FIGURES

xxiii

Figure 4.25 Variations of the moisture content of soil measured in the laboratory

along the o0 , o45 and o90 axes in relation to the installed PVD

(surcharge loading = 20 kPa, consolidation pressure = 40 kPa)....147


along the o0 , o45 and o90 axes in relation to the installed PVD

(surcharge loading = 32.5 kPa, consolidation pressure = 50 kPa).....148


along theo

0 ,o

45 ando

90 axes in relation to the installed PVD

(surcharge loading = 50 kPa, consolidation pressure = 80 kPa)148

Figure 4.28 Variations of the normalised equivalent radius of the smear zone in

relation to the in-situ vertical effective stress in laboratory...149

Figure 5.1 A schematic view of the contact kinematics at the mandrel-soil

interface using the master-slave concept155

Figure 5.2 Characteristics of a lightly overconsolidated soil where (a) the original

contact kinematics incorporate the concept of shear strength; (b) the shear

stress versus shear strain; (c) the contact constraints are in the context of the

penalty method in a normal direction; and (d) the contact constraints are in the

context of the penalty method in a tangential direction.....158

Figure 5.3 The axi-symmetric finite element model (a) geometry, mesh and the

boundary conditions; and (b) characteristics of the CAX8P element

incorporated into the analysis (after ABAQUS Analysis Users Manual

2007)..162


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LIST OF FIGURES

xxiv

Figure 5.4 The deformed mesh and state of excess pore pressure due to a (a) drain

shoe with a rapid edge transition; (b) drain shoe with a semi gradual edge

transition; and (c) drain shoe with a gradual edge

transition.....164

Figure 5.5 Constraints of physical contact and the deformed mesh associated with

the penalty parameters of (a)3

4105.1m

kN ; (b)

3

4105.2m

kN ; and

(c) 34

1075.3 m

kN ....165

Figure 5.6 Variations of (a) excess pore water pressure measured in the laboratory

and predicted numerically, and (b) the shear stress estimated in the finite

element model during installation of a PVD and withdrawal of the mandrel at

T3 and T6, as identified in Figure 4.5 (surcharge loading=20 kPa)..168












T3 and T6, as identified in Figure 4.5 (surcharge loading=32.5 kPa)...171


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LIST OF FIGURES

xxv

Figure 5.10 Variations of (a) excess pore water pressure measured in the

laboratory and predicted numerically, and (b) the shear stress estimated in the

finite element model during installation of a PVD and withdrawal of the

mandrel at T2 and T5, as identified in Figure 4.5 (surcharge loading=32.5

kPa)172




mandrel at T1 and T4, as identified in Figure 4.5 (surcharge loading=32.5

kPa)....173




mandrel at T3 and T6, as identified in Figure 4.5 (surcharge loading=50

kPa)....174





kPa)....175


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LIST OF FIGURES

xxvi





kPa)....176

Figure 5.15 Contours of excess pore pressure predicted numerically during

different stages of installing a mandrel when the (a) depth of installation=230

mm; (b) depth of installation=461 mm; and (c) depth of installation=690 mm

(surcharge loading=32.5 kPa)....177

Figure 5.16 Displacement of soil nodes 0.39 m below the surface and four times

the radius of the mandrel away, during mandrel penetration (based on the

finite element model) transposed into (a) radial movements; and (b) vertical

movements (surcharge loading=32.5 kPa).179

Figure 5.17 Development of the plastic zone and failed zone in the consolidometer

during installation of a mandrel-driven PVD according to the numerical model

when the (a) depth of installation= 230 mm; (b) depth of installation= 461

mm; and (c) depth of installation= 690 mm (surcharge loading=32.5

kPa)....180

Figure 5.18 Variations of numerically predicted normalised plastic shear strain

together with the moisture content of soil measured in the laboratory along the

o

0 ,o

45 ando

90 axes in relation to the installed PVD (surcharge loading = 20

kPa, consolidation pressure = 40 kPa)...181


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LIST OF FIGURES

xxvii



o0 , o45 and o90 axes in relation to the installed PVD (surcharge loading=32.5

kPa, consolidation pressure=50 kPa).181



o0 , o45 and o90 axes in relation to the installed PVD (surcharge loading=50

kPa, consolidation pressure=80 kPa).182

Figure 6.1 Location of the trial embankments in Muar clay region, Malaysia (after

Google Maps Malaysia 2010a)......................................................................187

Figure 6.2 The cross section of a test embankment and subsoil profile at Muar

clay region in Malaysia (after Indraratna and Redana 2000).........................188

Figure 6.3 Geotechnical properties of Muar clay (after Indraratna et al.

