Cavability control by hydraulic fracturing for top coal caving in hard thick coal seams

●A method of improving the top coal cavability by top coal and roof hydraulic fracturing is proposed.●Hydrofracturing technology of linear multi-hole and deep-hole retreating has been suggested.●The top coal caves along the roof-cut line of the hydraulic support.●The recovery ratio in shallow, hard, thick coal seam conditions reaches over 80%.●Hydrofracturing could replace blasting for improving the cavability of hard and thick top coal.

Insufficient breaking and falling of top coal is a technical problem in hard thick or super-thick coal seams during top coal mining. Based on top coal weakening by hydraulic fracturing and main roof hydraulic fracturing to induce coal crushing, a method of improving the top coal cavability by top coal and roof hydraulic fracturing is proposed. More specifically, linear multi-hole hydraulic fracturing and deep-hole retreating hydraulic fracturing methods for top coal control in top coal caving of hard thick coal seams are proposed. A set of high-pressure (60 MPa) hydraulic fracturing devices was developed. The hydraulic fracturing technology and devices were applied in the #3 coal seam in the Yulin District, where the coal is hard, the occurrence depth is shallow, and the ground pressure is not extreme. The top coal begins to fall when the working face advances 20 m from the setup room and begins to cave normally when the working face advances 40 m. After the top coal begins to cave normally, the recovery ratio in shallow, hard, thick coal seam conditions reaches over 80%, reaching the average level of top coal caving faces. These methods ensure efficient and safe top coal caving in the first coal face by control of the top coal and roof without explosive blasting. Compared to traditional methods of weakening the top coal and roof by blasting, hydraulic fracturing is simpler and safer and requires less work and lower costs (10% of the cost of explosive blasting).