i
COMPARISON OF BACK ANALYSIS AND MEASURED LATERAL
DISPLACEMENT OF CANTILEVER DIAPHRAGM WALL
FAUDZIAH BTE SHUKOR
A project report submitted in partial fulfilment of the
requirements for the award of the degree of
Master of Engineering (Civil-Geotechnics)
Faculty of Civil Engineering
Universiti Teknologi Malaysia
MAY 2009
iii
DEDICATION
T O MY DEAR HUSBAND AND FAMILY
iv
ACKNOWLEDGEMENT
Formost, I wish to express my sincere appreciation to my supervisor, Professor
Dr.Aminaton bt. Marto. She has given me her full patient, advice, guidance, comments
and her valuable time in assisting me to complete the project. She has assisted me in my
difficulties of getting the work done. Without her patience and guide I feel I may not be
able to complete the project in the time schedule. Thank you again to my dear Professor
Dr.Aminaton.
I would like also to express my gratitude to Ir.Law Kim Hing who has provided
me the data for the case study. He has also assisted me in trouble-shooting during the
running of the analysis.
My sincere thanks also to my other colleagues, friends and lecturers who have
some how or rather been directly or indirectly helped me in the project.
Finally, my gratitude to my husband and children who have given me courage and
strength to complete the project.
v
ABSTRACT
This study is aimed to evaluate the performance of a cantilever diaphragm wall
earth retaining system, constructed by staged excavation. The actual performance
extracted from geotechnical instrument is compared with the numerical method. The
numerical method is using Soil Hardening model. The simulation of the computer
analysis has been carried out using different soil stiffness parameter which have been
correlated from 2000N, 2500N, 3000N and 3500N values (with N is the Standard
Penetration Test values). The results obtained had been used to compare with the actual
performance of the field data. From the results of the finite element analysis, the obtained
lateral displacement profile is reasonably in close agreement when compared to the
instrumentation profile. Thus the soil stiffness parameters ( refE50 , ref
oedE , ref
urE ) which have
been used to correlate based on 3500N values are suitable for the soil at the site of Kenny
Hill formation, analysed in this project. The instrumentation data and the analyses have
yielded very useful information for deep basement construction in terms of the selection
of the soil parameters.
vi
ABSTRAK
Kajian ini adalah bertujuan untuk menilai prestasi sistem tembok penahan tanah
diaphragm kantilever dalam pembinaan pengorekan tanah secara berperingkat. Prestasi
sebenar tembok yang di perolehi dari alatan instrumentasi geoteknikal akan di
bandingkan dengan cara numerikal. Cara numerikal tersebut adalah mengunakan model
Hardening soil model. Kekuatan tanah yang di perbandingkani dengan nilai 2000N, 2500
N, 3000N dan 3500N (dimana N adalah nilai dari Standard Penetration Test) disimulasi
dengan mengunakan computer. Keputusannya digunakan untuk membuat perbandingan
dengan prestasi sebenar yang diperolehi dari data tapak. Keputusan dari simulasi
komputer , menghasilkan pergerakan tembok adalah menepati pergerakkan jika
dibandingkan dengan profil instrumentasi. Dengan itu parameter kekuatan tanah
( refE50 , ref
oedE , ref
urE ) dimana perbandingkan 3500N adalah bersesuaian untuk tanah Kenny
Hill formation.. Data dari instrumentasi dan analisis telah menghasilkan maklumat
penting dalam pemilihan parameter tanah untuk kerja pembinaan basemen.
vii
TABLE OF CONTENTS
CHAPTER
TITLE PAGE
1
TITLE PAGE
DECLARATION
DEDICATION
ACKNOWLEDGEMENT
ABSTRACT
ABSTRAK
TABLE CONTENTS
LIST OF TABLES
LIST OF FIGURES
LIST OF SYMBOLS
INTRODUCTION
1.1 Background of the Problem
1.2 Statement of Problem
1.3 Objectives of the Study
1.4 Scope and Limitation
1.5 Significant of the Study
i
ii
iii
iv
v
vi
vii
x
xi
xiii
1
1
2
3
3
4
2 LITERATURE REVIEW 5
2.1 Retaining Wall Type
2.2 Excavation Effects
2.3 Geotechnical Instrumentation
2.4 Previous Research In Deep Excavation
2.5 Elastic Properties of Soil
2.6 Applications OF Finite Element Analysis
2.6.1 Plaxis
2.6.2 Hardening Soil Model
5
5
7
7
12
14
16
18
viii
2.7 Design Approach
2.7.1 Stability Analysis
2.7.2 Free Earth Support Method and
Fixed Earth Support Method
2.7.3 Stress and Deformation Analysis
2.7.4 Settlement Induced by the
Construction of Diaphragm Wall
2.7.5 Excavation depth
2.7.6 Analysis of ground surface settlement
Induced by excavation
2.7.6.1 Peck’s Method
2.8 Mobilisation of Earth Pressure
2.9 Engineering properties of retained soils
2.9.1 Angle of Internal Friction
2.9.2 Cohesion
2.9.3 Unit Weight of soil
2.9.4 Active Earth Pressure
2.9.5 Passive Earth Pressure
2.9.6 At-Rest Pressure
21
21
24
24
27
28
29
29
31
31
31
32
33
33
34
35
3 RESEARCH METHODOLOGY 36
3.1 Introduction
3.2 Technical Literature
3.3 Data Collection
3.4 Parametric Study
3.5 Analysis of Data
36
36
36
37
37
ix
4 CASE STUDY OF DEEP EXCAVATION IN
KUALA LUMPUR
39
4.1 Introduction
4.2 Project Description
4.3 Stratigraphy Profile
4.4 Field Instrumentation
4.5 Stage Construction
4.6 Finite Element Simulation
4.6.1 2-D Modeling
4.6.2 Soil Parameters and constitutive model
4.6.3 Parametric Study
39
39
40
42
44
47
47
47
50
5 ANALYSIS RESULTS AND DISCUSSION 52
5.1 Introduction
5.2 Influence Soil Stiffness ( refE50 , ref
oedE )
52
52
6 CONCLUSION 61
6.1 Introduction
6.2 Conclusion
6.3 Recommendations
61
61
62
REFERENCES
63
x
LIST OF TABLES
TABLE NO. TITLE PAGE
2.1
2.2
2.3
2.4
4.1
4.2
4.3
4.4
4.5
4.6
4.7
4.8
5.1
5.2
5.3
Typical Values in Terms of Top Movement Caused
from Rotation of Basement Wall at its Base
Empirical Equations for Es (Bowles, 1998)
Ranges of Es for Various Soils (Bowles, 1998)
A Correlation between the Angle of Internal Friction
and the Standard Penetration Test (Winterkom and
Fang)
Simulation of Construction Work
Soil Parameters for Hardening Soil Model
Typical Parameters for Hardening Soil Model
Stiffness Soil Parameters for Hardening Soil Model
(2000N)
Stiffness Soil Parameters for Hardening Soil Model
(2500N)
Stiffness Soil Parameters for Hardening Soil Model
(3000N)
Stiffness Soil Parameters for Hardening Soil model
(3500N)
Soil Stiffness Input in Soil Hardening Model
Comparison of the Back Analysis in the Numerical
Simulation with the Actual Performance for
Horizontal Displacement at the Final Stage, at the
Soil Surface.
Extreme Bending Moment and Shear Force Values
for the Different Soil Stiffness
Vertical Displacement Values at Final Stage for the
Different Soil Stiffness
6
13
14
32
44
48
48
49
49
49
49
50
52
60
60
xi
LIST OF FIGURES
FIGURE NO. TITLE
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
Deformed Mesh for Analysis of Sheet Pile Wall
(Bakker and Brinkgreve 1991)
Hyperbolic Stress-Strain Relations in Primary
Loading Standard Drained Triaxial Test.
