Civil Engineering Reference
In-Depth Information
In practice, the lateral capacity of a pile group will seldom be a critical feature of
a design. In situations where there is a large component of lateral load, it will be
necessary to ensure that the maximum bending moments induced in the piles will not
lead to failure of the piles. The maximum bending moments will generally be estimated
on the basis of one of the methods described in section 5.2.3, without considering the
lateral capacity of the pile group directly.
5.2 Deformation of pile groups
Over the last two decades, advances in analytical techniques and careful experimental
research have combined to provide a much improved understanding of the manner
in which pile groups deform under load. The traditional approach of replacing the
pile group by an equivalent raft foundation in order to estimate settlements, has been
replaced by techniques where the group stiffness may be calculated in terms of the com-
bined stiffness of the individual piles, making due allowance for interaction between
piles in the group.
Analysis of the response of pile groups became feasible with the growth of numerical
methods of analysis, in particular the integral equation, or boundary element method
(Banerjee and Butterfield, 1981). Application of the numerical techniques to pile group
analysis has been described in detail by Poulos and Davis (1980), who provide useful
design charts showing how the response of a pile group is affected by parameters such
as the number and size of piles, the relative stiffness of the piles and the soil and so
forth. Butterfield and Douglas (1981) have produced a similar collection of charts
giving the stiffness of common forms of pile groups under vertical and horizontal
loading.
In practical terms, one of the most useful concepts to emerge from the analytical
work is the use of interaction factors. An interaction factor,
, is defined as the frac-
tional increase in deformation (that is deflection or rotation at the pile head) of a
pile due to the presence of a similarly loaded neighbouring pile. Thus, if the stiffness
of a single pile under a given form of loading is
k
, then a load,
P
, will give rise to
deformation
α
δ
, given by
δ
=
P
/
k
(5.7)
If two identical piles are each subjected to a load,
P
, then each pile will deform by an
amount,
δ
, given by
δ
=
(1
+
α
)
P
/
k
(5.8)
α
The value of
will of course depend on the type of loading (axial or lateral), and on
the spacing of the two piles and the pile and soil properties.
The use of interaction factors may be regarded as equivalent to superimposing the
separate deformation fields that each pile would give rise to by itself, in order to
arrive at the overall deformation. This approach, illustrated in Figure 5.6 for axial
loading, has been justified experimentally from carefully conducted tests on instru-
mented piles at field scale (Cooke
et al
., 1980), and also from model pile tests (Ghosh,
1975; Abdrabbo, 1976). As pointed out by Mylonakis and Gazetas (1998), the final
