The weakening of hard coal–rock mass is the core common problem that is involved in the top coal weakening in hard and thick coal seams, the hard roof control during the initial mining stage in the longwall mining face, and the hanging roof control in the gob of non-coal mine. Based on the characteristics of pulse hydraulic fracturing and constant pumping rate hydraulic fracturing, a weakening method for hard coal–rock mass by combining pre-pulse and constant pumping rate hydraulic fracturing is proposed. A complete set of equipment for the combined pulse and constant pumping rate hydraulic fracturing construction in the underground coal mine is developed. The pulse and constant pumping rate hydraulic fracturing technology and equipment were applied in the top coal weakening of the shallow buried thick coal seam. Compared with no weakening measures for top coal, the average block size of the top coal caving was reduced by 42% after top coal hydraulic fracturing. The recovery rate of the top coal caving mining face reached 85%, and it increased by 18% after hydraulic fracturing.

According to the conventional theory of hydraulic fracturing, the breakdown pressure decreases with an increase in the pore pressure. However, in the practice of hydraulic fracturing for gassy coal seam, the gas pressure needs to be overcome, and then pressure water could be injected into the coal seam. The pore pressure of gas leads to an increase in the water injection pressure. How does the pore pressure affect the hydraulic fracturing of unsaturated rock and its breakdown pressure? Experiments on hydraulic fracturing for unsaturated cement mortar samples (300 mm×300 mm×300 mm) with five degrees of pore pressures were conducted. The influence law of pore pressure on breakdown pressure of borehole radial fracture in hydraulic fracturing was studied with a large sized true triaxial hydraulic fracturing experimental system. The experimental results show that the hydraulic fracturing of rock is primarily controlled by the stress field, the initial pore pressure, and the water injection pressure. A critical pore pressure gradient exists for the initiation and propagation of hydraulic fracture. When the pore pressure gradient reaches a critical pore pressure gradient, rock fracture will occur. With an increase in the pore pressure, the breakdown pressure of rock increases, and the magnitude of increment of breakdown pressure increases as well. The subsequent propagation pressure of hydraulic fracture also increases with an increase in the pore pressure.

Pore pressure is disregarded in traditional laboratory rock hydraulic fracturing experiments, and the effect of pore pressure is not clear. An integrated experiment for seepage and hydrofracturing was established and used to perform sandstone hydraulic fracturing experiments under an initial evenly distributed pore pressure. The experimental results show that there is a positive correlation between the breakdown pressure and the pore pressure at the initiation stage. The data fitting results show that the breakdown pressure and pore pressure follow a linear growth trend. As the pore pressure increases, the acoustic emission energy at the moment of borehole wall fracturing correspondingly increases. After borehole wall fracturing, the reduced magnitude of the pumping pressure also increases, indicating that the initial rupture range is positively correlated with pore pressure. During fracturing propagation, the propagation range and opening of the fracture increase as the initial pore pressure increases within the same pumping time. During hydraulic fracturing, a pore pressure gradient is generated on both sides of the mineral particles. When the tensile stress or shear stress induced by the pore pressure gradient reaches the ultimate strength of the mineral particle bonding surface, the particle bonding surface breaks and opens. This experimental process is more similar to the actual hydraulic fracturing process of oil and gas reservoirs. These results provide a more comprehensive theoretical basis for resolving technical problems of unconventional oil and gas resource exploitation.