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 after most major earthquakes. Poor performance of pile foundations remains a great concern to the earthquake engineering community. As a result, pile behaviour in seismically liquefiable area is still an area of active research. This keynote lecture presents the understanding obtained from research carried out by our group. The research is based on scaled model tests (carried out in the shaking table at University of Bristol, Dynamic Centrifuge Tests carried out at Shimizu Corporation, Japan), element test on liquefiable soil samples (both laboratory research sands such as Toyoura, Silica No 8 from Japan, Red Hill, Leighton Buzzard as well as liquefiable sand samples from Ganges river, Brahmaputra rover sand, Assam sand, Sendai sand, Urayasu sand) and back analysis of well documented case studies of pile foundation performance. This paper also compares the current understanding as well theories of the pile failure in liquefiable soils and compares with codes of practice. It has been shown that the current theory of pile failure as mentioned in codes of practice is based on flexural mechanism where the 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 and needs consideration during design: (a) Piles are laterally unsupported slender columns (rather than laterally loaded beams) in liquefiable soils and therefore a pile may fail by buckling instability due to axial 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 pile supported structures increases and the damping of the whole system also increases. During the transient phase, tuning effect may occur if the period of the structure comes close to the period of the earthquake causing amplification of bending moment in the pile which may lead to dynamic failure. Provisions proposed by major international codes of practice for pile design including the Japanese Highway Code (JRA) are reviewed.