Variation of modal parameters of pile-supported structures in seismically liquefiable soils.

Multi-storey buildings and bridges built on loose to medium dense sands are often supported on pile foundations. During earthquakes, if these sandy soils are saturated, they tend to develop excess pore water pressure, which in extreme cases may lead to the so-called liquefaction condition. Over the past years, the seismic design of pile-supported structures in liquefiable soils has been a constant source of attention to the earthquake engineering community. Many seismic design codes advise practising engineers to design pile foundations against bending due to inertia and kinematic forces induced by the deformation of the surrounding soil. In the presence of liquefaction phenomena, Eurocode 8 (EN 1998-5:2004, 2004) recommends that “the side resistance of soil layers that are susceptible to liquefaction or to substantial strength degradation shall be ignored”. Similarly to the Eurocode 8, the Japanese Highway Code of practice JRA (JRA, 2002) suggests to design pile-supported structures considering two different loading conditions comprising: (a) kinematic loading exerted by the lateral pressure of the liquefied layer and any non-liquefied crust resting on the top of the liquefied deposit; (b) inertial force due to the oscillation of the superstructure. The code recommends engineers to check against bending failure considering inertia and kinematic forces separately.