The axial behaviour of piled foundations in liquefiable soil

Understanding the mechanisms by which any engineering structure resists load is an essential requirement for its consistent and reliable design. The axial resistance which can be mobilised by piled foundations in liquefiable soils when subjected to strong shaking remains highly uncertain, and a number of piled foundations have failed in strong earthquakes as recently as 2011 . The lack of visible foundation distress in many such cases indicates that failure can occur as a result of the loss of axial capacity during an earthquake, as opposed to the laterally-dominated failure modes which have been the focus of the research community for the last 20 to 30 years. In this thesis, a series of dynamic centrifuge experiments have been carried out to establish how the distribution of axial loads along the length of a pile changes during a strong earthquake. In each test, a 2 × 2 pile group was installed such that its tips were embedded in a dense sand layer which was overlain by liquefiable soil. The tests examine the effects arising from the hydraulic conductivity in the bearing layer, the influence of axial pile cap support and finally whether there are any differences in the behaviour of nominally jacked or bored piles under seismic loading. The pile cap has been shown to play a substantial role in supporting axial loads during strong shaking. In cases where the pile cap was unable to support axial load, the majority of the axial loading was carried as pile end bearing, with some shaft friction being mobilised in both the liquefiable and bearing soil layers as a result of relative lateral displacements between the soil and pile. However, where the pile cap is able to support axial loads, the settlement of the pile cap into the soil led to a dramatic transfer of axial load away from the piles and onto the pile cap. These results imply that where substantial excess pore pressures may be generated at the depth of the pile tip, then the pile caps must be able to support significant axial load. The increased effective stresses below the pile cap were responsible for the mobilisation of shaft friction on the section of pile within the liquefiable layer. However, these piles were unable to mobilise shaft friction in the bearing layer due to the reduced lateral loading on the piles. The axial behaviour of the piled foundations after the end of strong shaking is affected by the recovery of pile end bearing capacity and is therefore strongly dependent on the hydraulic conductivity of the bearing layer. The axial behaviour of nominally bored and jacked pile groups in liquefiable soil deposits are very different under seismic excitation, with the installation process of the latter substantially altering the soil conditions around the tips of the pile, such that in contrast to the bored pile groups, the jacked pile groups did not accumulate settlements until significantly after the strong shaking had commenced. These results imply that the method of installation is an important factor in the seismic response of a foundation, and may be more pronounced for real earthquakes where the number of strong shaking cycles may be more limited than those simulated in the experiments.