Site response analysis of liquefiable stratified ground comprising silt and sand: Numerical investigations

Abstract

Most of the times in situ ground comprises layers of saturated silt and sand. Present study investigates effect of thickness and position of these layers on the site response of the ground. The constitutive response of both materials has been modelled by pressure dependent multi yield material model that captures typical cyclic mobility response under undrained seismic loading. For the homogeneous sandy ground, hydraulic gradient along the depth was found to be constant and equal to the submerged unit wight of the soil (i.e., 6.86). However, when 0.5 m thick silt seam was introduced at different depths, hydraulic gradient increased to as high as 57. It is inferred that a water film under sustained high pressure may get formed beneath the silt seam. With passage of time, further increase in the hydraulic gradient across the silt seam was noted. A re-liquefaction of the overlying soil is likely to happen due to breaking of the silt seam and pressurized soil ejecta. Locally negative hydraulic gradients were also observed implying that in a soil domain there could be locally downward flow of water even though top soil gets liquified. In all cases, a zone of zero hydraulic gradient was observed near the bottom of the model. The maximum horizontal acceleration at the ground surface was found to be 0.10 g irrespective of the stratification of the ground. Wavelet coherence revealed that the acceleration record across the silt seam was in phase over entire shaking and for frequency up to 25 Hz. The input acceleration record and the acceleration record at the ground surface were found to be almost in no phase, for all cases. Spectral analysis revealed that liquefaction reduces the spectral accelerations.