Mechanical Response Characteristics and Law of Instantaneous Energy Conversion for Water-Bearing Coal–Rock Masses Subjected to Mining-Induced Stress

To investigate the characteristics of the responses of mechanical failure of coal–rock masses including their structures and karst fissures to energy conversion, the threshold characteristics of instantaneous energy conversion of coal–rock masses subjected to failure were revealed. This work took the evolution characteristics of water-bearing structures for gas outburst-prone coal–rock masses as the context for experimental research using a GCTS fluid–solid coupling triaxial servo-motor testing system. The results indicated that, under the effects of water weakening and moistening, the greater the degree of water saturation for coal specimens, the lower their peak strength, the larger the deformation experienced by coal specimens. As the water content increases, the failure characteristics of coal–rock masses tend to be simple, while the morphology characterized by small-sized cracks co-existed in the surrounding zone dominated by coalesced fissures is changed into simple coalesced fissure type, and the total crack propagation length increases accordingly. It is inferred that there is a certain correlation between the stress–strain curve of water-bearing coal specimens and the energy response characteristic curve. This finding implies that the energy conversion process of water-bearing specimens under increasing stress can be divided into four stages: the linear increases in elastic strain energy (stage I), the energy transfer and transformation (stage II), the energy transition (stage III), and the linear increases of dissipated energy (stage IV). Meanwhile, the available eigenvalue coefficient Ai is used to characterize the correlation between the aforementioned curves. Based on this result, a predictive model for the energy conversion curves of water-bearing specimens under mining-induced stress was established. The magnitude of energy erupted by coupled fluids within coal–rock masses under the driving coupled stresses including disturbance stress, hydraulic pressure, and gas pressure and potential degree of damage can then be predicted; consequently, prediction and early warning of compound dynamic disasters can be realized.