1994)..............................................................................................................190

Figure 6.4 Variations of field vane strength and cone resistance of Muar clay in

relation to depth (after Indraratna et al. 1992)...............................................190

Figure 6.5 The variations of plastic shear strain normalised by the rigidity index in

relation to the radial distance used for determining the extent of smear zone in

the case history of the Muar clay embankment in Malaysia..........................192

Figure 6.6 The cross section of the mandrel, the assumed elliptical cavity, the

elliptical smear zone, and the circular-equivalent smear zone for the case

history of the Muar clay embankment in Malaysia........................................193

Figure 6.7 Location of the development route proposed for the Sunshine

Motorway in Australia (after Google Maps Australia 2010b)...194


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LIST OF FIGURES

xxviii

Figure 6.8 Trial embankment built at the Sunshine Motorway site in Australia (a)

Planar view; and (b) Typical cross section (after Queensland Department of

Main Roads 1992)..195

Figure 6.9 Geotechnical properties of the layer of silty clay in the development

route proposed for the Sunshine Motorway (after Queensland Department of

Main Roads 1992)..197

Figure 6.10 The variations of plastic shear strain normalised by the rigidity index

in relation to the radial distance used for assessing the boundary of the smear

zone in the case history of the Sunshine Motorway in Australia in (a) soft silty

clay located 2.5-5 m below ground level; and (b) silty clay located 5-11 m

below ground level.199

Figure 6.11 The variation of the extent of the smear zone with depth in the case

history of the Sunshine Motorway in Australia.200

Figure 6.12 Location of the Second Bangkok International Airport site in Thailand

(after Google Maps Thailand 2010c).............................................................201

Figure 6.13 The cross section of a test embankment and subsoil profile at the

Second Bangkok International Airport site in Thailand (after Indraratna and

Redana 2000)...............................................................................................202

Figure 6.14 Geotechnical properties of the Second Bangkok International Airport

site (after Sangmala 1997).204

Figure 6.15 Initial void ratio, compression index, and overconsolidation ratio of

the Second Bangkok International Airport site (after Sangmala 1997).204


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LIST OF FIGURES

xxix

Figure 6.16 The geometry, boundary conditions and initial configuration of the

axi-symmetric numerical model analysing a case study of the Second Bangkok

International Airport...207

Figure 6.17 The finite element prediction of total pore water pressure at selected

locations, at the site of the Second Bangkok International Airport during

installation of a 12 metre long PVD and withdrawal of the mandrel at a

(a) depth = 1 m; (b) depth = 5 m; and (c) depth = 10 m...209

Figure 6.18 Variations of the normalised radius of the smear zone and the

isotropic overconsolidation ratio in relation to depth, at the site of the Second

Bangkok International Airport...210

Figure A.1 Normal compression line and swelling line (after Schofield and Wroth

1968)......248

Figure A.2 Critical state line in the qp : plane (after Schofield and Wroth

1968)......249

Figure A.3 The critical state line in the :lnp plane (after Schofield and Wroth

1968)......250

Figure A.4 The position of the initial state of a soil sample in the :lnp

plane...251

Figure A.5 Plastic potential and plastic strains for Cam clay model (after Roscoe

et al. 1963)..252

Figure A.6 Cam clay model (a) yield locus in the qp : plane; and (b) state

boundary surface (after Roscoe et al. 1963)...254


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LIST OF FIGURES

xxx

Figure A.7 Yield locus in qp : plane for the modified Cam clay model (after

Roscoe and Burland 1968).256


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LIST OF TABLES

xxxi

LIST OF TABLES

Table 2.1 Reduction in the discharge capacity of a PVD due to different

deformation modes (after Bergado et al. 1996a)......37

Table 2.2 Well resistance indices proposed by various investigators39

Table 3.1 The magnitudes of plastic shear strain and the associated ratio of the

cavity radius at the failure threshold of the elements of cavity wall for three

different experiments 109

Table 4.1 Specifications of the fast response pore pressure transducers utilised in

the laboratory studies (after DGSI Materials Testing Catalog 2009)....124

Table 4.2 Radial distance between the tip of each installed transducer and the

centre of the cell.125

Table 4.3 Properties of the reconstituted soft clay deposit..127

Table 5.1 Properties of the soft saturated clay used in the finite element

simulation...163

Table 6.1 The Cam clay properties for the soft silty clay located 8-18 m below

ground level at the Muar clay embankment in Malaysia (after Indraratna and

Redana 2000).191

Table 6.2 The Cam clay properties for the silty clay located 2.5-11 m below

ground level in the development route proposed for the Sunshine Motorway in

Australia (after Sathananthan 2008)...198

Table 6.3 The profile and parameters of the soil at the site of the Second Bangkok

International Airport in Thailand (after Indraratna and Redana 2000)..205


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LIST OF SYMBOLS

xxxii

LIST OF SYMBOLS

a Final radius of the piezocone (m)

0a Semimajor axis of the initial elliptical cavity (m)

1a Semimajor axis of the instantaneous elliptical cavity (m)

va Coefficient of compressibility ( kNm /

2)

wA Cross-sectional area of a drain (2

m )

0b Semiminor axis of the initial elliptical cavity (m)

1b Semiminor axis of the instantaneous elliptical cavity (m)

smearb Half-width of the smear zone in the plane strain condition (m)

wb Half-width of the drain in the plane strain condition (m)

B Half-width of the influence zone in the plane strain condition (m)

Skemptons pore pressure parameter

hc Horizontal coefficient of consolidation ( sm /

2)

uc Undrained cohesion (kPa)

v

c Vertical coefficient of consolidation ( sm /2

)

cC Compression index of soil

kC Slope of the line in ek:log plane, Index of permeability change

sC Swell index of soil

15D Diameter of soil particles corresponding to 15% passing (m)


