Finite element analysis of soil-structure interaction in disconnected piled raft foundations

In the last decades, the economical convenience of designing piled raft foundations (PR) has been proven. Further, recent engineering experiences put in evidence the suitability of an alternative configuration, in which the piles and the raft are disconnected (DPR) and a stiff granular layer is interposed. This choice markedly affects the raft-soil-pile interaction mechanism, and makes classical theories and design approaches unreliable. From an engineering standpoint, DPR can be conveniently designed with lower structural safety factors, since piles mostly serve as settlement reducers. In this work, 3D finite element elasto-plastic analyses have been performed to compare the deformation mechanisms and the performance of PR and DPR foundations. The overall nature of the soil-structure interaction mechanism has been investigated, as well as the structural response of pile elements. The influence of the h/d ratio for DPR has been parametrically discussed, where h is the thickness of the granular inter-layer and d the pile diameter. The differences in the PR and DPR responses are illustrated in terms of soil-structure interaction mechanisms and pile loading conditions. At variance with PR, vertically loaded DPR exhibit a peculiar distribution of the axial force, where, because of the negative soil-pile skin friction, the maximum axial force is not found at the pile head, but rather at a certain depth depending on the h/d ratio. Moreover, DPR setups lead to a significant decrease of the bending moment at the pile head.