Factors initiating crack formation during hydro-impulse loosening of outburst-hazardous coal seams

The article is dedicated to the exploitation of outburst-hazardous coal seams at significant depths. It is known that with the increase in mining depth in the gas-saturated coal-bearing massif, changes in its stress-strain state lead to specific manifestations of outburst-hazardous coal, such as spontaneous collapse, alteration of the aggregate state and structure of hydrocarbon compounds, changes in filtration properties, and so on. These features occur when compressive deformations are replaced by tensile (shear) deformations. There is a list of methods for impacting the massif to prevent the occurrence of gas dynamic phenomena. The main task of these methods is to create additional cracks throughout the thickness of the coal seam for effective gas filtration. Along with this, during mining operations, man-made factors arise. They create additional load and change the structure and properties of outburst-hazardous coal. The article deals with the factors of man-made impact on the heading part of the coal seam, which led to a decrease in the effectiveness of methods of preventing gas dynamic phenomena. The parameters of the processes that initiate the development of cracks around the filtration part of the well in the impulse mode of liquid injection, in the redistribution of stresses in the massif, and the case of self-destruction of coal in the heading part of the seam are considered. The theory of strength of materials is considered to establish their limit state and destruction. The paper presents the results of theoretical and experimental studies of the hydro-impulse loosening method for outburst-hazardous coal seams. The changes in deformation velocity were investigated under impulsive loading of coal relative to the modulus of elasticity in the range of 3∙102 MPa ≤ Е≤ 5∙102 MPa. It was established that all values of impulses of self-oscillations of liquid pressure are above the curve of the limit rate of development of deformations έ =10 s-1 at the value of the modulus of elasticity of coal E ≥ 3∙102 MPa. We concluded that during hydro-impulse impact modes with an injection pressure of more than 5 MPa, pressure pulses of ΔP ≥ 3 MPa with a frequency of f ≥ 0.8 kHz are created in the filtering part of the well. This leads to the development of shear deformations and initiates the cracks formation through-out the thickness of the coal seam. Research on hydro-impulse impact was conducted in the mines of the Donetsk Coal Basin. The Donbas region is characterized by the most complex mining-geological conditions, especially when working with outburst-hazardous coal seams. Keywords: gas dynamic phenomena, hydro-impulse loosening, сrack formation, coal-gas system.