Experimental Study on Tensile-Shear Fracture Behavior of Sandstone Under Vertically Non-Uniform Loading

Based on the mechanical characteristics of hard rock strata subjected to vertically non-uniform loading conditions in mountainous regions, gullies, and similar areas, this study investigates the fracture and failure patterns of hard rock strata under such conditions. A stepped rubber cushion loading method was employed to continuously apply distributed loads across the entire upper surface of the sandstone plates. Experiments were conducted to examine the fracture failure behavior of the sandstone plates under central vertically non-uniform loading with lateral clamping constraints. The displacement, stress, strain, and electromagnetic radiation characteristics of the sandstone plates were analyzed. The research results indicate the following: (1) The stepped rubber cushion loading method successfully achieves vertically non-uniform loading on the sandstone specimens. The vertical displacement of the sandstone plates increases with the number of stepped cushion layers. The average stress proportions in the thin, medium, and thick regions are 5.0%, 61.5%, and 100%, respectively. (2) Under vertically non-uniform loading, the fracture process of hard rock strata undergoes three stages: tensile failure, shear fissure development, and shear dislocation. During the tensile failure stage, a brief, stable-hinged micro-structure exists within the rock block. The entire process dynamically alternates between structural failure and the formation of temporary stable structures, accompanied by repeated fluctuations in stress states between decrease and recovery. Upon entering the shear fissure development stage, the frequency of stress fluctuations decreases, indicating a trend towards concentrated failure and instability. In the shear dislocation stage, shear fissures connect, resulting in the maximum stress drop throughout the entire failure process. (3) Under vertically non-uniform loading, tensile-shear fracture of hard rock strata occurs on the side experiencing greater vertical load. Correspondingly, in on-site engineering conditions within mountainous terrains, areas with increased vertical stress may be more prone to roof tensile-shear fracturing, thereby inducing mining tremors. These results may provide preliminary guidance for controlling rock bursts and mining tremors in mines in mountainous and gully areas, although further validation is required.