Piled raft foundations: design and applications

TLDR

Raft foundations alone may not meet design requirements, but strategically placed piles can enhance load capacity and reduce settlement, and piled raft behavior is characterized by a three‑stage design process. This paper discusses the philosophy of using piles as settlement reducers and the conditions under which such an approach may be successful. The design follows a three‑stage process: an initial approximate analysis of pile number versus load and settlement, a detailed assessment to locate piles and estimate piling requirements, and a refined analysis to confirm optimum pile placement and provide structural design data, with geotechnical parameters selected throughout. Typical piled raft applications are illustrated, showing that computed predictions agree with measured foundation behavior.

Abstract

In situations where a raft foundation alone does not satisfy the design requirements, 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 located, may improve both the ultimate load capacity and the settlement and differential settlement performance of the raft. This paper discusses the philosophy of using piles as settlement reducers and the conditions under which 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 considered as a three-stage process. The first is a preliminary stage in which the effects of the number of piles on load capacity and settlement are assessed via an approximate analysis. The second is a more detailed examination to assess where piles are required and to obtain some indication of the piling requirements. The third is a detailed design phase in which a more refined analysis is employed to confirm the optimum number and location of the piles, and to obtain essential information for the structural design of the foundation system. The selection of design geotechnical parameters is an essential component of both design stages, and some of the procedures for estimating the necessary parameters are described. Some typical applications of piled rafts are described, including comparisons between computed and measured foundation behaviour.