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Tschuchnigg Lebeau Validation Emb Pile

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FE-Analysis of piled and piled raft foundations

Jean-Sbastien LEBEAU

April - August 2008

AbstractIn the last few years the number of piled raft foundations especially those with few piles, has increased. Unlike the conventional piled foundation design in which the piles are designed to carry the majority of the load, the design of a piled raft foundation allows the load to be shared between the raft and piles and it is necessary to take the complex soil-struture interaction eects into account. The aim of this paper is to describe a nite element analysis of deep foundations: piled and mainly piled raft foundations. A basic parametric study is rstly presented to determine the inuence of mesh discretisation, of materials - loose or dense sand -, of dilatancy and interface elements. Then the behavior of piled raft foundations is analysed in more details using partial axisymmetric models of one pile-raft. We continue by preparing a more sophisticated 3D study to take into account the complex pilepile interaction which occured when the pile spacing is small. So the possibilies of employing the embedded pile concept as implemented into Plaxis 3D foundations is investigated. clues about the group eect are indicated. Finally, some

Key words: Piled raft foundation, piles, embedded pile, volume pile, hardening soil model

1

AcknowledgementsFirst of all I would like to express my gratefulness to Professor Helmut F. Schweiger for giving me the opportunity to work on geotechnical issues at the Institute for Soil Mechanics and Foundation Engineering of Graz University of Technology.

This paper was made possible by the great contribution of my supervisor Dipl.-Ing Franz Tschuch-

nigg. I am indebted to him for his friendly supervision and guidance throughout the period of mytraineeship. I deeply thank him because he conveyed me a better understanding of nite element modeling and analyses.

I also would like to thank my French professor, Yvon Riou for getting me in touch with the Institute. Finally, I would like to express my appreciation to all the people I met here who made my ve months stay in Austria very enjoyable.

2

Contents1 2 Introduction Preliminary studies2.1 Single pile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1 Presentation of calculations 2.1.1.1 2.1.1.2 2.1.1.3 2.1.1.4 2.1.1.5 2.1.2 Results 2.1.2.1 2.1.2.2 2.1.2.3 2.1.2.4 2.2 . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6 77 7 7 8 8 9 10 10 11 14 16 17 18 18 18 19 19 19 20 20

Geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Boundaries conditions . . . . . . . . . . . . . . . . . . . . . . . . . . Material properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . Meshes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Load control and calculation steps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mesh dependency . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Comparison between distributed loads and prescribed displacement Inuence of the interface coecient Rinter . . . . . . . . . . . . . . . Inuence of the dilatancy . . . . . . . . . . . . . . . . . . . . . . . .

Pile-raft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1 Presentation of calculations 2.2.1.1 2.2.1.2 2.2.1.3 2.2.1.4 2.2.1.5 2.2.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Boundaries conditions . . . . . . . . . . . . . . . . . . . . . . . . . . Materials properties . . . . . . . . . . . . . . . . . . . . . . . . . . . Meshes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Load control and calculation steps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

CONTENTS

CONTENTS

2.2.2.1 2.2.2.2 2.2.2.3

Mesh dependency

. . . . . . . . . . . . . . . . . . . . . . . . . . . .

20 21 22

Inuence of the interface coecient Rinter . . . . . . . . . . . . . . . Inuence of the dilatancy . . . . . . . . . . . . . . . . . . . . . . . .

3

Analysis of 2D models3.1 3.2 Single-pile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pile-Raft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1 3.2.2 3.2.3 Load-displacement curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Variations of Skin friction and Normal Stresses along the pile Analysis of the 3.2.3.1 3.2.3.2 3.2.3.3 3.2.3.4 3.2.3.5 3.2.4 3.2.5 . . . . . . . . .

2424 26 29 29 36 36 37 39 41 42 44 45 45 46 47

Kpp

factor

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Denition of

Kpp

Methodology to calculate

Kpp

. . . . . . . . . . . . . . . . . . . . . for dierent geometries: . . . . .

Comparison and evolution of Evolution of Evolution of

Kpp

Kpp Kpp

for dierent materials and dilatancy . . . . . . . . for dierent values of Rinter . . . . . . . . . . . . . .

Eciency of a piled-raft foundation in comparison with a raft foundation

Analysis of the pile behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.5.1 3.2.5.2 3.2.5.3 Base resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Skin resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4

Preliminary studies of 3D models4.1 Volume pile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.1 4.1.2 Finite element models Results 4.1.2.1 4.1.2.2 4.1.2.3 4.2 Embedded pile 4.2.1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4950 50 52 52 54 58 60 60 61

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Load-displacement curves . . . . . . . . . . . . . . . . . . . . . . . . Variations of skin friction . . . . . . . . . . . . . . . . . . . . . . . . Some remarks about parameters . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Embedded pile-raft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1.1 Finite element models . . . . . . . . . . . . . . . . . . . . . . . . . . 4

CONTENTS

CONTENTS

4.2.1.2 4.2.1.3 4.2.1.4 4.2.1.5

Embedded pile with Embedded pile with Embedded pile with

linear skin friction distribution

. . . . . . . . . . . . . .

63 69 73

multilinear skin friction distribution

layer dependent skin friction distribution

Comparison of the three options: Linear, multilinear and layer dependent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

5

Group eect5.1 Presentation of calculations 5.1.1 5.1.2 5.2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

8282 83 86 86 86 87 89 92 97

Geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Finite element model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.1 5.2.2 5.2.3 5.2.4 5.2.5 Vocabulary details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Load-displacement curves

Displacement proles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . More precise analysis of group 5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6

Conclusion

98

5

Chapter 1

IntroductionIn traditional foundation design, it is customary to consider rst the use of shallow foundation such as a raft (possibly after some ground-improvement methodology performed). If it is not adequate, deep foundation such as a fully piled foundation is used instead. In the last few decade, an alternative solution has been designed: piled raft foundation. Unlike the conventional piled foundation design in which the piles are designed to carry the majority of the load, the design of a piled raft foundation allows the load to be shared between the raft and piles and it is necessary to take the complex soil-struture interaction eects into account. The concept of piled raft foundation was rstly proposed by Davis and Poulos in 1972 and is now used extensively in Europe, particularly for supporting the load of high buildings or towers. The favorable application of piled raft occurs when the raft has adequate loading capacities, but the settlement or dierential settlement exceed allowable values. In this case, the primary purpose of the pile is to act as settlement reducer. The aim of this paper is to describe a nite element analysis of deep foundations: piled and mainly piled raft foundations. A basic parametric study is rstly presented to determine the inuence of mesh discretisation, of materials - loose or dense sand -, of dilatancy and interface elements. Then the behavior of piled raft foundations is analysed in more details using partial axisymmetric models of one pile-raft. We continue by preparing a more sophisticated 3D study to take into account the complex pilepile interaction which occured when the pile spacing is small. So the possibilies of employing the