Seepage and mechanical failure characteristics of gas⁃bearing composite coal⁃rock under true triaxial path

The failure of “coal⁃rock” system under the disturbance of deep mining is one of the main causes of dynamic disaster. In order to study the mechanism of the disaster，five group experiments were designed to study the mechan⁃ ics and seepage characteristics of specimens. The results show that the strength of specimens under the true triaxial path conforms to the Mogi⁃Coulomb criterion. Compared with other experimental paths，the strength of specimens under the true triaxial loading path is the largest，followed by the road is the composite loading and unloading，and the strength decreases under the transformation of σ2 and σ3. When the specimen is damaged，the permeability increases significantly under the true triaxial loading path，followed by the true triaxial unloading path. The permeability increa⁃ ses least under the true triaxial loading path，and the permeability of the specimen increases significantly when σ2 and σ3 are transformed. In addition，the composite specimen damage of coal or rock is regular. In the true triaxial loading tests，shear cracks appear in the σ1 -σ2 plane of coal. Under the other two paths，the shear cracks in the coal intersect at the interface and transform into tensile cracks，which extend to the rock mass，and the rock mass is frac⁃ tured. When σ2 and σ3 are transformed，the σ1 -σ3 plane is also damaged，and the damage degree of the specimen is intense. The cracks of σ1 -σ2 plane from coal to the rock appear across the interface extension，shear cracks still ap⁃ pear in coal，and the cracks intersect at the coal⁃rock interface and then turn into tensile cracks，which extend across the interface into the rock，and the rock appears splitting failure. When σ2 and σ3 are transformed and σ2 is less than σ3，both sides of the specimen (σ1-σ2 plane and σ1-σ3 plane) are damaged，and the damage state of the specimen is relatively complex.