Design of piled raft foundations

In many circumstances, the decision whether to incorporate piles into a foundation may be marginal, and will probably rest on the need to reduce settlements to an acceptable level. The optimal solution in such cases may be to consider a ‘mixed’ foundation, where the load-carrying capability of the raft is utilised and relatively few piles are used. This paper describes ways in which the overall stiffness of the foundation, and the load distribution between pile cap and piles, may be assessed in terms of the stiffness of the two components, with allowance for interaction between them. The first part deals with the situation where the piles are assumed to be loaded below their failure load, while the possible use of ‘plastic’ piles (loaded to their full capacity) beneath a raft foundation is discussed towards the end of the paper. The analytical ideas are illustrated by means of a case history of an instrumented piled foundation in stiff clay.