A method for coal seam pressure relief based on steel cone impact

This study aims to solve the mining safety problems caused by rock bursts in coal mines. With the geological mining conditions of the No.3 coal seam in the Wulihou coal mine in Zuoquan County, Jinzhong City, Shanxi Province as the engineering background, this study proposed a method for coal seam pressure relief based on steel cone impact. Specifically, based on the electromagnetic repulsion generated between energized coils, this study derived the functions of impact force produced when a magnetically driven coil-wound steel cone impacted the coal seam. Combining the theory on elastic mechanics, this study analyzed the stress field distribution, fracture types, and rupture ranges and degrees of the coal seam under the impact force. The results show that: (1) The impact force was positively correlated with AC voltage and the coil stage number but was negatively correlated with AC frequency and coil turns per meter. Voltage, the most significant factor in regulating the impact force, should be 660 V according to comprehensive consideration. (2) Calculations indicate that steel cones with a length and diameter of merely 1 cm and 1.28 cm each, respectively, along with matched two-stage accelerating coils with a total length of 2 cm, can achieve the miniaturization of the impact device for flexible arrangement in a borehole. (3) Under the action of the impact force, leaf-shaped zones subjected to extremely high stress occurred in the surrounding rocks along borehole cross sections. These zones exhibited stress concentration coefficients ranging from 4.0 to 16.7, an extension length 5 times the borehole radius, and extensional fissures. Outside these zones were high-stress zones, with stress concentration coefficients ranging from 1.7 to 3.3, an extension length up to 50 times the drilling radius, and the occurrence of compression-shear fissures. Therefore, the spacing between pressure relief boreholes should be 2.5 m. (4) Based on the lengths of pressure relief segments, two impact devices with a spacing of 2 m were installed in each borehole. Under the impact force, the surrounding rocks along the longitudinal sections of boreholes presented needle-shaped zones with extremely high stress. Elliptical zones with compression-shear fractures emerged among adjacent needle-shaped zones. Both types of zones jointly penetrated the 2-m-thick coal seam, achieving effective pressure relief on coal breaking. The results of this study lay a preliminary theoretical foundation for exploring technologies for coal seam pressure relief based on magnetically driven steel cone impact.