A CASE STUDY OF PRELOADING IN PEAT USING FINITE ELEMENT METHOD WITH PLAXIS SOFTWARE

AMANDA ANAK MANGAN

'fA 347 Bachelor of Engineering with Honors F5 (Civil Engineering) A484 2009/20102010

P.KHIDMAT MAKLUMAT AKADEMIK

111111111 IINfrnill 1111111 1000212981

UNIVERSITI MALAYSIA SARA WAK

RI3a

BORANG PENGESAHAN STATUS TESIS

ludul : A CASE STUDY OF PRELOADING IN PEAT USING FINITE ELEMENT METHOD WITH PLAXIS SOFTWARE

SESI PENGAJIAN: 2009/2010

Saya AMANDA ANAK MANGAN (HURUF BESAR)

mengaku membenarkan tesis • ini disimpan di Pusat Khidmat Maklumat Akademik, Universiti Malaysia Sarawak dengan syarat-syarat kegunaan seperti berikut:

I. Tesis adalah hakmilik Universiti Malaysia Sarawak. 2. Pusat Khidmat Maklumat Akademik, Universiti Malaysia Sarawak dibenarkan membuat salinan untuk

tujuan pengajian sahaja. 3. Membuat pendigitan untuk membangunkan Pangkalan Data Kandungan Tempatan. 4. Pusat Khidrnat Maklumat Akademik, Universiti Malaysia Sarawak dibenarkan membuat salinan tesis ini

sebagai bah an pertukaran antara institusi pengajian tinggi. 5. •• Sila tandakan ( ~ ) di kotak yang berkenaan

D SULIT (Mengandungi maklumat yang berdiujah keselamatan atau kepentingan Malaysia seperti yang termaktub di dalam AKT A RAHSIA RASMI 1972).

D TERHAD (Mengandungi mak1umat TERHAD yang telah ditentukan oleh organisasil badan di mana penyelidikan dijalankan).

o TIDAK TERHAD

Disahkan oleh

~. (TANDMANGAN PENULIS) (TANDATANGAN PENYELIA)

Alamat tetap: LOT 222 KAMPUNG

STAPOK, lLN BATU KAWA PROF. K. G. H. C. NIMAL SENEVIRATNE Nama Penyelia

93250, KUCHING

Tarikh: Tarikh:

CATATAN * Tesis dimaksudkan sebagai tesis bagi Ijazah Doktor Falsafah, Sarjana dan Sarjana Muda.

** lika tesis ini SULIT atau TERHAD, sila lampirkan surat daripada pihak berkuasalorganisasi berkenaan dengan menyatakan sekali sebab dan tempoh tesis ini perlu dikelaskan sebagai SULIT dan TERHAD.

APPROVAL SHEET

Final Year Project as follows:

Title: A Case Study Of Preloading In Peat Using Finite Element Analysis With

PLAXIS Software

Author: Amanda Anak Mangan

Matrics number: 13700

Read and approved by:

Proffesor Dr K. G. H. C. Nimal Seneviratne Date

Project Supervisor

-

A CASE STUDY OF PRELOADING IN PEAT USING FINITE ELEMENT

ANALYSIS WITH PLAXIS SOFTWARE

AMANDA ANAK MANGAN

This report is submitted to Faculty of Engineering, Universiti Malaysia Sarawak

(UNIMAS) as to fulfill the requirements of Bachelor Degree with Honours (Civil

Engineering) 2010

-~ -~-~-

...T 0 m~ love and special ones...

Famil~ - thanks for ~our

x

never end care, support, understand ing and concern.

Iwill a~wa~s love ~ou

,

ACKNOWLEDGEMENT

The author would like to thank Prof. Dr. K.G.H. C Nimal Seneviratne for the

constant supervision and guidance throughout the process of completing this

report.

The author would also like to thank all individuals whom have contributed to the

success of this project.

