Page 306 - Numerical Analysis and Modelling in Geomechanics

F.BASILE 287
soils investigation. This aspect represents a significant advantage over the t-z and
p-y curve approaches which are based on empirical parameters which may only
be derived from the results of pile load tests. However, in many practical
situations it is not possible to carry out such testing, at least in the preliminary
stages of design.
A summary of the main capabilities and limitations of some of the computer
programs discussed above is presented in Table 10.1.

Load distribution in pile groups

The distribution of load between piles in a group is of basic importance in design.
When a group of piles connected by a rigid “free-standing” cap (a common
assumption for this kind of problem) is subjected to a system of vertical loads,
horizontal loads and moments, the following features of behaviour play a major
role in the prediction of the load distribution between the piles:

1 Pile-to-pile interaction
Due to pile-to-pile interaction, groups of piles tend to deform more than a
proportionally loaded single pile. This is because neighbouring piles are
within each others’ displacement fields and hence the load per pile to
generate a given displacement is reduced for the central piles and increased
for the outer ones. Therefore, in a group of piles, the distribution of load is
not uniform, i.e. the corner piles carry the greatest proportion of load, while
those near the centre carry least. This feature of behaviour is commonly
modelled using the interaction factor approach (e.g. in MPILE and
DEFPIG). However, as discussed previously, this approximate method
suffers from some significant limitations.
2 Group stiffening effect
The simultaneous presence of all the piles within the soil mass has the
effect of “stiffening” the soil continuum. Therefore, the central pile of a
group (the most affected by the presence of the other piles) is subjected to a
reduction of the head deformation due to the greater stiffness of the
surrounding soil, “reinforced” by the presence of the other piles. This
increased stiffness of the central pile results in a higher proportion of the
applied load taken by the pile and hence the non-uniformity of load
distribution resulting from pile-to-pile interaction (Feature No. 1) is
reduced. It has been shown that these group stiffening effects are more
marked in a laterally loaded pile group than in an axially loaded one
(Burghignoli and Desideri, 1995; Basile, 1999), and they become more
significant for increasing the number of piles in a group.
It is therefore important to recognise that each pile interacts with the
surrounding soil with a twofold effect: on the one hand, the displacement of
the other piles tends to increase as a result of the stresses transferred to the