Study on Dynamic Evolution Law of Fracture and Seepage in Overlying Strata during Mining under Karst Aquifer

In view of coal seam mining under a karst aquifer, the roof of the coal seam weakens and seeps under the combined action of mine pressure and pore water, which accelerates the expansion of overburden fracture and eventually leads to the problem of roof water inrush accident. Based on the engineering background of the Daojiao Coal Mine, this paper establishes a fracture propagation model under hydraulic coupling through theoretical research, and reveals the fracture propagation conditions under hydraulic coupling before disturbance and the influence of stress change after disturbance on the critical water pressure change of fracture propagation. The fluid–solid similarity simulation test of mining under a karst aquifer is designed, and the influence of the combined action of water pressure and water–rock coupling on the propagation of mining-induced fractures in overlying strata is realized. Combined with the fluid–solid coupling numerical simulation test, the fracture evolution law and fracture seepage dynamic evolution law of mining under a karst aquifer are clarified. The results show that the stress intensity factor at the crack tip increases with the increase of the length a of the original crack, and decreases first and then increases with the increase of the dip angle β. In addition, a lower critical water pressure required for crack propagation results in easier expansion of the crack; according to the results of similar simulation experiment and numerical simulation experiment, the heights of caving zone and fracture development zone are approximately 8 and 35 m, respectively. The fractal dimension of fracture increases first and then decreases with the advance of working face, and finally tends to be stable; The pore water pressure of the rock above the goaf is obviously higher than that on both sides, and the water pressure monitoring curve shows the law of "high in the middle and low on both sides." In addition, the development height of the overlying rock fissures on both sides of the goaf is relatively high, resulting in a relatively high seepage rate. The seepage rate curve shows the law of "high on both sides and low in the middle."