Numerical Analysis of the Reuse of Piled Raft Foundations

Abstract

This thesis considers the reuse and the optimum design of piled raft foundations using numerical analysis. The solution proposed for reusing foundations is the design of a piled raft foundation capable of supporting successive buildings. A raft foundation, with a limited number of piles located strategically beneath the raft and utilising a high proportion of the ultimate load capacity, is considered the most efficient design when the raft foundation alone provides sufficient load capacity, but at the same time settlements are excessive. This is the case in high plasticity stiff clays such as London Clay. When building on old piles, these piles had to sustain a certain load from the previous structure over a period of time, during which the soil is subjected to the time-related processes of consolidation and creep. A time dependent constitutive model, capable of reproducing several aspects of the time dependent behaviour of soils including creep and isotach viscosity, is employed in the finite element analyses. The model parameters are calibrated with the results of laboratory tests on London Clay, whose time dependent behaviour in its natural state is characterised by isotach viscosity, rate dependent peak strength and rate independent critical state line. Preliminary analyses of single piles indicate that the consolidation processes in the ground result in little gain in capacity in the long term, whereas creep results in increased capacity. A back-analysis of documented piled load tests in London Clay is performed, where the effects of pile installation are quantified. Numerical model calibration is performed with the documented observations of a well instrumented piled raft foundation at Stonebridge Park, London. 3D parametric analyses of piled raft foundations are then performed, in order to assess the appropriate location and dimensions of piles and raft thickness. The performance of piled raft foundations with time is assessed parametrically at different levels of preloading. Optimisation of the performance of the piled raft foundation at Stonebridge Park is attempted by examining several pile arrangements, where a central pile support with a limited number of piles is more cost-effective than the original design.