Experimental Investigation on Shear Behavior of Intact Sandstones under Constant Normal Stiffness Conditions

The shear behavior of intact cubic sandstones was studied under constant normal stiffness (CNS) conditions using self-designed equipment. A series of direct shear tests on intact sandstone blocks was conducted with initial normal forces of 0, 20, 40, 60, and 80 kN at a constant normal stiffness of 400 kN/mm that came from the sandstone specimen. The results show that the deformation of intact sandstones at the prepeak elastic stage is independent of the boundary condition. The progressive shear failure process of intact sandstone specimens under the CNS condition can be divided into three stages: the prepeak stage including the prepeak elastic stage and the prepeak yielding stage; the postpeak shear wear stage; and the residual shear sliding stage according to the relationship between shear stress and normal stress. Under the condition of CNS, shear yielding failure occurs in the prepeak stage, while under the condition of constant normal load (CNL), no obvious shear yielding failure was observed. Under the condition of CNS, normal force would not stop increasing due to normal dilation after failure of some local positions in the shearing process until complete coalescence of the failure plane. The higher the initial normal force, the closer the peak shear strength point of specimens under the CNL condition approaches the yielding point of specimens under the CNS condition. Under the condition of CNS, the shear failure surface of intact sandstone is mainly formed by tension in the prepeak shear stage. The shear failure apparent friction angle at shear wear stage is smaller than the basic friction angle at residual shear sliding stage and decreases with the increase of the normal stress. The specimens under the CNS condition have a unified basic friction angle in the residual shear sliding stage regardless of the initial normal force.