NUMERICAL ANALYSIS OF SUBGRADE STABILIZATION IN SOFT SOIL

ENGKU SHAHRULERIZAL BIN ENGKU AB RAHMAN

A thesis submitted in fulfillment of the

requirements for the award of the degree of

Master of Engineering (Civil)

Faculty of Civil Engineering

Universiti Teknologi Malaysia

DECEMBER 2014

iii

DEDICATION

To my beloved mother, Raja Kamariah Binti Raja Kadir and father, Engku Ab Rahman

Bin Ku Junoh, my wife, Hazlina Binti Mahfidz and my daughter Tengku Puteri Nur

Aliyah, Engku Puteri Nur Ayesha and Engku Puteri Nur Afiqah.

iv

ACKNOWLEDGEMENT

In the name of Allah, the Most Gracious and the Most Merciful Alhamdulillah, all

praises to Allah for the strengths and His blessing in completing this master dissertation.

In preparing this thesis, I was in contact with many people, researchers, academicians,

and practitioners. They have contributed towards my understanding and thoughts. In

particular, I wish to express my sincere appreciation to my main thesis supervisor,

Assoc. Prof. Dr. Nazri Bin Ali, for encouragement, guidance, critics, motivation, and

friendship. Without his continued support and interest, this thesis would not have been

the same as presented here.

I would like to express my appreciation to the Public Work Department Sarawak,

especially to Central Material Laboratory Branch, for his support and help towards my

research and field data measurement. His kind co-operate and encouragement during

completion of this research are very much appreciated.

I am also very thankful to my wife, Hazlina Binti Mahfidz for her guidance,

advices and motivation. Thanks to all my Master Engineering (Civil) classmates;

Norasman, Mimi Sulastri, Azura, Yuhani, Munirah, Canarisa, Lim Chi Chien, , Tan Khern

Wee, Tang Hing Kwong, Chai Teck Jung, and Loo for their contribution, helps and support

for this research. The memories of team spirit and friendship throughout this journey will

never been forgotten. My sincere appreciation also extends to all my colleagues and

others who have provided assistance at various occasions. Their views and tips are

useful indeed. Unfortunately, it is not possible to list all of them in this limited space. I

am grateful to all my family members.

v

ABSTRACT

Soft soil also well known in geotechnical properties where low bearing capacity,

high water content, high compressibility, structural instability, lateral pressures,

differential settlement and long term settlement. In situ subgrade often not provided the

support required to achieve acceptable performance under traffic loading and

environmental demand. Previous similar projects are importance to determine the suitable

soil improvement method in relation to project. In this study, software application

approach by PLAXIS 2D in order to evaluate the behaviour of the subgrade stabilization

using an actual field data form Subsurface Investigation (S.I) work report and PLAXIS

2D Manual. The effectiveness of PLAXIS finite element to predict the settlement of soft

soil base on different type stabilization are successfully performed related to this matter.

vi

ABSTRAK

Tanah lembut dikenali dalam bidang kejuruteraan mekanik tanah dimana ia

mempunyai keupayaan galas yang rendah , kandungan air yang tinggi , kebolehmampatan

yang tinggi , ketidakstabilan struktur , tekanan sisi, dan pengukuhan yang pelbagai untuk

jangka masa yang panjang . Lapisan subgred seringkali dikatakan tidak dapat untuk

memberikan sokongan yang diperlukan agar mencapai prestasi yang boleh diterima

semasa pembebanan lalulintas dan permintaan alam sekitar. Projek-projek terdahulu yang

perlah dilaksanakan dan berada di dalam bidang yang ini amat penting bagi menentukan

kaedah membaik pulih tanah yang sesuai sehubungan dengan sesuatu projek . Dalam

kajian ini, dengan menggunakan perisian aplikasi PLAXIS 2D, kestabilan dan tingkah

laku tanah yang berada di lapisan subgred dapat dianalisis berdasarkan data sebenar di

lapangan yang diperolehi melalui kajian awal di tapak bina (SI). Keberkesanan perisian

