Instability and migration of overlying strata under coupled mining disturbance and fluid erosion

Severe instability of the overlying strata and surface subsidence occur at the S1234 longwall face of Ningtiaota Coal Mine under the combined effects of fluid erosion and hydro-mechanical coupling, while the underlying mechanisms remain unclear. Based on field geological investigation, micro-CT scanning, UDEC numerical simulation, and physical model tests were combined to systematically investigate the instability and migration of the overlying strata and the evolution of surface subsidence under hydro-mechanical coupling. Microstructural CT analysis revealed the controlling effect of heterogeneous pore distribution in sandstone on rock-mass strength. Discrete-element simulation reproduced the evolution of overburden fracture, stress redistribution, and displacement, while a 1:150 physical model reproduced the dynamic processes of excavation disturbance, intensified water inflow, rock-mass collapse, and surface subsidence. The results show that the water-wedge effect induced by aquifer seepage significantly weakens the rock structure, mining-induced stress redistribution drives the layered collapse of the overburden, and fluid erosion further promotes fracture development and expands the subsidence zone. At the fully developed mining stage, the maximum surface subsidence reached approximately 4.2 m, whereas after pre-drainage and pressure-relief drainage it decreased to approximately 3.2 m. Pre-drainage and pressure-relief drainage can reduce hydraulic disturbance in the aquifer and mitigate deterioration of the surrounding rock and the development of surface subsidence under mining conditions. The results provide a theoretical basis and technical support for safe coal mining, surface-subsidence control, and mine-water-hazard mitigation in water-rich mining areas.