Abstract: Because of the frequent occurrence of suddenly unload of rock stress in outburst or coal bump, this mechanical condition was used to established a simplified spall fracture model. The stress propagation in rock mass after the suddenly unload of rock stress was analyzed based on kinetic theory, the cause of spall fracture was obtained, and the developing process of spall fracture was reproduced by numeric simulation. The result shows that the main cause of spall fracture in coal bump is the unloading wave which comes into being after the suddenly unload of rock stress, then part of the unloading wave is reflected during the propagation process into rock mass, the incoming wave and the reflecting wave interact to produce a tension region and thus lead to the tensile failure of rock. The cause of spall fracture in coal/gas outburst is affected not only by the interaction between incoming wave and reflection wave, but also by gas; gas influence manifest itself in two aspects, high pressure free gas is involved in the fracture propagation in coal, and the discontinuous releasing of gas pressure along with the discontinuous occurrence of spall fracture may induced inward propagating unloading wave again.

Key words: coal and gas outburst, coal bump, spall fracture, mechanism, stress wave