(Brinkgreve et al,2004)
Definition of in Oedometer Test Results
(Brinkgreve et al,2004)
Overall Shear Failure Modes (a) Push-In and (b)
Basal Heave (Chang, 2006)
Free Earth Support Method (a) Deformation of
Rretaining Wall and (b) Distribution of Earth
Pressure (Chang, 2006)
Fixed Earth Support Method (a) Deformation of
Retaining Wall and (b) Distribution of Earth
Pressure (Chang, 2006)
Envelope of Ground Surface Induced by
Trenched Excavation (Clough and O’ Rourke,
1990)
Envelope of Ground Surface Settlement
Induced by Diaphragm Wall Construction (Ou
and Yang, 2000)
Relationship between Maximum Wall
Deflections and Excavation Depths (Ou
et,1993)
17
19
21
23
25
26
27
28
29
xii
2.10
2.11
2.12
2.13
3.1
4.1
4.2
4.3
4.4
5.1
5.2
5.3
5.4
5.5
5.6
5.7
Peck’s Method (1969) for Estimating Ground
Surface Settlement
Mohr Circle Diagram (Macnab,2002)
Rankine Diagram Active Pressure
(Macnab,2002)
Rankine Diagram Passive Pressure
(Macnab,2002)
Methodology Flow Chart
Diaphragm Wall Elevation and Llocation Plan
Geological Map of Malaysia
SPT-N Values Profile
Stages of Construction and Simulation on
Hardening Soil Model
Horizontal Wall Displacement versus Wall
Depth for 2000N
Horizontal Wall Displacement versus Wall
depth for 2500N
Horizontal Wall Displacement versus wall
depth for 3000 N
Horizontal Wall Displacement versus Wall
depth for 3500N
Comparison of Horizontal Wall Displacement
for 2000N, 2500N, 3000N and 3500N with
Measured Field Data,at the Final Excavation
Stage
Typical Profile for Computed Shear Force
Diagram for the Diaphragm Wall
Typical Profile for Computed Bending Moment
Diagram for the Diaphragm Wall
30
32
34
34
39
40
41
43
46
53
54
55
56
57
59
60
xiii
LIST OF SYMBOLS
A Cross sectional area
'c Effective cohesion
uc Undrained shear strength
d Distance from wall
E Young’s modulus
refE Young’s modulus for Mohr-Coulomb model
refE50 Secant stiffness in triaxial test
ref
oedE Tangent stiffness in oedometer test
ref
urE Unloading reloading stiffness
sE Young’s modulus of sand or clay
f Yield function
H Total depth of excavation
I Moment of inertial
pI Plasticity index
NC
oK oK value for normal consolidation
xk Permeability in horizontal direction
yk Permeability in vertical direction
L Width of excavation
m Power for stress level dependency stiffness
n Porosity
'P Mean effective stress
refp Reference stress for stiffness, p
ref = 100 kpa
xiv
q Deviator / Deviatoric stress
qc Tip resistance of cone penetration test
RL Reduced level
fR Failure ratio
erRint Interface reduction factor
SPT’N’, N Standard penetration test N value
u Pore water pressure
Lw Liquid limit
'φ Effective friction angle
uφ Undrained friction angle
ψ Dilatancy angle
τ Shear stress
σ 1 Effective stress
σ Total stress
'
3
'
,2
'
1 , σσσ Major, intermediate and minor principal stress, respectively
ν Poisson’s ratio
urv Poisson’s ratio for unloading reloading
aεε ,1 Axial strain
γ Soil unit weight
unsatγ Soil unit weight�
satγ Soil unit weight
1
CHAPTER 1
INTRODUCTION
1.1 Background of Problem
The most cost effective and practical method to support an excavation is to
slope back the sides of the excavation. The design requires only the soil properties of
the excavated area so as to determine the angle of reponse. The concept of the design
requires wider space for the proposed slope and the excavation may not be deep.
Another setback will be the excavated area is near to the adjacent building. The
subject of the construction for deep excavation become more complicated, when
there is no adequate space, slope cannot be accommodated and therefore an earth
retaining system is the solution. The three basic types of earth retaining systems are
cantilevered, braced or tied-back system.
The design of the retaining wall system can be from a simple empirical
method towards a more complicated complex computer analysis. Whatever the
method, the design aspects are the stresses, loads related to the wall system and the
effect of construction method. Other than those design methods, the designer past
experience is significant too. The designer must be equipped with the subject
literature, aided with the geotechnical journals and texts for him to apply an
appropriate solution from many options to excavation support problem.
However in the increasingly competitive environment where “value
engineering” is required followed by the reluctance of the client to invest in
“geotechnical cost”, the designer is required to produce a design with minimal cost.
Whatever the preferred solution from the many available solutions, the risks involved
have to be evaluated and any failures of the retaining wall must not happened. The
2
preferred solution must therefore produce safe and economical design taking all
aspects into consideration.
1.2 Statement of Problem
The occurrence of ground displacement due to excavation works in deep
excavation is based on many factors such as the stratigraphy, soil properties, lateral
earth pressure, the method of constructions, contractual matters, soil loads, water
table, seepage problem and workmanship. These factors need to be considered in the
design procedure to understand the ground response due to excavation. Related to the
deep excavation, other important aspect is the evaluation of the foundation for the
adjacent properties and the effects of the excavation on the serviceability of the
adjacent structures.
The subsoil stratigraphy is generally obtained by auger deep boring supported
with Standard Penetration Test result from borehole. With the data taken from the
multiple boreholes and then drawn in the geometry, can lead to understand the soil
stratigraphy. The soil properties such as friction angle, cohesive intercept, and
poisson’s ratio are required for any design method of earth retaining structure
whether using numerical or simplified method.
This data used in the design process is confirmed by the instrumentation
monitoring on the behavior of the earth retaining system. Both the predicted and
actual behavior of the retaining system must be the same during/and at end of the
construction period. The need to monitor the behavior of the earth retaining wall in
conjunction with the stage of excavation which is required to evaluate the actual
behavior is similar to the predicted or as designed. If otherwise immediate action is
necessary to resolve the problem cause by the unexpected behavior of the supported
soil. This may be due to the construction nearby causing the change in the water
level or wrong interpretation of soil data.
This study is aimed to evaluating the performance of a cantilever diaphragm
wall earth retaining system, constructed by staged excavation. The actual
3
performance extracted from geotechnical instrument will be compared with the
numerical method. The numerical method is using Soil Hardening model.
1.3 Objective of the Study
The objectives of the study are as follows:
i) To determine soil properties and the standard penetration test (SPT-N)
values of the soil layers at site location.
ii) To determine the correlation between the soil stiffness parameter with
the field SPT-N values by comparing the monitoring measurement
results with the values obtained from the finite element analysis result
using Hardening Soil model for the lateral displacement of cantilever
diaphragm wall.
iii) To determine the wall deflection using the previous researchers
correlation of soil stiffness and SPT-N values.
1.4 Scope and Limitation
This case study has been conducted on particular project, which is represent
by a development with a 7 m depth basement at a site somewhere in Kuala Lumpur.
The detail of the subsoil parameters, existing and proposed soil platform, the stages
of excavation, the retaining wall system and the on-site instrumentation data has been
used in the case study. The analysis has been carried out with numerical method
using Finite Element Analysis by the aid of computer program (Plaxis). The lateral
displacement results compare with the actual behaviour of the retaining wall system,
taken from the instrumentation data. However, the instrumentation data used was for
the horizontal deflection of the wall only. The limitation of using the Plaxis analysis
is limited to Soil Hardening Model using the available soil data obtained from the
available borehole results.
4
1.5 Significant of the Study
This study will be very useful to geotechnical engineers to be used as
reference for analyzing the stability of the retaining wall system with respect to the
lateral movement created during construction. The outcome of the study will show
that the design and construction method of the project will be used as guidance to
similar condition of other construction sites for prediction of horizontal movement.
The soil parameter for the individual soil type can be used as a guide to all designers
with similar condition. The design model is also significant whereby it can be used
for similar condition.
5
CHAPTER 2
LITERATURE REVIEW
2.1 Retaining Wall Type
Deep excavation earth retaining is divided into three systems namely
cantilever wall system, braced system and tied-back system. The designer need to
understand this system so as to evaluate the appropriate system to be adopted. The
choice of support system depends on the soil type, depth of excavation, availability
of space, budget, its advantage and disadvantage (Chang, 2006)
The cantilever wall system is most suitable at shallow depth where the lateral
earth pressure and water pressure is not a problem. Terzaghi and Peck (1967)
considered that excavation whose depth were less than 6 m are defined as shallow
excavations and those deeper than that as deep excavations. In related to the deeper
depth, a much reliable retaining wall system is needed, where it can be tied back or
strutted. In certain design, the construction adopts temporary tieback during the earth
excavation where it is released once the permanent lateral support is provided by the
structure. This type of construction is adopted in the top down construction
procedure.
2.2 Excavation Effects
There are many factors affecting the excavation works. The designer must
truly understand the factors both in theoretical and practical applications. It is a most
specialized work involving understanding of soil types and behaviour, soil forces and
6
subsurface profile. The parameters taken shall be most appropriate and to much
accuracy.
The earth retaining wall needs to be restrained at the top to avoid horizontal
soil yielding. Once the wall moves, it indicates the soil shear resistance occurs. The
studies done by Raj (1999) relate the top translation movement due to the rotational
response of the base of the wall with regard to the development of earth pressure.
Thus indicate that the lateral force is related to the active earth pressure. Typical
values in terms of top movement caused from rotation of basement wall at its base
are shown in Table 2.1.
Table 2.1: Typical Values in Terms of Top Movement Caused
from Rotation of Basement Wall at its Base (Raj,1999)
Soil and condition Amount of
translation
Cohensionless, dense
0.001 - 0.002H
Cohensionless, loose
0.002 - 0.004H
Cohesive, firm
0.01 - 0.02H
Cohesive, soft
0.02 - 0.05H
where H = height of basement wall in meter
7
2.3 Geotechnical Instrumentation
Geotechnical instrumentation is very significant in any deep excavation. It
monitors the response of soil and wall under excavation works at any stage. The
monitoring can be viewed carefully. The interpretation of the result is important to
the wall stability. If any problem occurred such as soil movement it can be
encountered and solved successfully during the construction period.
One of the commonly used instruments is the inclinometer. It measures the
deformation of the wall through the depth of the wall. It can be placed in the wall or
install in the subsoil immediately behind the wall. Usually it is extended 3 meter
below the wall toe to measure the movements. From the results obtained from the
inclinometer, the lateral movement profile can be used to validate the design and
make immediate measures for any mistakes or wrong assumptions done during the
initial design stage.
In wall monitoring, the instruments are used to monitor the deflection.
