Excavation effects on pile behaviour and capacity

Pile load tests are often carried out at the ground surface, prior to a basement excavation. The effects of stress relief due to excavation on pile stiffness and capacity are not considered. In this paper, a series of centrifuge model tests in dry sand are reported to investigate pile behaviour with and without simulating excavation effects. In-flight load tests on 10 instrumented single piles were carried out both at the ground surface and at the formation level after excavation. The effects of distinct pile-soil interfaces on pile behaviour are studied. The test results show that the capacity of a low-friction pile (a pile with a nondilatant interface) subjected to excavation is reduced by 16%-20% when different failure criteria are adopted. The reduction is due to a decrease in shaft resistance, which is proportional to the magnitude of stress relief resulting from excavation. The angle of friction mobilized at the ultimate state is the same for piles with and without stress relief. For a high-friction pile (a pile with a dilatant interface), however, capacity of the pile subjected to stress relief increases by 22%-44%. This results from an increase in shaft resistance, due to the higher magnitude of dilation of the interface soil when stress relief is induced by excavation. The governing mechanism of shaft resistance for a high-friction pile is distinct from that for a low-friction pile.