Three-Dimensional Centrifuge and Numerical Investigation of the Performance of Piled Foundations Subjected to Deep Excavation

To meet the demands for usable space in urban cities like Singapore, Hong Kong and London, the excavation of ever deeper basements has become the preferred solution. In order to minimise ground movements during excavation and to speed up the construction process, the top-down construction method is commonly adopted. With this construction method, piles are usually designed using results from pile load tests carried out at the ground surface and installed prior to excavation. The effects of stress relief due to the excavation on pile capacity and deformation characteristics are thus often neglected. In this study, centrifuge model tests were carried out to investigate the capacity and deformation of a single pile, an elevated 3 × 3 pile group and a piled raft with and without considering stress relief effects. For comparison purposes, in-flight pile load tests were conducted both at the ground surface and at the formation level after an in-flight simulation of a 20 m deep excavation. A non-dilatant pile-soil interface for modelling piles installed in normally consolidated clays and a dilatant pile-soil interface for simulating piles constructed in over consolidated clays and dense sands were adopted. In addition, discrete element modelling (DEM) of the two pile-soil interfaces was carried out to reveal the governing mechanisms and the changes in pile shaft resistance due to stress relief. Design implications for piled foundations underneath a deep excavation in non-dilatant and dilatant soils are discussed.