A Reconsideration of the Safety of Piled Bridge Foundations in Liquefiable Soils

Abstract

The collapse of piled foundations in liquefiable soil has been observed in the majority of recent strong earthquakes. This paper critically reviews the current understanding of pile failure in liquefiable deposits, making reference to modern design codes such as JRA (1996), and taking the well-documented failure of the Showa Bridge in the 1964 Niigata earthquake as an example of what must be avoided. It is shown that the current understanding cannot explain some observations of pile failure. The current method of pile design under earthquake loading is based on a lateral loading mechanism where inertia and drag due to slope movement (lateral spreading) induce bending in the pile, and where axial load effects are ignored. It is demonstrated here, however, that axial loads can be a dominant factor in collapse due to seismic liquefaction, due to the progressive onset of pile buckling when lateral soil resistance is removed. Additional design considerations based on the avoidance of buckling effects are formulated after back analysing fifteen case histories of pile foundation performance during past earthquakes, and verified using dynamic centrifuge modelling. Some practical implications of the omission of axial loads from previous design verifications are highlighted.