Evolution Characteristics of Sandstone Fractures under Stepped Cyclic–Constant Loadings Based on the Number of Cyclic Ladder Steps

As one of the most widely distributed media in the overlying strata of coal seams, sandstone is subjected to stepped cyclic–constant combined loads induced by mining and stopping operations. Nevertheless, the evolution characteristics of sandstone fractures under such stress paths remain unclear. Accordingly, triaxial compression tests are carried out in this paper. Multiple testing technologies including acoustic emission monitoring, three-dimensional fracture reconstruction, seepage simulation, particle size screening, and cross-section scanning are comprehensively adopted to clarify the fracture evolution characteristics of sandstone under different numbers of cyclic ladder steps m . The results showed that acoustic emission ring counts emerge densely and then gradually stabilize in each preliminary loading stage. Overall, as m increased, the proportion of high-frequency acoustic emission signals, b -value, and mass fraction of medium and small blocks after fragmentation all followed a trend of decreasing first and then increasing. The main fracture surfaces of sandstone were rougher at m = 1 and m = 4 . The fracture morphology of sandstone presented three stage variation characteristics with the change of m : numerous scattered fractures develop on both sides of specimens at m = 1 ; the structures surrounding main fracture surfaces become more regular at m = 2 and m = 3 ; obvious branch structures form on main fracture surfaces at m = 4 , which greatly improves fracture connectivity. The research findings help deepen the understanding of the influence of mining-induced stress on rock fracture evolution.