Piled raft foundations: design and applications

In situations where a raft foundation alone does not satisfy the design req uirements, it may be possible to enhance the performance of the raft by the addition of piles. The use of a limited number of piles, strategically loc ated, may improve both the ultimate load capacity and the settlement and di fferential settlement performance of the raft. This paper discusses the phi losophy of using piles as settlement reducers and the conditions under whic h such an approach may be successful. Some of the characteristics of piled raft behaviour are described. The design process for a piled raft can be co nsidered as a three-stage process. The first is a preliminary stage in whic h the effects of the number of piles on load capacity and settlement are as sessed via an approximate analysis. The second is a more detailed examinati on to assess where piles are required and to obtain same indication of the piling requirements. The third is a detailed design phase in which a more r efined analysis is employed to confirm the optimum number and location of t he piles, and to obtain essential information for the structural design of the foundation system. The selection of design geotechnical parameters is a n essential component of both design stages, and some of the procedures for estimating the necessary parameters are described. Some typical applicatio ns of piled rafts are described, including comparisons between computed and measured foundation behaviour.