True Triaxial Experimental Study of Disturbed Compound Dynamic Disaster in Deep Underground Coal Mine

With the increase of coal extracting depth, a considerable number of dynamic disasters display the co-occurrence and coupling effect of rockburst and coal–gas outburst, which is defined as the disturbed compound dynamic disaster. In this study, two loading modes were adopted to investigate disaster characteristics using true triaxial apparatus. The stress states of one freed and the other five stressed faces were introduced to simulate actual stress conditions. Two high-speed cameras were used to capture the disaster process. The mechanical and strength properties, failure modes, and ejection kinetic energy were analyzed. Results showed that the compound dynamic disaster mainly exhibited local grain ejection, fragment spalling, large-scale grain ejection, plate bending, and ultimately failure. The strength of the sample first increased and then decreased slowly with the increase in the intermediate principal stress. After failure, a V-shaped coal-burst pit was formed, which was approximately parallel to the intermediate principal stress and perpendicular to the free face. The grain ejection exhibited obvious characteristics of spatial sorting, and the grain size decreased with the distance from the free face. The kinetic energy showed little change with increase in the intermediate principal stress in the displacement loading mode; whereas, it first increased and then decreased in the stress loading mode. The pressurized gas promotes the development of coal cracks and fully fractures the coal rock. Under the combined actions of the elastic energy stored in coal mass and the internal energy of pressurized gas, compound dynamic disasters may occur. Gas extraction and coal seam elastic softening techniques can effectively reduce and prevent the occurrence probability of compound dynamic disasters.