Micromechanics of granular materials with capillary effects

Based on discrete element simulations and micromechanical calculations, this paper presents studies on the shear strength properties of unsaturated granular materials with capillary effects. Capillary forces are described on the local scale, depending on the water content, and are superimposed on the standard dry particle interaction that is currently described through an elastic–plastic relation. Simulations of triaxial compression tests at several water-content levels in the pendular regime are used to investigate the macroscopic consequences of this local description. The two methods are in rather good agreement, both reproducing the dependency of shear strength on water content, as classically observed in laboratory experiments. For the water-content levels considered, a nonlinear evolution of the cohesion with the water content is obtained, resulting in shear-strength saturation with increasing water content. Moreover, the computations allow a capillary stress tensor to be exhibited from capillary forces, directly related to the cohesion of the material. Finally, emphasising this capillary stress contribution, the generalised effective stress concept is reviewed, with a conclusion on the limitations of the classical macroscopic description of water effects in unsaturated granular materials.