A contribution to optimum design of piled rafts

The majority of piled raft foundations have been designed ignoring any contribution from the raft or pile cap, although it is well known that the raft plays an important role in the overall performance. In such conventional designs, the overall settlement of the foundation is likely to be very small, owing to the installation of more piles than are necessary. However, from an economical point of view, it is preferable that the foundation is designed in such a way that the average and differential settlements are limited to an acceptable level, but where the load-carrying capacity of the raft is taken into account. In this paper, a framework for a new design concept, in which piles are installed only beneath the central area of a relatively flexible raft to minimize the differential settlement, is presented through the results of an extensive parametric study. The so-called 'hybrid' approach, developed by Clancy, was used in the analyses. The study showed that piled rafts may be designed for negligible differential settlements by including a pile group over the central 16-25% (by area) of the raft, with the pile group stiffness approximately equal to that of the raft alone. The total pile capacity should be about 40-70% of the total applied load, depending on the pile group area ratio and the Poisson's ratio of the soil. The validity of the method is examined through the results of centrifuge model tests conducted by the authors.