The Law Of Coal And Rock Mass Crack Propagation Under The Influence Of Empty Hole Effect Under CO₂ Gas Blasting

Deep coal seam has the characteristics of high gas and low permeability.As one of the effective means to increase the permeability of coal seam,CO₂ gas blasting can significantly improve the efficiency of gas drainage.Therefore,exploring the mechanism of CO₂ gas blasting can ensure the safety and economy of this method applied in coal mine production sites.Based on elastic mechanics and fracture mechanics,the crack propagation mechanical model of CO₂ gas blasting coal mass is constructed in this paper.The relationship between the stress wave intensity and the crack propagation trace length is described,and the energy addition effect of blasting gas is explained.When the coal and rock mass without void is impacted by CO₂ gas,the tensile stress load distributed around the boundary is produced under the action of stress wave,which causes the coal seam to crack in the circumferential direction from the center.After absorbing the energy of the shock wave,the blasting gas expands and releases a certain amount of energy,which produces a synergistic effect on the stress wave,which promotes the full development of cracks around the blasting hole in coal and rock mass.The void effect is introduced to consider the influence of freedom on crack propagation.The physical tests of CO₂ gas blasting with and without voids were carried out respectively.The surface displacement of the sample was observed by an optical non-contact strain measurement system(VIC-3D),and the law of crack propagation around the burst hole in coal and rock mass was analyzed.The amplitude in the blasting process is collected by the ultrasonic monitoring system,and the attenuation coefficient is calculated to reflect the internal damage of coal and rock mass.The appearance of two ultrasonic amplitudes in the process of CO₂ gas blasting is not accidental,which reflects the substance of crack propagation promoted by the synergistic stress wave of detonating gas.The magnitude of stress can reflect the synergistic effect of stress waves and blasting gas.The greater the stress is,the stronger the synergistic effect is and the more sufficient the crack propagation is.The characteristic coincidence phenomenon of stress and attenuation coefficient in the peak period shows that the synergistic effect of stress wave and blasting gas is the strongest in this period.In the diagonal direction,the horizontal component of the stress intensity factor is basically the same as the vertical component,and the crack propagation of the specimen is the deepest,indicating that the closer the horizontal component is to the vertical component,the easier it is to crack.The void effect is induced by the increase of the free surface.In the process of CO₂ gas blasting,the superposition of compression wave and reflected wave produces radial tensile stress,which breaks through the compressive strength of rock mass.The pressure in the blasting cavity gradually reaches the peak,and an approximately straight line main crack channel is formed between the blasting hole and the empty hole.It plays a guiding role in the crack propagation of coal and rock mass.At this time,the stress and attenuation coefficient reach the peak,and the internal damage of coal and rock mass is the most severe.Under the condition of no empty hole and empty hole,LS-DYNA software is used to simulate and study the law of crack propagation around the blasting hole in coal and rock mass after CO₂ gas blasting,which vividly reflects the orientation of the empty hole.The evolution of crack propagation characteristics of coal and rock mass with holes under CO₂ gas blasting is numerically simulated.It is found that the main crack channel is formed in 0.13s,the stress of coal and rock mass reaches the peak value and the energy dissipation reaches the maximum in 0.18s.The negative correlation between energy and stress is consistent with the experimental results,which show that the void effect provides good guidance for the crack propagation of coal seam in the process of CO₂ gas blasting.With the organic combination of theoretical analysis,experimental exploration,and numerical simulation,this paper improves the basic theoretical system of CO₂ gas blasting,expounds the synergistic effect of blasting gas on stress waves,and reflects the characteristic law of crack propagation in coal seam affected by void effect,which can guide the safe and efficient application of CO₂ gas blasting technology in coal mine production site,and has important practical significance to further strengthen the permeability enhancement effect of coal seam and improve the efficiency of gas extraction.