Periodic softening–hardening damage mechanism of sandstone under cyclic water immersion

The stability of the surrounding rock in underground tunnels is vital for the safe extraction of coal. Groundwater seepage, in conjunction with rock–water interactions, triggers progressive degradation, which is a primary cause of surrounding rock instability. This degradation leads to damage, posing a serious threat to the safe operation of the coal mine. In this study, cyclic loading and unloading experiments and computed tomography (CT) scanning were conducted on sandstone specimens subjected to varying cyclic water immersion heights. The results demonstrate that increasing the cyclic immersion height and immersion height differential significantly reduces the ultimate bearing capacity of sandstone. This reduction corresponds to a diminished capacity for elastic energy storage and an accelerated rate of energy dissipation associated with crack propagation, as evidenced by an increasing proportion of dissipated energy. Combined with CT scanning analysis, both the three-dimensional fracture volume ratio and the fractal dimension rise with cyclic immersion height, indicating more complex fracture development. Additionally, the study reveals the periodic softening–hardening damage mechanism of sandstone under cyclic water immersion. During the cyclic water immersion process, the periodic loss and replenishment of moisture within the sandstone induce softening–hardening effects on its structure. This alternating process promotes progressive fracture initiation and propagation, accelerates internal damage accumulation, and leads to the degradation of mechanical integrity, ultimately rendering the sandstone more vulnerable to failure. These findings provide valuable insights for the study of surrounding rock stability in coal mine tunnels.