A STUDY ON GEOTECHNICAL BEHAVIOR OF STABILIZED PEAT SOIL WITH FINITE ELEMENT MODELING

SiiHeeYew

Master of Engineering 2012

Dedicated

To

Almighty GOD for HIS Glory;

To

My beloved family

11

ACKNOWLEDGEMENT

I wish to express my profound gratitude to my supervisors, Dr. Siti Noor Linda Taib

and Professor Nimal Senviratne for their discussions, invaluable comments and

constructive suggestions in reviewing this work. Without their continued criticisms

and sup'port based on their deep insight and vast experience in the field of

geotechnical engineering, this thesis would not have been the same as presented here.

Also, lowe many thanks to Associate Professor Prabir Kumar Kolay, who now

works for Southern lllinois University Carbondale, USA, for his valuable

suggestions, constant motivation and constructive discussions during the first year of

this research. I would like to extend my thanks to Mr. Ahmad Kamal, Miss

Dayangku Salma, Mr. Haji Affandi Othman and Mr. Chew. Their guidance and

advices on laboratory testing and finite element analysis are highly appreciated and

will never be forgotten. I wish to express my gratitude to Mr. Toh Thian Gek, the

engineer from the JKR Sarawak. His contributions is gratefully acknowledged.

The research presented in this dissertation was financially supported by the

ZAMALAH UNIMAS Scholarship Program,. provided by Universiti Malaysia

Sarawak. This program deserves special acknowledgement. Many thanks go to my

friends, Esther Lu, Winnie Wong and Lim Yek Chin for helping me in various ways

to achieve this goal. Finally, I dedicate this dissertation to my family, especially my

dearest mother. Her constant love, trust, understanding and encouragement follow

me everywhere.

III

ABSTRACT

(!eat or highly organic soils are well known for their high compressibility, natural

moisture content, low shear strength and long-term settlement. Yet, Sarawak has the

largest peat land area in Malaysia which is about 16,500 km2 or 13 % of the state

land. There is very little data available from Sarawak regarding the geotechnical

properties and behavior of local peat. In addition, the sampling of peat in undisturbed

state is extremely difficult particularly at depths below the watertab~

In the current study of Sarawak peat an undisturbed peat sampler was designed and

the sampling was executed in such a way that disturbance could be minimized. Peat

samples were collected from Matang area of Sarawak. The effect of stabilizing

natural peat with locally available ingredient, pond ash, was also explored. Pond ash

is created from the disposal of fly ash. The wet disposal of fly ash into an ash pond is

the most common fly ash disposal method. The compressibility characteristics of

local undisturbed peat soil are examined through oedometer and Rowe cell testing of

undisturbed samples. The experimental results were compared with available

published data on Malaysian peat.

The material parameters obtained from experiments were used to perform a finite

element analysis (PLAXIS) of shallow foundations placed on deposits of Sarawak

peat soil. Three depths of peat layers, typically found in Sibu area of Sarawak were

considered. The effect of stabilization was modelled by using a stability ratio (the

normalized depth of stabilized layer using layer thickness) of 0, 0.25, 0.50, 0.75 and

l.O. The model reveals that the use of pond ash in peat stabilization can significantly

increase the soil bearing capacity. The results revealed that the stabilization was very

IV

significant at the very beginning of 2S % of peat depth. It was apparent that the

working load of stabilized peat increased by 21 %, 28 % and 27 % in all 3 m, 7 m

and 10m peat with a PA dosage of 2S8 kg/m3 for 2S % of peat depth.

Furthermore, the use of PA in soil stabilization helps in reducing the pond volume

and achieving an environmental-friendly as well as a sustainable development of

natural resources.

v

ABSTRAK

Tanah gambut atau juga dikenali sebagai tanah yang mengandungi tanah organik

yang tinggi adalah sangat terkenal dengan kadar kemendapan yang tinggi, kandungan

kelembapan yang tinggi secara semula jadi, daya tahan ricih yang rendah, serta

menunjukkan kadar mendapan jangka panjang. Namun, Sarawak memiliki keluasan

tanah gambut terbesar di Malaysia, iaitu sekitar 16,500 km2 atau 13 % dari tanah

negara. Data yang terdapat di Sarawak mengenai sifat dan cirri-ciri geoteknik bagi

tanah gambut asli adalah sangat sedikit. Sebagai tambahan, pensampelan tanah

gambut dalam keadaan tak terganggu sangat sukar terutama pada kedalaman di

bawah muka air.

