PERFORMANCE PREDICTION OF PREFABRICATED VERTICAL DRAIN IN

SOFT SOIL USING FINITE ELEMENT METHOD

TEH ZHI HUAN

UNIVERSITI TEKNOLOGI MALAYSIA

PERFORMANCE PREDICTION OF PREFABRICATED VERTICAL DRAIN IN

SOFT SOIL USING FINITE ELEMENT METHOD

TEH ZHI HUAN

A project report submitted in partial fulfilment of the

requirements for the award degree of

Master of Engineering (Civil-Geotechnics)

Faculty of Civil Engineering

Universiti Teknologi Malaysia

JUNE 2015

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I dedicate this project report to my family especially my parents,

who never stop giving themselves their support and love.

iv

ACKNOWLEDGEMENT

First and foremost, I would like to express highest appreciation to my

supervisor, Dr. Rini Asnida Abdullah for her patiently guidance and constructive

suggestions throughout the preparation of this report.

I would also like to express my appreciation to management and engineers of

Sepakat Setia Perunding (Sdn) Bhd for providing all the necessary data to complete

this report.

My thankfulness also shows to my fellow coursemates and friends who

willing to help and share their knowledge along the way. I hope that they achieve

great success in their life.

Last but not least, my wholehearted appreciation goes to my family for their

unconditional support. Because of their support and encouragement, I able to

complete this project report.

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ABSTRACT

Soft soil ground improvement method by using preloading with prefabricated

vertical drain (PVD) had successfully applied in many soft ground construction

projects. Due to the complexity of today’s construction work, finite element method

could assist designers to model very complex subsoil and structural elements. In this

study, finite element analysis is performed to verify the effectiveness of modelling of

PVD in subsoil using computer software, Plaxis V8. The field settlement data were

collected at two particular locations and were analysed using Asaoka’s (1978)

method to estimate the ultimate settlement and back-calculated the coefficient of

horizontal consolidation for these two particular location. Thereafter, by using back-

calculated coefficient of horizontal consolidation, finite element analyses were

performed to predict the time rate settlement and compared against field settlement

results. From the time rate settlement curves generated by FEM, the ultimate

settlements were predicted using Asaoka’s (1978) method. It was found that the

predicted time rate settlement by FEM shows reasonable agreement with the actual

field settlement result. The ultimate settlement predicted from FEM is slightly lower

compared to actual field settlement monitoring result, but the degree of consolidation

settlement achieved were higher. In term of time required to achieve 90%

consolidation settlement after surcharge, FEM shows slightly advancement compared

to actual field settlement monitored. However, the advancement is only within 5%

which is in good agreement with the actual field monitored reading.

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ABSTRAK

Saliran tegak pra-fabrikasi (PVD) telah banyak digunapakai di kawasan tapak

tanah liat untuk memperkukuhkan tanah liat. Disebabkan kerumitan kerja pembinaan

hari ini, kaedah unsur terhingga (FEM) dapat membantu pereka untuk model tanah

yang sangat kompleks dan unsur-unsur struktur. Dalam kajian ini, analisis unsur

terhingga dilakukan untuk mengesahkan keberkesanan pemodelan PVD dalam tanah

dengan menggunakan perisian komputer, Plaxis V8. Rekod enapan di dua lokasi

telah dikumpulkan and dianalisis dengan kaedah Asaoka (1978) untuk meramal

enapan maksimum serta analisis-kembali nisbah pekali pengukuhan dalam arah

mendatar. Dengan nisbah pekali pengukuhan dalam arah mendater yang diperolehi,

Plaxis V8 digunakan untuk meramal enapan berlaku dengan masa dan dibandingkan

dengan rekod enapan tapak. Dari penyelesaian enapan berlaku dengan kadar masa

yang dihasilkan oleh FEM, kaedah Asaoka (1978) digunakan untuk meramalkan

enapan maksimum. Perbandingan enapan berlaku dengan masa diantara ramalan

FEM dengan rekod enapan di tapak menunjukkan perjanjian yang munasabah.

