Research on the Damage Evolution of the Confined Water Floor Cracks in the Deep Stope Based on the Macro–Micro Study

The combined action of mining disturbance and floor-bearing water pressure causes the primary meso-cracks of the coal seam floor to be damaged and expanded to varying degrees. Moreover, the compressive strength of the rock is reduced to varying degrees, resulting in the development of meso-cracks in the floor. Fissures in the macroscopic scale are formed, developed, and connected. In this work, a mesoscopic crack model of different dip angles of the floor stratum was established according to Griffith’s theory. Scanning electron microscopy (SEM) was used to scan and monitor the coal seam floor rock samples before and after mining. Using a rock rheometer, saturated and dry rock samples were subjected to triaxial compression tests under various confining pressures. FLAC3D software was used to simulate the production of fissures in macro-floor rock formations under the conditions of limited water mining. The results of the study demonstrated that microscopic natural fractures existed in the floor strata, with horizontal and oblique crack angles. Micro-cracks with horizontal dip and oblique angles appeared under the operation of mining disturbance and constrained water pressure, and cracks with varying dip angles appeared at the end of the original crack, according to SEM observations. The rock strength deteriorated to varied degrees as a result of the impact of floor water. The compressive strength of rock mass decreased in the saturated state, the rock mass displayed nonlinear failure in the dry state, and the compressive strength of the rock sample increased as the confining pressure increased. The failure depth and range of the floor increased with increase in the bearing water pressure of the floor during the simulated excavation operation.