Alat sampel tanah gambut yang tidak terganggu telah direka dan sampel telah

diambil dalam keadaan gangguan yang minima dalarna kajian ini. Sampel tanah

gambut dikutip dari kawasan Matang Sarawak. Kesan menstabilkan tanah gambut

semula jadi dengan ejen penstabil yang boleh didapati, abu kolam, juga dijelajahi.

Abu kolam terjadi daripada pelupusan abu. Pelupusan abu basah ke dalam kolam abu

adalah cara pembuangan abu yang paling biasa. Ciri-ciri ketermampatan tanah

gambut tak terganggu tempatan diperiksa melal ui oedometer dan Rowe ujian sel

dengan tanah gambut tak terganggu. Keputusan uj ian telah dibandingkan dengan data

sedia ada berkaitan tanah gambut dalam Malaysia.

Parameter bahan diperolehi dari eksperimen telah digunakan untuk menjalankan

anahsis komputer (PLAXIS) bagi asas cetek yang terletak atas deposit tanah gambut

Sarawak. Tiga kedalaman lapisan tanah gambut biasanya terdapat di kawasan Sibu

Sarawak dianggap. Kesan penstabilan dimodelkan dengan menggunakan nisbah

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kestabilan (kedalaman temonnal mengukuhkan lapisan menggunakan ketebalan

lapisan) 0, 0.25, 0.50, 0.75 dan 1.0. Keputusan mendedahkan bahawa penstabilan

adalah amat penting pada permulaan 25 % kedalaman tanah gambut. Beban kerja

untuk tanah gambut stabil telah dinaikkan menjelang 21 %, 28 % dan 27 % dalam

semua 3 m, 7 m dan 10m tanah gambut dengan dos PA sebanyak 258 kg / m3 untuk

25 % ·kedalaman tanah gambut.

Selain itu, penggunaan abu kolam dalam proses penstabilan tanah dapat membantu

dalam pengurangan isipadu kolam yang memenuhi konsep mesra alam serta

pembanguan lestari bagi alam semulajadi.

VII

P" I

TABLE OF CONTENTS

Page

TITLE

DEDICATION 11

TABLE OF CONTENT Vlll

LIST OF TABLES Xl

LIST OF FIGURES Xli

LIST OF APPENDICES XVll

ACKNOWLEDGEMENT 111

ABSTRACT IV

ABSTRAK VI

LIST OF SYMBO LS xv

CHAPTER 1 INTRODUCTION

1.1 Background 1

1.2 Problem Statement 3

1.3 Objectives of the Study 6

1.4 Scope of the Study 6

1.5 Structure of Thesis 8

CHAPTER 2 LITERATURE REVIEW

2.1 General 10

2.2 Soil Sampling 10

2.3 Index Properties 15

2.4 Soil Compressibility and Consolidation 17

Vlll

I 2.5 Consolidation Test 25

2.5.1 Conventional Oedometer Test 26

2.5.2 Rowe Cell Test 27

2.6 Compressibility Parameters 29

2.7 Peat Stabilization 32

2.8 Finite Element Analysis 34

2.8.1 Model Parameters 36

2.9 Summary 42

CHAPTER 3 METHODOLOGY

3.1 Introduction 43

3.2 Soil Sampling 44

3.3 Preliminary Test 47

3.4 Stabilized Peat 52

3.5 Consolidation Test on Undisturbed Peat Sample 53

3.5.1 Oedometer Test 53

3.5 .2 Rowe Cell Test 54

3.6 Simulation by Using Finite Element Analysis 61

CHAPTER 4 LADORA TORY RESULTS AND DISCUSSION

4.1 Introduction 65

4.2 Soil Properties Tests 65

4.3 Stabilized Peat 68

4.4 Consolidation Tests 73

4.4.1 Conventional Oedometer Test 73

IX

,..

