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Numerical Analysis of a Trial Embankment on Soft Clay

August 2011 Msc. Project Dissertation Pagei of 54

Declaration

I hereby certify that this work is my own, except where otherwise acknowledged, and that it has

not been submitted previously for a degree at this, or any other university.

Name: Dateme Ibifubara Abam

Signature:




August 2011 Msc. Project Dissertation Pageii of 54

Acknowledgements

I would like to express my sincere gratitude to the Almighty God for life, and for the privilege he

has given me to complete this program successfully. I also express my profound appreciation to

my supervisor Dr. Mohammed Rouainia for his time, guidance, experience and knowledge

throughout this work. I¶m also thankful to Stylianos Panayides for his time, patience,

encouragement, and continual support from the start right up to the very end of this project.

My deepest appreciation is directed to my employer, Engr Mayne David-West and his associate

partner Nathaniel Frank Iboroma for funding my studies and their sacrifice towards my progress

as an Engineer. I also thank my uncle Dr T.K.S Abam, and my tutors Colin Davie, Jean Hall, and

Gaetano Elia for helping me as well in my progress as a geotechnical engineer. Finally, I will

always be indebted to my parents Sir Prof & Dame Dr (Mrs) D.P.S Abam, for their sacrifices

throughout my life. I am where I am today because of you all. Thank you so much.




August 2011 Msc. Project Dissertation Pageiii of 54

Abstract

Structures built on soft clay materials over the years have been a major challenge in the

construction industry. Natural soft clays tend to have significant anisotropy of fabric

generated by plastic straining during laboratory tests. Anisotropy can influence plastic

behaviour for soft clays. A Kinematic Hardening Soil Model which accounts for structure

anisotropy and destructuration effects is implemented in the 3D commercial finite element

package PLAXIS to study the behaviour of an embankment founded on soft structured clay.

The Teven Road trial embankment constructed in Eastern Australia which was founded on

a soft soil deposit has been used as a case study in this project. Coupled analysis of excess

pore pressure dissipation and displacements have been done to study the embankment

behaviour. Long term settlement and excess pore pressure predicted from the finite

element analysis have been compared with measured data from the site. Parametric studies

have also been performed on the initial degree of structure, permeability anisotropy,

interpolation exponent, destructuration parameter to understand how they affect the model

prediction using coupled analysis.

K eywords: Anisotropy, constitutive model, embankment, initial structure, pore pressure,

settlement




August 2011 Msc. Project Dissertation Pageiv of 54

NOTATION

parameter controlling relative proportions of distortional and volumetric destructuration

stiffness interpolation parameter

Youngs Modulus

parameter controlling rate of loss of structure with damage strain

critical state stress ratio

mean effective stress

ratio of sizes of bubble and reference surface

parameter describing ratio of sizes of structure and reference surfaces

initial value of r

slope of swelling line in ln : ln p compression line

slope of normal compression line in ln : ln p compression plane

poisson¶s ratio

stiffness interpolation exponent




August 2011 Msc. Project Dissertation Pagev of 54

Table of Contents

Declaration ................................................................................................................................. i

Acknowledgements ...................................................................................................................ii

Abstract .................................................................................................................................. iii

NOTATION ............................................................................................................................. iv

List of Figures ......................................................................................................................... vii

List of Tables............................................................................................................................. x

1. Introduction ......................................................................................................................... 1

1.1 Background .................................................................................................................. 1

1.2 Project Aim .................................................................................................................. 3

1.3 Project Objectives ......................................................................................................... 3

1.4 Project Layout .............................................................................................................. 3

2. Literature Review ................................................................................................................ 5

3. Methodology ..................................................................................................................... 11

3.1 Teven Road Trial Embankment................................................................................... 11

3.2 Material parameters .................................................................................................... 12

3.3 3D Model ................................................................................................................... 14

3.4 Boundary Conditions and Model Assumptions ............................................................ 16

3.5 Calculations ................................................................................................................ 17

4. Parametric Study................................................................................................................ 18

4.1 Analysis of Embankment Behaviour on Soft Structured Clay ...................................... 18

4.2 Effect of Initial structure on the Finite Element Analysis prediction of Embankment

Behaviour .............................................................................................................................. 26




August 2011 Msc. Project Dissertation Pagevi of 54

4.3 Effect of Permeability Anisotropy on the Finite Element Analysis prediction of

Embankment Behaviour ........................................................................................................ 29

4.4 Effect of the Stiffness Interpolation Exponent on the Finite Element Analysis

Prediction of Embankment Behaviour ................................................................................... 31

4.5 Effect of the Destructuration Parameter (k) on the Finite Element Analysis Prediction of

Embankment Behaviour ........................................................................................................ 34

5. Conclusion ......................................................................................................................... 37

References ................................................................................................................................ 39

Appendix

Table A 1 Predicted Values from FE Analysis of Excess Pore Pressures at PC2-1 .................... 42

Table A 2 Predicted Values from FE Analysis of Excess Pore Pressures at PC2-2..................... 42



Table A 5 Predicted Values from FE Analysis of Vertical Displacements at Point A ................. 42