PLAXIS 2D dalam meramalkan kadar pengukuhan asas bagi tanah lembut melalui

pelbagai kaedah kestabilan tanah telah berjaya menyelesaikan permasalahan ini.

xi

LIST OF TABLE

TABLE NO. TITLE PAGE

1.1 Areas under peat in the various administrative divisions

in Sarawak sq. km (Singh et al, 1997)

3

3.1 Soil Properties. (S.I Work Report and Plaxis 2D

Manual)

33

3.2 Material Properties of the Geogrid. (Plaxis 2D Manual) 34

4.1 Analysis with Geogrid. 48

4.2 Analysis with Vertical Drain. 48

4.3 Analysis with combination Geogrid and Vertical Drain. 48

4.4 Analysis changed of PEAT layer to CLAY. 49

4.5 Analysis changed of PEAT layer to CLAY with

Geogrid.

49

4.6 Analysis changed of PEAT layer to CLAY with Vertical

Drain.

49

4.7 Analysis changed of PEAT layer to CLAY with

combination Geogrid and Vertical Drain.

50

xii

LIST OF FIGURES

FIGURE NO. TITLE PAGE

1.1 Distribution of peat in Sarawak (Land and Survey

Department Sarawak, 2011).

2

1.2 Site Location Jalan Matang Baru, Phase 1,Kuching 4

1.3 Site Location Jalan Matang Baru, Phase 1,Kuching 4

1.4 Site Location Jalan Matang Baru, Phase 1,Kuching 5

1.5 Site Location Jalan Matang Baru, Phase 1,Kuching 5

1.6 Arial view site location (Google Maps, 2014) 6

2.1 Two Dimensional Stress State. 9

2.2 Mechanical model to explain the process of consolidation. 14

2.3 Main window of the input program. 17

2.4 (a) 15-nodes and 12- stress points soil element, (b) 6-nodes

and 3-stress points soil element .(Brinkgreve et al., 2011).

19

2.5 Figure 2.5: Mohr Coulomb yield functions forming a

hexagonal cone. (Brinkgreve 2006).

24

2.6 Figure 2.6: The two dimension state of Mohr Coulomb

failure criterion. (Murty 2003).

24

2.7 2V:1H Method to Estimate Vertical Stress Increase as a

Function of Depth Below Ground (After Holtz and Kovacs,

1981)

28

xiii

3.1 Flow chart for the activities of the project. 31

3.2 Geometry of the main model. 32

3.3 Material sets of Model Cross-section. 34

3.4 Model Cross-section with generated mesh. 35

3.5 Model Cross-section with Deformed Mesh. 37

3.6 Model Cross-section with Total Displacement. 37

3.7 Model Cross-section with Total Stresses. 38

3.8 Model Cross-section with Effective Stresses. 38

3.9 Model Cross-section with Excess Pore Pressure. 39

3.10 Model Cross-section with Total Strains. 39

3.11 Initially the phreatic level and Installation of vertical drains. 41

3.12 Filling 2.0m thick, half of embankment. 41

3.13 Filling 2.0m thick, final embankment. 42

3.14 Model Cross-section with Deformed Mesh. 42

3.15 Model Cross-section with Excess Pore Pressure. 43

3.16 Model Cross-section with Excess Pore Pressure (shading). 43

4.1 Deformed Mesh. 45

4.2 Total Displacements. 45

4.3 Total Strains. 46

xiiii

4.4 Excess Pore Pressures. 46

4.5 Effective Stresses. 47

4.6 Analysis Model vs Total Displacement. 51

4.7 Percentage (%) of Total Displacement Reduced by Types of

Analysis.

51

4.8 Analysis Model vs Total Displacement. 52

4.9 Percentage (%) of Total Displacement Reduced by Types of

Analysis.

53

4.10 Embankment Model. 54

4.11

Embankment Model with Geogrid at 1

st embankment. 54

4.12

Embankment Model with Geogrid at 1

st

embankment and at

2nd

embankment.

55

4.13 Embankment Model with Vertical Drain. 55

4.14 Embankment Model with Vertical Drain and Geogrid at 1

st

embankment.

56

4.15 Embankment Model with Vertical Drain and Geogrid at 1st

embankment and 2nd embankment.

