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Excavation and earth surface processes (e.g., river incision) always induce the unloading of stress, which can cause the failure of rocks. To study the shear mechanical behavior of a rock sample under unloading normal stress conditions, a new stress path for direct shear tests was proposed to model the unloading of stress caused by excavation and other processes. The effects of the initial stresses (i.e., the normal stress and shear stress before unloading) on the shear behavior and energy conversion were investigated using laboratory tests and numerical simulations. The shear strength of a rock under constant stress or under unloading normal stress conforms to the Mohr Coulomb criterion. As the initial normal stress increases, the cohesion decreases linearly and the tangent of the internal friction angle increases linearly. Compared with the results of the tests under constant normal stress, the cohesions of the rock samples under unloading normal stress are smaller and their internal friction angles are larger. A strength envelope surface can be used to describe the relationship between the initial stresses and the failure normal stress. Shear dilatancy can decrease the total energy of the direct shear test under constant normal stress or unloading normal stress, particularly when the stress levels (the initial stresses in the test under unloading normal stress or the normal stress in the test under constant normal stress) are high. The ratio of the dissipated energy to the total energy at the moment failure occurs decreases exponentially with increasing initial stresses. The direct shear test under constant normal stress can be considered to be a special case of a direct shear test under unloading normal stress with an unloading amount of zero.

Experimental and Numerical Study on the Shear Strength and Strain Energy of Rock Under Constant Shear Stress and Unloading Normal Stress