Table A 6 Predicted Values from FE Analysis of Vertical Displacements at Point B ................. 43

Table A 7 Predicted Values from FE Analysis of Vertical Displacements at Point C ................. 43

Table A 8 Predicted Values from FE Analysis of Vertical Displacements at Point D ................. 43




August 2011 Msc. Project Dissertation Pagevii of 54

List of Figures

Figure 1.1. Model for destructuration of clays: (a) Cam-clay model: (b) bubble (yield surface)

and outer surface in bubble model; (c) reference surface, structure surface and bubble (yield

surface) for destructuration model; (d) deviatoric section through bubble and structure surface for destructuration model. This diagram has been reproduced from Fig 1. (D. Muir Wood 2000)..... 2

Figure 1.2. Project outline ........................................................................................................... 4

Figure 2.1. Finite element mesh and soil profile for the trial embankment (M.Rouainia 2005)..... 6

Figure 2.2 Longitudinal and cross sections of the Haarajoki test embankment. Cross section

35840, where no vertical drains were applied, is analysed. (Marcin Cudny 2003) ....................... 7

Figure 2.3. Finite element discretisation of the boundary problem. (Marcin Cudny 2003) ........... 7

Figure 2.4. Plan View of Trial Embankment Design. (N.Sivakugan 2005) .................................. 8

Figure 2.5. Construction History of Trial Embankment.(N.Sivakugan 2005) ............................... 9

Figure 2.6. Location of instrumentation below trial embankment (N.Sivakugan 2005) ................ 9

Figure 3.1. Soil strata beneath a section across Teven Road trial embankment .......................... 11

Figure 3.2. Construction sequence of the Teven Road embankment .......................................... 12

Figure 3.3. Plan view of the Teven road embankment on soft soil deposit. ................................ 15

Figure 3.4. 2D plan view mesh of Teven road trial embankment on the soft soil deposit ........... 15

Figure 3.5. 3D mesh of Teven road trial embankment on the soft soil deposit ........................... 16




August 2011 Msc. Project Dissertation Pageviii of 54

Figure 4.1. 3D representation of the deformed mesh of the trial embankment at the end of

consolidation period .................................................................................................................. 20

Figure 4.2. Excess pore pressure in the soil depost at end of each phase ................................... 21

Figure 4.3 Comparison of Finite Element analysis prediction for excess pore pressures at PC2-1,

PC2-2, PC3-3, and PC2-4 with measured data........................................................................... 22

Figure 4.4 Vertical displacements of the ground surface beneath the embankment at end of each

phase ........................................................................................................................................ 23

Figure 4.5. Horizontal displacements of the ground surface beneath the embankment at the end

of each face ............................................................................................................................... 24

Figure 4.6. Comparison of Finite Element analysis prediction for settlements at points A, B, C

and D with measured data ......................................................................................................... 25

Figure 4.7. Comparison of Finite Element analysis prediction for excess pore pressure dissipation

at PC2-1, PC2-2, PC3-3, and PC2-4 with measured data for different degrees of initial structure

................................................................................................................................................. 27

Figure 4.8. Comparison of Finite Element analysis prediction for settlements at Points A, B, C

and D with measured data for different degrees of initial structure ............................................ 28

Figure 4.9. Comparison of Finite Element analysis prediction for excess pore pressure dissipation

at PC2-1, PC2-2, PC3-3, and PC2-4 with measured data for different degrees of permeability

anisotropy. ................................................................................................................................ 29




August 2011 Msc. Project Dissertation Pageix of 54

Figure 4.10. Comparison of Finite Element analysis prediction for settlement at points A, B, C

and D with measured data for different degrees of permeability anisotropy ............................... 30

Figure 4.11. Comparison of Finite Element analysis prediction for excess pore pressure

dissipation at PC2-1, PC2-2, PC3-3, and PC2-4 with measured data for different values of . . 32

Figure 4.12. Comparison of Finite Element analysis prediction for settlement at points A, B, C

and D with measured data for different values of .................................................................. 33

Figure 4.13 Comparison of Finite Element analysis prediction for settlement at points A, B, C

and D with measured data for different values of destructuration parameter k ........................... 35

Figure 4.14. Comparison of FE prediction for settlement at points A, B, C and D with measured

data for different values of destructuration parameter k ............................................................. 36




August 2011 Msc. Project Dissertation Page x of 54

List of Tables

Table 3.1 Estimated Values used in the numerical analysis ....................................................... 13

Table 3.2 Coordinates for the boreholes shown in plan view of Teven road embankment .......... 14

Table 3.3 Summary of phase calculations .................................................................................. 17




August 2011 Msc. Project Dissertation Page1 of 54

1. Introduction

1.1 Background

Numerical simulations by means of the Finite Element Method have become a valuable

tool in geotechnical engineering to predict and to understand the behaviour of complex structures

and extensive research has been recently carried out in this area (Yildiz 2009). Embankments

and other construction works built on soft soil deposits have posed difficulties in

geotechnical design over the years. The behaviour of stress±strain for soft soils is very

complex as the soil response to loading the foundation is influenced by different basic

characteristics of natural soil behaviour which include creep, anisotropy and

destructuration (M. Karstunen 2006).

