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SETTLEMENT OF AN EMBANKMENT TREATED WITH PRELOADING AND

PREFABRICATED VERTICAL DRAIN OF EAST-COAST HIGHWAY PHASE 2

(A CASE STUDY)

NUR ANIS AD LINA BINTI MAT NOOR

Report submitted in partial fulfillment of the requirements

for the award of Bachelor of Civil Engineering

Faculty of Civil Engineering and Earth Resources

UNIVERSITI MALAYSIA P AHANG

JUNE 2012

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ABSTRACT

Soft soils are not suitable for construction of buildings or facilities without

having soil improvements. The soil improvements that are used in the chosen site are

Preloading and Prefabricated Vertical Drain (PVD). Analysis of settlement is

important to evaluate the site before the construction begins. The objectives of the

study are, to predict settlement of an embankment based on Terzaghi One­

Dimensional Consolidation analysis, to obtain the final settlement by using Asoaka's

method based on monitoring data and to compare the soil properties from the lab

data with back calculated based on the firn:il settlement. Using Terzaghi One­

Dimensional Consolidation Method, prediction of the total settlement is 287mm. The

data from monitoring record are used to plot Asoaka's Graph in order to get the

actual settlement which is 13mm, then being used to back calculate soil properties.

The soil properties those are back calculated based on field settlement are different

from those obtained from lab data. The differences can be seen by the correlation

between the field and lab, which are for the Coefficient of Vertical Consolidation, Cv

field = 0.04 Cv lab, the correlation for Coefficient of Horizontal Consolidation, ch

field = 0.04 ch lab, the correlation for the Compression Index, Cc field = 0.043 Cc lab

and the correlation of Recompression Index, Cr field = 0.06 Cr lab. This particular

correlation can be used as a design guideline to similar site condition.

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ABSTRAK

Tanah lembut tidak sesuai untulc pembinaan bangunan atau kemudahan tanpa

kaedah pembaikan tanah. Pernbaikan tanah yang digunakan di tapak yang dipilih

adalah Pra-Pembebanan dan Saliran Pugak Pasangsiap. Analisis pemendapan penting

untuk menilai tapak sebelum pembinaan bermula. Objektif-objektif kajian ini adalah,

untulc rneramalkan pemendapan tambakan berdasarkan Analisis Pengukuhan Satu

Dimensi Terzaghi, untulc mendapatkan pemendapan sebenar dengan menggunakan

kaedah Asoaka berdasarkan data pemantauan dan untulc membandingkan sifat-sifat

tanah dari data makmal dengan kiraan semula berdasarkan pemendapan sebenar.

Menggunakan Analisis Pengukuhan Satu Dimensi Terzaghi, pernendapan yang

diramalkan adalah 287mm. Data daripada rekod pemantauan digunakan untulc

mendapatkan Graf Asoaka untuk mendapatkan pemendapan sebenar iaitu 13mm dan

seterusnya digunakan untulc kiraan semula sifat-sifat tanah. Sifat-sifat tanah yang

dihitung semula berdasarkan pemendapan tapak adalah berbeza berbanding

pemendapan yang diperolehi di makmal. Perbezaan sifat tanah dapat dilihat daripada

korelasi di antara hasil kajian di tapak dan di makmal, di mana untuk Pekali

Pengukuhan Menegak, Cv tapak = 0.04 Cv makmal, korelasi Pekali Pengukuhan

Mendatar, ch tapak = 0.04 ch makmal, korelasi bagi Indeks Pemampatan, Cc tapak =

0.043 Cc makmal dan korelasi Indeks Pemampatan Semula, Cr tapak = 0.06 Cr

makmal. Korelasi ini boleh digunakan sebagai garis panduan reka bentuk untulc

tapak yang mempunyai keadaan sama.

TABLE OF CONTENTS

TITLE PAGE

DECLARATION

DEDICATION

ACKNOWLEDGEMENT

ABSTRACT

ABSTRAK

TABLE OF CONTENTS

LIST OF TABLES

LIST OF FIGURES

LIST OF SYMBOLS

LIST OF APPENDICES

CHAPTER!