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LIST OF SYMBOLS

xxxiii



0e Initial void ratio of soil

cse Void ratio of soil at critical state

Le Void ratio of soil at liquid limit

E Youngs modulus (kPa)

ng Minimum distance in the normal direction between an arbitrary point onthe slave surface and its projection on the master surface (m)

tg Relative displacement in the tangential direction between the two

partnered points (m)

G Shear modulus (kPa)

drH Thickness of the drainage path (m)

0i Threshold gradient below which no flow occurs

li Gradient required overcoming the maximum binding energy of mobile

pore water

rI Rigidity index of soil

0J Bessel function of the first kind of zero order

1

J Bessel function of the first kind of first order

k Coefficient of permeability (m/s)

0k Initial coefficient of permeability (m/s)

filterk Coefficient of permeability of the filter (m/s)

hk Horizontal coefficient of permeability (m/s)

hpk Horizontal coefficient of permeability in the plane strain condition (m/s)


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LIST OF SYMBOLS

xxxiv

Horizontal coefficient of permeability of undisturbed soil (m/s)

sk Horizontal coefficient of permeability in smear zone (m/s)

spk Horizontal coefficient of permeability in smear zone in the plane strain

condition (m/s)

soilk Coefficient of permeability of the soil (m/s)

vk Vertical coefficient of permeability (m/s)

eqvk . Equivalent vertical coefficient of permeability (m/s)

wk Coefficient of permeability of drain (m/s)

0K Coefficient of lateral earth pressure at rest

l Length of vertical drain (m)

vm Coefficient of volume compressibility ( kNm /

2)

M Slope of critical state line in the qp : plane

pn Isotropic overconsolidation ratio

N Specific volume of soil at 1=p kPa

OCR Overconsolidation ratio

50O Size larger that 50% of the fabric pores in a filter (m)

95O Apparent opening size of a filter (m)

p Total mean stress (kPa)

0p Initial total mean stress (kPa)

p Effective mean stress (kPa)

0p Initial effective mean stress (kPa)

cp Preconsolidation stress (kPa)

dundisturbehk


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LIST OF SYMBOLS

xxxv

yp Yielding stress under isotropic conditions (kPa)

0y

p Maximum isotropic preconsolidation stress (kPa)

iP Internal pressure of the cavity (kPa)

maxiP Maximum internal pressure of the cavity (kPa)

miniP Minimum internal pressure of the cavity to yield the soil elements adjacent

to the wall of cavity (kPa)

q Deviator stress (kPa)

wq Specific discharge capacity of a drain ( sm /

3)

wpq Discharge capacity of a drain in the plane strain condition ( sm /

3)

Theoretical discharge capacity of a drain ( sm /3

)

yq Deviator stress of the soil elements just after the initial yielding of the wall

of cavity (kPa)

r Instantaneous position of a soil element measured from the centre of cavity

(m)

Radial position measured from the centre of the cell (m)

0r Initial radius of an elliptical cavity (m)

Initial horizontal position of a soil node measured from the centre of

mandrel (m)

1r Instantaneous radius of an elliptical cavity (m)

ir Radius of the equivalent circular influence zone (m)

mr Equivalent radius of the mandrel (m)

( )mr Distance from the centre of cavity to the wall of cavity (m)

Equivalent radius of the mandrel (m)

)(requiredwq


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LIST OF SYMBOLS

xxxvi

pr Radial extent of the plastic zone (m)

Radius of plastic zone measured from the centre of the cavity (m)

smearr Radius of the smear zone (m)

wr Radius of the equivalent vertical drain (m)

0R Initial position of a soil element measured from the center of the cavity

(m)

us Undrained shear strength of the soil (kPa)

S Drains spacing (m)

Component of body force in a radial direction ( )3m

t Time (s)

Time increment (s)

ct Time required to establish contact between the mandrel shoe and the soil

elements at the initial wall of the cavity (s)

T Component of body force in a tangential direction ( )3m

hT Time factor related to the horizontal consolidation

hpT Time factor related to the horizontal consolidation in the plane strain

condition

vT Time factor related to the vertical consolidation

vT Modified time factor related to the vertical consolidation

u Pore water pressure (kPa)

U Component of displacement in a radial direction (m)

10U 10% of the degree of consolidation

U Overall average degree of consolidation


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LIST OF SYMBOLS

xxxvii

rU Average degree of consolidation due to radial flow

zr

U,

Average degree of consolidation at a depthzdue to radial flow

zrpU , Average degree of consolidation at a depthzdue to radial flow in the plane

strain condition

vU Average degree of consolidation due to vertical flow

V Component of displacement in a tangential direction (m)

rV Displacement rate of the soil elements in a radial direction (m/s)

tV Relative tangential velocity between the two partnered points (m/s)

vV Installation rate of the mandrel (m/s)

0Y Bessel function of the second kind of zero order

1Y Bessel function of the second kind of first order

pz Vertical distance between the tip of cone and the boundary of plastic zone(m)

Greek letters

Apex angle of the mandrel shoe (degree)