56

4.16 Displacement vs Type of Analysis at Selected Point of

Embankment at 100 Days of Settlement.

57

4.17 Displacement vs Type of Analysis at Selected Point of

Embankment at 300 Days of Settlement.

58

xivi

4.18 Displacement vs Type of Analysis at Selected Point of

Embankment at 500 Days of Settlement.

59

xvi

LIST OF SYMBOLS

ɛx, ɛy, ɛz - Normal strains

σx, σy, σz - Stress components

τxy, τxz, τyz - Shear-stress components

γxy, γxz, γyz - Shear strains

σ’ - Effective stress

σ - Total stress

u - Pore water pressure

φ - Friction angle

c - Cohesion

ψ - Dilatancy angle

E - Young’s modulus

ν - Poisson’s ratio

Eo - Initial modulus

γ - Dry Unit Weight

x - Displacement

EA - Normal Stiffness

θ - Angle

ρ - Density

vii

`TABLE OF CONTENTS

CHAPTER TITLE PAGE

DECLARATION ii

DEDICATION iii

ACKNOWLEDGEMENTS iv

ABSTRACT v

ABSTRAK vi

TABLE OF CONTENTS vii

LIST OF TABLES xi

LIST OF FIGURES xii

LIST OF SYMBOLS xvi

1 INTRODUCTION 1

1.1 Introduction 1

1.2 Problem Statement 2

1.3 Research Objectives 4

1.4 Research Scope 4

2 LITERATURE 7

2.1 Introduction 7

2.2 Soft Soil 7

viii

2.3 Stress, strain and strength in soil 8

2.4 Effective stress 9

2.4.1 The Principle Of Effective Stress 10

2.5 Settlements 11

2.6 The Process of Consolidation 13

2.6.1 Consolidation settlement

15

2.6.2 Secondary compression settlement

15

2.6.3 Distortion settlement

15

2.7 Finite Element Method 16

2.8 PLAXIS 2D 16

2.8.1 Geometry and elements 18

2.8.2 Soil element (Volume element) 18

2.8.3 Calculation types 19

2.8.4 Geogrid element 20

2.8.5 Drains 21

2.8.6 Interface element 21

2.8.7 Young modulus 22

2.8.8 Friction 22

2.8.9 Angle of Friction 23

2.9 Mohr Coulomb model 23

2.10 Settlement of soft soil under embankment 26

2.11 Settlement Impacts 27

2.12 Stress Distribution 27

2.13 Simple 2V:1H Method 28

ix

2.14 Theory of Elasticity 29

3 METHODOLOGY 30

3.1 Introduction 30

3.2 Research design and procedure 30

3.3 Data Collection 31

3.4 Modelling and analysis 32

3.4.1 Material Properties 33

3.4.2 Mesh Generation and Boundary Condition 34

3.4.3 Initial Condition 35

3.4.4 Consolidation analysis 36

3.4.5 Staged construction 36

3.4.6 Presentation of results 36

3.4.7 Calculation Type 40

3.4.8 Modelling Staged Construction 40

4 RESULT AND DISCUSSION 44

4.1 Introduction 44

4.2 Results and Discussion 44

5 CONCLUSION AND RECOMENDATION 60

5.1 Introduction 60

x

5.2 Conclusion 60

5.3 Recommendations for Continuity Study 61

REFERENCES 62

CHAPTER 1

INTRODUCTION

1.1 Introduction

In pavement engineering, either highway or runway as an infrastructure, a

pavement encompasses three important part namely traffic load, pavement and

subgrade. Pavement generally comprises granular materials with unbounded or

bounded materials located between traffic load and subgrade, distributing the load to

surface of subgrade. The construction of pavement on weak ground often requires

the soil to be improved in order to ensure the safety and the stability.

Government of Malaysia had been promises to construct a new Borneo new

link highway that can’t avoid this such area Sarawak coastal area. The ‘soft soil

problem’ such as the settlement of soft soil foundation has become one of the key

problems for foundation design. When a road embankment is constructed over soft

soil, there is a surcharges or an increase of stress in the soft ground including the

strain or settlement. If the surcharge load due to filling and constructions traffic load

is high near the ultimate bearing capacity of the supporting soft ground, excessive

yielding or plastic deformation in vertical and lateral direction of the soft ground will

occur and followed by tension crack, deep seated rotational slip when deformation is

large and extensive.

