Experimental Study of the Shear Strength Characteristics and Failure Mechanism of Intact Sandstone under True Triaxial Shear Conditions

Shear failure is a common phenomenon in deep engineering works, affecting both intact and structural rock masses. In particular, under high true triaxial unequal stresses (σ1 > σ2 > σ3), rocks exhibit significant disparities in the deformation, shear strength, and failure mechanism. Therefore, it is important to explore the shear behavior of rocks under true triaxial unequal stresses. This investigation utilizes the true triaxial shear apparatus to analyze the responses of the lateral and normal stresses to the shear strength and deformation of sandstone, and the resulting effects on the shear failure mechanism. The analysis reveals consistent patterns in intact sandstone shear behavior: dilation in the normal direction, and compression deformation in the shear direction. The lateral deformation transitions from dilation at no and low lateral stresses to dilation-compression at increasing lateral stress. Meanwhile, the effects of increases in initial normal and lateral stresses that induces compressive damage can lead to an overall increase in the internal friction angle and cohesion during the peak and residual stages. Both the peak and residual shear strengths of sandstone exhibit heightened sensitivity to normal stress. Overall, the normal stress primarily affects the strength and deformation of intact sandstone. Lateral stress plays a crucial role in influencing the shear failure mechanism of intact sandstone, including its shear friction characteristics and the roughness of the shear failure surface.