Seismic Requalification of Pile Foundations in Liquefiable Soils

Abstract

Pile-supported structures founded on liquefiable soils continue to collapse during earthquakes despite being designed with required factors of safety against bending due to lateral loads and axial capacity (shaft resistance and end-bearing). Recent research identified a few weaknesses in the conventional design approach: (a) when soil liquefies it loses much of its stiffness and strength, so the piles now act as long slender columns, and can simply buckle (buckling instability) under the combined action of axial load and inevitable imperfections (e.g. out-of-line straightness, lateral perturbation loads due to inertia and/or soil flow). In contrast, most codes recommend that piles be designed as laterally loaded beams; (b) Natural frequency of pile supported structures may decrease considerably owing to the loss of lateral support offered by the soil to the pile and the damping ratio of structure may increase to values in excess of 20 %. These changes in dynamic properties can have important design consequences. The immediate need is not only to rewrite the design code to incorporate these effects, particularly buckling instability but also to requalify and, if necessary, strengthen the existing important piled foundations in liquefiable soils. This paper aims to provide a methodology for carrying out requalification studies. A practical example is taken to show the application of the methodology.