ABSTRACT Practice has shown that increasing permeability through floor blasting is an effective approach for improving gas recovery rates in deep, soft coal seams. However, while existing studies have extensively examined the effects of in situ stress, little attention has been paid to the mechanisms within coal‐rock complexes. This study developed theoretical models for the propagation of blasting stress waves within coal‐rock complexes and for the stress distribution in the rock mass surrounding blast holes. Based on these models, both isotropic and anisotropic numerical models were established. The model was validated through field testing at a coal mine in Huainan, China. The results indicate that under isotropic in situ stress conditions, the circumferential stress around the blast holes increases with the increase in in situ stress, thereby inhibiting crack propagation. This inhibitory effect is proportional to the magnitude of the in situ stress and is more pronounced in the coal than in the rock. Under anisotropic in situ stress conditions, cracks tend to propagate preferentially along the direction of the maximum principal stress. In situ stress can reduce the peak intensity of blasting stress waves, limit their propagation range, and induce stress concentrations at the coal‐rock interface. Field engineering practice indicates that following roof blasting, the maximum increase in gas concentration in the overlying coal seam reaches 375%, and the maximum increase in pure gas flow reaches 1,546%; however, increased burial depth exerts a certain inhibitory effect on the efficiency of gas permeability enhancement. The findings of this study provide important theoretical support and practical engineering guidance for improving the gas‐permeability‐enhancing effects of floor blasting in deep coal seams.
Zhang et al. (Sat,) studied this question.