CHAPTER 5

CHAPTER 6

I 4.4.2 Rowe Cell Test 76

4.4.3 Summary 81

FINITE ELEMENT SIMULATION AND DISCUSSION

5.1 Introduction 84

5.2 Model to Back Analysis of Laboratory Consolidation

Behaviour 84

5.2.1 Model Parameters of the Virtual Rowe Cell Analysis 85

5.2.2 Numerical Simulation of Virtual Rowe Cell Test 86

5.2.3 Numerical Verification of the Relation between

Parameters 90

5.3 Model to Simulate Stabilized Peat Ground for Foundation 93

5.3.1 Model Parameters of the Stabilized Peat Soil 94

5.3.2 Numerical Simulation of the Stabilized Peat Ground 96

CONCLUSIONS AND RECOMMENDATIONS

6.1 Conclusion 105

6.2 Recommendation 107

REFERENCES 108

Appendix A - F 115

x

LIST OF TABLES

Table Page

2.1 Classification of peat based on degree of decomposition

(von Post, 1922) 16

2.2 Engineering properties for peat (West Malaysia) in previous literature 32

2.3 Peat parameters for the finite element analysis by previous researchers 39

3.1 Data for curve fitting (Head, 1998) 58

3.2 FE cakulation scheme for virtual Rowe cell test 63

4.1 Physical properties of peat sample 66

4.2 Published physical properties of West Malaysia peat 66

4.3 Physical properties of pond ash 69

4.4 Consolidation characteristics for Malihah peat from oedometer test 75

4.5 Consolidation characteristics for Kopodim peat from oedometer test 75

4.6 Consolidation characteristics for Malihah peat from Rowe cell test 79

4.7 Consolidation characteristics for Kopodim peat from Rowe cell test 79

4.8 Compressibility index for Malihah peat 81

4.9 Compressibility index for Kopodim peat 81

4.10 Comparison of compressibility index with published data 81

5.1 SSC model parameters for the virtual Rowe cell test 85

5.2 FE calculation scheme for virtual Rowe cell test 87

5.3 SSC model parameters for the unstabilized and stabilized peat 95

5.4 Mohr-Coulomb model parameters for the clay layer 96

5.5 FE calculation scheme for prediction of the optimum

partial stabilization 99

Xl

LIST OF FIGURES

Figure Page

1.1 Distribution of peat in Sarawak 4

1.2 Ground settlemertt caused poor drainage and road system in a

commercial lot, Sibu 4

2.1 Peat soa sampler by Redfield (1975) 13

2.2 Core sampler by Caldwell et al. (2005) 14

2.3 Primary consolidation settlement curve 20

2.4 Square root method 22

2.5 Logarithm of time method 23

2.6 Secondary consolidation settlement curve 25

2.7 Conventional oedometer apparatus 26

2.8 Rowe consolidation cell 28

2.9 Void ratio against logarithm of effective stress (e -log cr') curve 30

2.10 Values of the compression index and the natural water content for peats

in comparison to those for soft clay and silt deposits (Mesri et aI., 1997) 40

2.11 CalCc relationship for Middleton peat, UK (Mesri et aI., 1997) 41

3.1 Flow chart of the study 44

3.2 Studied locations in this research 47

3.3 PA sample collection at ash disposal field,

Sejingkat Thermal Power Plant, Kuching 53

3.4 Arrangement of Rowe cell and equipment for Rowe cell test 56

3.5 Valves connection in Rowe consolidation cell 56

3.6 Free strain loading 57

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3.7 Equal strain loading 57

4.1 Particle size curve for peat soil samples 68

4.2 MDD-OMC curves obtained from standard Proctor tests 69

4.3 Stress-strain curves obtained from DCS tests (28 days) 70

4.4 Comparison between average DCS - stabilizer dosage of original peat

and stabilized peat-PA specimens (28 days) 70

4.5 Comparison between average DCS - moisture content of original peat

and stabihzed peat-PA specimens 72

4.6 Deformation of Malihah peat under certain pressure in oedometer 74

4.7 Deformation of Kopodim peat under certain pressure in oedometer 74

4.8 Typical e-Iogp , curve for Malihah and Kopodim peat in oedometer test 76

4.9 Deformation of Malihah peat under applied pressure in Rowe cell 78

4.10 Deformation of Kopodim peat under applied pressure in Rowe cell 78

4.11 Rough organic matters found in Kopodim peat 79

4.12 Typical e-Iog p' curve based on height and volume changes for Malihah and

Kopodim peat in Rowe cell test 80

4.13 Comparison of empirical correlation between Cc and Wo for soils 82

4.14 Comparison of empirical correlation between Ca and Cc for soils 83

5.1 Closed flow and consolidation boundary of the model 87

5.2 Deformed mesh with extreme total displacement after 200 kPa 88

5.3 Displacement-time curve for the model 89

5.4 Comparison of model displacement from various ways of

calculations 90

5.5 Model displacement when cohesion as a variable 91

5.6 Model displacement when friction angle as a variable 91

XlII

5.7 Mohr's circle from PLAXIS Triaxial test for Malihah peat 93

5.8 Stress-strain curves for the unstabilized peat and stabilized peat that

obtained from DeS tests (28 days) 94

5.9 Model displacement when mesh fineness as a variable 98

5.10 Model displacement when geometry boundary as a variable 98