Friction angle between the contacting bodies

ij Kronecker delta

p Increment of the plastic strain

r Variations in total radial stress (kPa)

Variations in total tangential stress (kPa)

r Variations in total shear stress (kPa)


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LIST OF SYMBOLS

xxxviii

u Excess pore water pressure (kPa)

0u Initial excess pore water pressure (kPa)

ru Excess pore water pressure due to radial flow (kPa)

zu Excess pore water pressure due to vertical flow (kPa)

zru , Excess pore water pressure at any point (kPa)

ru Excess pore water pressure along the radial direction between wr and ir

(kPa)

i Strain

n Penalty parameter in the normal direction ( )3/mkN

rr Radial strain

Circumferential strain

zz Axial strain

t Penalty parameter in the tangential direction ( )3/mkN

p

V Plastic volumetric strain

Angle of internal friction of the soil (degrees)

ps Critical state angle of friction in a plane strain condition (degrees)

tc Critical state angle of friction in a triaxial compression condition (degrees)

te Critical state angle of friction in a triaxial extension condition (degrees)

Specific volume of the soil in the critical state at 1=p kPa

Shear strain

ij Shear strain


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LIST OF SYMBOLS

xxxix

rz Shear strain

e

q Elastic shear strain

p

q Plastic shear strain

w Unit weight of water (3

/mkN )

Stress ratio

Slope of elastic swelling line in the :lnp plane

Slope of normal compression line in the :lnp plane

Coefficient of friction between the master and slave surfaces

Plastic volumetric strain ratio

Pore water flow (m/s)

Poisons ratio

Polar angle (degrees)

Settlement (m)

final Final Settlement (m)

1 Total major stress (kPa)

2 Total intermediate stress (kPa)

3 Total minor stress (kPa)

ij Total stress (kPa)

n Normal stress at the contacting surfaces (kPa)

pr Total radial stress at the elastic-plastic boundary (kPa)

0 Initial internal pressure of the cavity, also the uniform pressure acting on

the soil boundaries at infinity (kPa)


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LIST OF SYMBOLS

xl

1 Effective major stress (kPa)

2 Effective intermediate stress (kPa)

3 Effective minor stress (kPa)

ij Effective stress (kPa)

0v Initial overburden stress (kPa)

fv Final overburden stress (kPa)

Shear stress across the interface (kPa)

ij Shear stress (kPa)

max Maximum shear stress that can be transferred at the interface between the

contacting surfaces (kPa)

failure Shear stress at failure (kPa)

Soil specific volume

Electrical resistance (ohms)


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CHAPTER 1 INTRODUCTION

1

1 INTRODUCTION

1.1 General

The population boom and associated development in metropolitan areas have

necessitated the use of soft clay land for construction purposes. These deposits are

normally characterised by low shear strength, high compressibility, and a low

coefficient of permeability, characteristics which make them a difficult

engineering exercise. This thesis presents the research conducted by the Author at

the University of Wollongong as a part of a continuous study program to

investigate the different aspects of ground improvement using vertical drains. As

shown in Figure 1.1, unacceptable differential settlements and severe damage may

occur due to the heterogeneity and high compressibility of the underlying soil if a

structure has been constructed before the ground was stabilised.

Unstable Soft Soil Layer Unstable Soft Soil Layer

Settlement

Damaged Structure

New Structure

Bearing Stratum Bearing Stratum

Unstable Soft Soil Layer Unstable Soft Soil Layer

Settlement

Damaged Structure

New Structure

Bearing Stratum Bearing Stratum

(a) (b)

Figure 1.1 Structure built on an unstable soft soil deposit (a) just after the

construction; and (b) after differential settlement


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2

Vibro-replacement, electro-osmotic, explosion based, and deep mixing, are some

stabilisation techniques used to mitigate unacceptable differential settlements of

underlying soft soil (Indraratna and Chu 2005) but the associated cost may

become excessive when the soft layer is very thick (15 m - 20 m). Pre-loading is

one of the classical techniques used to mitigate the effects of differential

settlement on structures and increase the shear strength of the soft soil deposits. In

this method, a surcharge load, usually in the form of an embankment that is equal

to or greater than the expected foundation loading, is applied to the layer of soft

soil until most of the primary consolidation has been achieved. Because the

magnitude of the surcharge load is limited by the failure criteria of the soil, the

loads are increased in stages as the shear strength of the soil amplifies. With thick

deposits of soft clay where the permeability is low, the time required to

consolidate the soil only with a surcharge is very long. Indraratna et al. (2005a)

stated that vertical drains combined with pre-loading are amongst the most

effective techniques known for accelerating consolidation and stabilising ground.

As illustrated in Figure 1.2, vertical drains reduce the drainage path and accelerate

the dissipation of excess pore water pressure generated from the application of

surcharge loads.

Drainage without PVDs Drainage with PVDs

Surcharge Surcharge

Vertical

DrainsSoft Soil

ImperviousLayer

Sand

blanket

Figure 1.2 Effect of vertical drains on providing short drainage paths


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According to Indraratna et al. (2005b) and Indraratna (2008), vertical drains in

particular:

(i) increase the shear strength of soft soils by decreasing the moisturecontent, which in turn reduces the void ratio

(ii) decrease the time required for the application of surcharge loads(iii) reduce differential settlement in the course of primary consolidation(iv) shorten the height of surcharge fill necessary to achieve the desired

compression when combined with vacuum

Figure 1.3 reveals the potential benefit of vertical drains to reduce the time

required for a specific degree of consolidation.

