Dynamic effects of deep-hole cumulative blasting in coal seam and its application

A model of cumulative blasting was established to study the mechanism of crack propagation and permeability increasing under cumulative blasting in a coal seam. The forming mechanism of shaped charge jet, the propagation characteristics of stress wave and detonation wave, the stress distribution characteristics of coal elements, and the mechanism of crack extension under cumulative blasting were investigated by theoretical analysis and numerical simulation. It is found that the propagation characteristics of detonation wave promote the forming of cumulative jet, and change the propagation characteristics of stress wave and the stress distribution characteristics of coal elements significantly. The stress peak values of coal in the major and minor cumulative directions are 1.10-1.29 times that of coal in the non-cumulative direction, and the stress peak value of coal in the major cumulative direction increases from 0.85 times that of coal in the minor cumulative direction to 1.06 times in the range of blasting effect, slowing down the attenuation rate of the stress peak value of coal and increasing the crack propagation velocity. Besides, field experimental results of cumulative blasting in a coal seam indicate that the average concentration of extracted gas after blasting is 1.58 times that before blasting, and cumulative blasting can promote crack propagation effectively.