Crack propagation and coalescence mechanism of double-hole cumulative blasting in coal seam

This paper focuses on the radius of coal failure zones under cumulative blasting with shaped charge. Based on the analysis of the mutual superposition effect of the explosion stress waves during the simultaneous detonation of two blastholes, a numerical analysis model of the double-hole cumulative blasting with linear shaped charge was established. Additionally, the propagation characteristics of the stress wave during the simultaneous detonation of two blastholes, stress state of the coal body, mechanism of coal crack propagation and coalescence, and influence of the stress wave superposition effect on crack propagation were evaluated. Results show that the stress wave superposition effect induces the formation of a pressure equalization zone in the partial region of the middle section of the two blastholes and its adjacent regions. This occurrence forces the radial cracks of the two blastholes to turn, and they cannot connect with each other, leading to the formation of a gap blank zone between the two blastholes. After the directional cracks generated under cumulative blasting load coalesce, the collision of the explosive gases produced from the two blastholes further promotes the expansion of the cracks in the directional crack coalescence zone and eventually penetrates the gap blank zone. Field test results of deep-hole cumulative blasting in coal seams show that the explosion stress waves from the blastholes in the opposite side promotes the propagation of the blasting-induced crack on the left or right side of the two blastholes. This propagation first increases and then decreases as it moves away from the blasthole. Between the two blastholes, the stress wave superposition effect from the two blastholes inhibits the propagation of the cracks in some areas, resulting in a W-like fluctuation in the degree of improvement of the gas drainage effect at different positions in the area between the two blastholes.