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1rrrrnnr11 0000073590
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.
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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)
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IV
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IX
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XI
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1
2
3
4
5
5
6
7
8
9
10
10
13
15
17
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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)
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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
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42
44
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51
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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
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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
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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
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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
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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.
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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.
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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.
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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.
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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
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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.
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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.
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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
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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 '.
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' --. ......... -.................................... .. . .. .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)
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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
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(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:
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kt c =....,....---,-
v (m.rw)
Where
k = permeability from field permeability tests
mv = coefficient of compressibility
Yw = density of water
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(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)