Material Strength, Capillary Effect and Fabric Anisotropies of Unsaturated Granular Materials in thePendular State

摘要

Water content is an important parameter affecting soil properties which is concerned in slope stability analysis and surface excavations.With a small amount of water adding into a granular material,generally less than 20％ degree of saturation,isolated water bridges are formed between soil particles in contact or within a small distance.This state is named as thependular state.The air-water interfacial energy and the air-water pressure difference (the matric suction) are acting through the liquid bridges as cohesive forces and the soil strength and dilatancy are increased.With the aid of the Discrete Element Method (DEM), this paper aims to investigate the relationship between material strength, capillary effect and fabric anisotropies in the wet granular material, which could enhance the fundamental principles of unsaturated soil mechanics.By implementing the capillary effect between spherical particles,unsaturated granular materials are simulated in DEM and the mechanical properties and the micro-mechanisms are investigated by triaxial compression tests.The directional statistical theory is employed and the internal fabrics and particle scale interactions are formulated by direction tensors.By decomposing the total stress into a contact stress tensor which is associated with mechanical contacts and a capillary stress tensor determined by capillary effect,a Stress-Force-Fabric (SFF) relationship is proposed for unsaturated granular materials.It links the the material strength with the mean contact and capillary force level, solid contact and water bridge intensity and the anisotropic effects in solid skeleton and water bridge network respectively.By applying the SFF theory,it is observed that the solid skeleton in wet granular materials is less anisotropic than that in a dry sample undertriaxial shearing.The water phase anisotropy evolves with the soil skeleton deformation but it is less significant than that in the solid phase.This work also explains that the capillary cohesion is mainly induced by the capillary bonding effect on coordination numbers and mean contact force levels.