Brahana et al (2007) concluded that the lateral deflection for anchored walls ranged
from 0.1 % to 0.5% of the wall height. Their result of study, described that from the
instrumentation data, suggest that changes of support has no effect on the earth
pressure. Their findings were that the trapezoidal earth pressure is still applicable to
be used in both temporary and permanent structure.
2.4 Previous Research in Deep Excavation
Knowing the history of failures caused by excavation is of great importance.
The designer is able to understand the unpredicted nature of soil behavior and
properties under the condition of unloading and loading. Hence the designer can list
the do and don’ts and what criteria are of significant and which criteria have been
ignored unpurposely.
Poulos and Chen (1996, 1997) and Poulos (2002) involved in the case study
where an excavation had caused the nearby building sitting on pile failed. The wall
8
of the excavation was not supported causing soil-pile interaction failed due to
excavation induced ground movement, causing additional forces and moments to the
adjacent area.
Gue and Tan (2004) described a case study of a construction site
experiencing base heaving and the sheet pile wall system moved excessively causing
the pile integrity damaged. Through the back analysis, the 12 m long sheet pile
cannot retain the soil when the excavated depth exceeds 3.5 m level. Introduction of
props at the top of the retaining wall would have avoided the failure.
Another type of failure was the collapse of the 25 m deep excavation due to
the buckling of the bracing (Magnus et al 2005). The cause of failure was due to the
conceptual technical error when using the complex numerical model. Even though
observation method was established during the construction, its weakness was that
the performance of the excavation was not checked regularly.
�
The observation method was introduced by Peck (1969) to enhance the
development of geotechnical engineering with the approach of “learn as you go”. It
is a valuable application where in any excavation; expectation of uncertainty is due
to be met. The predicted value need to be evaluated during the excavation by the
actual performance.
Ikuta et al (1994) described a case study on the observation method, of a top
down construction method. The first few monitoring reading for prediction of ground
movement was taken to be used to improve in the predictions. This would optimize
the cost of the whole construction works resulted from optimizing the design due to
the availability of the instrumentation data. The numerical method has solved to
clarify the important parameter related to excavation. Therefore, the application of
these numerical models has led to increasingly useful information, knowledge and
experience. Thus allowing geotechnical design to be safe yet economical for any
construction works under high risk environment.
Liew et al (2008) described a case history involving the construction of a
two-storey basement. The scope of work was to evaluate the condition of a distressed
9
temporary shoring structure, find the probable causes and then propose options for
remedial. The original sequence of works was that the temporary shoring support
which was made up of 12 m long steel sheet piles were driven from RL 59.0 m, to
facilitate the installation of contiguous bored pile (CBP) which is 16 m long and 750
mm diameter After the installation of CBP, excavation proceeded from RL 54.6 m to
52.0 m. At this stage, ground stresses at the adjacent area were observed. There were
ground subsidence, tension cracks and the CBP showed some deviation. The cause of
deviation of the CBP wall was due to the over excavation of the pile cap excavation
works, without the support of the planned raking strut. To confirm this cause, the
finite element simulation was carried out. The results showed that analyses agreed
with the measured wall displacement. The removal of the passive berm and the over
excavation had reduced the lateral resistance of the sheet pile and mobilised the
strength of the retaining wall from serviceability state to ultimate limit state
condition. The deviated sheet pile wall induced additional lateral force to the CBP
wall and the high induced force had damaged the CBP pile, which cause effects to
the ground displacement. Other findings from this case study were as follows:-
i. The existing backfilling over the natural valley had created
perched ground water regime which lead to unfavourable
behavior of backfill.
ii. Filling over valley without proper compaction result in
unstable backfill.
iii. Desk-study of pre-development ground contours of the site was
crucial so as to indentify potential geotechnical problems.
Freiseder et al (1999) carried out hardening soil model to analyze the
deformation behavior of an excavation for a 6.3 m deep underground car park. The
numerical analysis was carried out prior to the construction so as to refine the
analysis during construction. The data from the early stages of construction obtained
from the insitu site measurements were utilized. The results from the analysis and
site measurement were compared. The numerical model became a valuable tool to
10
assess the ground distress at any stage of critical phase. The site is located at
Salzburg, Austria which was next to the main station and existing underground
railway line. The allowable displacement should not exceed 10 mm at the top of
diaphragm wall or the railway line. Therefore finite element analysis was carried out
using the obtained data from the initial stage of construction. The aim was to refine
the analysis further. In the analysis, the underground railway line was modelled to
account for the stress redistributions (the underground railway line was completed
two years before). The displacements were set to zero. The model was performed
using undrained condition and consolidation analysis. The consolidation analysis
allowed the dissipation of excess pore water pressure matching the actual state of
construction.
Tan (2007) in his paper described a case history of incorrect use of numerical
modeling of the excavation system that result in the under design of the diaphragm
wall. In the paper he described the importance of understanding the difference
between the effects of change in volume due to volumetric shear distortion, The
Mohr coulomb model shows no volume change due to shear The stress path response
to undrained loading or unloading following the constant mean effective stress (p’)
path. The soil model will fail at the point where the initial p’ value at the start of the
unloading sequence meets the Mohr Coulomb failure criterion. As for soft clays
(normally consolidated) that tend to shear under drained condition, the volume will
change and contract. For the undrained loading / unloading condition, the soil is
prevented from contracting thus develop large excess pore water pressure within the
soft soil, consequence is that the effective stress path of the soft clay in undrained
shearing is to curve back from the constant p’ line. This was the aspect of the soil
behavior that was not incorporated in the plaxis design of the project. The
underestimate value of the strength of the soft clays resulted in under-prediction of
maximum wall deflection by a factor of 2. The other effect of this error, had led to
undersize of the diaphragm wall and also inadequately of the penetration wall depth.
This caused redundancy in the design, whereby plastic hinge occurred between 9th
and 10th
level strut. After the removal of the upper soil, the plastic hinge was formed
and yielded the soil and triggered the total collapse of the excavation support system.
11
Liew et al (2007) performed a study to evaluate the effectiveness and
economical of the proposal design / solution for the construction of a 3-level
basement excavation (11m depth). The basement design was using semi top-down
method with permanent ring slab at B1 level. The wall thickness is 600mm thick and
the depth was 15m. The 325mm thick B1 slab was utilized as the temporary prop to
surrounding diaphragm wall until excavation works had completed. The ring slab
also resisted the lateral load from diaphragm wall. In the study, computer
programmed, plaxis was carried out for the analysis of deformation problems both in
soil and rock. The model adopted was hardening soil model using effective stress
analyses. The soil material is assigned as undrained condition. Consolidation analysis
was to simulate the actual soil behavior from undrained to drained condition due to
excess pore water pressure. The effective Young’s Modulus (E’) and the effecting
unloading / reloading Young’s Modulus (E’ur) are correlated based on 2000N and
6000N respectively (as recommended by Tan et al (2001)). As for the wall interface
elements, Rinter of 0.67 and 0.5 are commonly used for sandy and silty soil
respectively. According to Gaba, A. R. et al Rinter of 0.8 can be adopted for concrete
cast against soil (diaphragm wall) provided that the wall is not subjected to high
vertical load which can cause the wall to settle relative to the soil. The results of the
study found that the lateral wall displacement as initial cantilever stage of excavation
adopted soil stiffness 3000N which matched computer analysis result with the
measured wall lateral displacement. For further excavation stage, the soil stiffness
reduced gradually to 1850N at the last stage of excavation works. Both the measured
wall displacement and the analysis results matched well which showed that the
maximum wall displacement is about 2% of the maximum excavation depth (11 m).
The study concluded that the effective Young’s Modulus (E’) and the effecting
unloading / reloading Young’s Modulus (E’ur) which are correlated based on 2000N
and 6000N respectively are suitable to be adopted in Kenny Hill Formation residual
soil.
James et al (2008) in his paper gave an account of the commonly adopted
steel embedded walls and steel shores in Hong Kong with an illustration from three
case histories. One of his findings was that when using sectional properties of the
steel for retaining earth system, was more consistent and uniform than that of the cast
in-situ reinforced walls, because it would eliminate the uncertainty in bending
12
stiffness of the wall and provided a good basis for design assumptions such as soil
stiffness. The soil stiffness in Hong Kong was correlated to SPT-N values.
According to James et al (2006), back analysis of wall deflection at Lok Ma Chah
(LMC) spur line was conducted by Pan et al (2006) and the result of the analysis was
that the elastic moduli were found to be 1500N for fill, 4000N for coarse alluvium /
completely decomposed granite. Another similar study had been done at the Hong
Kong Polytechnic University (HKPU) site, in which one of the back analysis of the
wall deflection, showed that the correlations of SPT-N values for soil stiffness are
1500N for fill and marine deposits, and 4000N for completely decomposed granite.
However the author advised that the application of these factors in the design of
excavation must be carefully examined since there are other factors attributing to
settlement.
Yee et al (2008) described three case studies in Malaysia for the excavations
works in problematic soils such as marine clays and ex-mining soils overlying
limestone. In all the three cases; the earth retaining support systems were using deep
soil mixing technique. One of the sites was at Kuala Lumpur and the project was a 12
storey condominium with 2 levels of basement car park. A dry deep soil mixing
technique was used to support the stress sloped excavation along the 3 sides of sites.
