Effect of Pile Spacing and Raft Thickness on the Behaviour of Piled-Raft Foundation—A Parametric Study Using FEM

Abstract

This paper presents a parametric study of piled-raft foundation under static loading in homogenous soft clay and loose sand. 3D finite element analyses have been carried out for conventional unpiled raft as well as piled rafts with 4 and 9 number of piles underneath the raft. A three-dimensional finite element package for soil and foundation is used for plane strain, linear elastic modelling of piled-raft system, and Mohr–Coulomb yield criterion is used to represent the two soil types as elastic-perfect plastic material. The loading parameters, length and diameter of circular piles, Poisson’s ratio and elastic moduli of raft, piles and soil are kept constant throughout the analyses. Focus is thereby on two parameters—pile spacing, i.e. S/D ratio and raft thickness. Variation in the peak values of settlements, bending moments, torsion, axial forces and shear forces are investigated in both raft and piles due to varying pile spacing and raft thickness in clay and sandy soil. The general conclusion of this study for providing a piled-raft foundation is to design a flexible/semi-flexible raft of optimum thickness and S/D ratio so that the deflections and force parameters on the raft and the piles are within permissible limits and the piles can effectively act as settlement reducers in addition to sharing of loads.