Low strength of the soft soil deposits limits the embankment height that may be

constructed on it with adequate safety for short term stability, and low permeability determines

large settlements that develop slowly as excess pore pressure dissipates in the soft deposit. Thus,

stability and time required for consolidation of the soft soil deposit are major considerations in

the design and construction of embankments over soft cohesive foundations (Yildiz 2009).

This project presents a numerical analysis of the ³Teven Road trial embankment´, whichwill be used as a case study in studying the behaviour of an embankment on soft clay deposit, by

comparing the excess pore pressures and settlements predicted from finite element analysis based

on laboratory results with field monitoring data from the instrumented embankment.

The Kinematic Hardening Structure Model (KHSM) developed by (Rouainia and D. Muir

Wood 2000) and implemented in the 3D commercial finite element package PLAXIS has been

used to model all the layers in the soil deposit. The KHSM is an extension of the well established

Modified Cam-Clay (MCC) model. Natural soft clays tend to have significant anisotropy of

fabric, developed during deposition and one-dimensional consolidation. During plastic

straining, as a result of the re-orientation of particles, the fabric anisotropy can change and

this inÀuences both elastic and plast ic behaviour of the material. The KHSM thus adds some

initial structure to the soft material which is lost progressively during plastic straining. The





KHSM introduces three surfaces which are the bubble surface, the structure surface and the

reference surface into the MCC model. The bubble surface moves with the current stress and

separates regions of elastic and plastic response. The structure surface stores the data about the

current anisotropy and magnitude of structure and acts as a bounding surface. This then reduces

to the reference surface as plastic straining occurs. The reference surface represents the

behaviour of the reconstituted material. All three surfaces change in size as plastic straining

occurs (D. Muir Wood 2000). Figure 1.1 shows the KHSM model for destructuration of clays.

Figure 1.1. Model for destructuration of clays: (a) Cam-clay model: (b) bubble (yield surface) and outer

surface in bubble model; (c) reference surface, structure surface and bubble (yield surface) for

destructuration model; (d) deviatoric section through bubble and structure surface for destructuration

model. This diagram has been reproduced from Fig 1. (Rouainia and D. Muir Wood 2000).





1.2 Project Aim

The aim of this project is to simulate the performance of a fully instrumented trial

highway embankment on thick soft clay deposit.

1.3 Project Ob jectives

In other to achieve the aim, the following objectives have been outlined for the project.

(i) Obtain material parameters to be used in the numerical analysis

(ii) Create a 3D finite element model of the embankment

(iii) Simulate the embankment construction on the soft soil deposit using the KHSM

constitutive model which accounts for anisotropy and destructuration effects in

finite element (FE) analysis.

(iv) Compare the settlements and excess pore pressures predicted from the FE analysis

and compare it with measured data obtained from the site.

1.4 Project Layout

This project consists of four chapters described in very brief terms below. A diagram of

the relationship between the chapters is shown in Figure 1.2. Project outline Chapter 1 gives a

background of the basic concepts, aim and objective of this project. Chapter 2 discusses related

work on this study. In Chapter 3, the geometry of the embankment and underlying soft soil

deposit is described. Following this is the soil parameters used in the numerical analysis for each

material in the soil strata of the deposit. Sequel to it is the layout and 3D model of the

embankment on the soil deposit. The boundary conditions used in the numerical analysis will

then be described, and finally the phase calculations following the construction sequence of the

embankment on site will be outlined. In Chapter 4, the embankment behaviour is analysed using

a coupled analysis of deformation and pore pressure dissipation. Parametric studies will be

conducted on the model to study the effect of effect of structure anisotropy, permeability





anisotropy, interpolation stiffness exponent, and destructuration parameter on the model

prediction of the embankment behaviour.

In Chapter 5 the conclusions drawn from the embankment behaviour, and parametric study will

be made.

Figure 1.2. Project outline





2. Literature Review

In an earlier study, (M.Rouainia 2005) investigated the influence of structure degradation

on the behaviour of embankments on soft soil. Using the fully instrumented trial embankments

which had been constructed on estuarine clay deposits on the Pacific Highway along the east

coast of Australia, the Kinematic Hardening Structure Model (KHSM) proposed by (D. Muir

Wood 2000) which is proficient in addressing the initial structure and destructuration process in

soft clays was implored to validate the settlement of the embankment by the finite element

method. The parameters which were used in the KHSM were calculated from a set of field and

laboratory data from soft soils in that region and in other to predict the consolidation behaviour

of the soft deposit, coupled analysis of excess pore pressure and displacements was used in the

finite element implementation. The left and right boundaries of the finite element mesh that was

used to describe the soils below the embankment shown in Figure 2.1 were restrained in the

horizontal directions, the bottom boundary was set to be undrained and the top boundary was set

to be free drained to zero pore pressure. As the embankment is symmetric, just half of the

embankment was simulated in the model and allowed to consolidate for 100,000 days. The

settlements at the ground surface as well as the excess pore pressures beneath the embankment

along some reference points were compared with the field measured data to access the level of

accuracy of the finite element model.

