Instability of slender pile groups during earthquake-induced liquefaction

Piled foundations in liquefiable layers often bear onto firm underlying layers (bedrock). Degradation of soil stiffness following the onset of liquefaction can lead to a large reduction in the lateral stiffness of the soil-pile system. Combined with high end-reaction forces from the bedrock, an axially loaded pile may approximate to a laterally unsupported slender column and may thus be vulnerable to instability phenomena such as buckling. This paper presents preliminary centrifuge test results on the instability of pile group foundations during earthquake-induced liquefaction. 2×2 aluminium-alloy model pile groups were embedded in a layer of loose sand, bearing on a rigid aluminium plate as a model of a rock-socketed pile group. A total of four different axial load combinations were tested, and the heaviest of these was found to fail during the earthquake. The pile groups were found to suffer displacement at the head prior to the earthquakes due to swing-up effects. These displacements are akin to initial imperfections observed in the field. These imperfections caused amplification of the initial displacements due to P-delta effects, leading to the formation of a ductile plastic failure mechanism and collapse, after a limiting pile-head deflection had been reached. This collapse was found to occur below the expected critical (Euler) load in the absence of soil support. The elasto-plastic post-buckling behaviour therefore means that the Euler load only provides an upper-bound to the true failure load. Based on the centrifuge test data, it will be shown that initial imperfections are a significant parameter that can cause failure of pile groups, even with a shallow liquefiable layer.