Bending, bearing and buckling: effects of axial load on pile response in liquefiable soils

The design of piled foundations to resist the effects of earthquake-induced liquefaction commonly focuses on lateral bending moments induced in the piles. Axial load in the piles is rarely considered, despite these loads often being substantial at the time of earthquake (including superstructural loads and any axial dynamic loads). Recent research has identified two key axial pile failure mechanisms in liquefying soil, namely instability (buckling) and bearing capacity failure. The former occurs due to the loss of lateral soil restraint to the pile which essentially behaves as an unsupported column. The latter is a result of a reduction in pile tip bearing capacity due to excess pore pressure rise within the bearing layer. This paper will describe these two competing modes of failure based on observations from dynamic centrifuge testing. An analytical solution to determine the elastic critical (buckling) load for floating piles will be presented, along with an analytical relationship describing the loss in pile bearing capacity. Both of these solutions allow for determination of the limiting load for a downwards-propagating liquefaction front. Finally, these relationships will be used to investigate the vulnerability of steel and reinforced-concrete piles of varying dimensions founded in saturated sandy soils during earthquake induced liquefaction.