Namun demikian, didapati bahawa enapan maksima yang diramalkan dengan FEM

adalah kurang daripada enapan rekod di tapak, tetapi darjah pengukuhan tanah yang

diperolehi dengan FEM adalah lebih tinggi. Dalam jangka masa yang diperlukan

untuk mencapai 90% pengukuhan tanah selepas surcaj, FEM menunjukkan

persingkatan masa berbanding dengan rekod enapan di tapak. Walau bagaimanapun,

persingkatan masa yang ditunjukkan oleh FEM hanya dalam likungan 5% yang mana

telahpun bersetuju dengan rekod enapan yang sebenar di tapak.

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TABLE OF CONTENTS

CHAPTER TITLE PAGE

DECLARATION ii

DEDICATION iii

ACKNOWLEDGEMENTS iv

ABSTRACT v

ABSTRAK vi

TABLE OF CONTENTS vii

LIST OF TABLES x

LIST OF FIGURES xi

LIST OF SYMBOLS

LIST OF APPENDICES

xiii

xv

1 INTRODUCTION 1

1.1 Introduction 1

1.2 Problem Statement 2

1.3 Objectives of Study 3

1.4 Scope of Study 4

1.5 Significance of Study 5

2 LITERATURE REVIEW 6

2.1 Preloading Technique in Ground Improvement 6

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2.2 Vertical Drains 7

2.3 Characteristic of Prefabricated Vertical Drain 9

2.4 Design of Prefabricated Vertical Drain (PVD) 9

2.4.1 Barron’s Equal Vertical Strain Hypothesis 9

2.4.2 Hansbo’s Theory 10

2.4.3 Plane Strain Consolidation Model 15

2.4.4 Plane Strain Equivalent Horizontal

Permeability by Lin et al.

19

2.5 Observation Method (Asaoka’s Method) 21

2.6 Factors Affecting Drain Efficiency 23

2.6.1 Smear Effecr 23

2.6.2 Well Resistance

2.7 Plane Strain Modelling of Soil and Vertical Drain

25

27

3 METHODOLOGY 30

3.1 Introduction 30

3.2 Review of Literature 32

3.3 Data Collection 32

3.4 Data Analysis 33

3.5 Description of the Case Study

3.6 Subsoil Condition at Proposed Site

3.7 Work Programme at Site

3.8 Instrumentation Monitoring Results at Site

34

35

39

41

4 RESULTS AND DISCUSSION 45

4.1 Introduction 45

4.2 Finite Element Modelling 45

4.2.1 Modelling of Subsoil 46

4.2.2 Modelling of PVD 47

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4.2.3 Stage Construction Modelling 48

4.3 Analysis using Asaoka’s Method 53

4.4 Finite Element Analysis 54

5 CONCLUSION AND RECOMMENDATIONS 60

5.1 Introduction 60

5.2 Conclusion

5.3 Recommendations

60

61

REFERENCES

Appendices A & B

63

66-71

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LIST OF TABLES

TABLE NO. TITLE PAGE

3.1 PVD Specification and Test Results 41

4.1 Subsoil parameter used for FEM analysis 47

4.2 Summary of back analysis by Asaoka’s Method 54

4.3 Comparison of settlement assessed by field

monitoring and FEM method

58

xi

LIST OF FIGURES

FIGURE NO. TITLE PAGE

2.1 Resulting settlement due to preloading 7

2.2 Schematic of soil cylinder with vertical drain 11

2.3 Schematic of PVD wih drain resistance and soil

disturbance

14

2.4 Conversion of an axismmetric unit cell into plane

strain

16

2.5 Axisymmetric radial flow 20

2.6 PVD in 2-D plane strain flow 21

2.7 Graphical presentation by Asaoka’s Method 22

2.8 Relationship between smear zone and sensitivity of

soil

25

2.9 Example of a plane strain problem 27

3.1 Flow chart of the study 31

3.2 Location of Project Site 34

3.3 Proposed Development Site 35

3.4 Atterberg Limits of Soil at Different Depth 36

3.5 (a) Undrained Shear Strength with depth 37

3.5 (b)