ii

Abstrak

Tanah gambut dikategorikan sebagai tanah bennasalah kerana mempunyai

kekuatan ricih rendah dankebolehmampatan yang tinggi, yang ini tidak sesuai

untuk pembinaan. Tanah gambut merangkumi 13% dari tanah Sarawak di mana

jalan-jalan utama sebahagian besar dibina.Oleh kerana itu, adalah perIu untuk I

mencari altematif yang sesuai untuk meningkatkan kekuatan kerana tanah sangat

mahal dan sangat terhad. Pembebanan awal menggunakan pasir telah menjadi

teknik yang popular perbaikan tanah dasar gambut untuk sub jalan di

Sarawak. Penelitian ini menyajikan kesan dari preloading di gambut dengan

menggunakan kaedah elemen finit menggunakan software PLAXIS untuk

menunjukkan pembinaan berperingkat dengan pembebanan awal tambak dan

perkaitan penurunan konsolidasi dan karakteristik disipasi tekanan pori. Dua kes

dibuat dengan ada perbezaan dalam keadaan batas dan berbanding semasa

menggunakan material yang sarna, tahap pembinaan dan anal isis kadar konsolidasi

serta mesh kekasaran dalam model Regangan. Dari kes-kes yang dianalisis, jelas

bahawa penyebarari tekanan air pori dengan keadaan had terbuka di bahagian

bawah tanah gambut, dan penurunan konsolidasi yang lebih cepat berbanding

dengan kes di mana keadaan had tertutup di bahagian bawah

gambut. Kesimpulannya, tekanan pori akan membubarkan asalkan ada syarat batas

terbuka dan tidak menyebabkan perbezaan besar dalam penurunan konsolidasi, dan

kekasaran yang berbeza dalam generasi mesh subbase akan memberikan kegagalan

mekanisma yang berbeza. Oleh kerana itu, parameter sebenar sifat gambut harus

digunakan untuk kaj ian di masa depan.

111

Abstract

Peat is categorized as problematic soil because it has low shear strength and high

compressibility, which are not suitable for construction. Peat covers 13% of

Sarawak land where most major roads are constructed. Therefore, it is necessary to

find suitable alternatives to improve the strength since nowadays lands are very

expensive and very limited. Preloading using sand has been a popular technique of

peat ground improvement for road sub base in Sarawak. This study presents the

effect of pre loading in peat using finite element method using PLAXIS software to

model stage construction of a preloading embankment and associated

consolidation settlement and pore pressure dissipation characteristics. Two cases

are generated with differences in the boundary conditions and compared while

using the same material properties, stage of constructions and consolidation

analyses as well as coarseness mesh in a plain strain model. From the cases

analyzed, it is clear that the pore pressure disperses with open boundary condition

at the bottom of peat, and the rate consolidation settlement faster compared to the

case where closed boundary condition at the bottom of peat. In conclusion, the

pore pressure will disperse provided there is open boundary condition and it does

not lead to large difference in consolidation settlement, and different coarseness in

the sub base mesh generation will give different failure mechanism. Therefore, real

parametric of peat properties should be used for future research.

'11 at Kltiiliuat M Nl El<.S TJ ~A .. , ~ wnar MadclUik.

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Table of Content

Page

Dedication

Acknowledgement ii

Abstract iii

List of Tables vii

List of Figures viii

Chapter 1

Chapter 2

INTRODUCTION

1.1 Introduction

1.1.1 Peat Soil

1.1 .2 PLAXIS Software 3

1.2 Problem Statement 3

1.3 Purpose of Study 3

1.4 Objectives 4

1.5 Scope of study 4

LITERATURE REVIEW

2.1 Definition of Peat 5

2.2 Peat Classification 9

2.3 Peat Deposits in Malaysia 11

2.4 Preloading 13

2.4.1 Preloading Techniques 13

2.4.2 Principles of Pre loading 17

2.5 PLAXIS Software 20

v

212.5.1 Short Review of the Features

Chapter 3 METHODOLOGY

283.1 General

293.2 Problem Arise

333.3 Staged Construction

3.4 Geometric Preparation 35

373.5 Generation of Initial Stress

383.5.1 Case Study One

433.5.2 Case Study Two

443.6 Data Analysis

443.7 Result

Chapter 4 RESULTS AND ANALYSIS

454. I General

Chapter 5 CONCLUSION AND

RECOMMENDATION

595.1 General

595.2 Conclusions

605.3Recommendation

61References

63Attachment I

vi

List of Tables

Table Page

62.1 Standard Unified Soil Classification

2.2 Malaysian Soil Classification system for 8

engineering purposes and field identification

(Organic Soils and Peat Part Only)

2.3 Classification ofpeat on the basis of 11

decomposition on the von Post scale.