The subsoil condition was that for the first 8m the soil comprised of loose silty sand
deposits and ex-mining soils. The underlain layer was the medium dense silty sand
layer overlying limestone bedrock. The water table was about 2 m from the ground
level. From the monitoring results, the wall deflection was less than 1% of the
excavation depth (without lateral props).
2.5 Elastic Properties of Soil
The stress – strain modulus sE is one of the elastic properties of soil. It can
be obtained from the slope (tangent or secant) of stress – strain curves from triaxial
tests. It can also be estimated from field tests. Triaxial tests, tend to improve the
value of sE , since the confining pressure “stiffens” the soil thus a larger initial
tangent modulus is obtained. Whilst unconfined compression tests tend to have a
13
conservative value of sE , i.e. the computed (usually initial tangent modulus) value is
small, thus �H being larger than the value obtained at in situ.
Due to high cost to obtain the data of sE value from the laboratory, the
standard penetration test (SPT) has been used frequently to obtain the stress-strain
modulus sE . According to Chang (2006), the equations in Table 2.2 give a number of
equations for the estimation of the sE values. Table 2.3 gives the ranges of sE for
various soil types.
Table 2.2: Empirical Equations for sE (Bowles, 1998)
Soil type SPT-N (kPa)
Sand sE = 500(N + 15)
(normally consolidated) sE = (15,000 ~ 22,000) In N
sE = (35,000 ~ 50,000) log N
Sand (saturated) sE = 250(N+15)
Sand (over consolidated) sE = 18,000+750N
Gravelly sand and gravel sE = 1,200(N + 6)
sE = 600(N + 15) N<15
sE = 600(N + 15)+2,000 N > 15
Clayey sand sE = 320(N + 15)
Silty sand
Soft Clay
sE = 300(N + 6)
Note
N - SPT-N value; I kPa = I kN/m2 = 0.1 t/ m
2
14
Table 2.3: Ranges of sE for Various Soils (Bowles, 1988)
Soil Type
sE
(Mpa)
Very Soft Clay
2 – 15
Soft Clay
5 - 25
Medium Stiff Clay
15 - 50
Stiff Clay
50 - 100
Sandy Clay
25 – 250
Silty Sand
5 - 20
Loose Sand
10 - 25
Dense Sand
50 - 81
Loose Gravel
50 - 150
Dense Gravel
100 - 200
Shale
150 - 5000
Silt
2 - 20
Notes:
The table only lists the possible range of sE .
sE of in situ soils is related to water conrent, density, stress history, etc
2.6 Applications of Finite Element Analysis
Applications of Finite Element started in the early 1970’s. Its applications
nowadays are tremendously used by geotechnical engineers to solve much complex
problems. Some of the applications are to predict ground movements, undertaking
parametric studies and to back-analyze past case-histories.
15
Palmer et al (1972) used finite element to evaluate the importance of different
parameters on the performance of a braced excavation at Oslo Subway Norway . A
unique technique by input many variables simulating various soil interaction and
behavior.. The soil deformation modulus was the most significant. The other
parameters were of nominal effect except for the wall and strut stiffness.
In the year 1977, Burland et al described the performance of the multi-
propped excavation at New Palace yard. The soil profile consists of sand and gravel
overlying stiff, fissured London clay. Their finding was that the cause of
approximately 50 % of the total ground movements both in vertical and horizontal
direction was due to the installation of the diaphragm wall and piles.
In the year 1983, Eisenstein et al performed the study on excavation in glacial
till, overconsolidated stiff, fissured silty-clay. The location was at Edmonton, Canada.
They ran finite element to analyze the lateral pressures, making many models on
stress-strain analysis-linear and non-linear elasticity using data derived from the
triaxial tests. Their observations were that the plane strain data and the stress path
using the triaxial data matched with the actual field performance of the
displacements. They described that the wall flexibility contribute to a reduction of
the lateral pressures and the ground movements.
Potts et al (1984) used finite element analysis to study the performance of a
propped retaining wall in deep excavation.. The study was on initial value of earth
pressures at rest. With a high value of Ko the prop force, bending moment were
substantial high thus exceeding the predicted value by the limit equilibrium
calculation. But with lower value of Ko, the bending moment and the prop forces
were smaller when compared with the value derived from the limit equilibrium
calculation which show a conservative value.
Parametric studies on the effect of soil/wall/prop stiffness and pre-excavation
earth pressures coefficient were investigated by Powrie et al (1991) on a deep
excavation of 9 m depth in stiff over consolidated boulder clay. The wall was
propped by a continuous slab. The analysis showed that the stiffness of the soil was
the major value that contributes to the deformations, not the wall flexural rigidity. As
16
for the bending moment, it was largely governed by the pre-excavation earth pressure
coefficient. The effect from the slab-wall connection was of lesser effect.
Ng and Lings (2000) performed simple models using a linear elastic-perfectly
plastic Mohr-Coulomb and a non-linear model Brick Model (Simpson, 1991) to
evaluate the effectiveness of these two models. The program simulated the
construction of a top down multi-propped construction in the stiff over consolidated
Gault clay at Lion Yard , Cambridge . The studies show that the Mohr-coulomb with
the “wished-in-place” wall was able to predict the deflection and the maximum
bending moment of the wall, in condition that the soil value was estimated perfectly.
The model could not estimate the ground deformation pattern. For the strut loads the
model estimated a very high value.
Law (2008) described the behaviour of a braced excavation at Kenny Hill
Formation. Finite element analyses were carried out to assess the effects of soil
stiffness parameters and the soil constitutive models using elastic-perfectly plastic
Mohr-Coulomb model and elasto-plastic Hardening Soil model implemented in
Plaxis. One of the findings was that the predicted wall deflections using the
correlation of 2500N was in good agreement with field measured data.
2.6.1 Plaxis
There are many computer aided programme in solving problem using finite
element numerical method. According to Burd (1999) the Plaxis programme started
at Delft University of Technology in early 1970’s when Peter Vermeer started to do a
programme of research on finite element analysis on the design and construction of
Eastern Scheldt Storm-Barrier in Netherland. He established the finite element code
called Elplast which was able to calculate the elastic-plastic plane using six-nodded
triangular elements, written in Fortran VV.
In the year 1982 Rene de Borst under the supervision of Pieter Vermeer ,
performed his master’s programme related topic on the analysis of cone penetration
test in clay. The study of axisymmetric led to the existence of Plaxis (Plasticity
17
Axisymmetry). The study was on a six-nodded triangles in the element. The setback
being the programme exhibit “locking” for incompressible model. This was later
improved by Sloan and Randolph (1982) highlighting the effect of “locking” was due
to the incompressibility constraints imposed to the nodal displacements. They had
made 15 – noded triangle thus increasing the number of nodes in the element. Their
findings benched the usage of 15-noded triangle as the simplest element for any
analysis in axisymmetric. This experiment had led De Borst and Vermeer to
implement the 15-noded triangle in Plaxis thus solving the problem of cone
penetrometer.
The development of Plaxis proceed with the problem to solve the soil-
structure interaction effects. This led to the study on beam element by Klaas Bakker
under the supervision of Pieter Vermeer. The outcome of the experiment using beam
element was applicable to flexible retaining wall and later application to the analysis
of flexible footings and rafts. Baker’s work formulated the implementation of 5-
noded beam element in Plaxis (Bakker et al (1990), Bakker et al (1991)). The 5-
noded beam element is compatible to the 15-noded triangular elements (has 5 nodes).
Baker’s work was novel for the invention of hybrid method introducing the
displacement of degree-of –freedom to the element behavior. The lack of degree of
freedom has made solution to reduce the number of variables thus simplified the
element . The early application of this is as shown in the Figure.2.1.
Figure 2.1: Deformed Mesh for Analysis of Sheet Pile Wall.
(Bakker and Brinkgreve 1991)
18
2.6.2 Hardening Soil Model
The Hardening Soil model is an advanced model used for simulating the
behavior of both stiff and soft soils (Schanz, 1998). Compared with an elastic –
perfectly model, the yield surface of the hardening plasticity model is not fixed in
principal stress space. It expands because of the plastic straining. The two types of
hardening are the shear hardening and compression hardening. In shear hardening,
the model is irreversible to strain due to primary deviatoric loading. As for
compression hardening, the model is irreversible to plastic strains due to primary
compression in oedometer loading and isotropic loading.
When soil are subjected to primary deviatoric loading, the soil shows a
decrease in its stiffness and develop an irreversible plastics strains. In a drained
triaxial test, the axial strain and deviatoric stress, can be expressed as a hyperbola.
This relationship between the axial strain and deviatoric stress, known as hyperbolic
model was initially formulated by Kondner (1963) and later by Chang et al (1970).
The model was then superseded by the Hardening Soil model. The Soil Hardening
model uses the theory of plasticity. It includes soil dilatancy, and the model has a
yield cap.
The distinct characteristic of the model are : plastic straining due to primary
deviatoric loading ( refE50 ), plastic straining due to primary compression ( ref
oedE ),
Elastic unloading / reloading (ref
urE ), failure according to Mohr-Coulomb model (c,φ
and ψ ), and stress dependent stiffness according to a power law (m).