Pre-loading without vertical drains

Pre-loading with vertical drains @ 1.5m spacing

Pre-loading with vertical drains @ 1m spacing


Pre-loading without vertical drains

Pre-loading with vertical drains @ 1.5m spacing



Figure 1.3 Potential benefit of vertical drains combined with pre-loading (after

Lau and Cowland 2000)

The installation of vertical drains creates a disturbed region known as the smear

zone where the structure of the clay layer is altered such that the horizontal

permeability is reduced and compressibility is increased (Indraratna and Redana

1998). The parameters required to characterise the smear effect are the extent of


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4

the smear zone and the ratio between the horizontal coefficient of permeability in

the undisturbed zone and that in the smear zone (Chai and Miura 1999).

Previously at the University of Wollongong, Redana (1999) analysed the effect of

smear in soft soils by converting the axi-symmetric (radial) permeability into an

equivalent plane strain model. The experimental studies of smear zone

propagation around vertical drains were conducted using a large-scale radial

drainage consolidometer. Then the simulation of smear effects in a 2-D plane

strain finite element model was performed using the modified Cam clay theory.

Subsequently, a multi-drain, plane strain analysis was carried out to study the

performance of the entire embankment stabilised by vertical drains, for a number

of case histories. Sathananthan (2005) developed a modified consolidation theory

incorporating vacuum pressure distributed linearly (trapezoidal) for axi-symmetric

and plane strain conditions. In addition, a new plane strain consolidation theory

was presented for non-Darcian flow. Based on the tests in a large-scale

equipment, the settlement of the soil stabilised by vertical drains was compared

with the proposed plane strain model. Several 2-D plane strain numerical analyses

were also performed to predict the failure height of the embankments under a

number of conditions such as, preloading, different geometry of embankments,

and various spacing of drains. Rujikiatkamjorn (2005) investigated the effect of

various factors such as, change of soil permeability and compressibility, variation

of vacuum pressure, well resistance and smear on consolidation of the soil around

vertical drains under vacuum preloading. Based on the results of the multi-drain,

plane strain analyses it was shown that the efficiency of the prefabricated vertical


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5

drains depends on the magnitude and distribution of vacuum pressure, and the

extent to which air is prevented from leaking. Subsequently, Rujikiatkamjorn

(2005) studied the length of vertical drains, anisotropic permeability of the soil

and vacuum pressure, and compared the reduction in consolidation time through

vacuum preloading with other available methods. Walker (2006) examined the

spatially varied properties of the soil in the smear zone, and developed linear and

parabolic variations in permeability to discuss the possibility of overlapping smear

zones. A nonlinear radial consolidation model was presented incorporating void

ratio dependant soil properties and non-Darcian flow. Thereafter, an analytical

solution was developed for multi-layered consolidation problems with vertical and

horizontal drainage using the spectral method. Models were verified against

analytical solutions available, laboratory experiments, and case histories.

As discussed by Bo et al. (2003), rectangular or rhomboidal mandrels are used

typically in the field. However, existing models consider a circular disturbed

region surrounding the mandrels. In this thesis the installation of mandrel-driven

prefabricated vertical drains (PVDs) and associated disturbance in cohesive soils

were investigated by developing a new elliptical cavity expansion theory (CET)

that predicts an elliptical smear zone around the PVDs. Large-scale laboratory

experiments and numerical modelling were conducted at realistic rates to study

the pattern of disturbed regions and factors affecting them. Furthermore, a number

of case histories taken from Malaysia and Thailand were analysed to examine the

associated soil disturbance during installation of prefabricated vertical drains.


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6

1.2 Application of Vertical Drains

Vertical drains come in two categories, displacement and non-displacement. Non-

displacement vertical drains involve the removal of soft soil and backfilling with a

more permeable material. Cylindrical cavities may be created by driving, jetting

or auguring with typical diameters of 200-450 mm (Hausmann, 1990), and are

usually filled with sand. Sand drains are susceptible to damage from lateral

movement of the ground. Improvements in installation and competitive

production costs over the past two decades have increased the popularity of

prefabricated vertical drains (displacement type vertical drains) over conventional

sand drains (Bo et al. 2003). Although there are several types of prefabricated

vertical drains (PVDs) on the market they basically consist of a plastic core

surrounded by a filter sleeve with a typical cross section of 100 mm x 4 mm

(Holtz 1987). Figure 1.4 demonstrates typical PVDs and cross sections of the

different types available on the market. Prefabricated vertical drains are normally

spooled out and threaded through a hollow mandrel that is generally rectangular

or rhomboidal (Bo et al. 2003). The free end of a PVD is attached to a shoe which

anchors it in stiffer clay and prevents soil entering the mandrel. The shoe may

vary from a simple reinforced steel bar of 10-20 mm in diameter to a thin mild

steel plate with a strip welded on as a handle (Karunaratne et al. 2003). Figure 1.5

illustrates the typical geometries of both mandrel and shoe. PVDs are usually

installed in a square or triangular pattern. Drains in a square pattern may be easier

to install in the field but a triangular layout reduces the zones of influence and

results in a more uniform consolidation between them (Indraratna et al. 2003).