From the results, the displacements predicted by the finite element analysis using the

KHSM were very good in the overall. However, the excess pore pressures were under predicted

as a result of variable boundary in-situ conditions. It was concluded that since in soft clay

materials, the excess pore pressures and settlements are connected, using models that account for

destructuration processes were necessary to correctly study geotechnical problems with related

materials (M.Rouainia 2005).





Figure 2.1. Finite element mesh and soil profile for the trial embankment (M.Rouainia 2005)

(Marcin Cudny 2003) investigated the possibilities of a multi-laminate constitutive model

which accounts for structural anisotropy and destructuration effects with the 2D finite element

code Plaxis to simulate the behaviour of a test road embankment constructed on soft soil deposits

at Haarajoki, Finland. A spatially distributed anisotropic overconsolidation was introduced in the

multi-laminate model which combines the strength anisotropy with the characteristic mechanical

process of destructuration. The longitudinal and cross sections of the Haarajoki test embankment

are shown in Figure 2.2. The construction of the embankment was completed within 3 weeks

and was done in multi-stages of 0.5 m using a gravel fill. Excess pore pressures of about 3 to 10

kPa were measured in the deposit before the construction began. The part of the embankment

without ground improvement (cross section 35840) was analysed in the study.





Figure 2.2 Longitudinal and cross sections of the Haarajoki test embankment. Cross section 35840,

where no vertical drains were applied, is analysed. (Marcin Cudny 2003)

Material parameters for the main soft soil were estimated from available laboratory test

results and previous parameter sets with only the initial values of state variables which represent

bonding and overconsolidation varying with depth. Phase calculations which followed the

construction sequence of 0.5m high multi stages for the embankment were set to consolidate for

a total period of 1920 days were computed as fully coupled static/consolidation analysis. The

finite element discretisation of the boundary problem is shown in Figure 2.3.

Figure 2.3. Finite element discretisation of the boundary problem. (Marcin Cudny 2003)





Measured and calculated vertical displacements were compared at the centre line and at

the points located 4 and 9 m from the centre line. Horizontal displacements were compared along

the vertical profile located 9 m from the centre line. From the results, the horizontal settlements

were overestimated but the vertical displacements were good. Considering an assumed simplicity

of the initial field conditions and use of average material parameters for the deep soft soil

deposit, the final accuracy of the simulations was deemed acceptable.

(N.Sivakugan 2005) presented a case history of a trial embankment that was built on soft,

organic clay in the Sunshine Coast Motorway in South East Queensland, Australia. An

assessment of the efficiency of vertical drains to reduce the time of consolidation of the clay

deposit was done by studying the settlements and pore pressure below the trial embankment on

the soft deposit with the installed vertical drains. The trial embankment constructed had

approximately 40m width and 90m length and was studied in three parts A, B and C (see Figure

2.4). Part B had no prefabricated vertical drains, while part A had prefabricated vertical drains

with triangular grid patterns spaced at 1m, and part C at 2m.

Figure 2.4. Plan View of Trial Embankment Design. (N.Sivakugan 2005)

The embankment fill was a granular material which was placed in stages with the

construction sequence shown in Figure 2.5. The left side of the embankment was instrumented

with the locations shown in Figure 2.6 to obtain relevant data.





Figure 2.5. Construction History of Trial Embankment.(N.Sivakugan 2005)

Figure 2.6. Location of instrumentation below trial embankment (N.Sivakugan 2005)

The numerical analysis was performed using the finite difference program, FLAC, and

settlements predicted on the basis of a fully coupled Biot consolidation model. The bottom

boundary was considered to be rigid as the sand layer below the clay deposit was dense so that

any settlements connected with it could be ignored, while the lateral boundaries of the finite

difference mesh were positioned at 150 m from the centre of the embankment, and fixed in the

horizontal direction to minimize the effect. Both the top and bottom surfaces of the clay were

assumed as free draining, and the water table at the ground surface.





From the results obtained in the numerical analysis, the settlements and pore pressures

compared well with the field measured data. Furthermore, it was observed that the horizontal and

vertical permeability values on each of the models from field measurements were smaller than

those obtained from the laboratory testing.





3. Methodology

3.1 Teven Road Trial Embankment

The Teven Road trial embankment on the soft soil deposit which will be used as a case

study in this project is shown in Figure 3.1 The base of the trial embankment has a 54m width

and 84m length with a total height of 1.6m reached over a period of 69 days following a

construction sequence shown in Figure 3.2. The soil strata consist of four layers. The first is a

2m thick crust clay slightly sandy layer which is unsaturated and overconsolidated. Following

this layer is a soft estuarine clay layer about 8m thick. The third layer which is 2m thick is a

dense sand layer which is fine to medium grained and the last layer is a firm to stiff clay layer

which is 19m thick. The water level was measured as 1m below ground level. Locations of

settlement plates and piezometers are also shown in Figure 3.1.

