Effects of loading rate on the compound dynamic disaster in deep underground coal mine under true triaxial stress

The compound dynamic disaster is an abrupt and violent energy-releasing process under the combined action of gas and geological tectonic stress in deep underground coal mines. To investigate the occurrence mechanism of compound dynamic disasters, a novel experimental approach was designed and the one free-face true triaxial tests with different loading rates were conducted. Results shown that the compound dynamic disaster can be classified into several stages: initial quiet period, local particles and fragments ejecting, mid-term quiet period, external coal plate bending accompanied by gas-emitting, dynamic disaster occurring process, and the final re-stabilized period. The presence of the gas can significantly facilitate the development of cracks and enhance the kinetic energy, thus increasing the probability of the compound dynamic disasters. Under the one free-face true triaxial engineering stress conditions, the mechanical properties (e.g. deformation and strength) of coal samples are obviously rate-dependent, and the peak stress exhibits an approximate logarithmically increase trend with increasing loading rate. After failure, several tensile-shear fractures approximately parallel to the intermediate principal stress direction and perpendicular to the free face are generated. And an arc-shaped coal-burst pit on the free face of coal sample is observed, and the pit volume increases with the increase of the loading rate. Moreover, the particle ejection kinetic energy increases gradually as the loading rate increased, suggesting a higher loading rate can enhance the intensity of the disaster. The findings may provide guidance for the prevention and control of the compound dynamic disasters in deep underground coal mines.