Design of pile foundations in seismically liquefiable areas: Theory and Codes of Practice

Collapse and/or severe damage to pile-supported structures are still observed in liquefiable soils aftermost major earthquakes. Poor performance of pile foundations remains a great concern to theearthquake engineering community. As a result, pile behaviour in seismically liquefiable area is still anarea of active research. This keynote lecture presents the understanding obtained from research carriedout by our group. The research is based on scaled model tests (carried out in the shaking table atUniversity of Bristol, Dynamic Centrifuge Tests carried out at Shimizu Corporation, Japan), elementtest on liquefiable soil samples (both laboratory research sands such as Toyoura, Silica No 8 fromJapan, Red Hill, Leighton Buzzard as well as liquefiable sand samples from Ganges river, Brahmaputrarover sand, Assam sand, Sendai sand, Urayasu sand) and back analysis of well documented case studiesof pile foundation performance. This paper also compares the current understanding as well theories ofthe pile failure in liquefiable soils and compares with codes of practice. It has been shown that thecurrent theory of pile failure as mentioned in codes of practice is based on flexural mechanism wherethe lateral loads on the pile (due to inertia and/or lateral spreading) induce bending failure. However,the current understanding identifies few other mechanisms which may control the pile behaviour andneeds consideration during design: (a) Piles are laterally unsupported slender columns (rather thanlaterally loaded beams) in liquefiable soils and therefore a pile may fail by buckling instability due toaxial load alone given the inherent imperfections in a pile; (b) During the process of liquefaction (i.e.soil changing from being solid-like material to a fluid like material), the fundamental period of pilesupported structures increases and the damping of the whole system also increases. During the transientphase, tuning effect may occur if the period of the structure comes close to the period of the earthquakecausing amplification of bending moment in the pile which may lead to dynamic failure. Provisionsproposed by major international codes of practice for pile design including the Japanese Highway Code(JRA) are reviewed.

کلیدواژه ها:

Liquefaction ، Seismic area ، Soil-pile interaction ، Codes of practice ، Case studies