Figure 3.1. Soil strata beneath a section across Teven Road trial embankment





Figure 3.2. Construction sequence of the Teven Road embankment

3.2 Material parameters

The model parameters for the finite element analysis have been obtained from a previous

study (Allman) which was determined from a set of field and laboratory data from soft soils in a

nearby site and listed in Tabl e 3.1. In this study, only the soft estuarine clay layer has been

modelled using the structure model in the KHSM. All other layers in the soil deposit have been

modelled with the bubble model, while the embankment is modelled with the Mohr-coulomb

model.





Table 3.1 Estimated Values used in the numerical analysis

Table 3.1 cont. Estimated Values used in the numerical analysis





3.3 3D Model

The 3D model of the trial embankment on the soft soil deposit was built in such a way

that simulating the staged construction sequence of the trial embankment on site shown in

Figure 1.1 could be achieved. This was done by creating a plan view of the embankmentconstructed in stages, and inserting boreholes at the vertices to describe the soil profile

and depths as shown in Tabl e 3.1. Figure 1.1 shows the plan view of the embankment on

the deposit. The coordinates for the outer boundaries and embankment are shown in

Tabl e 3.2

Table 3.2 Coordinates for the boreholes shown in plan view of Teven road embankment





Figure 3.3. Plan view of the Teven road embankment on soft soil deposit.

Figure 3.4. 2D plan view mesh of Teven road trial embankment on the soft soil deposit





Figure 3.5. 3D mesh of Teven road trial embankment on the soft soil deposit

3.4 Boundary Conditions and Model Assumptions

The boundary conditions for the problem were such that pore water was allowed to move

across the left and right boundaries in other not to affect horizontal flow conditions, and the

displacements were fixed. The bottom boundary is set to undrained with zero displacement. At

the ground surface, displacements were not restricted and free drainage conditions applied.

Boundary effects were controlled by choosing an area such that significant influence on the

results will be avoided. The global coarseness of the mesh size for the model was set to medium

for the horizontal element distribution, and fine in the vertical element distribution in other to

ensure reasonably accurate results as computing problems hindered the use of a fine mesh

entirely. Total number of elements was 10,160. Figure 3.4 and Figure 3.5 show the 2D and 3D

mesh of the Teven road embankment on the soft soil deposit.





3.5 Calculations

The calculations consisted of seven phases which was done in accordance with the

construction sequence on site earlier discussed. The entire height of 1.6m of embankment was

raised in stages, and allowed to consolidate in between stages.Tabl

e 3.3 shows a summary of the phase calculations, procedure, and phase time completion. The total consolidation time was

100,000days.

Table 3.3 Summary of phase calculations





4. Parametric Study

Coupled analysis of the dissipation of pore pressure and the displacement of the ground

surface beneath the embankment has been adopted in the finite element (FE) analysis to predict

the consolidation behaviour. Excess pore pressure ± time plots obtained from the FE analysis

were done at points PC2-1, PC2-2, PC3-3, and PC2-4 to analyse the dissipation of the excess

pore pressures in the soil deposit and compared with measured data. PC2-1, PC2-2 and PC2-4

are located at 3m, 5m, and 10m deep respectively from the centre of the embankment on the

ground surface, while PC3-3 is 7m deep, 15m away from the centre of the embankment as can be

seen in Chapter 3, Figure 3.1. Similarly, settlement-time plots obtained from the FE analysis

were done at points A (centre of the embankment), B (15m from the centre of the embankment),

C (toe of the embankment) and D (7m from the toe of the embankment) to study embankment

behaviour and compare the displacements of the ground level under the embankment with

measured data. These can be seen in Chapter 3, Figure 3.1.

Parametric studies have been carried out on the KHSM to understand the model

prediction of the embankment behaviour using the coupled analysis. The effect of initial degree

of structure, permeability anisotropy , destructuration parameter , and stiffness

interpolation exponent on the FE analysis prediction have been investigated and also

compared with measured data from the field.

4.1 Analysis of Embankment Behaviour on Soft Structured Clay

The 3D deformed mesh from the model prediction at the end of the phase calculations

following the construction sequence is shown in Figure 4.1 The dissipation of excess pore

pressures in the soil deposit is shown at the end of each phase in Figure 4.2 while the excess pore

pressure predicted from FE analysis are shown in appendix, Tabl e A 1± Tabl e A 4. The excess

pore pressure ± time plots from the FE analysis are compared with measured data and shown in

Figure 4.3. The measured data which was compared with the FE analysis prediction have been





obtained from a previous study by (Sheng 2006, et al ) using win-dig software and are shown in

appendix Tabl e A 9

As can be seen from Figure 4.2, the dissipation of the excess pore pressure begins and

continues at the end of each phase as the embankment is constructed. The pore pressuredissipation increases at end of each loading stage and hits its peak value at the end of

construction (phase 5). At the end of the consolidation period (phase 6), the pore pressure in the

soil deposit is completely dissipated. As can be seen from Figure 4.3, the FE analysis generally

over predicts the pore pressure dissipation apart from the FE analysis prediction at PC2-2,

although the general behaviour of the pore pressure is some-what same as that of the measured

data. This could be as a result of highly variable boundary conditions in situ. Again we notice

that a sharp peak value occurs in PC2-1, but smooth peak values in PC2-2, PC3-3, and PC2-4.