The formulation of the Hardening Soil model is the hyperbolic relationship
between the vertical strain, 1ε , and the deviatoric stress q, in primary triaxial loading.
The drained triaxial test established yield curves, described by the following
Equations (2.1) and (2.2):
ai qqI
q
E
I
/1
−=− ε for qfq < (2.1)
19
f
iR
EE
−=
2
2 50 (2.2)
whereby aq is the asymptotic value for shear strength, iE is the initial
stiffness, qf is the ultimate deviatoric stress.
The relationship of these equations is plotted in Figure 2.2. The parameter
50E is the confining stress dependent stiffness modulus for primary loading and is
described by the Equation (2.3):
m
ref
ref
pc
cEE �
�
�
�
��
�
�
+
−=
φ
σφ
cot
cot'
3
5050 (2.3)
where refE50 is a reference stiffness modulus corresponding to the reference
confining pressureref
p .
aq
1− ε
Figure 2.2: Hyperbolic Stress-Strain Relations in Primary Loading for a
Standard Drained Triaxial Test (Brinkgreve et al, 2004).
Using plaxis, the ref
p is taken as 100 stress units as a default setting. The
actual stiffness depends on the minor principal stress, '
3r which is the confining
pressure in a triaxial test. '
3r is negative for compression. The value of stress
dependency is given by the power m. Simulation for soft clays, the power m is taken
E50
1 Eur
1
axial strain devia
toric s
tress I�
1 - �
3I
Failure line
fq
Asymptote
20
as equal to 1.0. Janbu (1963) reports values of m around 0.5 for Norwegian sands and
silts. Von Soos (1980) reports values ranges 0.5 < m < 1.0.
In Equation (2.1), the ultimate deviatoric stress fq , and the quantity aq are
defined as follows :
( )φ
φφ
snrcq f
−−=
1
sin2cot
1
3 ffa Rqq /= (2.4)
The minor principal stress, 1
3r , is usually negative. The ultimate deviatoric
stress fq is derived from the Mohr-Coulomb failure criterion. As value of deviatoric
stress, q is equal to ultimate deviatoric stress fq , the Mohr-Coulomb failure criterion
is satisfied, thus perfectly plastic yielding occurs. This is described in the Mohr-
Coulomb model. Failure ratio fR is the ratio between fq and aq , which is smaller
than 1. For plaxis, fR is equal 0.9 as the default setting.
The stress paths for unloading and reloading used stress dependent stiffness
modulus as follows .
m
ref
ref
ururpc
cEE �
�
�
�
��
�
�
+
−=
φ
σφ
cot
cot'
3 (2.5)
where ref
urE is the reference Young’s Modulus for unloading and reloading, with
respect to the reference pressure ref
p . In most cases it is appropriate to set ref
urE
equal to 3 refE50 ;this is the default setting used in Plaxis.
As 50E have ben defined by Equation (2.2) the oedometer stiffness has to be defined.
The equation as follows in Equation (2.6)
( )mpc
cref
oedoed refEE+
−=
φ
σφ
cot
cot'
1
(2.6)
21
When oedE is the target stiffness modulus as shown in Figure 2.3. ref
oedE is a tangent
stiffness at a vertical stress of refp=−
'
1σ .
1σ−
ref
oedE
ref
p 1
1ε−
Figure 2.3: Definition of ref
oedE in Oedometer Test Results
(Brinkgreve et al, 2004).
2.7 Design Approach
Chang (2006) expressed that failures or collapse of excavations are disastrous
and must be avoided. The failure may arise from the stress on the support system
exceeding the strength of the material, or from the shear stress in soils exceeding the
shear strength etc. Therefore in excavation design, various analyses have to be
performed as follows.
i) Stability Analysis
ii) Stress and Deformation Analysis
2.7.1 Stability Analysis
The method of analyzing whether the soils at the excavation are able to bear
the stress generated by excavation are called stability analysis. This includes the
shear failure analysis, sand boiling analysis and upheaval analysis. When the shear
stress at a point exceeds or equals to the shear strength of the soil at the point, the
22
point is in failure limit state. If all these failure points are connected a failure surface
is formed, which will cause collapse or failure to the excavation. This kind of failure
is called overall shear failure. The failure modes can be in form of push in or based
heave as shown in Figure 2.4. Push-in is caused by earth pressures and both the sides
of retaining wall are reaching the limiting state. The soil moved towards the
excavated zone until reaching the full-zone failure.
This analysis views the earth retaining wall as a free body, where equilibrium
between the external and internal forces is achieved. When push-in occurs, there are
different extents of movements of the embedded part of retaining wall. This varies
the earth pressure on the wall. To analyze this earth pressure, fixed earth support
method and the free earth support method are adopted.
The basal heave is formed from the weight of the soil outside the excavation
exceeding the bearing capacity of the soil below the excavation bottom. The soil
move causing the excavation bottom to heave so much that cause the excavation to
collapse (Figure 2.4).
Therefore when analyzing the basal heave, several possible heave failure
surfaces are analyzed, but the smallest factor of safety is the most probable critical
failure surface. When basal heave occurs, the soil surrounding the bottom of
excavation heaves. If the soil is a soft clay, the earth pressure on both sides of the
wall will reached the limiting state.
23
a) Wall Bottom “Kick-Out”
b) Failure Surface
Figure 2.4: Overall Shear Failure Modes (a) Push-In and (b) Basal Heave
(Chang, 2006)
24
2.7.2 Free Earth Support Method and Fixed Earth Support Method
For the push-in failure, the two methods to analyse these failures are free
earth support method and fixed earth support method. The free earth method assumes
that the embedment depth of the earth retaining wall is allowed to move under the
influence of lateral earth pressure is in the limiting state and the profile of the earth
pressure is as shown in Figure 2.5. As shown in Figure 2.6 the fixed earth support
method assumes that the embedment length of the retaining wall fixed at a point
below the excavation surface. Rotation occurs at this fixed point. Therefore when the
lateral earth pressure on the retaining wall is in the limiting state, the earth pressure
around the fixed point on the two sides of the wall, may not reach the active or
passive pressure, as shown in Figure 2.6. For a cantilever wall, the analysis method
of free earth support is not applicable because in the free earth support method there
is not fixed point at the embedded length of the wall. Therefore the passive and
active forces on the wall are not in equilibrium. As for the strutted wall, the free earth
support method can be applied. The strutted forces on the wall together with the
passive and active forces are in equilibrium. If applying the strutted wall using the
fixed earth support method, the wall depth will be too deep and not economical.
2.7.3 Stress and Deformation Analysis
The stress and deformation due to excavation, occured from either
unbalanced forces or construction defects. The greater the unbalanced forces, the
larger the movement of the soils. Therefore stress analysis is required for the design
of structural component. The deformation analysis is to analyze the wall deflection
and soil movement due to the excavation works so that adjacent buildings are not
affected.
Both the stress and deformation analysis methods for excavation apply
simplified and numerical method. As for the simplified method, it is in the form of
monitoring results of excavation from many case histories in which the results are
sort out into stress and deformation characteristic of wall and soils. These
characteristics are significant to assist in understanding the actual behavior of
excavation. The field measurement results also represent the results of every
25
important element on deformation. Thus, this can help in predicting for similar
excavation projects, in terms of soils condition and construction methods and designs.
a) Deformation of Retaining Wall
b) Distribution of Earth Pressure
Figure 2.5: Free Earth Support Method (a) Deformation of Retaining Wall and (b)
Distribution of Earth Pressure (Chang, 2006).
26
a) Deformation of Retaining Wall
b) Distribution of Earth Pressure
Figure 2.6: Fixed Earth Support Method (a) Deformation of retaining wall and (b)
Distribution of Earth Pressure (Chang, 2006)
27
.
2.7.4 Settlement Induced by the Construction of Diaphragm Wall
Clough and O’Rourke (1990) showed that the ratio of the maximum
settlement (due to the construction of diaphragm walls) to the depth of excavation is
0.15% according to field monitoring results as shown in Figure 2.7. Therefore it is
important to monitor soil settlement within the diaphragm wall so as to protect
adjacent properties.
Ou and Yang (2000) studied the settlement results from a construction site
using diaphragm wall at Taipei Rapid Transit System. The study showed that the
maximum settlement induced by a single panel is 0.05 Ht % ( Ht is the depth of the
excavation) and for several panels the settlement was 0.07 Ht % (Figure 2.8). The
maximum amount of total settlement was 0.13 Ht % and it occurred 0.3 Ht from the
wall. This value of total maximum settlement was less than that of Clough and
O’Rourke’s (1990) envelope (0.15 Ht %). The settlement was not significant at 1.5 -
2 Ht from the wall.
Figure 2.7: Envelope of Ground Surface Settlements Induced by Trenched
Excavation (Clough and O’Rourke, 1990)
28
Figure 2.8: Envelope of Ground Surface Settlement Induced by the Diaphragm
Wall Construction (Ou and Yang, 2000)
2.7.5 Excavation Depth
Ou et al (1993) studied the relationship between the deformation of
excavation and its depth (see Figure 2.9) many case histories showed that that the
deformation of the wall increases with the increased of the depth of excavation. As
for soft clay the deformation of the wall is greater than in sand. The maximum
deformation (d hm )can be calculated as in Equation (2.7)
d hm =(0.2-0.5%) Hc ( 2.7)
Where cH = excavation depth
The upperbound of the d hm value is applied for soft clay and lower bound is for
sand clay.