Vertical drains may also be installed dynamically with a vibrating or drop


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7

hammer, or statically. According to Indraratna et al. (2003), a dynamic installation

disturbs the surrounding soil more than the static method. Common installation

rigs at a site are shown in Figure 1.6.

Filter sleeve

Plastic core

Filter sleeve

Plastic core

Plastic coreFilter sleeve

Plastic coreFilter sleeve

Figure 1.4 Prefabricated vertical drains (a) typical front view (after Global

Synthetics 2010); and (b) a cross section of different types (unit: mm, after

Chai et al. 2004)

(b)

(a)


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8

Mandrel

Shoe

Shoe Plate

Shoe

Mandrel

Mandrel

Mandrel

Shoe

Shoe Plate

Shoe

Mandrel

Mandrel

Figure 1.5 Typical geometries of mandrel and shoe (after Saye 2001)

Installation rigs

Rows of PVDs

Sand blanket

Installation rigs

Rows of PVDs

Sand blanket

Figure 1.6 Common installation rigs on site (after Menard 2010)


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9

A typical scheme of prefabricated vertical drains system is presented in Figure

1.7. It is necessary to remove vegetation and surface debris, and then grade the

ground before installing PVDs. A sand blanket is commonly laid onto the soil to

expel water from the vertical drains and act as a working platform for installing

rigs. To further facilitate drainage, horizontal drains may be implemented on the

surface. Monitoring and evaluating the performance of embankments by field

instrumentation is vital in order to control any geotechnical issues that occur

during the course of construction, to record the rate of settlement, and to verify the

design parameters. According to Bo et al. (2003), field instrumentation may be

divided into two groups by considering the construction phases. The first group

are used to prevent sudden failure during construction stages; inclinometers,

settlement plates, and piezometers fit into this category. The second group are

used to monitor/record changes in settlement and excess pore water pressures over

time, during the loading stages. Multi-level settlement gauges and piezometers

belong to this category.

Installation rig

PVDsPeripheral trench

Installation rig

PVDsPeripheral trench

Figure 1.7 General scheme of a system of prefabricated vertical drains combined

with pre-loading (after Geo-Technics America Inc 2010)


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10

In addition to the smear effect, the well resistance, and drains not being saturated

are also the factors that adversely influence the efficiency of PVDs and retard the

consolidation process. The resistance to water flowing along the PVD is known as

the well resistance. Although the deep installation of drains and their limited

discharge capacity contribute to the well resistance factor, this issue can be

ignored in the case of modern PVDs where the discharge capacity is usually high

enough and the drains will not kink during installation. The gap between dry PVD

and the mandrel during installation contributes to the non-saturated-drain

phenomenon. This issue adversely affects the compression of soft soil but need

only be considered in the early stages of consolidation because it diminishes as the

soil consolidates and the PVD becomes saturated.

1.3 Application of Cavity Expansion Theory

Cavity expansion theory studies the changes in stresses, displacements and pore

pressures attributable to the expansion and contraction of cavities. In

geomechanis, cavity expansion in soil and rock is a fundamental problem. The

theory has been extensively applied in the areas of in-situ soil testing, deep

foundations, tunnels and underground excavations, and wellbore instability.

In-situ soil testing is broadly used in geotechnical engineering. The pressuremeter

and cone penetrometer are among the most commonly used in-situ soil testing

devices. The mechanical action created by these devices is similar to the

expansion of a cavity, and hence the cavity expansion theory has been used in the

interpretation of the measured data to obtain the soil properties (e.g. Wroth 1984,


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11

Clarke 1995, Lunne et al. 1997, Yu and Mitchell 1998). It is generally assumed

that the pressuremeter tests can be simulated as the expansion or contraction of an

infinitely long cylindrical cavity in soils. This assumption then makes it possible

to develop analytical correlation between the cavity expansion curves and the soil

properties, such as the shear modulus, in-situ total horizontal stress, and for clays

undrained shear strength and the coefficient of horizontal consolidation. The

similarity between the cone penetration and cavity expansion was first discussed

by Bishop et al. (1945). Predicting the cone resistance using the cavity expansion

theory can be achieved by developing the theoretical solutions of the limit

pressure for the cavity expansion in soil, and then correlating these limit pressures

to cone resistance. According to Yu and Mitchell (1998), because the cavity

expansion theory considers the effect of soil stiffness, compressibility and

dilatancy, and horizontal stress due to penetration it provides a more accurate

prediction of cone resistance than that obtained using the bearing capacity theory.

While small-strain cavity expansion solutions are only required for the

interpretation of the self-boring pressuremeters, large-strain solutions have been

developed to derive soil properties from the results obtained by cone

pressuremeters (e.g. Yu et al. 1996, Houlsby and Withers 1988).

