Abstract：In order to study the occurrence mechanismin of penetration stability failure of coal containing gas,it was supposed that the outburst be a process of coal mass as a form “sublayer”pushing forward. Based on the totall stress method,a simplified calculation formula of the “critical improved gas pressure gradient”was given. According to the formula,the critical “improved gas pressure gradient ” is directly proportional to shearing strength,and is inversely proportional to the uncovering coal circular section radius or the short side of rectangular width. Based on the formula,some phenomena and influence factors on the process of the outburst were analyzed. Some conclusions were drew that were the thickness of the falure sublayer increasing with the decreasing of the gas pressure gradient and the velocity of the failure sublayer pushing foward no a constant value.

Keyword：gas pressure gradient;tensile strength;shear strength;outburst of coal and gas;