Coal damage mechanism in the developing process of coal and gas outburst

Based on the damage analysis of elliptical aperture, the mechanism of coal damage in the developing process of coal and gas outburst was researched. The results show that the damage to coal by gas is mainly caused by the concentrated tensile stress appearing near the endpoint of the pores. Fractures in coal, gas pressure, ground stress and the tensile strength of the coal matrix are the major controlling factors of this kind of damage. When the ground stress releases abruptly and the gas pressure is high, tensile failure will occur around the endpoint of the small pores due to gas pressure, and the coal may be broken up like powder; this is called pulverization. Otherwise, when the gas pressure is low, the tensile stress can only occur around the endpoint of the large pores and fractures due to gas pressure, the fractures in coal extend and link together, the fracture extension direction is statistically perpendicular to the direction of the minor principal stress. This kind of damage is shown as the stratified spall around the outburst hole. Supported by the National Basic Research Program (973) of China (2005CB221503); the Key Project of Natural Science Foundation of China (50534080)