2.4 Areas under peat in the various administrative 12

divisions in Sarawak (km2)

3.1 Material properties of the road embankment and 36

subsoil

vii

List of Figures

Figure Page

1.1 Peatswamp land forest in Sarawak (Department of 2

Irrigation and Drainage Sarawak, 2009)

2.1 An excavator sunk into the soft soil 5

2.2 Principles of preloading 14

2.3 Settlement vs time 15

2.4 Vacuum system 16

2.5 Vertical stress profile (a) initial in situ condition, 18

(b)conventional surcharge and (c) vacuum-induced

surcharge

3.1 Flow to conduct the project 31

3.2 Flow of conducting the staged construction using 32

PLAXIS

3.3 Situation of typical road embankment on soft soil 34

3.4 Geometry model in the Input window (PLAXIS) 35

3.5 Generated mesh 37

3.6 Closed consolidation condition on left and right 39

3.7 Initial stress generation 40

3.8 Construction of first layer of embankment 41

3.9 Phase input data 42

3.10 Point of selection for curve generation 43

4.1 Plot ofdeformed mesh 45

Vlll

4.2 Plot ofhorizontal displacements (arrows) 46

4.3 Plot of horizontal displacements (contour lines) 47

4.4 Plot ofvertical displacements (arrows) 47

4.5 Plot of vertical displacements (contour lines) 48

4.6 Plot ofhorizontal increments (arrows) 48

4.7 Plot ofvertical increments (arrows) 49

4.8 Plot of effective stresses (principal directions) 49

4.9 Plot of horizontal displacements (arrows) 50

4.10 Plot ofvertical displacements (arrows) 50

4.11 Plot of horizontal increments (arrows) 51

4.12 Plot ofvertical increments (arrows) 51

4.13 Plot of excess pore pressures (shadings) 52

4.14 Plot of effective stress (principal directions) 53

4.15 Plot ofgroundwater head (contour lines) 53

4.16 Active pore pressure vs time 54

4.17 Increment of steps in meter vs time 55

4.18 Steps increment vs time 56

4.19 Case 2 Closed consolidation condition 57

CHAPTER!

INTRODUCTION

1.1 Introduction

Land development is constantly growing, in line with the increasing demand

and population of the country. Construction of infrastructure over soft soil such as

peat is a need to solve the increasing demand and decreasing land availability.

Since peat soil is categorized as problematic soil, it is essential to study the

settlement of soil prior to construction. Therefore, this project will concentrate on

the effect preloading, especially road constructed on peat soil.

In the past few years, there is increasing number of researches been done to

model the constructions over soft soils. These modelling and case study will be

elaborated further in Chapter 2.

1.1.1 Peat soil

Peat is categorized under soft soil and organic soil. According to Huat et. al

(2004), this type of soil covered about 3.0 million hectares or 8.0% of the

Malaysian land. Although problematic, engineers believed that there are ways to

improve the soft soil condition, appropriate for construction. The peat soil

properties will be discussed further in Chapter 2 as well.

Figure 1.1 shows the distribution of roads constructed on peaty area. Most

major cities in Sarawak are located by the river bank, areas where soft soil such as

sandy and peat swamp is the main ground. History tells that the riverside

economic growth is much better due to the ease in transportation . Nowadays,

people prefer to travel either on land or by air. Therefore, it is important to

improve the road facilities in Sarawak.

t SOJTH CHI~lA SEA

KAllMANnN

E3 EXISTING ROAD

• PEATSOlS

Figure 1.1 Peatswamp land forest in Sarawak (Department of Irrigation and Drainage Sarawak, 2009)

2

1.1.2 PLAXIS software

PLAXIS is a geotechnical software, developed to assist in analyzing the

deformation and stability in geotechnical engineering problems. Geotechnical

applications require advanced structured models for the simulation of the non­

linear and time-dependent behaviour of soils. Furthermore, since soil is a multi

strata material, special procedures are required to deal with hydrostatic and non­

hydrostatic pore pressures in the soil. Although the modelling of the soil itself is

an important issue, many geotechnical engineering projects mix up the modelling

of structures and the interaction between the structures and the soil. PLAXIS is

equipped with special features to deal with the numerous aspects of complex

geotechnical structures. These features will be elaborated further in Chapter 2.

1.2 Problem statement

Preloading, also known as precompression is a method involving placement

and removal of dead load (Bell, 1993). The problem arise from this method is time

dependent. PLAXIS is designed to overcome this method by analyzing the soil

behaviour prior to construction.

1.3 Purpose of study

The study aims to use PLAXIS to demonstrate the effect of preloading to

improve the road construction method over sub grade that consists of peat soil.