3.6 (a)

3.6 (b)

3.6 (c)

3.6 (d)

3.6 (e)

Sensitivity of soft clay with depth

Initial Void Ratio

Compression Index

Recompression Index

Compression Ratio

Recompression Ratio

37

38

38

38

38

38

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3.7 (a) Coefficient of vertical consolidation 39

3.7 (b) Pre-consolidation Pressure 39

3.8 Ground Treatment Layout Plan 40

3.9 Instrumentation Layout Plan 42

3.10 Settlement Monitoring Result at RSG-7 43

3.11 Settlement Monitoring Result at RSG-8 44

4.1 Modelling of Construction Sequence at RSG-7 48

4.2

4.3

4.4

4.5

4.6

4.7

4.8

4.9

4.10

Modelling of Construction Sequence at RSG-8

Asaoka’s plot of RSG-7

Asaoka’s plot of RSG-8

Vertical settlement predicted by FEM consolidation

analysis at RSG-7

Vertical settlement predicted by FEM consolidation

analysis at RSG-8

Comparison of time rate consolidation settlement

between FEM and field monitoring result at RSG-7

Comparison of time rate consolidation settlement

between FEM and field monitoring result at RSG-8

Asaoka plot of RSG-7 by FEM predicted

consolidation settlement

Asaoka plot of RSG-8 by FEM predicted

consolidation settlement

51

53

54

55

55

56

56

57

58

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LIST OF SYMBOLS

Cc - Compression index

Cr - Recompression index

ch - Coefficient of horizontal consolidation

cv - Coefficient of vertical consolidation

CR - Compression ratio

RR - Recompression ratio

dm - Equivalent diameter of mandrel

dw - Equivalent diameter of the drain

De - Diameter of equivalent soil cylinder

e - Void ratio

F(n) - Vertical drain spacing factor

Fr - Well resistance factor for vertical drain

Fs - Smear effect factor

kh - Soil horizontal permeability

ks - Horizontal permeability of the soil within the smear zone

khax - Horizontal permeability of undisturbed zone in axi-symmetric unit cell

khpl - Horizontal permeability of undisturbed zone plane strain unit cell

ksax - Horizontal permeability of smear zone in axi-symmetric unit cell

Pc - Preconsolidation pressure

qw - Discharge capacity of PVD

Th - Dimensionless time factor for radial consolidation

hU - Degree of consolidation in horizontal direction

β1 - Slope in Asaoka’s plot

∆ - Difference

φ - Friction angle of soil

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c - Cohesion of soil

E - Young Modulus of soil

ψ - Angle dilatancy of soil

ν - Poisson’s ratio

λ* - Modified compression index

κ* - Modified swelling index

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LIST OF APPENDICES

APPENDIX TITLE PAGE

A

B

Field Monitoring Records

Calculation of Plane Strain Permeability

67

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CHAPTER 1

INTRODUCTION

1.1 Introduction

Soft soils present the unique challenges to geotechnical professionals due to

its poor bearing capacity and exhibit large settlement when subjected to loading. In

time of urbanization, rapid growth of population and increasing demand of

development such as houses, offices and infrastructures has resulted the construction

activities on soft ground which comprises of compressible soil become unavoidable.

The construction activities which normally building the platform will create external

loading to the soft ground. The additional loading imposes to the soft compressible

soil would causes significant magnitude of consolidation settlement over a

considerable period of time. Thus for the development projects of building the

platform or embankment on top of soft soil, the post construction settlement would

be of primary concern.

To minimize the post construction settlement and differential settlement,

ground improvement technique by preloading with prefabricated vertical drain are

most widely selected technique to resolve this problem. This method is generally

used due to its cost effective and fast installation of prefabricated vertical drain.

