This study investigated contact distribution and force anisotropy associated with elliptical particles in granular soils within the pendular state of unsaturated soils, employing the discrete element method. The high cost of determining the micromechanical factors through laboratory tests justifies the use of this method. The macromechanical behavior of unsaturated granular soils depends on interparticle contact characteristics and liquid bridge behavior. The findings indicated that as the degree of saturation increased, both the shear strength and the anisotropy of the normal and shear forces initially rose before subsequently declining. Notably, the contact normal anisotropy exhibited minimal variation with changes in saturation. Furthermore, it was observed that as confining pressure increased at a specific eccentricity and degree of saturation, the associated anisotropies exhibited a continuous increase. In this context, as the eccentricity of the particles increased, the peak shear strength and its corresponding anisotropies initially increased and then decreased. Conversely, residual soil strength showed a consistent increase in shear strength and anisotropy with rising eccentricity.

While the shear behavior of granular soils is directly related to the microstructure of contacts which often leads to the coaxiality between Cauchy stress and Satake fabric tensors, it is generally accepted by the geomechanics and geotechnical engineering community that the capillary effects are isotropic. At low saturation levels, however, the pore fluid tends to form interparticle menisci that can also manifest an anisotropic structure, which may result in the development of anisotropic capillarity in wetted granular media. To study the interplay between the solid grain contacts and the liquid bridges at the micro-scales, this study adopts a coupled discrete element method that utilizes a linear contact model combined with a capillary model, and explores their effects by conducting a series of numerical experiments. The distributions of contact and capillary force orientations during the experiment are further investigated to better understand how their alignments affect the global response of the granular assembly subjected to a deviatoric loading. The results indicate that the global shear stress response is not only affected by the contact fabric but also by the network of liquid bridges, and we also observe that the particles may lose contact while the pendular menisci may not be destroyed during the elastic unloading.