State Key Laboratory of Water Resource Protection and Utilization in Coal Mining,National Institute of Low Carbon and Clean Energy
School of Mechanics and Civil Engineering,China University of Mining and Technology
Shendong Coal Group Co.,Ltd.
School of Energy and Mining Engineering,China University of Mining and Technology of Mechanics and Engineering,Liaoning Technical University

The underground reservoir technology for water resources storage and recycling has provided a new wayin the mining areas of western China. one of the scientific issues for the safety and stability of underground reservoirsin a coal mine is the mechanism and quantitative characterization of sudden changes in the permeability characteristicsof coal and rock under mining disturbance. Taking the underground reservoirs in the Daliuta coal mine as anexample,this study carried out a two⁃dimensional physical similarity model test of the fracture evolution of rock stratumbetween upper and lower coals,and the triaxial cyclic loading and unloading seepage coupling tests. In the processof the working face of the lower coal seam advancing towards the upper reservoir,the tensile and shear failure occurredin the middle stratum,and the cracks gradually extended to the direction of the reservoir dam. Using the digital im⁃age correlation method,it was observed that the fracture network of the rock mass as the dam foundation was connectedfrom discrete local deformation to macro cracks,which penetrated into the coal pillar dam. The power⁃law distribu⁃tion characteristics of mining fracture networks were quantitatively analyzed from the perspective of geometry and topol⁃ogy,and it was confirmed that the fracture evolution has connectivity probability in space. The connection of frac⁃ture network of rock strata between upper and lower coals under an underground reservoir is gradual,which is differentfrom the rapid expansion process of fracture in rock samples. The volume expansion of coal caused by mining stressis 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 permeabilityincrease. The permeability measured by a transient method was calculated by the non⁃Darcy permeability formulabased on the fractional calculus theory,which has better sensitivity than the conventional calculation method. The per⁃meability upheaval of rock often coincides with the behaviors of the sudden drop of stress,the expansion of volumestrain,and the sudden increase of fracture signal. It was further revealed that the essence of permeability is the topolo⁃gical connection of the fracture networks,and the mechanical and topological definitions of permeability upheaval wereput forward. Based on the percolation theory,the connectivity of the fracture networks is regarded as the jumping be⁃havior from the continuous phase to the discontinuous phase,and the percolation model of jumping permeability in coaland rock mass was established. It was verified by stress⁃seepage tests and creep⁃seepage tests that the model is effec⁃tive and simple,which lays a foundation for the mathematical description of the mechanical behavior in permeabilityupheaval.