CHAPTER2

INTRODUCTION

1.1 Introduction

1.2 Problem Statement

1.3 Objectives

1.4 Scope and Limitation

LITERATURE REVIEW

2.1 Introduction

2.2 Settlement

2.2.1 Immediate Settlement

2.2.1 Primary Consolidation

2.2.3 Secondary Compression Settlement

2.3 Consolidation

2.3.1 One Dimensional Consolidation Theory

2.3.2 Degree of Consolidation

2.3.2.l Compression Index (Cc)

2.3.2.2 Recompression Index (Cr)

Page

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1

1

2

3

4

5

5

6

7

8

9

10

10

13

15

17

2.3.3 Terzaghi Theory of Consolidation 18

2.4 Pre-loading 19

2.5 Prefabricated Vertical Drains (PVD) 22

2.6 Prefabricated Vertical Drains and Preloading 27

2.7 Laboratory Test 27

2.7.1 One-Dimensional Consolidation Test 28

2.8 Settlement Prediction and Interpretation by 30

Observational Methods

2.8.1 Hyperbolic Method 30

2.8.2 Asaoka Method 31

2.9 Soil Investigation 32

2.9.1 Purpose of a Soil Investigation Program 34

2.10 Soil Improvement 34

2.10.1 Vibroflotation 35

2.10.2 Dynamic Compaction 35

2.10.3 Stone Columns 36

2.10.4 Compaction Piles 36

2.10.5 Compaction Grouting 36

2.10.6 Drainage Techniques 37

2.11 Field Instrumentation 37

2.12 Settlement/Heave Monitoring 37

2.12.1 Settlement Plate/Platform 38

2.12.2 Remote Settlement (Gauge Monitoring 39

Tubes)

2.12.3 Inductive Coil Gauge (Deep Settlement 39

Monitoring)

2.12.4 Borehole Extensometer (Deep Settlement 40

Monitoring)

2.12.5 Horizontal Inclinometer (Settlement 40

Monitoring)

CHAPTER3

CHAPTER4

CHAPTERS

METHODOLOGY

3.1 Introduction

3.2 One Dimensional Terzaghi Method

3.3 Consolidation Analysis Using Hansbo Method

3.4 Back Calculation of Soil Properties

3.5 Comparison of Result

3.6 Summary

ANALYSIS AND RESULTS

4.1 Introduction

4.2 Magnitude of settlement and time taken to reach 90%

consolidation by using Terzaghi's and Hansbo 's

Method

4.3 Analysis by Using Asoaka's Method

4.4 Analysis of ch by using Asoaka's Method

4.5 Back Calculation of Field Soil Characteristics

CONCLUSION AND RECOMMENDATION

5 .1 Introduction

5.2 Conclusions

5.3 Recommendations

42

42

44

44

44

45

45

46

46

47

48

49

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51

51

51

52

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LIST OF TABLES

TABLE NO TITLE PAGE

2.1 Correlation for Compression Index, Cc 16

2.2 Compression and Recompression of 17

Natural Soils

2.3 Time Factor as function of percentage 19

of consolidation

4.1 Predicted Settlement and Time Taken 48

4.2 Total Settlement from Asoaka Plot 48

4.3 Back Calculated Field Soil Properties 50

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LIST OF FIGURES

FIGURE NO TITLE PAGE

2.1 Influence Chart for Vertical Stress Embankment Loading - Infinite Extent 8

2.2 Consolidation Settlement 11

2.3 Concept of Pre-Loading and Surcharge 21

2.4 Odometer test for measuring consolidation behaviour 29

2.5 Notation and Terminology used for Oedometer Compression Curves

29

2.6 Hyperbolic Method to Predict Future 30

Settlement

2.7 Graphical Method of Asoaka 31

3.1 Methodology of Study 43

4.1 Simplified Soil Profile 46

e

Cc

Cr

(J 0

n

LIST OF SYMBOLS

Void ratio

Initial void ratio

Compression index

Recompression index

Coefficient of vertical consolidation

Coefficient of horizontal consolidation

Thickness of compressible layer

unit weight

Initial effective stress

Increment stress for filling material

Settlement

Time factor

Degree of Consolidation

Time interval

Equivalent diameter of soil cylinder

Equivalent diameter of drain

Drain spacing ratio

x

XI

LIST OF APPENDICES

APPENDIX TITLE PAGE

A Specification for Prefabricated Vertical Drains (PVD) 53

B Borehole Log for Borehole 11 54

c Borehole Location 55

D Generalised Soil Profile (Mainline) from CH.94,500 to 56

f'.H.9.S.200 E Settlement calculation using One

Dimensional Terzaghi's Analysis 57

F Sample Calculation of Settlement Using One Dimensional Consolidation in Theory Terzaghi 58

