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NUMERICAL MODELLING OF 
ROCK SOCKETED PILES
SUDHEERA SENANEE ILLANGAKOON KODAGODA
library
UNIVERSITY Or MORATUWA, SRI LANKA 
MORATUWA
This thesis is submitted to the Department of Civil Engineering of 
the University of Moratuwa in partial fulfillment of the 
requirements for the Degree of Master of Engineering
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University of Moratuwa
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Department of Civil Engineering 
University of Moratuwa 
Sri Lanka nr
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December, 2008
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DECLARATION
This thesis is a report of research carried out in the Department of Civil Engineering, of 
University of Moratuwa, between January 2000 and December 2001. Except where 
references are made to other work, the contents of this thesis are original and have been 
carried out by the undersigned. The work has not been submitted in part or whole to any 
other University or an institution for a higher degree.
Signature of the Candidate 
Sudheera Senanee Illangakoon Kodagoda 
Department of Civil Engineering 
University of Moratuwa
Certified,
Supervisor
Prof. U. G. A. Puswewala.
ABSTRACT
The two basic types of pile foundations are skin friction piles and end- bearing piles. 
Friction piles resist the applied vertical load by utilising the shear stresses developed along 
the pile-soil interface, with the end-bearing at the tip of pile contributing a small proportion 
of the total pile resistance. End-bearing piles resist the applied vertical loads mainly by end­
bearing generated at the tip of pile.
There are many instances where end-bearing piles / piers are constructed with their tip 
located inside a cavity made in rock strata: such piles are referred to as rock-socketed piles. 
The resistance developed by rock-socketed piles and piers is partially due to the shearing 
stresses generated at the pile rock interface on the wall of the rock cavity and partially due to 
the end bearing at the tip of the pile. The load that can be carried by the rock socket is 
usually underestimated in the design practice due the lack of guidance in relevant codes.
This thesis presents the findings of the research focused on computational modeling of rock 
socketed piles conducted to investigate the behavior of such piles.
Axisymmetric finite element analyses were conducted to determine the percentages of load 
resisted by interface shear along the socket wall and end-bearing at pile tip, for various 
socket length/pile diameter ratios. Different interface conditions were modelled by varying 
cohesion and friction parameters input to the interface element, as well as by assuming 
perfect connection between pile and rock material.
The finite element results obtained are compared with some available field test data to verify 
the models. Finally a field test procedure is suggested for deriving load carrying capacity of 
the socket.
Findings and further expansion of this research program would be of immense benefit to 
state and private organisations involved in deep foundation design and construction. It will 
highlight whether the use of rock-socketed piles can be economically advantageous over the 
simple end bearing piles with no sockets, and indicate optimum geometries of rock sockets.
ACKNOWLEDGEMENT
I wish to express my deep respect and gratitude to my supervisor Prof. U. G. A. Puswewala of the 
Department of Civil Engineering, University of Moratuwa, who initiated the research and guided and 
assisted throughout in completing the same.
The Science and Technology Personnel Development Project of the Ministry of Science and Technology, 
Sri Lanka and the Asian Development Bank who funded this research program are acknowledged
Thanks are due to University of Moratuwa which provided all the facilities through its material and staff 
resources.
The Central Engineering Consultancy Bureau and its relevant staff officers are appreciated for granting 
leave for the relevant period to conduct and complete the research.
I am thankful to my husband Mr. G. N. Kodagoda who assisted me in many ways to complete the 
research.
While thanking all who have assisted me for conducting the research I would like to dedicate this thesis to 
my parents Mr. I. D. S. Illangakoon and Mrs. W. Illangakoon who have been supporting and encouraging 
me for studies throughout my life.
CONTENTS
DECLARATION
ABSTRACT
ACKNOWLEDGEMENT
Page
CHAPTER 1-INTRODUCTION
1.1 Back ground
1.2 Rock socketed piles
1.3 Significance of the Research
1.4 Numerical Methods
1.5 Objective of the Research
1.6 Literature Review
1.7 Methodology
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2
3
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4
5
CHAPTER 2-FINITE ELEMENT IDEALISATION OF ROCK SOCKETED 
PILES
2.1 Finite Element Program FEAP
2.2 The Program Used for Analysis - FEAP
2.3 Pre Processor for FEAP-AUTOFEAP
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2.4 Finite Element Idealisation
2.4.1 Geometry
2.4.2 Boundary Conditions
2.4.3 Forces and stresses
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9
2.4.4 Meshes without Interface Element 9
2.4.5 Interface Element 10
2.4.6 Meshes with Interface Element 
2.5 Selection of Material Properties
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CHAPTER 3-NUMERICAL ANALYSIS OF ROCK SOCKETTED PILES 
WITHOUT INTERFACE ELEMENT
3.1 Problems Analysed
Example 1: For the socket geometry of L/D=l 
Example 2: For the socket geometry of L/D=2 
Example 3: For the socket geometry of L/D=3 
Example 4: For the socket geometry of L/D=4
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3.2 13
3.3 15
3.4 16
3.5 17
CHAPTER 4-NUMERICAL ANALYSIS OF ROCK SOCKETTED PILES 
WITH INTERFACE ELEMENT
4.1 Introduction to Analysis
4.2.1 For Socket Geometry L/D=l and Cs=105, Er/Ec= 0.5 to 5
4.2.2 For Socket Geometry L/D=2 and Cs=105, Er/Ec= 0.5 to 5
4.2.3 For Socket Geometry L/D=3 and Cs =105, Er/Ec= 0.5 to 5
4.2.4 For Socket Geometry L/D=4 and Cs =10*, Er/Ec= 0.5 to 5
4.3 For Socket Geometry L/D=4 and Cs = 106, Er/Ec= 2
4.4 For Socket Geometry L/D=4 and Cs = 107, Er/Ec= 2
4.5.1 For Socket Geometry L/D=l and Cs = 108, Er/Ec= 0.5 to 5
4.5.2 For Socket Geometry L/D=2and Cs = 108, Er/Ec= 0.5 to 5
4.5.3 For Socket Geometry L/D=3and Cs = 108, Er/Ec= 0.5 to 5
4.5.4 For Socket Geometry L/D=4 and Cs = 108, Er/Ec= 0.5 to 5
4.6 Variation of stress with bond modulus, Cs for L/D=4
4.7 Variation of bond modulus along the rock socket
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CHAPTER 5-DISCUSSION OF NUMERICAL RESULTS AND VERIFICATION
5.1 Variation of shear stress along the rock socket interface
5.2 Comparison with Field Data and Verification 
5.3Implementaion in Real Situations
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34
CHAPTER 6-CONCLUSION 35
LIST OF REFERENCES
APPENDIX
APPENDIX-1
Verification of the Finite Element Program
APPENDIX-2
User Instructions for Graphical Interface AUTOFEAP and Technical Details
APPENDIX-3 
Sample Date Output File