BUCKLING OF PILES DURING EARTHQUAKE LIQUEFACTION

Collapse of pile-supported structures by formation of plastic hinges in the piles is observed in the majority of recent strong earthquakes despite the fact that a large factor of safety is employed in their design. Studies show that when failure occurred in structures, they most often resulted due to loads that have been overlooked by the designer or considered as secondary; rather than inadequate factor of safety. Recent research into the pile failure mechanism has shown that there is also a fundamental omission of a load effect in the seismic pile design in liquefiable areas. The current codes of practice for pile design such as Euro code 8, NEHRP 2000, JRA1996 and IS1893 is based on a bending mechanism where lateral loads due to inertia or slope movement (Lateral spreading) induces bending failure in the pile. These codes omit considerations necessary to avoid buckling of a pile due to the axial load acting on it during soil liquefaction due to the diminishing confining pressure surrounding the pile. It is needless to mention that irrespective of lateral loads imposed by the earthquake, a pile has to sustain the axial load acting on it when the surrounding soil is at its lowest possible strength and stiffness owing to liquefaction. The provisions in the current codes are inadequate and buckling needs to be addressed. It must be mentioned here that buckling is the most destructive form of failure and it occurs suddenly. Bending and buckling require different approaches in design. Bending is a stable mechanism and is dependent on strength whereas buckling is dependent on geometric stiffness and is almost independent of strength. Designing against bending would not automatically suffice the buckling requirements. In pile design, to avoid buckling there is a need to have minimum diameter depending on the depth of the liquefiable soil, typically length to diameter ratio of about 12 in the likely liquefiable zone. Thus there is a need to reconsider the safety of the existing piled foundations designed based on the current codes of practice. This paper discusses the practical implications and new research needs.