This study investigated the mechanical behavior of a wet granular soil at low moisture content in which isolated pendular water bridges (menisci) at the interface of the solid particles give rise to capillary forces in addition to existing interparticle contact forces. We derived a single effective stress tensor encapsulating evolving liquid bridges, packing, interfaces, and water saturation. Apart from the fact that the stress due to contact forces is dependent on the fabric, we found that the so-called suction (capillary) stress arising from liquid bridges is inevitably direction dependent, i.e., anisotropic. The latter is at odds with the common belief that capillary stress is isotropic. We demonstrate that capillary stress is a function of the distribution of liquid bridges, degree of saturation, as well as particle packing, and thus provide an adequate effective stress definition to describe both the constitutive behavior and strength of unsaturated media. We conducted discrete element modeling simulations of triaxial compression tests of pendular-state granular samples at different matric suctions to verify the anisotropic nature of the capillary stress and resulting strength contributions to validate the proposed effective stress equation.

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