288 Pile groups under compressive loading
Randolph(1.1)pointed out that where the ratio of the width of a pile group to the pile
length is greater than unity the pile cap contributes significantly to the load transfer from
the superstructure to the soil. Hence, the stiffness of a piled raft where the piles are
arranged to cover the whole foundation area will be similar to that of the raft structure
without the piles. Thus by concentrating the piles in the central area and using shorter
piles (or no piles) around the edges, the bending moments due to dishing of the raft are
considerably reduced. In the case of a uniformly loaded foundation area analyses show
that piles of length greater than 70% of the foundation width situated over the central 25%
to 40% of the raft area are required. Hence, instead of conventionally spreading the piles
uniformly over the whole foundation area, as little as 30% to 50% of the cumulative length
of all the piles is needed.
Load distribution between the piles is achieved through the continuous pile cap which
must be designed to be stiff enough to achieve this. With perfect optimization differential
settlement can be reduced to zero. The analysis to achieve optimization is complex
and involves interaction factors discussed in Section 5.4. A computer is required to perform
the necessary calculations. It is also necessary to check that the stress is not excessive on the
shafts of the central piles which are designed to carry a high proportion of the load.
5.11 References
5.1 POULOS, H. G. Pile behaviour – theory and application, Geotechnique, Vol. 39, No. 3, 1989,
pp. 365–415.
5.2 BURLAND, J. B. and WROTH, C. P. General report on Session 5: Allowable and differential settle-
ments of structures, including settlement damage and soil structure interaction, Proceedings
of the Conference on Settlement of Structures, Cambridge, 1974, Pentech Press, London, 1975,
pp. 611–54.
7006005004003002001000Pile load, kNErection of super structure begun(^13)
/^2
storeys
(^16)
/^2
storeys
16 storeys end of construction
12 storeys
Corner
Edge loads
and live
Full dead
Interior
1974 1975 1976
Figure 5.42Load distribution on piled raft in London Clay (after Cooke et al.(5.36)).