This can partly be attributed to the value of initial degree of structure, used in the modelling

the soft clay layer. The destructuration parameter could also be another factor as low values of

slows down the rate at which structure is lost with continuing strain and hence smoothens the

peak value (Rouainia and D. Muir Wood 2000).

The vertical and lateral displacements of the ground surface predicted from FE analysis

are shown at the end of each phase in Figure 4.4 and Figure 4.5. The vertical displacements

predicted from FE analysis are shown in appendix, Tabl e A 5± Tabl e A 8 while the settlement ±

time curves from the FE analysis are compared with measured data and shown in Figure 4.6.

As can be seen from Figure 4.5, the FE analysis predicts maximum horizontal

displacements of about 91mm either side of the embankment at the end of the consolidation

period. Also notice the potential failure surfaces developed on either side of the embankment in

Figure 4.5(f). As can be seen from Figure 4.6 the FE analysis predicts about 445mm maximum

vertical displacement of the ground surface beneath the embankment occurring at the centre of

the embankment (point A) which reduces as the distance moves away from the centre. The FE

analysis also predicts heaving of the ground surface of about 12.7mm at point D. In general, the

displacements predicted from the FE analysis compares very well with the measured data

although the model accuracy reduces as we move away from the embankment.





Figure 4.1. 3D representation of the deformed mesh of the trial embankment at the end of

consolidation period





Figure 4.2. Excess pore pressure in the soil depost at end of each phase

(a) end of phase 1, height of embankment = 0.5m, ,

(b) end of phase 2, height of embankment = 0.5m, ,

(c) end of phase 3, height of embankment = 1.0m, ,

(d) end of phase 4, height of embankment = 0.5m, ,

(e) end of phase 5, height of embankment = 1.6m, ,

(f) end of phase 6, height of embankment = 1.6m,





Figure 4.3 Comparison of Finite Element analysis prediction for excess pore pressures at PC2-1, PC2-

2, PC3-3, and PC2-4 with measured data





Figure 4.4 Vertical displacements of the ground surface beneath the embankment at end of each phase





(e) end of phase 5, height of embankment = 1.6m,

,






Figure 4.5. Horizontal displacements of the ground surface beneath the embankment at the end of

each face





(e) end of phase 5, height of embankment = 1.6m, ,






Figure 4.6. Comparison of Finite Element analysis prediction for settlements at points A, B, C and D

with measured data





4.2 Effect of Initial structure on the Finite Element Analysis prediction of Embankment

Behaviour

The effect on the prediction of the soil behaviour using FE analysis due to the amount of

structure in the KHSM was investigated. The initial structure in the soft estuarine clay layer

(layer 2) which is modelled with the structure model in the KHSM is initially removed .

Subsequently, the amount of structure in the layer is gradually increased for values of

The FE prediction of the dissipation of excess pore pressure and deformation in the

soil deposit is analysed and compared for all assumed values of as well as measured data.

These are shown in Figure 4.7 and Figure 4.8 (Data obtained from FE analysis have been put in a

CD attached to this project.)

As can be seen from Figure 4.7 and Figure 4.8 we observe that when the initial structure

is non-existent , the FE analysis greatly over predicts the excess pore pressure and

displacements beneath the embankment. However as the initial degree of structure increases, the

FE analysis prediction of the displacements and dissipation of excess pore pressure improve, and

then converge at . Further increase in the initial degree of structure gives virtually the

same FE analysis prediction at this stage. We can thus conclude that the structure surface in the

KHSM is quite large at , and thus cannot degrade significantly. Hence further increase in

the initial degree of structure will not produce different results. Furthermore, the FE analysis

prediction of vertical displacements compares well with the measured data for an initial degree

of structure,





Figure 4.7. Comparison of Finite Element analysis prediction for excess pore pressure dissipation at

PC2-1, PC2-2, PC3-3, and PC2-4 with measured data for different degrees of initial structure





Figure 4.8. Comparison of Finite Element analysis prediction for settlements at Points A, B, C and D

with measured data for different degrees of initial structure





4.3 Effect of Permeability Anisotropy on the Finite Element Analysis prediction of

Embankment Behaviour

The effect of the permeability anisotropy on the FE analysis prediction of soft soil

behaviour under the trial embankment was investigated as its importance in the pore pressure

estimation and rate of post settlements cannot be over-emphasized. The permeability anisotropy

ratios in all the layers were varied using values of respectively. The

results shown in Figure 4.9 and Figure 4.10 display how the measured data from the field

compare with the predictions from FE analysis of the excess pore pressures and displacements of

the ground surface. (Data obtained from FE analysis have been put in a CD attached to this

project).


PC2-1, PC2-2, PC3-3, and PC2-4 with measured data for different degrees of permeability anisotropy.