29
Figure 2.9: Relationship between Maximum Wall Deflections and
Excavation Depths (Ou et al,1993)
2.7.6 Analysis of Ground Surface Settlement Induced by Excavation
To predict the ground surface settlement there are four empirical formulas for
the solution as follows (Chang, 2006):
i) Peck’s method
ii) Bowle’s method
iii) Clough and O’Rourke’s method
iv) Ou and Hsieh’s method
2.7.6.1 Peck’s Method
Peck (1969b) proposed a method to predict excavation – induced ground
surrface settlement based on field measurement. The case studies were at Chicago
and Oslo sites, where he established curves relationship between the ground surface
settlement (dv) and distance from the wall (d) (Figure 2.10).
In this method the soil is divided into three categories which is related to the
soil characteristics as follows:-
30
Type I - sand and soft to still clay
Type II - very soft to soft clay
1. Limited depth of clay below the excavation bottom
2. Significant depth of clay below the excavation bottom but cbb NN <
Type III very soft to soft clay to a significant depth below the excavation
bottom and cbb NN ≥ .
Where bN is the stability number of soil ,is defined as γ ue SH / , where γ is
the unit at of soil, eH is the excavation depth, uS is the undrained shear strength of
soil, and cbN is the critical stability number against basal heave.
Peck’s method had results of case histories before 1969. The wall employed
was mostly steel sheet piles or soldier piles. Therefore the application of these
relation curves is not applicable to all excavation, but at present it is still being used
by engineers to ground surface settlement.
Figure 2.10: Peck’s Method (1969) for Estimating Ground Surface Settlement
31
2.8 Mobilisation of Earth Pressure
In the design of embedded retaining wall, the distribution and intensity of
mobilised earth pressure is important. It will affect the overall stability of the
retaining wall, the bending moments and the prop / anchor forces.
For a cohensionless soil, c’ = 0, and the limiting effective stress σ h’ is
defined as follows:
σ h’ = Ka . σ v’ or Kp . σ v’ (2.8)
σ v’ = � . (z-u) (2.9)
Where z is the depth below ground surface, u is the pore water pressure, � is
the bulk unit weight, Ka and Kp are the active and passive earth pressure coefficients.
2.9 Engineering Properties of Retained Soils
It is necessary to understand the basic concepts of the engineering properties
of soil retained by the earth retaining wall (Macnab, 2002). The properties are
derived both by measurement, some developed and other are derived. However they
form the input data for empirical and analytical methods.
2.9.1 Angle of Internal Friction
The angle of internal friction defines the increase in shear strength of the soil
with increasing confining pressure. It is derived by plotting a series of triaxial tests as
Mohr-Circles diagram. The asymptote of several Mohr-Circles is called the Mohr-
Coulomb envelope (see Figure 2.11). The angle of internal friction is the slope of the
Mohr-Coulomb envelope. It is most obvious in cohesionless soil especially sands and
gravels. For soft cohesive soil such as clay it will approach to zero. The angle of
friction can be obtained from cone penetration tests, or laboratory test derived from
undisturbed sample taken from the field. A correlation between the angle of internal
friction and the standard penetration test is as shown in Table 2.4
32
Figure 2.11: Mohr Circle Diagram (Macnab, 2002).
Table 2.4: A Correlation Between the Angle of Internal Friction and
the Standard Penetration Test ( Winterkom and Fang)
2.9.2 Cohesion
Cohesion is a property exhibited in fine grained soils such as clays and silts.
It is formed by the atomic attractive force tend to create shear strength in the
material with the absence of confining pressure. In the Mohr-Coulomb envelop, the
intercept at the shear strength parameter. It is pronounced in cohesive soil. In soft
clays the ø is almost negligible.
33
Sometimes cohensionless soil will stand vertically when cut. It exhibit the
characteristic of cohesive soil. Actually the sands and gravels are bounded together
by capillary attraction forces because of the presence of moisture content. The
cohesion are weak and this phenomenon is called the apparent cohesion. This type
of cohesion can also be due to the result of particle cementation caused by the
mineralogy or thixotropic action. The thixotropic action is due to the previous high
stress history of the soil. However the apparent cohesion will permit a vertical face
of an excavated cohensionless soil for a short period. The cohesion disappears once
the exposed soil dries out.
2.9.3 Unit Weight of Soil
It is the special volume of soil and expressed as weight per unit volume.
2.9.4 Active Earth Pressure.
Rankine in his theory of earth pressure is defined as a wedge of soil which
would move if not restrained. When a face was cut on soil, a wedge which was
defined by an angle measured from the vertical axis 45° - ø/2 from the toe of the
wall was caused by gravity trying to more upward and downward outlined in
Figure 2.12.
The gravitational force was counteracted by the shear stresses along the
line AB. The unbalance force is the function of the soil weight. Therefore the
coefficient of active earth pressure is derived as in Equation (2.10)
Ka = tan ² ( 45° - ø/2) ( 2.10 )
34
Figure 2.12: Rankine Diagram-Active Pressure (Macnab, 2002)
2.9.5 Passive earth pressure.
Rankine’s theory described that if a force was applied to a face of soil to
make it back into its original position, the force equilibrium would be the
summation of the gravitational load of the failure wedge plus the shear stress along
the line CD ( Figure 2.13). Therefore the coefficient of passive earth pressure is
defined as in Equation (2.11)
Kp= tan ² ( 45° + ø/2) (2.11)
Figure 2.13: Rankine Diagram-Passive Pressure (Macnab, 2002).
35
2.9.6 At-Rest Pressure
The at-rest pressure is function of the weight of the soil. The at rest
condition is when the excavated face of the soil is in place without the use of the
shear strength of the soil. The coefficient of at-rest earth pressure is designated as Ko
is derived approximately by Equation (2.12).
Ko = 1 – sin ø (2.12)
36
CHAPTER 3
RESEARCH METHODOLOGY
3.1 Introduction
The study consists of 4 phases. Firstly was to develop a case study
considering what the aims and objectives. Following that a literature reviews of the
said case study. Secondly, data collection for the said case-study has been carried out.
Thirdly, parametric study has been carried out to validate the analysis with the actual
performance at site. Finally, a conclusion has been for the study. Figure 3.1 shows
the flow chart of the study.
3.2 Technical Literature
The technical literature has been captured from geotechnical journals,
conference papers, previous thesis and relevant text books related to the study. The
literature review described in Chapter 2, highlight the factors to be considered in
deep excavations where the soil-structure interaction plays an important element to
be considered, other than the soil parameter itself. The literature also described the
causes of failure from past case-history, which will be a guide to present and future
designer pertaining to design of retaining wall.
3.3 Data Collection
The data was collected from a professional designer who was involved in the
project, for the construction of a Basement at Jalan Kia Peng. The data collected
were as follows:
37
i) Subsurface soil profile from borehole report, SPT-N value of different
stratigraphy soil layers, ground water profile and laboratory results.
ii) Monitoring data from instrumentation which comprised of inclinometer
readings for measuring the lateral displacement.
iii) Other details such as location map, layout of basement, diaphragm wall
details and layout of instrumentation.
�
3.4 Parametric Study
�
Parametric study has been carried out to correlate the soil-stiffness
parameters with SPT-N values using the Hardening soil model. By back-analysis
these parameters, the horizontal displacement of the wall has been obtained. The
results are compared with the actual displacement at the site. �
�
�
3.5 Analysis of Data
�
The SPT-N value obtained from the soil investigation report has been
analyzed to obtain the average SPT-N value in each soil layers. The SPT-N value is
used to correlate the soil stiffness i.e. at 2000N values, 2500N values, 3000N values
and 3500N (with N is the Standard Penetration Test values) values, in the Hardening
Soil model.
�
From the four models, the results of the lateral horizontal displacement has
been analysed and compared with the instrumentation result. The validation is to
observe which of the four numerical models give good agreement with the actual
performance of the wall. This choosen model gives the soil stiffness ( refE50 �
ref
oedE ��
ref
urE ) that can be adopted to same soil condition. Finally, a conclusion is made from
the whole study of the performance of the earth retaining wall with respect to the
back analysis of this case-study using Hardening Soil model.
38
Figure 3.1: Methodology Flow Chart
METHODOLOGY
Literature Review
• Journal , conference &
papers
• Thesis
Proposal Writing & Presentation
• Introduction
• Objectives of the study
• Literature review
• Research methodology
Data Collection
• Soil investigation report
• Field instrumentation data
• Site layout plan and detail drawing
Parametric Study
• Back analysis the data obtained to evaluate
the input soil parameters E values are in
good agreement with the actual results.
• Input the different E values of soils to the
model.
Study of Field Measurements
Data
• Data obtained is evaluated
Comparison Study
• Evaluate of numerical result and field data.
• Comparison of the E values of the different soil layer.
Conclusion
• Comments and recommendations
Completion of Study and Preparation for Presentation &
Submission of Report
39
CHAPTER 4
CASE STUDY OF DEEP EXCAVATION
4.1 Introduction
The case study is the construction of a cantilever diaphragm wall as the
retaining wall system for the basement. Data obtained from the field borehole test i.e.
the SPT-N values are evaluated and used to correlate with the parameter for soil
stiffness values, 2000N values, 2500N values 3000N values and 3500N values.
