An uncoupled approach for estimating seismic-induced pore water pressures in liquefiable sandy soils

Proper evaluation of seismic-induced excess pore water pressures in saturated sandy soils is still an open issue, which can be tackled with fully coupled to uncoupled approaches. The former are more accurate but computationally onerous, while the latter require seismic demand and pore pressure build-up to be computed in two successive steps, typically employing simple constitutive assumptions. Starting from the work by Seed et al. (1975), this paper presents a novel uncoupled procedure to compute excess water pressures developing in a 1D soil column under partially drained conditions, when subjected to horizontal seismic excitations. Fundamental modifications are introduced to account for: non-uniform distribution of equivalent loading cycles; soil stiffness degradation; and modification of the frequency content of ground motion due to pore pressure build-up. The approach was implemented in Matlab via the Finite Difference Method and validated against both fully coupled Finite Element analyses and one centrifuge test. An extensive parametric study was also performed for a two-layer soil column, by varying the thickness and hydraulic conductivity of the shallow layer, as well as the seismic input. The good agreement with both numerical and experimental data demonstrates that key features of liquefaction are well-captured by the proposed uncoupled approach.