5.11 Typical geometry model of3 m peat depth with 25 % stabilization 100

5.12 Typical deformed mesh of 3 m peat depth with 25 % stabilization 101

5.13 Applied load against model displacement of 3 m peat depth 102

5.14 Applied load against model displacement of 7 m peat depth 102

5.15 Applied load against model displacement of 10m peat depth 102

5.16 Working load against Stabilized depth ofpeat ground 104

XIV

LIST OF SYMBOLS

a ' v

A

c

e

Gs

H, Ho ­

Swelling index

Initial mass

Initial volume

Final vol ume

Vertical coefficient of consolidation

Secondary compression index

Excess pore pressure

Bulk unit weight

Volumetric strain

Modified swelling index

Modified creep index

Effective vertical stress

Total vertical stress

Area of sample

Cohesion

Compression index

Void ratio

Initial void ratio

Initial specific gravity

Initial height of consolidating soil layer

Permeability coefficient in horizontal direction

Permeability coefficient in verticaJ direction

xv

p Effecti ve stress

PA Pond ash

S Degree of saturation

tp Time to reach end of primary consolidation

ts Time to reach end of secondary compression

Tv Vertical theoretical time factor

U Pore water pressure dissipation

V Volume

Wo Initial moisture content

Wf Final moisture content

"fS81 Saturated unit weight

"funsal Unsaturated unit weight

be Settlement due to consolidation

A. * Modified compression index

v Poisson's ratio

p Density

Pd Initial dry density

a'o Initial vertical stress

af Final vertical stress

Friction angle

'I' Dilatancy angle

t Time

z Coordinate in the vertical direction

XVI

LIST OF APPENDICES

Appendix Page

A Designed and fabricated undisturbed peat soil sampler 115

B Rowe cell assembly, ancillary connections and test procedure 120

C Typical result for soil index properties determination

(Taman Malihah) 130

D Typical result for Rowe consolidation cell test (Taman Malihah) 134

E Comparison of model displacement in various PLAXIS simulations 155

F Typical Nang Sang I Teku Link Road land profile 156

XVII

CHAPTER!

INTRODUCTION

t.t Background

Soft compressible fibrous peat deposits can be found in many places in

Sarawak including the capital city of Kuching. Peat soil is classified as highly

organic with organic content more than 75% and represents the extreme form of soft

soil. It originates from disintegration of plant and organic matters when they

accumulate more quickly than they humidify (decays) (Duraisamy et al, 2007). Peat

has certain characteristics including high natural moisture content, high

compressibility and water holding capacity, low specific gravity, low bearing

capacity and medium to low permeability (Wong et al., 2008). For geotechnical

purposes, peat's degree of decomposition or humification system is often classified

to 3 classes (ASTM D5715, 2006): I. Fibric or fibrous (least decomposed). II. Hemic

or semi-fibrous (intermediate decomposed). m. Sapric or amorphous (most

decomposed).

Peat ground has caused many difficulties for civil engineers for many years

particularly in constructing civil engineering structures on them. Any structure on a

peat soil is likely to be affected by the slip failure of the ground, or the residual

settlement that could go on for a long time even after the structure is put into service.

Secondary consolidation will continue for a long period after construction and the

peat deposits will not return to their original state although minimal rebound is

possible if the weight is removed or the water level increased. In addition, settlement

and upheaval or horizontal displacement of the ground around the structure may also

have an effect on adjoining structures adversely (Murtedza et al., 2002). Therefore,

peat soil is generally considered unsuitable for foundations or any other loading in its

natural state.

The utilization of peat land in Malaysia is currently quite low. However, the

pressure to utilize these peat areas for housing developments, industrial sites and

embankments for new roads will increase with increase in the population and

resulting growth of urbanization and other developments. With the increasing

demand for land development in Sarawak it is not always possible to avoid

construction on this problematic soil. Thus, resolving of the peat soil problem has

become one of the urgent engineering issues locally.

Edil (2003) summarizes various construction methods that can be applied in

peat and organic soils, namely replacement with suitable alternative soil; ground

improvement and reinforcement to enhance soil strength and stiffuess such as stage

construction and preloading (with/without vertical drains or with vacuum

2

consolidation); stone columns, piles, or by reducing construction loads by using fill;

and deep stabilization method by using chemical admixtures such as cement, lime

and fly-ash.