2

Preloading on soft soil refers to the process of compressing the soft

compressible soil layer under applied vertical stress (preload) prior to the

construction and placement of final construction load. Prefabricated vertical drains

consist of a flat or cylindrical plastic core wrapped in a geotechnical fabric, and

allow water to drain up through the centre of the drain. The main purpose of vertical

drain is to shorten the drainage path of soft compressible soil layer and thus increase

the rate of consolidation settlement.

Baron (1947) has produced the ‘equal strain hypothesis’ which presume no

differential settlement will take place for prefabricated vertical drain treated ground.

The ‘equal strain hypothesis’ by Baron (1947) has been extended to include the

effect of smear and well resistance by Hansbo (1981). Today, analytical method by

Hansbo (1981) is widely used in the design of vertical drains. Field observation of

the settlement monitoring results always gives the best understanding of the soft soil

behaviour through the settlement trend. The magnitude of final settlement can be

estimated based on the field data by Asaoka’s method, hyperbolic method and

viscosity method, and thus the degree of consolidation at any time could be assessed.

Asaoka’s method (1978) is the most popular method to predict the final settlement as

proven in previous study that this method gives good prediction of final settlement

compared to actual final settlement at site. Recently, some researchers such as

Indraratna and Redana (1997), Lin et al. (2000) and Chai et al. (2001) had

successfully established the derivation of the axisymmetric condition of vertical

drain to plane-strain condition to be employed in finite element analysis.

1.2 Problem Statement

Soft soil ground improvement method by using preloading with prefabricated

vertical drain had successfully applied in many soft ground construction projects.

3

Easy and fast installation, properties standardisation, and cost effective compared to

other ground improvement method make this method gaining popularity. For a given

construction site which comprises of thick layer of soft soil, using the analytical

approach by Hansbo (1981), the design of vertical drain would very much depend on

the pattern of installation of vertical drain either triangular grid or square grid. Beside

the analytical approach, designer could also carry out the design of vertical drain

using finite element method or empirical method which depends on the preference

and proficiency of the designers.

Due to the complexity of today’s construction work, finite element method

could assist designers to model very complex subsoil and structural elements and

thus finite element method is playing more and more important role in design works.

However, the accuracy of field data and laboratory test result is of the major

parameter inputs to reflect the real condition of the site which is crucial in design that

later could match back the field monitoring results.

The comparison between the field monitoring results and finite element

analysis would help designers to get better understanding of the real soil behaviour

compared to finite element modelling. Hence, suitable tolerance could be allowed in

future design work using finite element method. As such, the effectiveness of

modelling of prefabricated vertical drain in soft soil using finite element method

must be evaluated.

1.3 Objectives of Study

The objectives of this study are stated in following:

4

1. To understand the effectiveness of modelling of prefabricated vertical

drain in soft soil using finite element method through literature review.

2. To analyses and compare the settlement of embankment in prefabricated

vertical drain treated soft ground obtained by finite element analysis and

field instrumentation monitoring.

3. To establish an approach for predicting a reliable 90% consolidation

settlement by using finite element analysis.

1.4 Scope of Study

This study is based on the project involved building an embankment for a

mixed development in soft ground area in Kuala Langat District, Selangor. The field

monitoring only limited to settlement monitoring which obtained from rod settlement

gauges.

Finite element analysis is carried out using commercial software, Plaxis v8.

2D plane strain modelling using Mohr Coulomb (MC) model and Soft Soil (SS)

model are used to model the constitutive subsoil properties. The permeability

matching derivation by Lin et al. (2000) is used in this study to obtain the

equivalence between axisymmetric behaviour of the vertical drain to plane strain

condition in Plaxis modelling. Asaoka’s method will be used to predict the final

settlement for settlement data obtained from instrumentation and finite element

analysis. The time required for 90% consolidation between finite element analysis

and field instrumentation monitoring is compared.

5

1.5 Significant of Study

This study presents the performance prediction of PVD in soft soil using

finite element analysis. It is aimed at verifying the effectiveness of modelling of

PVD in soft soil by appropriate conversion technique from three dimensional to two

dimensional. Comparison between field monitoring settlement result and finite

element analysis enabled the designer to predict the time rate settlement in future

project, thus the resting period for 90% consolidation settlement could be assessed.

