Effect of coupling excess pore pressure and deformation on nonlinear seismic soil response

Abstract

The excess pore pressure (\(\Delta p_w\)) generation and consequent reduction in effective stress lead to the softening of a liquefiable soil deposit that can alter ground motions in terms of amplitude, frequency content and duration. However, total stress models, which are the most currently used, do not take into account coupling of excess pore pressures and soil deformations. To assess this effect, two analyses were made: (1) a Biot hydraulic and mechanical computation of a saturated soil deposit with coupling pore pressures and soil deformations and (2) a mechanical computation of a decoupled model with same initial behaviour. Both analyses were performed with a fully nonlinear elastoplastic multi-mechanism model. As \(\Delta p_w\) depends on the soil properties, two soils were analysed: loose-to-medium and medium-to-dense sand. The results regarding the profile of maximum accelerations and shear strains, the surface accelerations and their corresponding response spectra are analysed. The mean values of the normalized response spectra ratio of surface accelerations between the coupled and decoupled model show a deamplification of low and high frequencies (i.e. at frequencies lower than 1.0 Hz and higher than 10 Hz) that tend to increase with the liquefaction zone size. Coupling of \(\Delta p_w\) and soil deformation is therefore of great importance to accurately model the ground motion response. On the contrary, while peak acceleration predictions could be conservative, the amplification on the low frequencies could be largely underestimated which could be highly prejudicial for flexible buildings.