Experimental Research On Microwave Irradiation To Reduce Burst Tendency Of Coal

With the gradual development of coal mining in my country,the frequency of rock bursts in coal mines has increased sharply.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 bursts,and the microwave fracturing weakening method is a new way to solve this problem.In order to explore the feasibility and effectiveness of the microwave fracturing weakening method to reduce the burst tendency of hard coal,a coal mass microwave fracturing weakening test is carried out based on the self-developed microwave irradiation test device and C46.106 MTS electro-hydraulic servo test machine.The influence of microwave irradiation on the parameters such as dynamic failure time,impact energy index,elastic energy index,uniaxial compressive strength and longitudinal 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 and fracturing effect of the burst tendency of the coal under microwave irradiation is studied,and the selective heating effect of the microwave on the minerals in the coal,the relationship between the coal damage factor and the microwave parameters are analyzed,and the microwave fracturing is revealed the mechanism of the weakening method to reduce the burst risk of the surrounding rock of the roadway.Research results have shown :(1)The microwave fracturing weakening method can effectively reduce or eliminate the burst tendency of coal.After microwave fracturing of coals,the peak value load of coal samples dropped significantly,and the stress drop in the post-peak section of the stress-strain curve increased,which reduced the total energy accumulated before the peak value and was released slowly by stages.The coal mass changed from brittle failure to plastic deformation;simultaneously,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 four burst tendency indicators of the coal mass are reduced to varying degrees.(2)The effect of the microwave fracturing weakening method on the burst tendency reduction of coals is manifested as an obvious energy threshold phenomenon;under the condition of equal microwave energy,the combination of high power and short time energy is more conducive to weaken the burst tendency of coals.(3)The selective heating effect of microwave on the various minerals in the coal mass causes the uneven temperature distribution of the coal mass,and the surface temperature difference of the coal samples has an exponential function relationship with the microwave energy.The existence of the coal temperature difference is bound to cause thermal fracturing of the coal mass,which is manifested as the structure breaking of the coal samples and the tensile failure of the particle bands,which causes the surface of the coal samples to appear multiple longitudinal cracks and branch cracks perpendicular to it.(4)Research results shows a remarkable correlation between the burst tendency and the P-wave velocity of the coal sample.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.(5)The mechanism of microwave fracturing weakening coal mass to prevent rockburst of coal roadways is to increase the radius of the critical plastic softening zone where rockburst 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,before the peak softening modulus of coals stress-strain curve is significantly obviously decreased,which leads to an increase in the critical load for rockburst,thereby reducing the possibility of rockburst in roadways.This paper contains 49 figures,18 tables and 55 references.