Study on the Coal Damage and Fracture Mechanism under Multiple Actions of Blasting Stress Wave

Controlled blasting technology is an effective way to enhance coalbed methane recovery, but the theoretical research has fallen behind engineering practice seriously. Therefore, in this work, based on the damage and fracture mechanics theory, the coal damage process and mechanism under the multiple actions of the blasting stress wave (BSW) were analyzed firstly. Then, the simulation experiment of coal damage accumulation was designed and carried out; during the experiment, the ultrasonic velocity of the samples was measured and the damage values were calculated. Under the multiple actions of BSW, the initial radial cracks around the blast-hole wall or in the area near the explosion source are formed under the action of tangential tension of BSW and the internal pressure of the blast-hole, and under the combined actions of stress wave, gas, and in situ stress, the primary cracks in the middle and far area of the explosion source expand and form the mesoscopic cracks because of the damage accumulation at the crack tip. And then the mesoscopic cracks grow and intersect to form a macroscopic crack. Finally, the macroscopic cracks expand and intersect with the initial radial cracks, the samples break suddenly under the load far below their fracture toughness because of the damage accumulation effects. Hence, the research results have guiding significance for better understanding and application of controlled blasting technology to enhance the CBM recovery.