Stress-force-fabric relationship for unsaturated granular materials in pendular states

This paper explores the particle-scale origin of the additional shear strength of unsaturated granular materials in pendular states induced by the capillary effect by applying the stress–force–fabric (SFF) relationship theory to unsaturated granular material stress analysis. The work is based on discrete element simulations with the particle interaction model modified to incorporate the capillary effect. By decomposing the total stress tensor into a contact stress tensor originating from contact forces and a capillary stress tensor due to the capillary effect, the directional statistics of particle-scale information are examined. The observations are used to support the choice of the appropriate analytical approximations for the directional distributions associated with the solid skeleton and water bridges. The SFF relationship for unsaturated granular materials is formulated, and is shown to match the material stress state with good accuracy and is used to interpret the material strength in terms of the relevant microparameters. Macro and micro observations are carried out on both relatively dense and loose samples in triaxial shearing path to the critical state. The capillary force remains nearly isotropic during triaxial shearing. Anisotropy in the water bridge probability density, however, develops alongside the anisotropy in contact normal density, which decreases when the suction level decreases and the water content increases. The anisotropy effect in the water phase is much smaller than the solid skeleton, and a coupling effect with the solid phase makes the fabric anisotropy in wet materials smaller than that in dry materials. Combined with the SFF function, the increased solid coordination numbers and mean contact forces by the water bridge effect are more important factors for the suction-induced shear strength.