Pile Design and Construction Practice, Fifth edition

Pile groups under compressive loading 277

occurs on piles installed in groups but the addition to the working load on each of the piles
in the group is not necessarily more severe than that calculated for the isolated pile. The
basis for calculating the negative skin friction as described in Section 4.8.1, is that the ulti-
mate skin friction on the pile shaft is assumed to act on that length of pile over which the
fill and any underlying compressible clay move downwards relative to the shaft. The mag-
nitude of this skin friction cannot increase as a result of grouping the piles at close centres,
and the total negative skin friction acting on the group cannot exceed the total weight of fill
enclosed by the piles. Thus in Figure 5.36a

total load on pile group working load (5.42)

where  is the unit weight of fill, and Dis the depth over which the fill is moving down-
wards relative to the piles.
Where the fill is underlain by a compressible clay, as in Figure 5.36b

total load on pile group working load (5.43)

where Dis the total thickness of fill,  is the unit weight of compressible clay, and Dis
the thickness of compressible clay moving downwards relative to the piles.
It should also be noted that the negative skin friction acting on the piles in the group does
not increase the settlementof the group caused by the working load on the piles. If the filling
has been in place for a long period of years any underlying compressible soil will have been
fully consolidated and the only additional load on the compressible soil causing settlement
of the group is that from the working load on the piles. However, if the fill is to be placed
only a short time before driving the piles, then any compressible soil below the fill will
consolidate. The amount of this consolidation can be calculated separately and added to the

BL(DD)

(BLD)

Working load

D

D

D

Bearing

stratum

for piles

L×B L×B

(a) (b)

Compressible

clay

Fill

Fill

Figure 5.36Negative skin friction on pile groups in filled ground (a) Fill overlying relatively
incompressible bearing stratum (b) Fill placed on compressible clay layer.