After a gas drainage event causes different degrees of initial porosity in the coal seam, the heterogeneity of the coal mass becomes much more obvious. In this paper, soft coal testing samples with different degrees of heterogeneity were prepared first by a new special experimental research method using hydrogen peroxide in an alkaline medium to generate oxygen. Then, a series of mechanical tests on the soft coal mass samples were carried out under multiple factor coupling conditions of different heterogeneities and confining pressures. The results show that with a low strength, the ductility failure characteristic and a kind of rheology similar to that for soft rock flow were reflected for the soft coal; i.e., the stress-strain curve of the coal mass had no apparent peak strain and residual strength. An interesting phenomenon was found in the test process: there was an upwardly convex critical phase, called the brittle-ductile failure transition critical phase, for the heterogeneous soft coal mass between the initial elastic compression phase and the ductile failure transition phase in the stress-strain curve of the coal mass. An evolution of the brittle-ductile modulus coefficient of the soft coal was developed to analyze the effect of the internal factor (degree of heterogeneity) and external factors (confining pressure) on the transition state of the brittle-ductile failure of soft coal. Further analysis shows that the internal factor (heterogeneity) was also one of the essential factors causing the brittle-ductile transition of soft coal.

Coal and rock burst are one of the main dynamic disasters that affect coal mine production. In this paper, the burst structural model of the rock-coal-bolt (RCB) system and the burst tendency criterion are established on the background of deep thin coal seam mining. Uniaxial and triaxial mechanical tests under different stress states are carried out on RCB specimens with different angles. Combined with thermal imaging, the mechanical behavior of the inclined RCB specimen under uniaxial loading is discussed. The results show that the burst tendency of the RCB specimen increases with the angle. The stress-strain curves of some uniaxial and triaxial test specimens show two or more peaks, and the thermal imaging evolutionary process shows that the cracks of the coal and rock develop from shear to tension shear cracks. There is a further development of fracture and energy accumulation between the first and second peaks in the stress-strain curve of the specimen. Therefore, the failure degree of the second peak of the specimen may be stronger than that of the first peak. Additionally, the established stiffness coefficient and burst energy index can better describe the burst tendency of the RCB specimen under different stress states. The results show that the burst tendency of the RCB specimen under the triaxial test is much higher than that of the uniaxial test. In other words, it also explains that the essence of the burst failure of the surrounding rock in the roadway is the initial instability induced by the inside surrounding rock in the roadway. Moreover, the burst tendency is the largest when the rock and coal combination angle is 15°, and the burst damage range may also be increased by the failure of internal coal and rock mass.

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