These values are used in Hardening Soil model to obtain the values of refE50 , ref
oedE
and ref
urE . The refE50 is equal to ref
oedE and the values ref
urE is equal refE 503 . These
inputs are used in the modeling and the results of lateral displacement are compared
with the actual data from the site. The comparison some how are evaluated whether
good agreement between the modeling and actual values, after evaluating the soil
stiffness values.
4.2 Project Description
The case study is the construction of 30 storey building with 2 storey parking
area. The site is located at Kuala Lumpur in a Kenny Hill Formation. The
excavation work has 8 stages of construction. The retaining wall system is a
cantilever reinforced diaphragm with wall thickness of 600 mm and the total depth
being 17 m. The final embedded length is 10 m. The layout of the site is as shown in
Figure 4.1.
40
7 m excavated area
10 m
diaphragm wall
Figure 4.1: Diaphragm Wall Elavation and Location Plan
4.3 Stratigraphy Profile
The geological map of Kuala Lumpur in Figure 4.2 shows that the site is on
the Kenny Hill Formation. The Kenny Hill formation is from the Permian to
Carboniferous in age. It consists of a series of horizontal interbedded shales, phyllite
Excavation area
KLCC
Inclinometer I 3
Diaphragm wall
LOCATION PLAN
PPPPLANPLAN
41
and quartzite, whereby it has experience the process of extensive weathering giving
the product of thin residual soil followed with a very thick weathered rock layer. In
between, quartz intrusions have made their way but in small thickness (Tan et al,
2007).
From the soil investigation report indicates that the top 6 m consist of
alluvial deposits of very loose to loose silty sand with SPT-N values of 8.
Underneath the layer is the medium dense silty sand with 3 meter thickness, having
SPT-N values of 15. The layer underneath the medium dense silty sand (6 m
thickness) is the dense silty sand of SPT-N values of 60. Underneath the layer is the
hard-strata of Kenny Hill formation with SPT-N value of 100. This hard strata is of
clayey silt material. The soil SPT-N values are as shown in Figure 4.3.
Figure 4.2: Geological Map of Malaysia
42
4.4 Field Instrumentation
Extensive instrumentation equipments were installed at the site. For this study,
the data obtained from inclinometers I-3 is used to verify the results obtained from
back-analysis by the numerical modeling.
The inclinometer I-3 was installed to monitor the lateral displacements during
and after construction. The inclinometer was located behind the wall.
43
Figure 4.3: SPT-N Values Profile
44
4.5 Construction Stage
The stages of construction in the excavation works are as shown in Table 4.1
These sequences of work are simulated in the back analysis. The cantilever wall is
17m length with 10 m embedded into the hard stratum.
Table 4.1: Simulation of Construction Work
Stage Depth (m) of Excavation Work Description
2 1.5 Excavate 1.5 m depth
3 1.5 Installation of 17 m wall
4 2.5 Excavate 1.0 m depth
5 3.5 Excavate 1.0 m depth
6 5.5 Excavate 2.0 m depth
7 6.5 Excavate 1.0 m depth
8 7.5 Excavate 1.0 m depth
9 8.5 Excavate 1.1 m depth
Stage 2
Excavation works to 1.5 m depth
Stage3
Installation of 17 m diaphragm wall
45
Stage 4
Excavation works to 2.5 m depth
Stage 5
Excavation works to 3.5 m depth
Stage 6
Excavation works to 5.5 m depth
46
Stage 7
Excavation works to 6.5 m depth
Stage 8
Excavation works to 7.5 m depth
Stage 9
Excavation works to 8.5 m depth
Figure 4.4: Stages of Construction and Simulation on Hardening Soil Model
47
4.6 Finite Element Simulation
Numerical modeling using the computer-aided program ‘PLAXIS’ was used
to simulate the sequence of stage excavation. The back analysis was performed to
verify the behavior of the wall in terms of horizontal displacement.
4.6.1 2D Modeling
The model was a 2-D analysis under plain strain conditions (Brinkgreve,
2002) utilizing 15 node elements. The model boundary conditions were fixed by
standard fixities whereby the side boundary, the x-direction is fixed and the y-
direction is free to move vertically. The bottom boundary was fixed from movement
in x and y direction. As for the initial stresses, it used gravity loading.
The model, as shown in Figure 4.4, was built as per site construction
sequence, with the removal of elements the same as the stages done on site. At initial
stage, the excavation was to a depth of 1.5 m below ground level followed by the
installation of the 600 mm thick diaphragm wall. This wall was activated as a beam
element in the model. Next the stages of excavation were made by removal of the
excavated element in the model. The processes of removal of the elements were
repeated to a depth of 7 m from the top of the wall (8.5 m below the existing ground
level.
4.6.2 Soil Parameters and Constitutive Model
The soil profile shows loose weathered soil before reaching the hard stratum.
The choice of model is the hardening soil-model. The soil properties were taken from
the laboratory test results. The SPT-N values for each layer of soil were taken as
average value after plotting them as shown in figure 4.3. The data for the soil
parameters are shown in Table 4.2 to Table 4.7.
48
Table 4.2: Soil Parameters for Hardening Soil Model
Symbol Unit S1 S2 S3 S4
SPT’N’ Blows/300mm 8 15 60 100
'c kPa 3 5 8 12
'φ [ ]0 28 30 33 35
urv [ ]− 0.2 0.2 0.2 0.2
Note: S1 soil at 1st layer
S2 soil at 2nd
layer
S3 soil at 3rd
layer
S4 soil at 4th
layer
Table 4.3: Typical Parameters for Hardening Soil Model
Symbol Unit S1 S2 S3 S4
ψ [ ]0 0
0 0 0
xk m/s 1 x 10-6
1 x 10-6
1 x 10-7
1 x 10-7
yk m/s 1 x 10-6
1 x 10-6
1 x 10-7
1 x 10-7
m [ ]− 0.5 0.5 0.7 0.7
refp kPa 100 100 100 100
NC
nK [ ]− 0.5 0.5 0.455 0.426
fR [ ]− 0.9 0.9 0.9 0.9
erRint [ ]− 0.8 0.8 0.8 0.8
49
Table 4.4: Stiffness Soil Parameters for Hardening Soil Model (2000N)
Symbol Unit S1 S2 S3 S4
refE50 kPa 1.60 x 10
4 3 x 10
4 1.2 x 10
5 2 x 10
5
ref
oedE kPa 1.60 x 104
3 x 104 1.2 x 10
5 2 x 10
5
ref
urE kPa 4.8 x 104
9 x 104 3.6 x 10
5 6 x 10
5
Table 4.5: Stiffness Soil Parameters for Hardening Soil Model (2500N )
Symbol Unit S1 S2 S3 S4
refE50 kPa 2.1 x 0
4 3.75 x10
4 1.5 x 0
5 2.5 x 10
5
ref
oedE kPa 2.1 x104
3.75 x104 1.5 x 0
5 2.5 x 10
5
ref
urE kPa 6.3 x 04
1.125x105 4.5 x10
5 7.5 x 10
5
Table 4.6: Stiffness Soil parameters for Hardening Soil Model (3000N)
Symbol Unit S1 S2 S3 S4
refE50 kPa 2.4 x 10
4 4.5 x 10
4 1.8 x 10
5 3 x 10
5
ref
oedE kPa 2.4 x 104
4.5 x 104 1.8 x 10
5 3 x 10
5
ref
urE kPa 7.2 x 104
1.35 x 105 5.4 x 10
5 9 x 10
5
Table 4.7: Stiffness Soil Parameters for Hardening Soil Model (3500N)
Symbol Unit S1 S2 S3 S4
refE50 kPa 2.8 x 10
4 5.25 x 10
4 2.1 x 10
5 3.5 x 10
5
ref
oedE kPa 2.8 x 104
5.25x 104 2.1 x 10
5 3.5 x 10
5
ref
urE kPa 8.4 x 104
1.57 x 105 6.3 x 10
5 10.5 x 10
5
50
The values for the Young’s Modulus were obtained by using a correlation of
the blow count from the SPT-N values as shown in Table 4.4 to Table 4.7. The
Young’s Modulus have been taken with the correlation based on 2000N values,
2500N values, 3000N values and 3500N values respectively.
The wall interface elements erRint of 0.8 was adopted as the wall (concrete cast
in soil) was not subjected to vertical load which will cause the wall to settle relative
to the soil. erRint is the ratio of crtitical state angle of shearing Ø’crit over the angle of
shearing at peak, Ø’peak.
4,6.3 Parametric Study
After a basic model has been created, parametric study is commenced by
simulating the back analysis with the same value for certain input parameters and
changing the value for certain input parameters.
In the back analysis, the soil stiffness has a considerable influence on the
lateral soil movement. The soil stiffness has to be modified in order to reasonably
match the measured ground movement. The soil stiffness adopted in the back
analysis using hardening soil model is tabulated in Table 4.8
Table 4.8: Soil Stiffness Input in Soil Hardening Model
Analysis
No
refE50
(Kpa)
ref
oedE
(Kpa)
refref
ur EE 503=
(Kpa)
1
2
3
4
2000N
2500N
3000N
3500N
2000N
2500N
3000N
3500N
6000N
7500N
9000N
10500N
Note : SPT-N value from soil investigation report.