3

1.4 Objectives

To achieve the mentioned alms stated above, it is essential to fulfill the

objectives as follows:

i) To understand the effect of preloading on peat soil as the sub

grade.

ii) To demonstrate the effect of preloading in peat using PLAXIS

software.

iii) To use PLAXIS and finite element method to determine

deformation and failure due to preloading and relate them to soil

properties.

1.5 Scope of study

The scope ofthis project is to study the behaviour of peat upon preloading,

and the application of finite element method in solving peat deformation. The

analysis is based on research and reports obtained, from publish literature and also

from relevant party and authority.

This study will focus on the analysis of effect of preloading on peat using

finite element code, PLAXIS. The software used will be PLAXIS 2D-Version 8.

4

CHAPTER 2

LITERATURE REVIEW

2.1 Definition of peat

In the engineering point of view, peat is recognized as highly compressible

type of soil. Engineers are reluctant to construct on peat lands because of the

difficulty to access the site and other problems related to unique characteristics of

peat. Peat poses high moisture content, other than very high and long term

settlement. These characteristics usually raise problems from preconstruction to

the post construction. One such problem is shown in Figure 2.1.

Figure 2.1: An excavator sunk into the soft soil

5

Geologist identifies the peat soil as a separate soil entity and the Unified Soil

Classification System (USC), which is adopted by the American Society for

Testing and Materials (ASTM) as the standard classification of soils for

engineering purposes and has been described as having been accepted in

international geotechnical communication. It has a major division called Highly

Organic Soils (pt) which refers to peat, muck and other highly organic soils

(ASTM, 2007).

Table 2.1: Standard Unified Soil Classification

Group Group nameMajor divisions symbol

clean gravel GW

well graded gravel,

gravel <5% smaller fine to coarse gravel

> 50% ofcoarse than no. 200 Coarse GP poorly graded gravel

fraction retained Sieve grained

soils on No.4 (4.75 gravel with GM silty gravel ·

more than mm) sieve >12% fines GC clayey gravel 50%

well graded sand, fine retained on SW No.200 sand clean sand to coarse sand

(0.075 mm) 2: 50% ofcoarse SP I

poorly-graded sand sieve fraction passes sand with SM silty sand

No.4 sieve >12% fines SC clayey sand

ML silt. . morgamc

CL claysilt and clay

Fine grained liquid limit < 50 organic OL

I

organic silt, organic clay

soils more than MH

silt of high plasticity,

50010 passes elastic silt

No.200 inorgahic

clay of high plasticity, sieve silt and clay I CH

fat clay liquid limit 2: 50

orgamc I

OH organic clay, organic silt

HighJy organic soils Pt Peat

Malaysia termed her own soil classification, suitable for the local use, a

proposed from the Public Works Department, Malaysia together with (Jarret,

1991). According to Jarret, peat soil in Malaysia can be subdivided into three

categories, which differs in their organic content respectively. This organic

content is three-point scale classification based on fiber content resulting from

decomposition and humidification according to the U.S Department of Agriculture

(USDA). A fiber is defined as > 0.15mm. The organic fiber content has been

referred to as the fabric of the organic soil. An appreciation of the constituent

matter and its attributes like orientation aids in the constitutive modelling of this

soil type for basic understanding of the mechanical behaviour.

7

00

'able 2.2: Malaysian Soil Classification system for engineering purposes and field identification (Organic Soils and Peat Part Only)....... .....II I...-........ ~... I) 57 rd.

~o~.,... ~II'MP----... ...........~=.. ,......,ILIOHTLYORGANIC 140 9OU.S .... ORGANIC Noa...-SlLT P&ATS 00

Sli",Orpnic SOILS

<JjFo ao ,,.,.CLAY Clioar.­eyo-.3. 50-70

2Oko Co CEo 10-90 :090

ORGASIC OROA."JC SOILS 0

SOILS Qrsaftje CGmcnt 3)%...75%

II......,. ar...- SIL(........ 1iIce Co)

~ 0rpNe CLAY of1_ plMticky SliIhdJ Or.,- CLAY of ................dclty lIilhdY o.anlc CLAY or...... ..iQo II....,. Or.wc CLAY ofwry hip plMlicil)' ~!rJ'..~ CLAY or ........Iyiii tiCilY

Subdivision ofOrpnic Soih is cil&cuk•• ntidler die humirlCalion _u _ rdlUlo Ix IboIlL "'weh a "'ties. Ale,," is die proa.w. 0IM0me of IUbdivision l.mna 10 dMa'iplion aac:h .. -Fillrous ORCiA!'IlIC SOIL" Or"~ ORGA.'f1C SOILoflntermedJale PlastEitv.

