Study on Mine Earthquakes Mechanism and Ground Vertical Well Hydraulic Fracturing Shock Absorption in Thick Hard Rock Mine

Mine earthquakes are serious disasters in coal mines, especially in extremely thick hard strata. This study investigates the occurrence mechanism of fracture-type mine earthquakes in thick hard strata. Hydraulic fracturing by ground vertical well was used for shock absorption. Dongtan coal mine was taken as a case study. Field investigation, theoretical analysis, industrial tests, and field monitoring were used for revealing the mechanism. First, the mechanical model of extremely thick, hard strata under horizontal concentrated stress was established. The fracture step equation and energy release equation of extremely thick hard rock were derived by semi-inverse solution and variational method. Then, the mechanical model of extremely thick hard rock after hydraulic fracturing by ground vertical well was established. The relationship between the spacing of the ground vertical well and the maximum magnitude of mine earthquakes was deduced. The fracturing well in the 6306 working face was designed for controlling the maximum mine earthquake magnitude. Results show that the increases in the breaking distance of the thick hard rock layer led to an increase in the released energy during the fracture, and an enhancement of the magnitude of the mine earthquake. By applying hydraulic fracturing technology using the ground vertical shaft, the occurrence frequency and total energy of mine earthquakes above 1.5 ML in the 6306 working face decreased by 54.55% and 81.22% than that in 6304 working face, and reduced by 70% and 84.98% than that in 6305 working face. Hydraulic fracturing technology by ground vertical well can significantly reduce the frequency of fracture-type and the total energy of mine earthquakes in extremely thick and hard strata. However, it can not prevent and control the occurrence of back-transition mine earthquakes and slip-type mine earthquakes. The obtained results can provide a basis for the fracture-type mine earthquake mechanism and fracturing shock absorption technology in coal mines with super-thick hard strata.