Experimental investigation on decreasing burst tendency of hard coal using microwave irradiation

How to effectively change the physical and mechanical properties of coal and rock mass is an urgent problem to be solved in the field of prevention and control of rock burst, and the microwave fracturing-weakening method is a new way for solving this problem. To explore the effectiveness of the microwave fracturing-weakening method to reduce the burst tendency of hard coal, a test of microwave fracturing-weakening on coal is carried out based on the self-developed microwave irradiation device and C46. 106 MTS electro-hydraulic servo experimental machine. The influence of microwave irradiation on the parameters such as dynamic failure time, impact energy index, elastic energy index, uniaxial compressive strength and P-wave velocity of coal is studied, and the optimal microwave parameter range to reduce the burst tendency of coal is obtained. On this basis, the damage-fracturing effect of the coal with burst tendency under microwave irradiation is studied. Also, the relationship between the selective heating effect of the microwave on the minerals in the coal, the coal damage factor and the microwave parameters are analyzed. The mechanism of reducing burst risk of the surrounding coal of the roadway is revealed by using microwave fracturing-weakening. Research results show that the method of microwave fracturing-weakening can effectively reduce or eliminate the burst tendency of coal. After microwave fracturing on coals, the peak load of coal samples drops significantly, and the stress drop in the post-peak section of the stress-strain curve increases, which reduces the total energy accumulated before the peak and is released slowly by stages. The coal changes from brittle failure to plastic deformation. Simultaneously, the coal samples before the peak of the load-time curves shows the characteristics of multiple steps and graded drops, and the elastic energy stored in the coal mass is reduced, so that the burst tendency indicators of the coal are reduced to some degrees. The effect of the microwave irradiation on the burst tendency reduction of coals is manifested as an obvious energy threshold phenomenon. Under the condition of equal microwave energy, the energy combination of high power-short time is more conducive to weaken the burst tendency of coals. The selective heating effect of microwave on the various minerals in the coal results in thermal cracking of the coal, which is manifested as appearing multiple longitudinal cracks and branch cracks perpendicular to it on the surface of the coal samples. Research results show a remarkable correlation between the burst tendency and the P-wave velocity of the coal samples. The greater the microwave energy, the greater the reduction in P-wave velocity of the coals, and the smaller the burst tendency of the coals. The mechanism of microwave fracturing-weakening on coal to prevent the rock-burst of coal roadway is to increase the radius of the critical plastic softening zone where rock-burst occur, causing the high stress in the surrounding rock of the roadway can be transferred to the deep part, thereby improving the ability of the surrounding rock plastic softening zone of the roadway to dissipate the energy released by the elastic zone. Simultaneously, the pre-peak softening modulus of coals is significantly decreased, which leads to an increase in the critical load for rock-burst, thereby reducing the possibility of rock-burst in roadways. © 2021, Editorial Office of Journal of China Coal Society. All right reserved.