The shaft friction and end bearing capacity of driven piles in soils can be studied

by cavity expansion theory. This analysis is a large-strain problem which involves

high nonlinear nature of the material and geometry. During the deep installation of

a pile in soil, much of the soil is displaced predominantly outwards in the radial

direction. Measurements of the radial displacements of soil near the pile


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12

mid-depth taken from field data presented by Cooke and Price (1978) the model

tests of Randolph et al. (1979) show that the radial movement of the soil can be

predicted by cylindrical cavity expansion theory. Nystrom (1984) highlighted that

the simple one dimensional cavity expansion model can predict the behaviour of

piles similar to more complicated two dimensional finite element methods. The

installation of a pile can reasonably be modelled as an undrained loading case

when the pile is driven rapidly into the ground. Cavity expansion theory can be

used to study (Yu 2000):

(i) The installation of a pile as the expansion of a cylindrical cavity fromzero radius to the radius of the pile. The shaft friction may then be

estimated by the changes of stress in soil surrounding the pile obtained

from this analysis.

(ii) End bearing capacity of a pile from the limit pressure of a sphericalcavity in a semi empirical way.

Bishop et al. (1945) and Hill (1950) discussed that the pressure required to create

a deep hole in an elastic-plastic material is relative to that essential to expand a

cavity of the same volume and under the same conditions with no friction. Gibson

(1950) was the first to express the end bearing pressure of a deep foundation as a

function of the limit pressure of a cavity. It is worth noting that while the end

bearing capacity of piles in clays in the long term drained condition is much larger

than its value in the short term undrained condition, the short term capacity of the

pile is necessary to be large enough to prevent a failure immediately after

installation, and the settlements essential to mobilise the long term capacity of the

pile may be beyond the tolerance defined by the serviceability criteria. Therefore,


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13

it is a standard practice to consider an undrained condition for clays when

evaluating the end bearing capacity of the piles (Fleming et al. 1985).

Tunnelling and underground excavations involve reducing the in-situ stresses

along the excavated circumference through removal of geomaterials from their

primary locations, and therefore can be simulated by unloading cavities from a

state of in-situ stress. Evaluation of tunnelling-induced settlements has

conventionally been based on an empirical relationship suggested by Peck (1969)

with the assumption that the profile of transverse surface settlement follows a

normal probability curve. The theoretical approach to this problem includes

simple cavity unloading solutions (e.g. Pender 1980, Lo et al. 1984, Ogawa and

Lo 1987, Mair and Taylor 1993) and nonlinear elastic-plastic finite element

methods (e.g. Ghabousssi et al. 1978, Rowe and Kack 1983, Clough et al. 1985,

Lee and Rowe 1990, Rowe and Lee 1992), both of which involve simulating

construction of the tunnel by inferring the tractions that are acting around the

surface of the tunnel before excavation and then removing these tractions.

Lo et al. (1980) and Ogawa and Lo (1987) showed that the plane strain cylindrical

cavity unloading solution after applying a correction factor, can be used to

estimate the soil displacements measured around tunnels. Mair and Taylor (1993)

pointed out that the spherical cavity unloading solutions can be applied to study

the soil behaviour around an advancing tunnel heading. According to Yu (2000),

while the cavity unloading solution in an infinite medium can be used to assess

the displacement of the tunnel wall, it tends to underestimate the surface

movement significantly for shallow tunnels. This inconsistency is mainly because


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the cavity unloading in an infinite soil mass does not consider the effect of the free

ground surface. Sagaseta (1987) and Verruijt and Booker (1996) investigated the

unloading of a cavity in a half space considering the effect of the free ground

surface and derived analytical solutions for displacements. Stability of the tunnels,

on the other hand, is to ensure that the geomaterials surrounding the tunnels do

not collapse as a result of insufficient internal pressure of support. Cavity

unloading solutions dealing with stability mainly follow Caquot and Kerisel

(1966) by assuming that collapse of a tunnel will occur when the plastic zone

reaches the ground surface. Mair (1979) showed that the stability of tunnels

predicted by the cavity expansion solution agree with the results of centrifuge

tests. Sloan and Assadi (1993) found that the cavity expansion solutions are

usually very similar to the rigorous upper and lower bound stability solutions.

Wellbore instability during drilling in petroleum engineering is a key problem that

can be analysed by the cavity expansion theory. According to Bradley (1979) and

Santarelli et al. (1986), wellbore instabilities as a result of stress can be divided

into:

(i) Reduction of hole size as a consequence of ductile yield of the rock,(ii) Enlargement of hole size due to fracture or rupture of the brittle rock,

and

(iii) Unintentional hydraulic fracturing caused by excessive pressure ofmud

Kulhawy (1974) discussed that the pre-yield and pre-peak stress-strain properties

of some rocks are nonlinear and the elastic properties are a function of the


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pressure. Santarelli et al. (1986) carried out a numerical study on the stresses

acting on a borehole using a nonlinear elastic model with pressure dependent

Youngs modulus. Lekhnitskii (1963) and Wu et al. (1991) presented the elastic

solutions for the expansion of a thick walled cylinder with a cross-anisotropic

model. Wu and Hudson (1991) investigated the effect of stress-induced anisotropy

on wellbore stability, and showed that it has a very significant effect on the

distribution of elastic stress around a borehole. While Carter and Booker (1982)

assumed incompressibility for the pore fluid and particles in developing a semi

analytical poroelastic solution for the time dependent displacements and stress

around a long circular opening in a saturated elastic medium, Detournay and

Cheng (1988) included the compressibility of the pore fluid and particles in their

analysis. Charlez (1997) developed plastic analysis of the wellbore instability

problem by using the critical state models. Yu and Rowe (1999) presented an

analytical solution for cavity contraction in critical state materials to study the

borehole stability.

