Dynamic behavior and micro–meso scale fracture mechanisms of sandstone under long-term water immersion

With the large-scale construction and long-term operation of underground reservoirs in coal mines, the problem of surrounding rock stability under long-term water immersion has become increasingly prominent. Therefore, clarifying the dynamic response of rocks under long-term immersion and impact loads is of great theoretical significance for ensuring the safe operation of coal mine reservoirs throughout their entire life cycle. In this study, sandstone samples were immersed in water for 0, 3, 30, 90 and 180 days respectively. The microstructure characterization and dynamic tests were carried out by using nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), computed tomography (CT), and an improved separated Hopkinson compression bar (SHPB) system. The results show that: (1) The combined results of SEM, NMR and CT tests indicate long-term impregnation reduces the proportion of micropores and increases the proportion of mesopores and macropores. The total dissolved solids (TDS) concentration in the impregnating solution increases with time, and the pH value of the solution first rises and then decreases with the extension of the immersion time. (2) The faster the impact velocity, the greater the frequency and amplitude of the stress wave generated. The water saturation gradually increases with immersion, weakening the discontinuity of acoustic impedance and reducing the reflection coefficient at the interface. (3) The increase in impact velocity intensifies fragmentation and promotes the development of complex fracture networks. With the extension of immersion time, the expansion of clay minerals such as montmorillonite will generate expansion stress, thereby creating micro-cracks, which leads to a higher fractal dimension of the crack network and eventually stabilizes over time. (4) Under the action of impact loads, the proportion of intergranular fractures increases with the extension of immersion time. However, at high impact speeds, the relative strength contrast between the cementing material and quartz particles leads to an increase in transgranular fracture. Overall, this study provides guidance for the safe design and operation of underground reservoirs in coal mines.