Understanding the microscopic moisture migration in pore space using DEM simulation

The deformation of soil skeleton and migration of pore fluid are the major factors relevant to the triggering of and damages by liquefaction. The influence of pore fluid migration during earthquake has been demonstrated from recent model experiments and field case studies. Most of the current liquefaction assessment models are based on testing of isotropic liquefiable materials. However the recent New Zealand earthquake shows much severer damages than those predicted by existing models. A fundamental cause has been contributed to the embedded layers of low permeability silts. The existence of these silt layers inhibits water migration under seismic loads, which accelerated liquefaction and caused a much larger settlement than that predicted by existing theories. This study intends to understand the process of moisture migration in the pore space of sand using discrete element method (DEM) simulation. Simulations were conducted on consolidated undrained triaxial testing of sand where a cylinder sample of sand was built and subjected to a constant confining pressure and axial loading. The porosity distribution was monitored during the axial loading process. The spatial distribution of porosity change was determined, which had a direct relationship with the distribution of excess pore water pressure. The non-uniform distribution of excess pore water pressure causes moisture migration. From this, the migration of pore water during the loading process can be estimated. The results of DEM simulation show a few important observations: (1) External forces are mainly carried and transmitted by the particle chains of the soil sample; (2) Porosity distribution during loading is not uniform due to non-homogeneous soil fabric (i.e. the initial particle arrangement and existence of particle chains); (3) Excess pore water pressure develops differently at different loading stages. At the early stage of loading, zones with a high initial porosity feature higher porosity changes under the influence of external loading, which leads to a larger pore pressure variation (increase or decrease) in such zones. As the axial strain increases, particle rearrangement occurs and final porosity distribution has minor relationship with the initial condition, and the pore pressure distribution becomes irregular. The differences in the pore pressure development imply that water will migrate in the pore space in order to balance the pore water pressure distribution. The results of this simulation offer an insight on the microscale water migration in the soil pore space, which is important for holistic description of the triggering of soil liquefaction in light of its microstructure.