Earth pressures against- and stability of

retaining structures

Sigurur Mr Valsson

Faculty of Civil and Environmental Engineering University of Iceland

2011

Earth pressures against- and stability of

retaining structures

Sigurur Mr Valsson

30 ECTS thesis submitted in partial fulfillment of a

Magister Scientiarum degree in Civil Engineering

Advisors

Arne Kavli Dr. Arne smund Skotheim

Dr. Sigurur Erlingsson Dr. Torgeir Dssland

Faculty Representative Dr. Bjarni Bessason

Faculty of Civil and Environmental Engineering

School of Engineering and Natural Sciences

University of Iceland

Molde, June 2011

Earth pressures against- and stability of retaining structures

30 ECTS thesis submitted in partial fulfillment of a Magister Scientiarum degree in Civil

Engineering

Copyright 2011 Siguur Mr Valsson

All rights reserved

Faculty of Civil and Environmental Engineering

School of Engineering and Natural Sciences

University of Iceland

Hjararhaga 6

107, Reykjavk

Iceland

Telephone: 525 4000

Bibliographic information:

Sigurur Mr Valsson, 2011, Earth pressures against- and stability of retaining structures,

Masters thesis, Faculty of Civil and Environmental Engineering, University of Iceland,

pp. 176.

Printing: Samskipti

Reykjavik, Iceland, June 2011

Abstract

The objective of the presented research is to investigate methods used to calculate earth

pressures against- and the stability of retaining structures.

The classical methods of calculating earth pressures are covered in detail. They are named

after their creators Charles-Augustin de Coulomb and William John Macquorn Rankine. A

special interest is taken in a variant of Rankines theory developed at Norges teknisk-

naturvitenskapelige universitet (NTNU) that incorporates cohesive properties of soils as

well as roughness between the structure and the soil material. Both methods are then

tested against each other in order to investigate if a clear link exists between the two. The

methods are tested against proposed design values of earth pressure coefficients according

to the design standard Eurocode 7.

Hndbok 016 is a design handbook published by the Norwegian Road Administration (n.

Statens vegvesen). In it is a chapter dedicated to the design of retaining structures and

bridge abutments. The methods proposed to design retaining structures are used to design

cantilevered structures of various heights and with various foundation- and backfill

materials based on a few geometrical constraints.

The structure designs created with methods in Hndbok 016 are modeled in the finite

element program PLAXIS to test if the same material factors of safety could be derived

that way.

The results from this study are then discussed and commented upon.

v

Table of Contents

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

List of Tables ..................................................................................................................... xvi

Symbols ............................................................................................................................. xvii

Abbreviations .................................................................................................................... xix

Acknowledgements ........................................................................................................... xxi

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

2 Theory .............................................................................................................................. 2 2.1 Vertical pressures .................................................................................................... 2

2.1.1 In situ vertical stresses ................................................................................... 2

2.2 Strength parameters of soil ...................................................................................... 3

2.2.1 Undrained shear strength, Su analysis ............................................................ 3

2.2.2 Drained shear strength, - analysis ............................................................. 5 2.2.3 Deciding when to use which model ............................................................... 7

2.3 Horizontal earth pressures ....................................................................................... 8

2.3.1 At rest horizontal earth pressures ................................................................... 8

2.3.2 Smooth vertical walls and stress fields - Rankine zones ............................... 9

2.3.3 Rough walls and stress fields - Prandtl zones .............................................. 11

2.3.4 Roughness and partially developed stress fields .......................................... 12

2.3.5 Coulombs theory of earth pressures against retaining walls ...................... 19

2.4 Bearing capacity of foundations ............................................................................ 21

2.5 Failure modes of retaining structures .................................................................... 28

3 Comparison of methods used to calculate lateral earth pressures ........................... 30

3.1 Linking the roughness number, , and the roughness angle, . ............................. 30 3.2 Active earth pressures on walls leaning into the backfill ...................................... 37

3.3 Comparison of lateral earth pressures methods to method recommended by

Eurocode 7 ............................................................................................................. 41

3.3.1 Active pressures according to Eurocode 7 ................................................... 42

3.3.2 Passive pressures according to Eurocode 7 ................................................. 47

4 Design methods proposed by Hndbok 016 ............................................................... 52 4.1 Requirements ......................................................................................................... 52

4.2 Cantilever walls ..................................................................................................... 59

4.2.1 Calculation procedure .................................................................................. 60

4.2.2 Assumptions and constraints........................................................................ 61

4.2.3 Calculated cases ........................................................................................... 61

4.2.4 Critical base width for L-shaped cantilever wall ......................................... 62

4.2.5 Effect of varying the toe width of cantilever walls ...................................... 65

vi

4.2.6 The effect of backfill inclination ................................................................. 73

5 Analysis of stability using PLAXIS .............................................................................. 76 5.1 Finite element method in PLAXIS ......................................................................... 76

5.2 Application of PLAXIS to evaluate method proposed by HB016 ......................... 77

5.2.1 Material models ........................................................................................... 78

5.2.2 The Phi/c reduction method in PLAXIS ................................................. 79

5.2.3 Modeling and documenting failures in soil masses ..................................... 80

5.3 Results from PLAXIS simulations ......................................................................... 85

5.3.1 Blasted rock foundation horizontal terrain ............................................... 85

5.3.2 Gravel foundation horizontal terrain ........................................................ 87

5.3.3 Clay or clayey silt foundation horizontal terrain ...................................... 89

5.3.4 Blasted rock foundation inclined terrain .................................................. 91

5.3.5 Gravel foundation inclined terrain ............................................................ 93

5.3.6 Clay or clayey silt foundation inclined terrain ......................................... 94

5.3.7 Summary of PLAXIS simulation results ..................................................... 95

6 Conclusions .................................................................................................................... 97 6.1 Study of earth pressures ......................................................................................... 97

6.2 Structure design with hand calculation methods .................................................... 97

6.3 Evaluation of proposed designs using PLAXIS ..................................................... 98

6.4 Geometry of simulated failure surfaces ............................................................... 100

References ......................................................................................................................... 101

Appendix A: Stress on an arbitrary plane ..................