Failure analysis of rock strata between upper and lower coals under underground reservoir in coal mine and its critical percolation model of jumping permeability

The underground reservoir technology for water resources storage and recycling has provided a new way in the mining areas of western China. one of the scientific issues for the safety and stability of underground reservoirs in a coal mine is the mechanism and quantitative characterization of sudden changes in the permeability characteristics of coal and rock under mining disturbance. Taking the underground reservoirs in the Daliuta coal mine as an example, this study carried out a two-dimensional physical similarity model test of the fracture evolution of rock stratum between upper and lower coals, and the triaxial cyclic loading and unloading seepage coupling tests. In the process of the working face of the lower coal seam advancing towards the upper reservoir, the tensile and shear failure occurred in the middle stratum, and the cracks gradually extended to the direction of the reservoir dam. Using the digital image correlation method, it was observed that the fracture network of the rock mass as the dam foundation was connected from discrete local deformation to macro cracks, which penetrated into the coal pillar dam. The power-law distribution characteristics of mining fracture networks were quantitatively analyzed from the perspective of geometry and topology, and it was confirmed that the fracture evolution has connectivity probability in space. The connection of fracture network of rock strata between upper and lower coals under an underground reservoir is gradual, which is different from the rapid expansion process of fracture in rock samples. The volume expansion of coal caused by mining stress is much greater than that of sandstone under the combined effects of in-situ stress, initial mining stress, cyclic loading, and unloading. As a key stratum of hard rock, sandstone is conducive to inhibiting fracture expansion and permeability increase. The permeability measured by a transient method was calculated by the non-Darcy permeability formula based on the fractional calculus theory, which has better sensitivity than the conventional calculation method. The permeability upheaval of rock often coincides with the behaviors of the sudden drop of stress, the expansion of volume strain, and the sudden increase of fracture signal. It was further revealed that the essence of permeability is the topological connection of the fracture networks, and the mechanical and topological definitions of permeability upheaval were put forward. Based on the percolation theory, the connectivity of the fracture networks is regarded as the jumping behavior from the continuous phase to the discontinuous phase, and the percolation model of jumping permeability in coal and rock mass was established. It was verified by stress-seepage tests and creep-seepage tests that the model is effective and simple, which lays a foundation for the mathematical description of the mechanical behavior in permeability upheaval.