51
In the simulation, the results from the correlation between the soil stiffness
( refE50 , ref
oedE , ref
urE ) with the SPT-N values are evaluated for its effect on the wall
horizontal displacement. These results are then compared with the actual lateral
deflection results obtained from the field monitoring measurement.
52
CHAPTER 5
ANALYSIS OF RESULTS AND DISCUSSION
5.1 Introduction
The back analysis results presented in this chapter included the lateral
displacement of the soil due to excavation.
5.2 Influence of Soil Stiffness (ref
E50 ,ref
oedE )
As described in the earlier chapter, the influence of soil stiffness ( refE50 , ref
oedE )
is very significant in the lateral movement of the wall. This has been simulated in the
parametric study by taking the values of the individual SPT-N values of the soil layer
multiplying by the factors 2000N, 2500N, 3000N and 3500N respectively.
Table 5.1 : Comparison of the Back Analysis in the Numerical Simulation with the
Actual Performance for Horizontal Displacement at the Final Stage, at
the Soil Surface
Excavation
Stages
Displacement results from different soil stiffness parameters
(m)
Instrument
Data
No I-3(m)
2000 N 2500 N 3000N 3500N
5
6
7
8
9
5.33 x 10-3
14.65 x 10-3
22.03 x 10-3
31.82 x 10-3
44.05 x 10-3
4.96 x 10-3
13.14 x 10-3
19.54 x 10-3
28.64 x 10-3
39.57 x 10-3
4.83 x 10-3
12.62 x 10-3
18.53 x 10-3
27.57 x 10-3
37.23 x 10-3
4.62 x 10-3
12.06 x 10-3
18.12 x 10-3
25.62 x 10-3
35.59 x 10-3
*
*
*
*
40 x 10-3
Note: * no data available
53
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0
8.5
9.0
9.5
10.0
10.5
11.0
11.5
12.0
12.5
13.0
13.5
14.0
14.5
15.0
15.5
16.0
16.5
17.0
0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040 0.045 0.050
Horizontal Wall Displacement (m)
Wa
ll D
ep
th (
m)
stage3
stage 4
stage 5
stage 6
stage 7
stage 8
stage 9
Final Excavation Level
Figure. 5.1: Horizontal Wall Displacement versus Wall Depth
for 2000N
54
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0
8.5
9.0
9.5
10.0
10.5
11.0
11.5
12.0
12.5
13.0
13.5
14.0
14.5
15.0
15.5
16.0
16.5
17.0
0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040
Horizontal Wall Displacement (m)
Wall D
epth
(m
)stage 3
stage 4
stage 5
stage 6
stage 7
stage 8
stage 9
Figure 5.2: Horizontal Wall Displacement versus Wall Depth
for 2500N
55
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0
8.5
9.0
9.5
10.0
10.5
11.0
11.5
12.0
12.5
13.0
13.5
14.0
14.5
15.0
15.5
16.0
16.5
17.0
0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040
Horizontal Wall Displacement (m)
Wall D
epth
(m
)
stage 3
stage 4
stage 5
stage 6
stage 7
stage 8
stage 9
Final Excavation Level
Figure 5.3: Horizontal Wall Displacement versus Wall Depth
for 3000N
56
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0
8.5
9.0
9.5
10.0
10.5
11.0
11.5
12.0
12.5
13.0
13.5
14.0
14.5
15.0
15.5
16.0
16.5
17.0
0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040
Horizontal Wall Displacement (m)
Wa
ll D
ep
th (
m)
stage3
stage 4
stage 5
stage 6
stage 7
stage 8
stage 9
Final Excavation Level
Figure 5.4: Horizontal Wall Displacement versus Wall Depth
for 3500N
57
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.05
Horizontal displacement (m)W
all
Dep
th (
m)
2000N
2500N
3000N
measured
3500N
Figure 5.5: Comparison of Horizontal Wall Displacement for
2000N, 2500N, 3000N and 3500N with measured field data,
at the final excavation stage
Table 5.1 compares the horizontal displacement at the surface, of the
measured data obtained from geotechnical instrument with data obtained from the
finite element analysis. The comparisons start from the fifth to the final stage of
excavation. The values of horizontal displacement decreases as the soil stiffness
increases. The measured displacement matches the displacement for soil stiffness at
2500N, but actually the profile for 3500N matches the profile for the measured data.
58
The maximum displacement at the surface for the measured data at the end of the
excavation works shows a value of 40 mm. Therefore the maximum translation of
the wall with the height of the wall (7 m) is 0.57 %. For the theoretical value
adopting the 3500N soil stiffness, whereby the maximum horizontal displacement is
35.59 mm, the maximum translation is 0.51%. These values of maximum translation
of the wall both theoretically and actual are categorized under cohesive and firm soils.
The lateral displacement results extracted from the analysis of the wall using
soil stiffness parameters ranges from 2000N, 2500N, 3000N and 3500N are shown in
Table 5.1 and Figure 5.1 to Figure 5.5. By discussing the four concepts of
deformation, analysis based on different soil stiffness (Figure 5.1 to Figure 5.5) show
that the increase in the soil stiffness will result in the decrease of the wall horizontal
displacement. Referring to Figure 5.5 at which the theoretical calculated wall
displacement based on the four different soil stiffness parameter has been plotted
against wall depth and compared to the actual wall lateral movement obtained from
the geotechnical instrument. The profile for 3500N matches with the actual lateral
deflection at the excavation depth at 0.5 m to 7.5 m. The measured values for the
lateral displacement at the excavation depth of 7.5 m onwards, are smaller than the
analysis values. These may possibly indicate that the soil is under higher stiffness
than the adopted stiffness value. At the top surface that is from the ground surface to
the excavation depth of 0.5 m, the curve for the lateral displacement obtained from
the measured data deviate significantly. These may possibly indicate some
disturbance.
Thus the value for the horizontal displacement for soil stiffness 3500N is in
good agreement with the field measurement. This shows that the design and
construction method of the project will be appropriate to similar condition of other
construction sites for prediction of horizontal movement. The soil stiffness parameter
for the individual soil type can be used as a guide to all designers with similar
condition.
59
94.00 96.00 98.00 100.00 102.00 104.00 106.00 108.00 110.00 112.00 114.00 116.00 118.00
20.00
22.00
24.00
26.00
28.00
30.00
32.00
34.00
Shear forcesExtreme shear force -190.24 kN/m
Figure 5.6: Typical profile for computed shear force diagram for the
diaphragm wall
92.00 94.00 96.00 98.00 100.00 102.00 104.00 106.00 108.00 110.00 112.00 114.00 116.00
20.00
22.00
24.00
26.00
28.00
30.00
32.00
34.00
Bending momentExtreme bending moment -792.30 kNm/m
Figure 5.7: Typical profile for computed bending moment diagram for the
diaphragm wall
60
Table 5.2: Extreme bending moment and shear force values for the different soil
stiffness
Soil Stiffness Extreme bending moment
kNm/m
Extreme shear force
kN/m
2000N 792.30 190.24
3000N 738.94 182.49
3500N 728.76 180.96
Table 5.3: Vertical displacement values at final excavation stage for the different
soil stiffness
Soil Stiffness Vertical displacement
(1 x 10-3
m )
2000N 3.95
2500N 3.10
3000N 2.36
3500N 2.25
Figure 5.6 shows that the typical profile for shear force diagram and Figure
5.7 shows the typical profile for bending moment diagram for the diaphragm wall.
Table 5.2 shows the values for the shear force and the bending moment diagram for
the different soil stiffness. Table 5.3 shows the values for the vertical displacement
for the different soil stiffness. From the tables and figures it shows that the values for
the bending moment, shear force and displacement decreases as the soil stiffness
increases. This is a normal phenomenon because the soil strength influences many
aspects of the excavation works. The computed shear and the bending moment
diagram can then be adopted for the design of the wall.
61
CHAPTER 6
CONCLUSION
6.1 Introduction
The simulations of the computer analysis using different soil stiffness
parameter have been correlated from 2000N, 2500N, 3000N and 3500N values. The
results are to compare with the actual performance of the field data.
6.2 Conclusion
Based on the results of the analysis, the following conclusions can be made:
i) From the results of the finite element analysis the obtained lateral
displacement profile shows reasonably close agreement with the
lateral movement obtain from the instrumentation. The soil stiffness
parameters ( refE50 , ref
oedE , ref
urE ) which have been used to correlate based
on 3500N values are suitable for the soil condition.
ii) The maximum obtained translation of the wall system with height of
7 m is 0.57% for the actual wall movement and 0.51% for the
theoretical calculated wall displacement adopting soil stiffness of
3500N. These values of maximum translation of the wall both
theoretically and actual are categorized under cohesive and firm soils
iii) The instrumentation data and the analyses have yielded very useful
information for deep basement construction in terms of the selection
of the soil parameters.
62
iv) The Hardening Soil model which has been adopted in the analysis is
appropriate for Kenny Hill residual soils.
6.3 Recommendations
The result of the study has basically modeled an excavation sequence of work
with soil stiffness parameters matching the actual site performance. The model can
be further improved by varying erRint . This parameter is also significant to simulate
the soil-structure interaction behavior so as to give a more accurate result of wall
movement related to the soil behind it.
63
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