1.2 Problem Statement

Sarawak has the largest peat land area in Malaysia which is about 16,500 km2

or about 13% of the state land, of which about 90% of the peat deposit is more than 1

m in depth below ground surface (Mutalib et ai., 1991). Peat occurs mainly between

the lower stretches of main river courses (basin peats) and in poorly drained interior

valleys (valley peats). They are found in the administrative divisions of Kuching,

Samarahan, Sri Aman, Sibu, Bintulu, Miri and Limbang (Jamaludin, 2002). Figure

1.1 shows the distribution of peat land in Sarawak given by Jabatan Pertanian

Sarawak 200l. The fluctuation of the water table due to excessive rainfall provides

suitable conditions for the accumulation of peat deposits in these areas which are

poorly drained. Figure l.2 illustrates the ground settlement caused by poor drainage

in Sibu town which is normally a serious problem. According to Duraisamy et al.

(2007), ground subsidence on peat land in Sibu town has resulted in depressions

disrupting the natural patterns of drainage. This scenario causes further problem

resulting in unhealthy stagnation of water in many parts of the town.

3

t DISTRlaUTION Of' PEAT SOILS IN SARAWAK

so Oi SEA

• PUTfOU

Figure 1.1: Distribution of peat in Sarawak (Source: Jabatan Pertanian Sarawak 2001)

Figure 1.2: Ground settlement caused poor drainage and

road system in a commercial lot, Sibu.

4

Pu at Khidmat Maldu at Abdemi~ UNIVERSm MALAYSIA SARAW,

As mentioned above, peat soils in Sarawak have caused many problems

1stduring development and construction. As reported by New Straits Times on

February 2010, Sarawak was to receive RM3.4 billion to build and upgrade roads

and to implement power supply projects under the National Key Results Area

(NKRA) pertaining to improving basic infrastructure in the rural areas. Rural and

Regional Development Minister, Datuk Seri Mohd Shafie Apdal acknowledged that

it was 10 times more expensive to implement infrastructure projects in Sarawak

compared to Peninsular Malaysia due to the difficult terrains and presence of soft

soils in many places. Piling has been adopted by some local contractors to overcome

the peat problem however it is expensive in many cases.

There are very few experiences gained from full scale trial embankments in

this region on soft soil. Besides, engineers are reluctant to construct on peat because

of the difficulties of accessing the site and high construction cost (Hashim and Islam,

2008). There are a few researches conducted on the behavior of peat and the

development of soil improvement method for construction on peat soil in West

Malaysia. Nevertheless, only a few literature is available about peat soil in East

Malaysia particularly Sarawak region. Thus, a study of the geotechnical behavior of

natural peat soil in Sarawak needs to be carried out and this research study IS

concentrating on the peat soil samples collected from Matang area, Sarawak.

5

1.3 Objectives of the Study

The objectives of this study are as follows:

1. To compare the index properties of the undisturbed and stabilized peat

soil;

II. To determine the geotechnical properties (e.g., compaction, consolidation,

permeability, unconfined compression strength) on the undisturbed and

stabilized peat soil samples via experimental works;

Ill. To investigate the effect of partial ground stabilization on the performance

of foundations in peat soils in Sarawak by finite element modelling (FEM)

using PLAXIS software;

IV. To select a suitable soft soil model and validate its applicability to be used

in analysing of problems involving unstabilized and stabilized local peat;

v. To determine the optimum partial stabilization depths for typical peat-clay

profiles in Sarawak for shallow foundations.

1.4 Scope of the Study

The study focuses on the determination of the geotechnical characteristics of

peat soil found in Matang area, Kuching. The interpretation of the results of the study

is limited to:

6

1. Undisturbed peat samples were obtained from Taman Malihah and Taman

Kopodim, Matang, Kuching using the undisturbed peat sampler (procedure

outlined in Appendix A, pp. 115).

11. The specimens used for the identification of index properties, classification,

Proctor test, unconfined compression strength test and conventional

oedometer test are remoulded samples.

Ill. Identification of index properties of peat include: water content, specific

gravity, sieve analysis and acidity.

IV. Classification of peat was made based on degree of humification (von

Post), fiber content and organic content.

v. Evaluation of compressibility of the peat was limited to pnmary

consolidation and dissipation of excess pores water pressure was

constrained in vertical direction.

VI. Evaluation of shear strength of the peat was made based on finite element

simulation in PLAXIS.

VII . Finite element model parameters such as saturated and unsaturated unit

weight of the undisturbed peat and stabilized peat were detenruned using

standard Proctor test. While, compressibility parameters were based on the

results of Rowe cell test and unconfined compression strength test

respectively.

Vlll. Soil permeability parameters, cohesion, friction angle, dilatancy angle and

Poisson's ratio of the undisturbed peat and stabilized peat were selected

arbitrarily as according to previous researchers due to the limitation of

laboratory equipments.

7