Study on propagation law of cracking and permeability enhancement caused by blasting in deep high-gas coal seams

Aiming at the problem that the crack extension law of deep high-gas coal seams is not clear during blasting and permeability enhancement under the action of ground stress, firstly, according to the propagation and superposition of blasting stress wave, the stress distribution law around the blast hole under the coupling effect of ground stress and blasting was theoretically analyzed. Then, the crack extension characteristics of single-hole blasting under different ground stress conditions were investigated through laboratory tests, on the basis, the numerical simulation was carried out to investigate the crack extension mechanism and the penetration process of double-hole blasting under different lateral stress coefficients, and combined with the ImageJ image recognition software and LS-PrePost software, the effect of ground stress on the degree of crack development was characterized using crack density and extension length as a quantitative index. Finally, based on the results of laboratory test and numerical simulation, the layout scheme of coal seam blasting holes for cracking and permeability enhancement considering the influence of ground stress was proposed. The results of single-hole blasting laboratory test shown that, the ground stress can reduce the tensile stress induced by the blasting load and thus inhibit crack extension. Under the condition of non-hydrostatic ground stress, the tensile stresses orthogonal to the direction of the larger principal stresses were weakened, resulting in the inhibition of crack extension in this direction, causing the cracks to preferentially extend in the direction of higher stresses. The numerical simulation results of double-hole blasting shown that, when the lateral stress coefficient was not 1, the blasting crack expansion was directionality, and the main crack tended to expand in the direction of higher ground stress, resulting in an elliptical shape of the blast crack area, and the direction of the long axis and the larger in-situ stress were consistent. According to the results of laboratory tests and numerical calculations, it was advisable to drill holes along the direction of large ground stress to improve the effect of fracturing and permeability when blasting in deep coal seams. The research results are of great significance for understanding the crack expansion mechanism and optimizing the blast hole layout scheme when blasting for permeability enhancement in deep high-gas coal seams.