Pile Design and Construction Practice, Fifth edition

276 Pile groups under compressive loading

The equivalent raft is assumed to have a rough base and is divided into four equal rec-
tangles and the settlement computed for the corner of each rectangle from equation 5.41.
The settlement at the centre of the pile group is then four times the corner settlement.

5.6 Pile groups in filled ground

The problem of negative skin friction or drag-down on the shafts of isolated piles embedded
in fill was discussed in Section 4.8. This drag-down is caused by the consolidation of the fill
under its own weight, or under the weight of additional imposed fill. If the fill is underlain
by a compressible clay the consolidation of the clay under the weight of the fill also causes
negative skin friction in the portion of the shaft within this clay. Negative skin friction also

1.0

0.9

0.8

0.7

0.6

0.5

0 0.5 1.0 1.5

H/B

Correction factor FB

FB

settlement for rough base

settlement for smooth base

2.0 2.5

Figure 5.34Correction factors for roughness of base of foundation (after Meigh(5.23)).

1.0

0.9

Rock surface

GL

D

D/B

B

012345678910

0.8

0.7

Correction factor

F

D

Figure 5.35Correction factors for depth of embedment of foundation below surface of rock (after
Meigh(5.23)).