Optimizing foundation performance: the impact of raft on piled raft foundation in sand

Conventional piled foundation designs tend to be overly conservative since the beneficial role of the raft is often neglected by assuming the piles to be the only part of the structure interacting with the soil. The contribution of the raft to the global response of the foundation is particularly important in the case of "large"piled rafts, where the pile length is comparable to the raft width. This configuration, although not theoretically optimal, is common for existing foundations of bridges and high-rise buildings. Although the beneficial effect of raft-pile-soil interaction on both bearing capacity and stiffness is generally acknowledged, simple and reliable approaches recognized by design codes are not yet available. Aiming at providing simple tools for designing large piled rafts under static and dynamic/cyclic loads, in this paper a numerical study is presented, with preliminary finite element analyses performed to examine the mechanical behaviour of a "large"piled raft under vertical centred loads and positioned on a dry sand layer. This paper presents the findings of this study, comparing the performance of the piled raft to the corresponding pile group and unpiled raft, and highlights the importance of considering the presence of rafts in the design of piled foundations.