A micromechanical study of unsaturated granular material using DEM simulation and X-ray CT

In unsaturated soil, capillary bridges develop at the air-water interface within the pore space due to the cohesion deficiency on the side of the air phase. The interaction between capillary bridges and the soil particles make the mechanics of unsaturated soil complicated. Using the discrete element method (DEM), the behaviour of unsaturated granular materials can be predicted by including local interactions in numerical simulations. Many recent studies have been carried out to investigate the global soil properties with water-induced stresses homogeneously distributed in a granular medium. However, the quantity and spatial distribution of capillary bridges, which depends on the anisotropy and packing density of the particle arrangement, have not been well investigated. To better understand the mechanical properties of unsaturated soils, triaxial test simulations were carried out using particle assemblies with different matric suctions, varied water distribution models and various confining stresses. Numerical testing simulation of dry triaxial tests was firstly used for calibration of the micro-mechanics of the numerical granular medium. Toroidal approximations for capillary bridges (between solid particles) were then adopted in the DEM simulations using the iterative method for estimating the geometries of the menisci. A series of water distribution models were presented for numerical specimens with different volume fractions that were permissible for capillary bridge formation. By conducting the triaxial test simulations, almost identical stress-strain behavior was observed for all distribution models with the same magnitude of liquid volume fraction. Higher stress ratios and increased dilatancy could also be achieved using specimens with a larger liquid volume fraction. Moreover, the results showed that higher shear strength and larger deformation appeared to be generated in an unsaturated granular medium when the same amount of water was more widely distributed. To validate the geometric information for the capillary bridges in the numerical model, X-ray CT was conducted using three unsaturated sample specimens consisting of spherical glass beads. Statistical image analysis was carried out using Avizo Fire v8.1 software. The merging of capillary bridges with liquid clusters leading to the formation of larger liquid ganglia, was observed when the saturation degree reached a critical limit. Capillary bridges with non-constant curvatures could develop between two solid particles without locating symmetrically to the axis passing through the particles’ centroids. These observations also appeared to show the development of liquid rings at the particle-particle interface due to the prominent interface area. The microscopic studies conducted here suggested that the strength of an unsaturated material could be significantly influenced by the coordination numbers of both water and (granular) solid. It was observed that the matric suction could dominate the water contact coordination number when the liquid phase was uniformly distributed. In this case, the ratio of coordination numbers between water and solid may thus be used as an efficient indicator for shear strength. The results also showed that the water contact coordination number appeared to be strongly dependent on the magnitude of liquid volume fraction. When the saturation degree is constant, the larger coordination number ratio may also imply a wider distribution of capillary effects