Collapse of structures resting on piled foundations is still observed after strong earthquakes despite the fact that a large factor of safety is employed in their design. It has been identified that the assumed failure mechanism underlying the current design methods cannot explain some of the observed seismic pile failures. In this paper a new theory of pile failure in liquefiable soils is described and is compared with the current design methods. First, the theory of pile failure based on buckling instability is described introducing the concept of effective length of pile in liquefiable zone. The main postulate of this theory is that if piles are too slender, they require lateral support from the surrounding soil if they are to avoid buckling instability. This lateral support can fall to near zero due to seismic liquefaction and a slender pile may buckle. Detailed centrifuge testing, in-depth study of field case records and analytical studies form the basis of this theory. Next, the design methods of Eurocode 8, JRA (1996) and NEHRP (2000) are examined with respect to this theory of pile failure. It has been shown that the current codes of practice for pile design omit considerations necessary to avoid buckling of fully embedded piles in liquefiable soils. These codes should be modified to address buckling. It is proposed that the slenderness ratio of pile (i.e. the effective length of pile/minimum radius of gyration) in the liquefiable soils be kept below 50 i.e. piles should have length to diameter ratio of about 12 in the likely liquefiable zone to avoid instability failure.
展开▼