PEATS PEAT PI Plf HI.H3 Flbric: or FlbtooI Pclu. ~ K4-H6 Hemic or Mocter.ely lJieeompoled

Orpnic lUI H7-HIO P-. Concent SIpric or AmoqJhout ,. more ..... n 7$%

u.uaII)< Vft)' .-Ie 10 .,... i. 1IOIour ...... _of ......­...,. Ibc YiIibIc. Oftea_ .....dwe GIpIIic s_IL

~ _""" 10black il'I eolour. ~"halow ....iIy.. -.. • .... MljoIily of~.il Grpnic SO if ~dIc whole ~I will Ibc reoopi.ble"'.1 ICmir1e.More likely 10 IUN."II IIrOapy if hilihlV humiRed

Note: The mcthodoiosy for estimatins the Orsanic Content and Degree of Humific:ation UBin, the Van Post

In the world of forestry, peat swamp forests are waterlogged forests growing

on a layer of dead leaves and plant material up to 20 meters thick. They comprise

an ancient and unique ecosystem characterized by water logging, with low

nutrients and dissolved oxygen levels in acidic water regimes. Their continued

survival depends on a naturally high water level that prevents the soil from drying

out to expose combustible peat matter. This harsh waterlogged environment has

led to the evolution of many species of flora uniquely adapted to these conditions.

Unlike engineers and geologist, the people in the forestry field appreciate the water

logging condition in peat.

Peat properties enables it to accommodate the need of peat swamp forest to

produce a dynamic link between land and water, a transition zone where the flow

of water, the cycling of nutrients and the energy of the sun combine to produce a

unique ecosystem of hydrology, soils and vegetation (United Nations Development

Programme (UNDP) & Ministry of Natural Resources and Environment Malaysia,

2008). These swamps provide a variety of goods and services, both directly and

iDdirectly, in the form of forestry and fisheries products, energy, flood mitigation,

water supply and groundwater recharge.

U Peat classification

In engineering practice, the peat classification and division is based on visual

inspection of the structure and consistency and on the squeezing test according to

von Post (1924), (see Table 2.3). The following is a distinctive description of the

caaegorized peat:

9

• Fibrous peat is low-humified and has a distinct plant structure. It is brown

to brownish-yellow in colour. If a sample is squeezed in the hand, it gives

brown to colourless, cloudy to clear water, but without any peat matter.

The material remaining in the hand has a fibrous structure. (Degree of

decomposition on the von Post scale; H I-H4.)

Pseudo-fibrous peat is moderately humified and has an indistinct to

relatively distinct plant structure. It is usually brown. If a sample is

squeezed in the hand, less than half of the peat mass passes between the

fingers. The material remaining in the hand has a more or less mushy

consistency, but with a distinct plant structure (H5- H7).

• Amorphous peat is highly humified. The plant structure is very indistinct

or invisible. It is brown to brown-black in colour. If a sample is

squeezed in the hand, more than half of the peat mass passes between the

fingers without any free water running out. When squeezing, only a few

more solid components, such as root fibres and wood remnants can be felt.

These constitute any material remaining in the hand, (H8-HIO).

10

Table 2.3: Classification of peat based on the decomposition of fibre content according to the von Post scale.

Dllipation Group Description

Fibrous peat HI-H4 Low degree of decomposition. Fibrous

structure. Easily recognizable plant

structure, primarily of white mosses.

Psc:udo-fibrous peat H5-H7 Intennediate degree of decomposition.

Recognizable plant structure.

Amorphous peat H8-HIO High degree of decomposition. No

visible plant structure. Mushy

consistency.

Peat deposits in Malaysia

Peat as mentioned earlier is classified as one of the major problematic soils

found in Malaysia. Over 3.0 million hectares of Malaysia is covered with peat

(Huat et al. 2004). The areas covered with peat deposits are found where the

conditions are favourable for their fonnation (Duraisamy, Bujang, & Azlan, 2007).

For example on the west coast of Malaysian Peninsular, the deposits are fonned in

depressions consistin~ predominantly of marine clay deposits or a mixture of

marine and river deposits especially in areas along river courses. In Figure 1.1 the

bmation are found near the coastal area. There are two types of peat deposit, the

Ihallow deposit usually less than 3 m thick while the thickness of deep peat deposit

in Malaysia exceeds 5 m.

In the past decade the development of peat land in Malaysia is almost

avoidable although construction on marginal land such as peat has become

11