Method

G Calculation for Consolidation Analysis using Hansbo' s Method 60

H Sample Calculation of Consolidation Analysis Using 62 Hansbo's Method

I Calculation of Asoaka's Analysis 64

J Sample Calculation for Asoaka's Method 66

K Asoaka's Plot 69

L Settlement Record for Settlement Plate 71

1

CHAPTER I

INTRODUCTION

1.1 INTRODUCTION

In general, preloading and vertical drain are the most suitable for

predominantly fine-grained, inorganic high water-content and low-strength soils.

Typically, the site essentially are normally consolidated or only lightly over­

consolidated. The consolidation settlement of soft clay subsoil creates a lot of

problems in foundation and infrastructure engineering. The settlement of soft clay

soils also creates problems during the construction and after the construction has

done. Apart from low shear strength, the primary consolidation would take long time

to be complete due to its low permeability. To shorten this consolidation time,

vertical drains are installed together with preloading on a constructed embankment

that lay on soft clay layer. Vertical drains are artificially-created drainage paths

which can have a variety of physical characteristic.

Preloading techniques, particularly when used together with vertical drains,

are similar to other ground improvement techniques. They have great potential

benefit in a number of situations in geotechnical practice. Preloading with vertical

drains requires time for the dissipation of excess pore pressure and the settlement to

occur. This may be an important consideration in the overall design.

The function of Prefabricated Vertical Drain (PVD) is to allow drainage to

take place in both vertical and horizontal directions over a much shorter drainage

2

path so that the rate of consolidation can be accelerated and the consolidation time

can be greatly reduced. The primary use of PVD is to accelerate consolidation to

greatly decrease the settlement time of embankment over soft soils.

However, for a PVD to perform its function, an extra surcharge needs to be

used during this process. The surcharge will create extra stresses that will induce

extra pore pressure to dissipate easily via PVD this increasing settlement is a short

period. PVD are band shape (rectangular cross section) products consisting of a

geotextile filter material surrounding a plastic core.

1.2 PROBLEMSTATEMENT

Soil is a foundation for all type of construction structure which is located on

earth. For sites underlain by deep layers of fill or soft or loose soils, conventional

practice was to either remove and replace the unsuitable soils or bypass them with

expensive deep foundations. Today, in-situ improvement is a viable alternative and

in most instances proves to be the most economical means to mitigate an undesirable

situation.

Structures can stand firmly on a soil, and it needs a great strength to support

loads within the structure. The problem of stability and settlement of the soft soil are

the major challenge to engineer. In some areas in this country where soft' clay are the

major part of the soil, constructing a highway on it could be complicated and design

plan must carefully be made.

The high settlement of the soft clay is due to the one of the major factor that

is high compressibility properties of soft clay. This is happened from the fact that

soft clay are finer in particles and being too cohesive with the presence of water.

High settlement could affect the movement of the whole structure and it would be

ended up with the cracks and landslides.

3

The presence of water could have made the soil become unstable. It is

because, soft clay have the lowest value of permeability where water are hard to get

through it particles and this is the reason why soft clay have a high moisture content.

The soil particles have high tendency to bond closely with one another that make soft

clay become easily compressed when it's undergoing compaction activity.

Because of the weaknesses of the soft clay, the improvement of the soil

should be done. Some of the soil properties need to be assumed based on literature

review and previous projects because it cannot be obtained from the Site

Investigation. If the assumptions of soil properties are not accurate, it will affect to

the embankment and PVD design. To get the 90% consolidation time for the clay to

settled, it will take long time and to reduce the consolidation time, soil improvement

should be done.

1.3 OBJECTIVES

From the problem statements that are stated before, the objectives of the

study has been identified. The objectives of this study are:

1. To predict settlement of an embankment based on Terzaghi 1-D

consolidation analysis.

11. To obtain the final settlement by using Asaoka' s method based on

monitoring data.

111. To compare the soil properties that obtained from lab data with back

calculated based on the final settlement.

