Pile Design and Construction Practice, Fifth edition

Chapter 5
Pile groups under compressive
loading
5.1 Group action in piled foundations
The supporting capacity of a group of vertically loaded piles can, in many situations, be
considerably less than the sum of the capacities of the individual piles comprising the group.
In all cases the elastic and consolidation settlements of the group are greater than those of
a single pile carrying the same working load as that on each pile within the group. This is
because the zone of soil or rock which is stressed by the entire group extends to a much
greater width and depth than the zone beneath the single pile (Figure 5.1). Even when a pile
group is bearing on rock the elastic deformation of the body of rock within the stressed zone
can be quite appreciable if the piles are loaded to their maximum safe capacity.
Group action in piled foundations has resulted in many recorded cases of failure or excessive
settlement, even though loading tests made on a single pile have indicated satisfactory
performance. A typical case of foundation failure is the single pile driven to a satisfactory
set in a compact or stiff soil layer underlain by soft compressible clay. The latter formation
is not stressed to any significant extent when the single pile is loaded (Figure 5.2a) but when
the load from the superstructure is applied to the whole group, the stressed zone extends
down into the soft clay. Excessive settlement or complete general shear failure of the group
can then occur (Figure 5.2b).
The allowable loading on pile groups is sometimes determined by the so-called
efficiency formulae, in which the efficiency of the group is defined as the ratio of the
Stressed
zone
(a) (b)
Heavily
stressed zone
Figure 5.1 Comparison of stressed zones beneath single pile and pile group (a) Single pile
(b) Pile group.