In condition where problematic surfaces carry risks for engineering

constructions, surfaces can be adapted as expected with various soil improvement

methods. Bored pile, soil injection, stone column, vibro compaction, jet grouting

dewatering, compaction, preloading with and without vertical drains, grouting, deep

mixing, deep densification and soil reinforcement are common methods used for this

2

aim. Previous similar projects are of importance in order to determine the suitable

soil improvement method in relation to project. By using PLAXIS 2D, with support

by an actual field data form site investigation (S.I) work report can generate the best

way of subgrade stabilization.

1.2 Problem Statement

Soft soil present several challenges for the geotechnical engineer as they pose

problems related to stability and settlements. Soft soil covers the majority of West

Coast and East Coast of Peninsular Malaysia and in Sarawak, the largest state in

Malaysia, has the biggest reserve of soft soil such as peat land. For a several decades

engineer try to avoid those such area but since the growth of population and

development, the peat and soft soil has to be use for infrastructure such as housing

estate and new roadway.

Sarawak has approximately 1,657,600 ha. of tropical peat that covers 13% or

the total land area (12.4 million ha.). It is the largest area peat land in Malaysia. It

constitutes nearly 63% of the total peat land of the country. More than 80% of the

peat more than 2.5m depth (Khing, 2014).

Figure 1.1: Distribution of peat in Sarawak (Land and Survey Department Sarawak,

2011)

3

Table 1.1: Areas under peat in the various administrative divisions in

Sarawak sq. km (Singh et al,1997)

With the rapidly development of highway and building construction, the

settlement of soft soil foundation has become one of the key problems for foundation

design. Construction over peat deposits always creates special problems due to the

poor engineering properties of peat soil. Excessive settlements comprise the major

problem associated with the highly compressible nature of peat soil. Low shear

strength often causes stability problems in peat soils when they are subjected to

external loading. Creep settlements that occur after the construction of the road

pavement will lead to significant problems (Ismail Bakar, 2014).

Division Area (sq. km)

Kuching Administration Division 23,059

Samarahan Administration Division 192,775

Sri Aman Administration Division 283,076

Sibu Administration Division 540,800

Sarikei Administration Division 169,900

Bintulu Administration Division 146,121

Miri Administration Division 276,579

Limbang Administration Division 25,300

Total 1,657,600

4

Figure 1.2: Site Location Jalan Matang Baru, Phase 1,Kuching

Figure 1.3: Site Location Jalan Matang Baru, Phase 1,Kuching

Excessive and uneven settlement is very often to be characterizing by

pavement rutting, distortion, undulation or lateral tilt, change of chamber or super

elevation on road surfacing. Pavement distortion or localizes depression may result

in pavement surface rutting and ponding. These problems can end up in damage in

drains, culverts, buried utilities and traffic hazard to road user (Alvin Lopez, 2010).

5

Figure 1.4: Site Location Jalan Matang Baru, Phase 1,Kuching

Figure 1.5: Site Location Jalan Matang Baru, Phase 1,Kuching

6

1.3 Research Objectives

The aim of this research study is to identify the most suitable stabilization

method for subgrade in soft soil which will be cost effective. There are three

objectives proposed in this research study: -

i. To develop model of subgrade using PLAXIS 2D.

ii. To monitor the displacement and settlement of subgrade in soft soil.

iii. To propose ground improvement of subgrade and monitor the settlement.

1.4 Research Scope

This research based on the specific scope in order to ensure the precision of

the research area and also to achieve research objectives. The site location limited

for Jalan Matang Baru, Phase 1, Kuching, Sarawak. The input data, such as soil

properties are based on site investigation (S.I) work report, sourced from the Central

Material Laboratory, JKR Sarawak used to generate the best way of subgrade

stabilization by PLAXIS 2D.

Figure 1.6: Arial view site location (Google Maps, 2014)

62

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