Figure 4.10. Comparison of Finite Element analysis prediction for settlement at points A, B, C and D

with measured data for different degrees of permeability anisotropy

As can be seen from Figure 4.9 at PC2-2, we observe that as increases, the peak

values of pore pressure predicted from the FE analysis do not change much although complete

dissipation in the soil deposit is achieved at different times. This can be easily understood as a

more permeable material will consolidate faster than the less permeable ones. At points PC3-3,

and PC 2-4, the peak values predicted from FE analysis increase as reduces. Furthermore,

the FE analysis over predicts the pore pressure at PC2-1, PC3-3, and PC2-4.





In Figure 4.10 at points A and B, the plot shows that the settlements predicted from the FE

analysis compares well with the measured data on site for permeability anisotropy ratio of

.

4.4 Effect of the Stiffness Interpolation Exponent on the Finite Element Analysis

Prediction of Embankment Behaviour

The effect of the stiffness interpolation exponent parameter on the FE analysis prediction

of the soft soil behaviour beneath the trial embankment was investigated. Assumed values of

were used in all the layers while the values of the destructuration strain

parameter and stiffness interpolation parameter remained fixed. The results of the prediction from FE analysis for all assumed values of were compared with measured data and

are shown in Figure 4.11 and Figure 4.12. (Data obtained from FE analysis have been put in a

CD attached to this project).

In Figure 4.11, the differences in predicted peak values for all assumed values of the

interpolation exponent are small. It is also observed that while the FE analysis predicts a

smaller peak value of excess pore pressure for a low value of than for a high one, the higher

values of consolidate faster than the lower ones. For , FE analysis predicted a peak

value of at point PC3-3 after 133.5days, and after consolidating for 1000days,

while for , the predicted peak value from FE was after 133.5days, and

after 1000days. Again, the peak values of the excess pore pressure at PC3-3, and PC2-4

appears smoother than those at PC2-1, and PC2-2

In Figure 4.12 the settlements predicted from FE analysis are almost same for all values

of after construction, but begin to change slightly as the clay consolidates after 800 years at

Point A. After this period, as reduces the settlement increases. This can be can attributed to the

fact that as controls the way in which plastic stiffness falls as the bubble yield surface

approaches the structure surface and hence the rate at which plastic strains develop, reducing the

value of increases the plastic hardening modulus and hence reduces the plastic strains and

slows the destructuration process (Rouainia and D. Muir Wood 2000).





The FE analysis still over predicts the dissipation of excess pore pressure at all the points

except at PC2-2. As can be seen from Figure 4.12, the FE analysis compares well with measured

data at points A, and B for . Further increase in gives very little difference in the

settlements predicted from FE analysis.


PC2-1, PC2-2, PC3-3, and PC2-4 with measured data for different values of .





Figure 4.12. Comparison of Finite Element analysis prediction for settlement at points A, B, C and D

with measured data for different values of





4.5 Effect of the Destructuration Parameter (k ) on the Finite Element Analysis Prediction

of Embankment Behaviour

The effect of the destructuration parameter on the FE analysis prediction of the soft soil

behaviour beneath the embankment was investigated. The parameters and were

maintained in all the layers while assumed values of were used in soft

estuarine clay layer which is modelled with the structure model of the KHSM. The results of the

prediction from FE analysis for all assumed values of were compared with measured data and

are shown in Figure 4.13 and Figure 4.14. (Data obtained from FE analysis have been put in a

CD attached to this project).

As can be seen from both plots, we observe that as increases, the prediction from FE

analysis doesn¶t change. This can be attributed to the fact that since the calculation type used

which was earlier mentioned in Chapter 3 is consolidation rather than plastic, it is possible that

due to long periods of consolidation, plasticity is not achieved hence there is no loss of structure

in the material.

However, in the event that the value of the parameters and which control the rate at

which plastic strains develop were able to cause enough plastic strains in the structured layer for

destructuration to occur, it is expected that increase in will lead to rapid loss of structure in the

material (Rouainia and D. Muir Wood 2000). Thus the predicted peak values for pore pressure

dissipation from FE analysis may have increased with increasing values of





Figure 4.13 Comparison of Finite Element analysis prediction for settlement at points A, B, C and D

with measured data for different values of destructuration parameter k





Figure 4.14. Comparison of FE prediction for settlement at points A, B, C and D with measured data

for different values of destructuration parameter k





5. Conclusion

The construction of embankments on soft soil deposit is a major problem in geotechnical

engineering. Low permeability, low strength and high deformations are among the characteristics

displayed by natural soft soils which make construction on this material very difficult.

The Kinematic Hardening Soil Model (KHSM) developed by (Rouainia and D. Muir

Wood 2000) has been implemented in finite element analysis and used to study the behaviour on

a trial embankment on soft structured clay. Parametric studies were also conducted on the initial

degree of structure, permeability anisotropy ratio, interpolation exponent and destructuration

parameter in the KHSM model to understand how they affect the prediction from FE analysis of

the embankment behaviour. The results from the coupled analysis of excess pore pressure and

displacements of the ground surface gave the following conclusions

(1) The settlements predicted from FE analysis is generally in good agreement with the

measured data obtained from the field. However, the excess pore pressures are generally

overestimated. This has been attributed to variable in-situ conditions.

