Stress in Wet Granular Media with Interfaces via Homogenization and Discrete Element Approaches

The nature of the stress tensor for an unsaturated pendular-state granular medium is investigated by following two micromechanical approaches. Firstly, a stress tensor is analytically derived through stress-homogenization of the medium with internal surfaces being explicitly incorporated in addition to the solid, liquid, and gaseous volumes. As such, the derivation identifies a surface stress tensor associated with the liquid–gas interface endowed with distributed surface tension forces. Secondly, numerical simulations of unsaturated conditions are pursued within the discrete element method (DEM) which can only consider resultant point forces, whereas actual internal forces are indeed distributed in nature, e.g., the liquid pressure which acts over the wetted surfaces. Despite this shortcoming, stress descriptions provided by these two fundamentally distinct approaches are found to be equivalent for unsaturated media subjected to mechanical and hydraulic loading in the pendular regime. Moreover, both approaches indicate that the capillary stress, interpreted as the part of the total stress representing the combined effects of the liquid and gas phases and interfaces, is driven by the microstructure and is thus generally nonspherical.