Analysis of non-connected piled raft foundations (NCPRF) with cushion by finite element method

For the optimum design of Piled Raft Foundations (PRF), the concept of settlement reducer piles has been applied. However, when these piles are connected to the raft, a high stress may develop in the relatively small number of piles. Therefore, an alternative approach is to disconnect the piles from the raft forming a new system called a non-connected or disconnected piled-raft foundation (NCPRF or DCPRF). To facilitate load transfer mechanism from the raft to the soft to medium subsoil, a cushion, which is a compacted coarse grained soil layer beneath the raft, has been used. In this study, three dimensional finite element analyses via ABAQUS software is performed and applied to a few PRF cases including those reported in the literature and some cases of monitored and instrumented high rise buildings. The analysis consists of an investigation on factors such as: pile disconnection, cushion, raft thickness, and pile arrangement on the performance of NCPRF. Results indicate that when the non connected piles are used, the maximum axial stress decreases in the piles and the neutral plane lowers down its location. Cushion height and stiffness have a major influence on the stress ratio of piles to the total external load applied to the mat. Also, increasing the raft thickness and concentrating longer piles in the central raft area affect significantly on the reduction of settlements particularly the differential settlement.