Experimental study on tension-shear mechanical behavior of sandstone

Rock excavation and river incision result in stress unloading in one or more directions, which leads to the redistribution of stress fields of rock masses in the influenced zone. The rock masses in the unloading zone commonly bear tension-shear stress, making the failure of unloading rocks exhibit obvious tensile property. However, due to the limitation of test technology, it is still quite difficult to study the shear mechanical behavior of rock under the action of normal tensile stress. In this study, a tension double-shear test auxiliary device that can be used by conventional direct shear testing machine is designed to convert the normal compressive force to the tensile one. The direct shear tests on sandstone under tensile normal stress are carried out using this device. The results show that under the tension-shear stress, the curves of shear stress versus shear displacement before peak stress only show two deformation stages of initial nonlinear deformation and linear deformation (namely without yield stage before peak). The shear strength and shear stiffness of sandstone gradually decrease with the increase of the normal tensile stress. Compared with the Mohr-Coulomb criterion, the Hoek-Brown criterion is better in describing the strength characteristics of sandstone under tensile shear stress. Besides, as the normal tensile stress increases, the rupture surface of rock specimen has more obvious tension and is straighter in shape.