(2) The effect of the initial degree of structure is evident on the prediction from FE analysis

of the settlements of the ground surface as an increase in the degree of structure results in

a better prediction of the settlements from FE analysis which continues until the structure

surface becomes large enough and cannot degrade further. At this point, the settlements

predicted from FE analysis is in good agreement with measured data from the field.

(3) Permeability anisotropy ratio has an effect on the maximum values of pore pressure

dissipation predicted from FE analysis. An increase in the permeability anisotropy ratio

results in lower peak values of pore pressure dissipation predicted from FE analysis. In

this study, the settlements predicted from FE analysis compares well with measured data

from the field for a permeability anisotropy ratio of 2.





(4) After long periods of consolidation, the stiffness interpolation exponent affects the

predicted settlements from FE analysis as an increase in results in a reduction of the

predicted settlement at ground surface beneath the centre of the embankment. This takes

place mainly at the centre of the embankment (point A).

(5) The destructuration parameter does not affect the prediction from FE analysis as the

calculation type used in this study is consolidation as against plastic strain. Hence due to

long periods of consolidation, plasticity is not reached so there is no loss of structure in

the material.

In the overall, we can conclude from this study that using constitutive models that

account for effects of initial structure and destructuration to analyse construction on natural soft

clays is important as they improve the prediction of the behaviour of the structure on the soft soil

deposit.





References

Allman, D. S. a. M. A. "Numerical Analysis of a Trial Embankment on Soft Clay."

Borges, J. L. (2004). "Three-dimensional analysis of embankments on soft soils incorporatingvertical drains by finite element method." Computers and Geotechnics 31(8): 665-676.

Cudny, M. and P. A. Vermeer (2004). "On the modelling of anisotropy and destructuration of

soft clays within the multi-laminate framework." Computers and Geotechnics 31(1): 1-22.

D. Muir Wood, M. R. (2000). "A kinematic hardening constitutive model for natural clays with

loss of structure." Geotechnique 50(2): 153-164.

Jin-Chun Chai, N. M., and Shui-Long Shen (2002). "Performance of embankments with and

without reinforcement on soft subsoil." Can. Geotech. J. 39: 838-848.

M. Karstunen, C. W., H. Krenn, F. Scharinger, H. F. Schweiger (2006). "Modelling the

behaviour of an embankment on soft clay with different constitutive models." InternationalJournal for Numerical and Analytical Methods in Geomechanics 30(10): 953-982.

M.Rouainia, D. S. J. Z. (2005). "Influence of sturcture degradation on the behaviour of embankments on soft soil." 13th ACME Conference: University of Sheffield.

Marcin Cudny, H. P. N. (2003). "Numerical analysis of a test embankment on soft ground usingan anisotropic model with destructuration." Int. Workshop on Geotechnics of Soft Soils - Theory

and practice, Schweiger, Karstunen & Cudny (eds.).

N.Sivakugan, B. R. (2005). "Observed and predicted behaviour of clay foundation responseunder the Sunshine Motorway trial embankment." International Conference on Soil Mechanics

and Geotechnical Engineering.

Shen, S.-L., J.-C. Chai, et al. (2005). "Analysis of field performance of embankments on softclay deposit with and without PVD-improvement." Geotextiles and Geomembranes 23(6): 463-

485.





Sheng, W. H. S. F. D. B. D. S. a. D. (2006). "Finite-Element Parametric Study of theConsolidation Behaviour of a Trial Embankment on Soft clay " International Journal of

Geomechanics 6(5): 328-341.

Yildiz, A. (2009). "Numerical analyses of embankments on PVD improved soft clays."Advances in Engineering Software 40(10): 1047-1055.





APPENDIX





Table A 1 Predicted Values from FE Analysis of Excess Pore Pressures at PC2-1

Table A 1 cont. Predicted Values from FE Analysis of Excess Pore Pressures at PC2-1






Table A 4 Cont. Predicted Values from FE Analysis of Excess Pore Pressures at PC2-4

Table A 5 Predicted Values from FE Analysis of Vertical Displacements at Point A

Table A 5 cont. Predicted Values from FE Analysis of Vertical Displacements at Point A





Table A 6 Predicted Values from FE Analysis of Vertical Displacements at Point B

Table A 6 cont. Predicted Values from FE Analysis of Vertical Displacements at Point B

Table A 7 Predicted Values from FE Analysis of Vertical Displacements at Point C

Table A 7 cont. Predicted Values from FE Analysis of Vertical Displacements at Point C

Table A 8 Predicted Values from FE Analysis of Vertical Displacements at Point D

Table A 8 cont. Predicted Values from FE Analysis of Vertical Displacements at Point D

Table A 9 Measured data from instrumentation on site of Excess pore pressure at PC2-1, PC2-2, PC3-

3, and PC2-4

Table A 10 Measured data from instrumentation on site of vertical displacements at Points A, B, C and

D




Table A 10 cont. Measured data from instrumentation on site of vertical displacements at

Points A, B, C and D

final project dissertation - copy

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