Mitigation of liquefaction risk in layered soils via stone column drains: numerical study and novel uncoupled approach

This paper investigates the effectiveness of vertical gravel drains for liquefaction mitigation in stratified soil deposits, emphasising the overlooked hydro-mechanical interaction with adjacent non-liquefiable layers. A comprehensive series of fully coupled 3D finite element analyses was first conducted in the OpenSees framework, modelling a unit cell within an indefinite drain system. Different spacing ratios, soil types, and seismic inputs were examined to provide generality and robustness to the study. The main outcome from the numerical analyses is that gravel drains significantly reduce both the peak excess pore water pressure and the duration of high pore pressures, with the hydraulic conditions imposed by the overlying non-liquefiable layers proving critical, particularly near layer interfaces. To quantify mitigation effectiveness, a new integral, dimensionless parameter was proposed, which conveys both the magnitude and dissipation time of the excess pore water pressures. As a further outcome, this study extends to axisymmetric conditions a 1D uncoupled approach recently proposed for assessing free-field liquefaction, incorporating improvements to capture non-uniform cyclic loading and frequency variations induced by pore pressure build-up. The methodology couples a nonlinear total stress seismic response analysis with an iterative excess pore pressure computation using the Stockwell transform, implemented via a Finite Difference scheme in Matlab. Successful validation against the benchmark fully coupled 3D analyses proves that the uncoupled approach can be effectively adopted with low computational cost.