Effect of in-situ stress on the cracking and permeability enhancement in coal seams by deep-hole cumulative blasting

With the gradual development of coal mining to deeper levels, the in-situ stress of coal seams shows an increasing trend, resulting in a gradual decrease in permeability, and the stress state of the coal and rock mass and the properties of the surrounding rock also change. The mechanical properties and mechanical parameters of coal and rock mass greatly differ between depths, which influences the cracking and permeability enhancement effect of coal seam deep-hole cumulative blasting. Aiming at the problem of the increasing permeability of coal seams by deep-hole cumulative blasting under in-situ stress, on the basis of an analysis of the stress field of the surrounding rock and the stress of the blasting crack surface, the process of cumulative blasting and crack development characteristics under different confining pressures were numerically simulated. Through field tests of cumulative blasting under different buried depths, the influence of in-situ stress on the cracking and permeability enhancement effect of coal seam deep-hole cumulative blasting was discussed. The results show that the role of in-situ stress differs greatly between the stages of radial crack expansion of coal seam deep-hole cumulative blasting. Before blasting, the stress state and deformation characteristics of the borehole surrounding rock are determined by borehole shape and in-situ stress. In the initial stage of cumulative blasting, the impact of cumulative blasting on the surrounding rock is obviously stronger than in-situ stress. Therefore, the expansion direction of blasting cracks in the initial stage is mainly determined by the cumulative structure, and directional cracks are formed along the opening direction of the cumulative charge groove. With the crack extension, the blasting effect is gradually weakened, and the in-situ stress is dominant. The surrounding rock of the borehole produces tangential compressive stress under the in-situ stress, which limits the radial crack expansion of blasting. Meanwhile, the coal cracks that are not collinear with the principal stress gradually deflect toward the direction of the maximum principal stress under the action of strong shear stress. When the equivalent dynamic stress produced by blasting cannot continue to compress the coal, the elastic strain energy accumulated in the surrounding rock of the borehole begins to release toward the blasting center, causing the coal to crack and produce new cracks. In addition, the crack expansion range in different directions is controlled by the lateral pressure coefficient. When the vertical principal stress is constant, the crack range toward minimum principal stress further decreases with increasing lateral pressure coefficient.