Dynamic effects during alternate infiltration and drainage of unsaturated granular media studied using the Lattice Boltzmann Method

Dynamic effects in connection with multi-phase or unsaturated water flow in soils are often observed experimentally. They generally arise as temporarily delayed changes in the water content or in the outflow. Possible mechanisms leading to such effects during infiltration have not been investigated in detail. Experimental investigations on sand columns have shown that for cyclic hydraulic conditions, i.e. the alternate infiltration and drainage of water, dynamic effects may even result in an accumulation of water within the pore structure, which is denoted here as hydraulic ratcheting. This cyclic accumulation of water may lead to catastrophic consequences due to the associated increase in pore water pressure. Although this hypothesis is supported by experimental evidence, a microscopic insight into the underlying processes and mechanisms is needed in order to predict it. In this study, numerical simulations based on the Lattice Boltzmann Method (LBM) are conducted to examine these microscopic effects that are beyond the scope of traditional models. In the LBM, the fluid is represented by a discrete mesh used to solve the fundamental Boltzmann equation. By adding cohesive and repulsive body forces into the LBM mesh, multi-component and multi-phase fluids and capillarity interaction with solids can be realistically represented. With this tool, a fully interacting unsaturated soil, composed of three phases: solid particles, water and air, is modelled as shown in Figure 1. The air and water interact by forming bubbles, and capillarity bridges are formed around closely-packed particles. By harmonically changing the pressure conditions on the bottom boundary, the cyclic behaviour related to the hydraulic ratcheting is simulated. Extensive simulations of random configurations explore the effects of the soil geometric features (particle size distribution, mean constriction size and porosity) and the frequency of infiltration on the cyclic water accumulation. The results can be used in the development of constitutive predictive models dealing with this complex phenomenon.