Interaction of piled foundation with a rupturing normal fault

Abstract

Post-seismic observations in the 1999 Kocaeli earthquake in Turkey have indicated that piled foundations may be less suitable than stiff mat foundations in defending a structure against a major normal fault rupturing underneath. This paper explores the interplay of such a rupture, as it propagates in a moderately dense sand stratum, with an embedded two by four pile foundation (typical of common highway overpass bridges). An experimentally validated numerical scheme and constitutive law for sand are utilised in the analysis, with due attention to realistically modelling the non-linear pile–soil interface and the structural inelasticity of the piles. Parametric results identify and elucidate the development of different rupture mechanisms as a function of the exact location of the group relative to the fault and of the magnitude of the tectonic displacement (the fault offset). It is shown that even for a moderate fault offset (less than 0·5 m), lightly reinforced piles will fail structurally, while also forcing the pile cap and the bridge pier on top to undergo substantial rotation and displacement. Even heavy reinforcement might not prevent potentially disastrous displacements. Pile inelasticity is unavoidable and should be acceptable as part of a ductility-based design. However, despite the possible survival of the piles themselves, letting them reach the limit of their ductility capacity may lead to large cap rotation and displacements, which are likely to impose severe demands on the superstructure. Piled foundations may indeed be inferior to rigid raft foundations in protecting a structure straddling an active seismic fault, but with few notable exceptions.