4

1.4 SCOPE AND LIMITATION

This project is using the site investigation and settlement monitoring data

collected from the construction of East-Coast Highway Phase 2, from CH. 91,800 to

CH. 103,240. The study was based on SI data collected from 1 borehole along the

CH. 94,500 to CH. 95,200. The borehole log data are used to determine general soil

profile. The case study is focusing on settlement criteria only.

5

CHAPTER II

LITERATURE REVIEW

2.1 INTRODUCTION

The construction of building, roads, bridge and harbours on soft clay are

facing the higher risk for settlement and stability problem. This has become the main

geotechnical problem in soft clay engineering. Brand & Brenner stated that soft clay

is defined as clay that has the shear strength less than 25kPa Pl. Soft clay cause many

problem for geotechnical engineers since it is highly compressible, high liquid limit

and high plasticity.

Clay. is define as soils particles having sizes below 2µm which can be

determine at site by its feel that is slightly abrasive but not gritty and clay also feel

greasy 121. Clays are flake shape microscopic particles of mica, clay minerals and

other minerals .

Clay is a common type of cohesive soil which has small particle size that

cannot be separated by sieve analysis into size categories because there are no

practical sieve can be made with the so small opening. Clay is said as a sub­

microscopic mineral particle size of soil which has the fine texture. When is clay

present in dominant proportions compare with silt and sand, the soil is described as

having a fine or heavy texture. Fine textured soils are plastic and sticky when wet but

hard and massive when dry.

6

Clay is said to be surface active which means that much happen on their

surface, and clay minerals are cohere to each other and adhere to larger minerals

particles. Their surface can absorb and holds water, organic compounds, plant

nutrients ion and toxic ions.

2.2 SETTLEMENT

Settlement of the subsoil supporting the embankment will take place during

and after filling. In carrying out stability analyses, it is necessary to estimate the

magnitude of settlement which occurs during construction so that the thickness of the

fill can be designed to ensure stability. An iterative process is required in the

estimation of settlement because the extra fill (more load) required to compensate for

settlement will lead to further settlement.

The two-dimensional consolidation can be solved numerically using solutions

ed by Terzaghi One Dimensional Consolidation analysis. The result of

settlement can be conveniently divided into three stages:

1. Initial/Immediate Settlement, Si

11. Primary Consolidation Settlement, Sc

iii. Secondary Compression, S5

The calculation of the total settlement is:

ST = S, + Sst + S;

Where:

ST = total settlement,

Sc = primary consolidation settlement,

Ss = secondary compression settlement,

S; = immediate settlement

(2.1)

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2.2.1 Immediate Settlement

The deformation of dry soil and of moist and saturated soils without any

change in the moisture content, it will cause the immediate settlement to happen. The

immediate settlement calculations are generally based on equations that derived from

the theory of elasticity.

Excess pore pressure will set up in the clay during the application of the load,

but relatively little drainage of water will occur since the clay has a low permeability.

Estimation of initial settlement can be carried out using elastic displacement

theory as:

Si= :L-1 (I.q)dh

Eu (2.2)

Where,

Si = immediate settlement

q = Applied Stress I Pressure on the subsoil

dh = Thickness of each layer

Eu = undrained elastic modulus of clay (Young's Modulus or

modulus of elasticity)

I = Influence factor

8

A useful chart is given by Osterberg and Shown in Figure 2.1. The chart

allows estimation of the initial settlement of the embankment.

.-..

" ..? 0 ~

" .... ~02011--~--1_._~fl'f-l----+...,..q-+--1+~,__-1..-1.....Lu...;~ :> :::: t:: -O·/'fl't----+-.:.....+-+++-i+...'--+--<.I-~~ q :Unit loo d

d-z .. f ·q

'001 ) 1 'oal{) 2 '64:00

of z

Figure 2.1: Influence Chart for Vertical Stress Embankment Loading - Infinite Extent

2.2.2 Primary Consolidation Settlement

With time, the excess pore water pressure dissipate as drainage occurs and he

clay undergoes further settlement due to volume changes as stress is transferred from

pore pressure due to effective stress. The rate of volume change and corresponding

settlement is governed by how fast the water can drain out of the clay under the

induced hydraulic gradients.

