Objective  The gradual depletion of shallow coal resources drives a shift to the mining of deep coal seams. However, the complex deep mining environment significantly increases the potential risks and severity of coal and gas outbursts. Therefore, there exists an urgent need to investigate the mechanisms underlying the mechanical instability of coals under deep mining, aiming to provide guidance for the safe mining of deep coal mines.

Methods  By integrating physical experiments, numerical simulations, and theoretical analysis, this study investigated coal and gas outbursts under burial depths of 500 m, 1000 m, and 1600 m and established a multi-field coupling model that incorporates the coal deformation, gas flow, and coal damage fields. Furthermore, it revealed the mechanisms underlying the mechanical instability of coals during coal and gas outbursts under deep mining.

Results and Conclusions  Physical experiment results indicate that during coal and gas outbursts, the coal seams exhibited a rise in the pressure relief rate as the burial depth increased, with the relative outburst intensity increasing to 29.05%, 38.05%, and 42.70% under burial depths of 500 m, 1000 m, and 1600 m, respectively. The post-outburst temperature of the coal seams was found to decrease significantly as the burial depth increased, with the maximum temperature drops reaching 0.17℃, 0.37℃, and 0.55℃ under burial depths of 500 m,1000 m, and 1600 m, respectively. During coal and gas outbursts, the migration velocity of pulverized coals was observed to increase with the burial depth, reaching peaks of up to 22.08  m/s, 22.87  m/s, and 26.58 m/s under burial depths of 500 m, 1000 m, and 1600 m, respectively. Overall, a greater burial depth corresponded to a stronger outburst dynamic. Numerical simulation results reveal that with decreasing lateral pressure coefficient and increasing gas pressure during coal and gas outbursts, the coal damage zone evolved from a semicircular shape to a butterfly shape while propagating primarily in the vertical direction. The deflection angles of the damage cavity increased to 8.1°, 19.0°, and 34.4° under burial depths of 500 m, 1000 m, and 1600 m, respectively. Under the latter two burial depths, the maximum permeability occurred at the exposed surface of coals, reaching 10.8 times and 47.5 times the initial permeability, respectively. The maximum seepage force was also identified at the exposed surface, reaching up to 3.62 MPa/m, 7.74 MPa/m, and 11.93 MPa/m under burial depths of 500 m, 1000 m, and 1600 m, respectively. Based on the variation characteristics of vertical stress, permeability, and seepage force during the transition from moderately deep to ultra-deep mining, the coal seams can be divided into a rapid change zone, a fluctuation zone, and a stable zone. Under deep mining, the coals were subjected to high in-situ stress and high gas pressure, manifested as high vertical stress and high seepage force, respectively, which led to coal failure, an expanded plastic zone, and significantly elevated outburst risks. Further theoretical analysis of the mechanical properties of deep coals demonstrates that the coal and gas outbursts experienced four mechanical evolution stages: preparation, initiation, development, and termination. The analytical results systematically reveal that the mechanical instability of deep coals was jointly governed by high in-situ stress and high seepage force. This finding provides a new theoretical perspective for the mechanical evolution process of deep coal and gas outbursts. Given the damage characteristics of deep coals, along with the high seepage force in their fluctuation zones, it is necessary to implement intensified measures to enhance pressure relief and permeability in the prevention engineering of coal seam and gas outbursts at depth. These efforts will help break the coupling between the in-situ stress concentration and high gas pressure accumulation and reduce the energy that induces deep coal and gas outbursts in fluctuation zones.