Evaluation of variation of permeability in liquefiable soil under earthquake loading

Liquefaction phenomenon is usually accompanied by large amounts of settlement owing to disruption of soil structure. In addition to that, large settlement also occurs by a significant increase in soil permeability during seismic excitation. To properly simulate the post-liquefaction settlement, it is important to take the compressibility properties of the liquefied sand as well as the permeability increase into account. Using initial permeability coefficient in the course of simulation of liquefaction leads to underestimation of settlement. In addition to that, using unrealistic values for permeability may cause erroneous predictions of other aspects of soil behavior. Therefore, an accurate simulation of pore pressure generation and dissipation and consequent settlement during liquefaction requires incorporating the actual variation of permeability in the numerical model. In this paper, variation of soil permeability during liquefaction and its effects on soil seismic response is studied using a fully coupled dynamic analysis. Having a realistic mechanism for simulation of soil skeleton response using a well-calibrated critical state two-surface plasticity model, the focus of attention in this paper is on the effects of permeability variation on the behavior of liquefied grounds. Numerical simulations are performed using parts of Modified OpenSees Services (MOSS) library, a set of coupled finite elements, constitutive integration procedures, and material models from the UC Davis toolset along with a novel implementation of variable permeability. Two relationships are proposed for considering the variations of permeability coefficient in the process of liquefaction. The proposed relationships have been implemented into the model and applied for simulation of a centrifuge experiment. Comparison of the numerical results and experimental measurements reveals that there is a direct relationship between (variable) permeability coefficient and excess pore pressure ratio in all build-up, liquefaction, and dissipation phases.