A coupled continuum-discrete fluid-particle model for granular soil liquefaction

Saturated granular soils consist of mixtures of mineral particles, forming a porous matrix, and fluid (usually water) filling the pores. These soils exhibit a broad range of response patterns depending on confining pressure, level of deformation and pore fluid pressures. Large pore fluid pressures, typically caused in the field by either static groundwater condition or vibrations due to earthquakes, lead to stiffness degradation and possibly liquefaction. Liquefied soils are marked by a transition from a solid condition to a state with at least some fluid-like properties and characteristics. The mechanisms of this transition as well as the characteristics of liquefied soils are still not fully understood, especially at the micro-mechanical level. A continuum-discrete hydromechanical model was utilized in this study to analyze the coupled meso-scale pore fluid flow and micro-scale solid phase deformation of saturated granular soils. The fluid motion was idealized using averaged Navier-Stokes equations, and the discrete element method was employed to model the solid particles. The fluid-particle interactions were provided by established semi-empirical relationships. The proposed approach was validated using published experimental results of pore fluid seepage induced by hydraulic gradients through the skeleton of granular soils. Numerical simulations were conducted to analyze the impact of critical and over-critical upward pore water flow on three-dimensional sandy soil deposits within rigid containers of various heights. The conducted simulations provided valuable information on a number of salient micro-scale mechanisms of granular media liquefaction under quicksand conditions. Computer simulations were also performed to assess the liquefaction of granular deposits due to a dynamic earthquake-type base excitation. Periodic boundaries and a high gravity field were used in order to reduce the number of particles, required to achieve a realistic simulation, to a computationally manageable size. These simulations confirmed the macroscopic aspects of liquefaction as observed in experimental physical modeling and case histories of granular soil liquefaction and provided a valuable insight into the microscopic characteristics of soil liquefaction. The proposed hydromechanical model was shown to be an effective tool to investigate the response mechanisms of saturated granular soils when subjected to extreme loading conditions.