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STATENS GEOTEKNISKA INSTITUTSWEDISH GEOTECHNICAL INSTITUTE

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Calculating long-termsettlement in soft clays with special focus on theGothenburg region

MATS OLSSON

Swedish Geotechnical InstituteSE-581 93 Linkping

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D e p a r t m e n t o f C i v i l a n d E n v i r o n m e n t a l E n g i n e e r i n gD i v i s i o n o f G e o E n g i n e e r i n g

C H A L M E R S U N I V E R S I T Y O F T E C H N O L O G YG t e b o r g , S w e d e n 2 0 10

Calculating long-term sett lement in soft clays with special focus on the Gothenburg region

MATS OLS S O N

T H E S I S F O R T H E D E G R E E O F L I C E N T I AT E O F E N G I N E E R I N G

Calculating long-term settlement in soft clays

- with special focus on the Gothenburg region

MATS OLSSON MATS OLSSON, 2010 ISSN 1652-9146 Lic 2010:3 Department of Civil and Environmental Engineering Division of GeoEngineering Chalmers University of Technology SE-412 96 Gteborg Sweden Telephone + 46 (0)31 772 10 00 www.chalmers.se Chalmers reproservice Gteborg, Sweden 2010

Abstract

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Calculating long-term settlement in soft clays

- with special focus on the Gothenburg region

MATS OLSSON Department of Civil and Environmental Engineering Division of GeoEngineering Chalmers University of Technology

ABSTRACT

Long-term settlement in clay constitutes an engineering challenge in road design

and construction in areas with deep deposits of soft clay. Soil improvement and

construction of building foundations or embankments can be quite complicated

and expensive in such areas. Construction costs need to be balanced against high

maintenance costs. In order to do this optimally, there is a need to predict long-

term settlement with a high degree of accuracy.

Two different test sites were chosen for back-calculation, a test embankment at

Ndinge and a groundwater lowering at Kaserntorget. There was also one

hypothetical test site.

In this thesis a short description is presented of the fundamental behaviour of soft

clays with regard to compressibility as well as a short explanation of the theory

for the three different models that has been used within this thesis Embankco,

GS Settlement and the Soft Soil Creep model.

Soil parameter determination for long-term settlement analysis is discussed

together with some of the inherent complications. For the IL oedometer test the

study shows that if the time for the load stage of interest is not sufficiently long

the evaluated creep parameter could be misleading. Back-calculation of CRS

oedometer test, using the Soft Soil Creep model, is performed for this model and

a procedure is suggested.

The outcome of the analysis shows that all three models produce similar results

for the hypothetical case. For the two test sites in question, both GS Settlement

and the Soft Soil Creep model were capable of predicting the measured

settlement with acceptable accuracy. The Embankco program was only used for

the hypothetical case.

Keywords: Soft clay, creep, test sites, long-term settlement.

Abstract

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Acknowledgements

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ACKNOWLEDGMENTS

The work presented in this thesis was conducted at the Division of GeoEngineering at Chalmers University of Technology, under the supervision of Professor Claes Aln. Financial contributors were the Swedish Road Administration, SGI and Chalmers University of Technology, who are greatly acknowledged for their support. I wish to thank my supervisor Professor Claes Aln for initiating this project and for his great interest and guidance during this project. I also wish to thank Professor Gran Sllfors for all his support and interesting discussions during the project. Special thanks to my co-supervisor and colleague Per-Evert Bengtsson at SGI for his critical examination, support and guidance throughout the project. I would also like to thank my employer, SGI, for giving me the opportunity to dedicate the last three years to this project. I would like to thank Tyrens in Gothenburg for providing me with valuable data. I would also like to express my appreciation to all my colleagues and friends for their support and encouragement and to Anna-Karin for her love, patience and support. Gteborg, April 2010 Mats Olsson

Acknowledgements

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Table of contents

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TABLE OF CONTENTS

ABSTRACT ................................................................................................iii

ACKNOWLEDGMENTS ..........................................................................v

TABLE OF CONTENTS..........................................................................vii

LIST OF NOTATIONS .............................................................................xi

1. INTRODUCTION...............................................................................1

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

1.2 Research objectives.......................................................................1

1.3 Scope of Work ...............................................................................2

1.4 Limitations ....................................................................................2

2. FUNDAMENTAL BEHAVIOUR OF SOFT CLAYS WITH

REGARD TO COMPRESSIBILITY................................................5

2.1 Introduction...................................................................................5

2.2 Natural state of soft clays .............................................................5 2.2.1 Influence of ground water changes............................................................ 6

2.3 Yielding of soft clays .....................................................................7 2.3.1 Strain rate effects....................................................................................... 8

2.3.2 Temperature effects................................................................................. 11

2.4 Consolidation of soft clays .........................................................12 2.4.1 Theory of consolidation........................................................................... 14

2.4.2 Delayed consolidation ............................................................................. 15

2.5 Models for consolidation ............................................................17 2.5.1 General .................................................................................................... 17

2.5.2 Taylors model.......................................................................................... 17

2.5.3 The Isotache model ................................................................................. 17

2.5.4 The Bjerrum model ................................................................................. 18

2.5.5 The time resistance concept .................................................................... 19

2.6 Overconsolidated conditions ......................................................20

3. PROGRAMS FOR CALCULATING TIME-DEPENDENT

BEHAVIOUR ....................................................................................23

3.1 Embankco....................................................................................23

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3.1.1 Soil model ............................................................................................... 23

3.1.2 Calculation method ................................................................................. 24

3.2 GS Settlement ..............................................................................25 3.2.1 Soil model ............................................................................................... 25

3.2.2 Calculation method ................................................................................. 28

3.3 Soft Soil Creep model .................................................................30 3.3.1 Soil model ............................................................................................... 30

3.4 Relationships between model parameters ..................................35

4. DETERMINATION OF SOIL PARAMETERS ...........................37

4.1 Introduction.................................................................................37

4.2 Determination of soil parameters for settlement analysis .........37 4.2.1 Evaluation of the creep parameter from laboratory tests......................... 38

4.2.2 Creep parameters at the preconsolidation stress empirical .................. 39

4.2.3 The Chalmers model ............................................................................... 40

4.2.4 Modelling laboratory tests....................................................................... 44

4.3 Discussion ....................................