A micromechanical interpretation of the capillary effect of unsaturated granular material in a pendular state

The micro-mechanism of the capillary strengthening effect is studied in this paper with the aid of Discrete Element Methods (DEM) and the Stress-Force-Fabric (SFF) relationship. For an unsaturated granular material in pendular regime, pore water is discontinuously distributed at contact points in the form of capillary bridges, leading to the capillary force being the sum of the force component induced by air-water pressure difference on the section of the bridge neck and the force component induced by surface tension. Such a formulated contact model has been implemented in the DEM simulations. Directional statistical theory has been applied to study the statistical features of the contact interactions, the water bridges and the capillary forces. The impact of water bridges on the strength of unsaturated granular material has been interpreted through the SFF relationship to explain the capillary strengthening effect. Specimens with more uniform water distributions were found of higher shear strength, suggesting the impact for water distributions, a topic for more research.