Study on the Stress Field and Crack Propagation of Coal Mass Induced by High-Pressure Air Blasting

High-pressure air blasting (HPAB) is one type of physical blasting technique that enhances the extraction rate of coalbed methane by impacting the coal mass with high-pressure gas to create cracks within it. First, based on the physical and mechanical parameters of the simulated coal rock mass, the RHT constitutive model of the coal rock mass was established, and its parameters were determined. Then, the laws of crack propagation and stress wave decay in coal induced by high-pressure air blasting were revealed by comparing the effect with that of equivalent explosive blasting. Next, the HPAB experiment was simulated to explore the coal crack propagation law under in-situ stress conditions. Finally, the HPAB experiment was carried out and the results of this experiment were compared with the numerical simulation results. The results indicate that the crack propagation induced by high-pressure air blasting is considered as two major stages, i.e., the crack initiation and crack propagation stage induced by the stress wave and the crack stable propagation stage induced by the duration high-pressure gas. In the case of equal energy, the peak stress wave of high-pressure gas is smaller, decays more slowly and has a longer action time, compared to explosive blasting. Therefore, the number of initial random cracks in coal mass induced by high-pressure air blasting is less, and the range of crack propagation induced by high-pressure air blasting is larger. When lambda = 0 (lambda is the ratio of the horizontal in-situ stress to the vertical in-situ stress), the in-situ stress in the coal seam can promote the propagation of vertical cracks but inhibit the propagation of horizontal cracks. When lambda = 0.5 and 1, the in-situ stress inhibits the propagation of both horizontal and vertical cracks.