1.4 Objectives and Scope of the StudyThe main objective of this research is to better understand the disturbance and

pore water pressure in soft soils due to the installation of mandrel-driven PVDs.

The disturbed regions surrounding the mandrels and the stresses distributed in

soils predicted from the analytical and numerical models were compared with the

results of the laboratory tests. A number of case histories were also studied to

evaluate the significance the models developed in practical situations. In

particular, this research is aimed at:


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1. Developing a new elliptical CET and characterising the soil around a PVD

Mandrel penetration into soft saturated clays and the resulting disturbance were

studied analytically with a new elliptical cavity expansion theory (CET). The

elliptical CET developed uses modified Cam clay parameters to address the

analysis of PVDs being installed in soft clay deposits in undrained conditions. The

pattern of distribution for excess pore pressure, calculated analytically, was

verified using the data available in the literature.

2. Installing PVDs at a realistic rate and assessing the factors affecting the smear

A number of large-scale laboratory tests incorporating field conditions and the

effects of confining pressures were performed. In particular, the installation of

PVDs under different surcharge loads was accomplished using a mandrel-driving

machine at a penetration rate within the range of usual practices. The variations of

pore water pressure during installation of a PVD and withdrawal of the mandrel

were monitored. The extent of the smear zone in the large-scale consolidometer

was determined using the results of moisture content tests.

3. Simulating the installation of PVDs using the finite element method

A numerical simulation of the mandrel installation process was achieved using the

ABAQUS finite element software package. A series of coupled analyses, which

took into account the large-strain formulation, were performed. The deformation

and movement of soil were predicted as the mandrel was pushed into the soft soil.

The variations of pore water pressure are illustrated at different locations during

installation of a PVD and withdrawal of the mandrel.


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4. Applying the models developed to case histories

The analytical formulation and finite element model established were applied to

case histories from the Muar clay region in Malaysia, the Sunshine Motorway in

Australia and the Second Bangkok International Airport in Thailand. The results

demonstrate that the models developed are applicable to field conditions.

1.5 Organisation of the DissertationThe significance of stabilising layers of soft soil, the privilege of a system of

vertical drains, and the goals of the present research are covered in this

introductory chapter. Chapter 2 presents a detailed literature review associated

with vertical drains where the characteristics of prefabricated vertical drains and

the factors influencing their efficiency are discussed in depth. It also describes

consolidation theories coupled with vertical drains, and the plane strain modelling

that is essential for analysis in field conditions. It focuses on the analysis of soil

disturbance associated with the installation of mandrel-driven PVDs.

Chapter 3 identifies the analytical theory developed in this research to investigate

mandrel penetration and its resulting disturbance in soft saturated clays. It also

introduces a new elliptical cavity expansion theory (CET) that incorporates the

modified Cam clay parameters to study the installation of PVDs into soft clay

deposits. This formulation accounts for the rate of mandrel penetration and the

time factor for predicting the internal pressure in the cavity, and the corresponding

stresses and excess pore pressure in the soil while driving the mandrel. A more

realistic elliptical smear zone based on the elliptical CET is also expressed.


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Chapter 4 presents the large-scale laboratory tests conducted in this research. The

installation of prefabricated vertical drains, incorporating the field conditions, was

performed using a mandrel-driving machine capable of working at rates within the

range of usual practices. By using pore pressure transducers capable of a fast

response, the variations in pore water pressure during installation of a PVD and

withdrawal of the mandrel were examined. Finally, a criterion based on the extent

of smear zone measured in the large-scale consolidometer was developed to

interpret the results of this study.

Chapter 5 discusses the finite element modelling of mandrel installation using the

ABAQUS software package. By incorporating the large-strain formulation and the

penalty method, a number of coupled analyses were completed. The changes in

pore water pressure at different locations during various stages of the simulation

were compared with the quantities measured in the laboratory. The extent of

smear zone was studied according to the numerical simulation of the mandrel

installation.

Chapter 6 provides case histories from the Muar clay region in Malaysia, the

Sunshine Motorway in Australia and the Second Bangkok International Airport in

Thailand. The layers of soft soil were examined to characterise the smear zone

associated with the installation of prefabricated vertical drains. The results of

these analyses which incorporated the models developed in this research were

then compared with the data published previously.


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Chapter 7 draws conclusions from the present research and offers

recommendations for future investigations. A list of references followed by

Appendices appears at the end.


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CHAPTER 2 LITERATURE REVIEW

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2 LITERATURE REVIEW

2.1 Installation and Monitoring of Prefabricated Vertical Drains

On major projects it is imperative to have a prediction of the required length of

prefabricated vertical drains as

Documents

Analytical and Numerical Study of Soil Disturbance Associated Wit