Abstract
Piled raft is commonly used as foundation for high rise buildings. The design concept of piled raft foundation is to minimize the number of piles, and to utilize the entire bearing capacity. High axial stresses are therefore, concentrated at the region of connection between the piles and raft. Recently, an alternative technique is proposed to disconnect the piles from the raft in a so called unconnected piled raft (UCPR) foundation, in which a compacted soil layer (cushion) beneath the raft, is usually introduced. The piles of the new system are considered as reinforcement members for the subsoil rather than as structural members. In the current study, the behavior of unconnected piled rafts systems has been studied numerically by means of 3D Finite Element analysis via ABAQUS software. The numerical analysis was carried out to investigate the effect of thickness and stiffness of the cushion, pile length, stiffness of foundation soil, and stiffness of bearing soil on the performance of the unconnected piled raft. The results indicate that when unconnected piles are used, the axial stress along the pile is significantly reduced e.g. the axial stress at head of unconnected pile is decreased by 37.8% compared with that related to connected pile. It is also found that the stiffness and thickness of the cushion, and stiffness of foundation soil have considerable role on reduction the settlement.
Highlights
The piled raft is a composite foundation that comprises three parts; raft, piles and soil
An elastoplastic numerical analysis was performed by (Eslami et al, 2012), showed that the thickness and the stiffness of cushion affects the location of the maximum axial stress along piles. (Sharma et al, 2011), showed that using the cushion will adjust the load-sharing ratio evenly among the piles. (Hor et al, 2015) employed a three dimensional finite element method via Plaxis to model the complex interactions of the disconnected piled raft taken into account the load transfer in the interposed cushion layer and along the pile
The results obtained from this study showed that, the performance of unconnected piled raft for heavy load structures is better compared with connected system. (Ata et al, 2015) performed a parametric study on unconnected piled raft, they showed that unconnected piled raft may be considered as an economical alternative to the connected piled raft
Summary
The piled raft is a composite foundation that comprises three parts; raft, piles and soil. To overcome the problem of connection reaction an alternative approach has been proposed where unconnected piles are used and provided an interposed cushion layer between the raft and the piles The piles in such condition will behave as soil stiffeners instead of as structural members (Wong et al, 2000), (Burghignoli et al, 2007), (Jamiolkowski et al, 2009). The study show that, a much lower factor of safety against structural failure of the piles can be used by disconnecting the piles from the raft This may be due to the fact that the piles can be considered as soil reinforcement members strengthen the subsoil rather than as structural members carrying the applied load (Liang et al, 2003) examined numerically in the elastic regime many parameters such as the thickness and stiffness of the cushion layer, the ratio of length to diameter of the pile, and the elastic modulus of piles. The present parametric study aims at identifying the most important parameters which affect the performance of the unconnected piled raft foundation such as; thickness and stiffness of the cushion, length of the piles, stiffness of bearing soil, and stiffness of foundation soil
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