Estimation of Overall Settlement of Piled Rafts

Abstract

Estimation of overall (or average) settlement and differential settlement are the two most critical issues in the design of moderate to large sized raft and piled foundations. Recent studies have shown that a central pile group can largely eliminate differential settlements, but it is still necessary to estimate the average settlement of the combined pile group and raft foundation. This paper presents a simple method of estimating the overall stiffness of piled rafts in a non-homogeneous soil with finite depth, based on the method of Clancy and Randolph (1996). Although the method enables the estimation of the piled raft’s stiffness even when piles are installed beneath the full raft area, particular emphasis is placed on the situation where the pile support is limited to the central region of the raft. The method combines the so called ‘equivalent pier method’ (Poulos and Davis, 1980) and the ‘flexibility matrix method’ (Randolph, 1983). In the presented method, a piled raft is replaced by a ‘capped pier’. Firstly, a simple method of estimating the overall stiffness of stubby piers in a homogeneous or a non-homogeneous soil is presented. The estimated stiffnesses are compared with those calculated by a finite difference approach (FLAC, Itasca Corporation, Version 3.22). The comparison shows that the simple method gives approximate overall stiffness of piers with a wide range of slenderness ratios (length/radius) and pile-soil stiffness ratios. Secondly, the applicability of the equivalent pier method to pile group analysis is examined for homogeneous soil conditions. The calculated stiffnesses are compared with those obtained by FLAC, and the so called ‘hybrid’ method (HyPR: Clancy, 1993) where the existence of individual piles is considered. The applicability of the ‘capped pier’ method to piled raft analysis is then examined. The results show the presented simple method gives satisfactorily accurate stiffness of pile groups and piled rafts without any long computations. Finally, a short example of the application is presented for a centrifuge model piled raft. It was found that the presented method gave close overall stiffness to those obtained by observations, and by alternative rigorous calculations.