Floor Damage Evolution in Coal Mine Reservoirs

Addressing the severe risks of floor instability and leakage in underground coal mine reservoirs in Western China under coupled mining and hydraulic pressures, this study developed a fully coupled stress-damage-seepage numerical model. Incorporating rock heterogeneity based on the geological conditions of the Shendong mining area, the model systematically simulates the evolution of floor damage under varying storage pressures, burial depths, mining heights, and lithologies. The simulation results demonstrate that storage pressure is the primary driver for deep damage propagation via a “hydraulic wedging” mechanism, governed by a critical activation threshold of 1.0 MPa. Specifically, before the water storage pressure reaches 1.0 MPa, the floor damage remains dormant and basically unchanged, stagnating at a shallow level. However, once the pressure exceeds this 1.0 MPa threshold, it overcomes the effective confining stress, abruptly shifting the failure mode from shallow discrete fracturing to deep penetrating failure, which is accompanied by an order-of-magnitude surge in permeability. Furthermore, a dimensionless sensitivity analysis reveals that burial depth and lithology strongly govern the failure path and depth. Notably, mudstone possesses a significantly lower intrinsic permeability, and even when subjected to damage, its water barrier performance remains superior to that of sandstone because its localized plastic shear characteristics highly restrict permeability mutations. In contrast, brittle sandstone is highly susceptible to tensile cracking and the formation of deep penetrating seepage channels. Additionally, mining height demonstrates weak sensitivity to the depth of floor damage due to an “equivalent unloading” mechanism, which validates the technical feasibility of constructing underground water reservoirs in ultra-thick coal seams. These findings provide a vital theoretical foundation for the scientific site selection of underground reservoirs and the precise determination of operational water level thresholds to ensure long-term stability.