9

One dimensional primary consolidation settlement can be estimated using the

expression:

n [ Cr 0"'1

p Cc o-' vf? Sc= L --log--+--loa-- i

i=! 1 + eo 0"'1

VO 1 + eo t:> 0"'1

VC

Where,

Sc = Consolidation Settlement Magnitude

a ' vo = Initial vertical effective stress

a' vf = Final vertical effective stress

= <>' vo + L'-.a' v 2:: o" vc

a' vc = Preconsolidation Pressure I Yield Stress

Hi = Initial thickness of incremental soil layer i of n.

ea = Initial voids ratio

Cc = Compression Index

Cr = Recompression Index

2.2.3 Secondary Compression Settlement

(2.3)

Even after complete dissipation of the excess pore pressures and the effective

stresses are about constant, there will generally be further volume changes and

increased settlement which is termed as Secondary Compression.

Where:

Ss = Secondary Compression Settlement

Ca= secondary compression index

(2.4)

ep = void ratio at the end of primary consolidation

H1 = Initial thickness of incremental soil layer, i of n.

t = time for calculation

2.3 CONSOLIDATION

10

Consolidation is the process by which an increase in stress causes water to

flow out of the soil accompanied by volume reduction. Consolidation is a process

that occurs in clay and silt. With sands and gravels, pore water drainage from the

voids occurs almost instantaneously as load is applied and is not normally referred to

as consolidation. Any process which involves decrease in water content of a

saturated soil without replacement of water by air is referred to consolidation.

2.3.1 One Dimensional Consolidation Theory

Consolidation settlement is calculated based on the value of either the

coefficient of volume compressibility (rnv) or the compression indices (Cc and Cr).

Considering the layer of saturated clay of thickness H shown on Figure 2.2. Due to

construction, the total vertical of stress in an element layer of thickness dz at depth z

is increased by change in stress, f1(J '.

11

' --. ......... -.................................... .. . .. .6 ........ ....... ,. .. ..... ........ .... .

1 I t I I t I * i 1 J<r'

<!: t t t '

ul i

Figure 2.2 Consolidation Settlement

The one-dimensional consolidation theory, where the excess pore water

pressure (u), depth within the clay layer (z) and time (t) are related by the following

governing differential equation of layer i in any kth time section is [JJ:

OU o2u -=C-ot v oz2

Where:

Cv = Coefficient of vertical consolidation

z = depth within clay layer

t = time related

u = excess pore water pressure

(2.5)

Where Cv is the coefficient of consolidation and is defined by:

k c =--....,... v (mvrJ

Where:

Cv = Coefficient of vertical consolidation

mv = Volume of compressibility

r w =Unit weight of water

12

(2.6)

Additional stress (~11 ') will results in the increase of corresponds to l11 ' - l10 '

and a decrease in void ratio corresponds to ~e = e0 - e1.

By knowing the ratio of the change in void ratio to the change on the

effective stress the settlement of normally consolidated clay due to change of stress

(~l1 ') is given as

Where:

Sc = Settlement

Cc = Compression index

H = Thickness of compressible layer

l1' = Initial effective stress

~()'= Stress from the filling material

(2.7)

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2.3.2 Degree of Consolidation

The average degree of consolidation as a function of time factor for

Terzaghi's theory of consolidation by vertical flow can be expressed as:

c For Tv=~ < 0.2

H o

U, = J- : , ex{ ":T· J

Where:

Uv = Degree of consolidation

Tv = Time factor

Cv = Coefficient of vertical consolidation

H = Thickness of compressible layer

(2.8)

(2.9)

(2.10)

The coefficient of consolidation, Cv, can be obtained from oedometer tests at

the levels of effective stress similar to those anticipated under embankment loading.

Another reliable way to determine Cv is from field in-situ permeability tests together

with mv from laboratory oedometer consolidation tests:

kt c =....,....---,-

v (m.rw)

Where

k = permeability from field permeability tests

mv = coefficient of compressibility

Yw = density of water

14

(2.11)

The use of field values of k will give better representative effects of large

scale soil structure and permeability, not able to be reflected in laboratory tests. Since

the permeability and compressibility of the soil reduce with increase in effective

stress (under embankment loading), the value of Cv should be modified to reflect the

state of stress over the period during which settlement rates are being calculated.

Primary Consolidation Settlement as a function of time

U = 81 xlOO s

Where:

U = degree of consolidation (% ),

S = ultimate primary consolidation settlement,

S1 = primary consolidation settlement at time t

(2.12)