Volume 6 – Geotechnical Manual, Site Investigation and Engineering ...

Chapter 9 FOUNDATION ENGINEERING
In this case, the pile rotation at ground surface, Ө, equals zero and the fixing moment, M f , and
lateral deflection, δ h , are given by the following expression:
M f = - 0.375H(o.5L c)
ρ c '
(9.27)
δ h = (E p/G c ) 1
7
0.27- 0.11
H
(9.28)
ρ c 'G c ρ c '
0.5L c
Where:
δ h
Ө
G c
L c
E po
= lateral pile deflection at ground surface
= pile rotation at ground surface
= characteristic shear modulus, i.e. average value of G* over the critical length L c of
the pile
E
= critical pile length for lateral loading = 2r o
G
= equivalent Young’s modulus of pile = 4E pI p
r o
4
c ’ = degree of homogeneity over critical length, L c = G* 0.25Lc
G c
G* = G(1+0.75v s )
G* 0.25Lc = value of G* at depth of 0.25L c
v c = Poisson’s ratio of soil
G = shear modulus of soil
H = horizontal load
M = bending moment
E p I p = bending stiffness of pile
= pile radius
r o
The lateral deflection of a fixed-head pile is approximately half that of a corresponding free-head
pile.
9.4 PILE GROUP
9.4.1 General
Piles installed in a group to form a foundation will, when loaded, give rise to interaction between
individual piles as well as between the structure and the piles. The pile- soil-pile interaction arises
as a result of overlapping of stress (or strain) fields and could affect both the capacity and the
settlement of the piles. The piled foundation as a whole also interacts with the structure by virtue
of the difference in stiffness. This foundation-structure interaction affects the distribution of loads
in the piles, together with forces and movements experienced by the structure.
The analysis of the behaviour of a pile group is a complex soil-structure interaction problem.
The behaviour of a pile group foundation will be influenced by, inter alia:
(a) Method of pile installation, e.g. replacement or displacement piles,
(b) Dominant mode of load transfer, i.e. shaft resistance or end- bearing,
(c) Nature of founding materials,
(d) Three-dimensional geometry of the pile group configuration,
March 2009 9-45