REFERENCES

Arulrajah, A., Nikraz, H., Bo, M. W. (2003). Factors affecting field settlement

assessment and back-analysis by the Asaoka and Hyperbolic methods.

Australian Geomechanics, 38, No.2.

Arulrajah, A., Nikraz, H., Bo, M. W. (2004). Factors affecting field instrumentation

assessment of marine clay treated with prefabricated vertical drains. Geotextiles

and Geomembranes, 22 (2004), 415-437.

Arulrajah, A., Nikraz, H., Bo, M. W. (2005). Finite element modelling of marine clay

deformation under reclamation fills. Ground Improvement, 9, No.3, 105-118.

Asaoka, A (1978). Observational Procedure of Settlement Prediction. Soils and

Foundation, Vol. 18, No. 4, Dec. 1978, Japanese Society od Soil Mechanics and

Foundation Engineering. pp. 87 – 101.

Barron, R. A. (1948). Consolidation of fine-grained soils by drain wells. Trans.

ASCE, 113, 718–742.

Chai, J.C., Berdago, D.T. and Shen, S.L. (2013). Modelling prefabricated vertical

drain improved ground in plane strain analysis. Proceedings of Institution of

Civil Engineers on Ground Improvement. May 2013, 166, 65-77.

Chai, J.C., Miura, N. (1999). Investigation of Factors Affecting Vertical Drain

Behaviour. Journal Of Geotechnical And Geoenvironmental Engineering, ASCE,

125:216-226.

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Chen, C.S. (2004). Use of Prefabricated Vertical Drain to Expedite the Consolidation

Settlement. Seminar and Exhibition on Building on Geosynthetics, jointly

organized by Association of Consulting Engineers Malaysia and International

Geosynthetics Society – South-East Asia Chapter, 30 June and 1 July 2004, Pan

Pacific Hotel, Kuala Lumpur.

Hansbo, S. (1979). Consolidation of clay by band-shaped prefabricated drains.

Ground Engineering, 12, No.5, 16-25.

Hansbo, S. (1981). ‘‘Consolidation of fine-grained soils by prefabricated drains.’’

Proc. 10th Int. Conf. Soil Mech. and Found. Engrg., Vol. 3, International Society

of Soil Mechanics and Foundation Engineering, Stockholm, 3, 677–682.

Hansbo, S., Jamiolkowski, M, Kok, L. (1981). Consolidation by Vertical Drains.

Geotechnique, Volume 31, Issue 1

Hird, C.C., Pyrah, I.C. and Russell, D. (1992). Finite element modelling of vertical

drains beneath embankment on soft ground. Geotechnique. 42, No.3, 499-511.

Indraratna, B., Bamunawita, C., Redana, I., McIntosh, G. (2003). Modelling of

Prefabricated Vertical Drains in Soft Clay and Evaluation of Their Effectiveness

in Practice. Journal of Ground Improvement, 7(3), 2003, 127-138.

Indraratna, B. and Redana, I. (2000). Numerical modelling of vertical drains with

smear and well resistance installed in soft clay. Can. Geotech. J., 37: 137-145.

Lin, D. G., Kim, H. K. and Balasubramaniam, A. S. (2000), Numerical Modeling of

Prefabricated Vertical Drain, Geotechnical Engineering Journal, Vol. 31, No. 2,

August, pp. 109-125.

Moseley, M.P. and Kirsch, K. Ground Improvement. 2nd. ed. London and New York:

Spon Press. 2004.

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Plaxis (2002) Plaxis Finite Element Code for Soil and Rock Analyses, 2D-Version 8.

Plaxis BV, Delft, The Netherlands.

Sathananthan, I. (2005). Modelling of vertical drains with smeared in soft clay.

Doctor Philosophy. University of Wollongong, Australia.

U.S. Department of Transportation, Federal Highway Administration (1986).

Prefabricated Vertical Drains, Vol. 1: Engineering Guidelines. FHWA/RD-

86/168.