Objective With the gradual depletion of shallow coal resources, the complex deep mining environment has significantly increased the risk and severity of coal and gas outburst. It is urgent to investigate the mechanical instability mechanism of coal under deep mining to ensure the safe mining of deep coal seams.

Methods A combination of physical experiments, numerical simulations, and theoretical analyses was adopted to conduct physical simulation tests of coal and gas outbursts at different burial depths of 500 , 1000  and 1600 m. A multi-field coupling model of coal deformation field, gas flow field, and coal damage field was established to reveal the mechanical instability mechanism of outburst coal under deep mining conditions.

Results and Conclusions  (1) Physical simulation results of outbursts showed that with increasing burial depth, the pressure relief rate of coal seams accelerated, and the relative intensity of outburst increased to 29.05%, 38.05% and 42.70%, respectively. Meanwhile, the temperature drop of coal seams after outbursts became more significant, with maximum temperature reductions reaching 0.17 , 0.37 and 0.55  ℃, respectively. The migration velocity of outburst coal particles was also higher, with peak velocities of 22.08, 22.87 and 26.58 m/s, respectively. Overall, a greater burial depth corresponded to a stronger dynamic outburst phenomenon. (2) Numerical simulations of outbursts revealed that with a decreasing lateral pressure coefficient and increasing gas pressure, the coal damage zone evolved from a semicircular shape to a butterfly shape and propagated mainly in the vertical direction. The deflection angle of the damage cavity increased to 8.1°, 19.0° and 34.4°, respectively. At burial depths of 1000  and 1600 m, the maximum permeability appeared at the exposed face, which was 10.8 and 47.5 times the initial permeability, respectively. Meanwhile, the maximum seepage force existed at the exposed face and increased with burial depth, reaching 3.62 , 7.74  and 11.93 MPa/m, respectively. (3) According to the variation characteristics of vertical stress, permeability, and seepage force during the transition from medium-deep to ultra-deep mining, coal seams can be divided into rapid change zone, fluctuation change zone, and stable zone. Under deep mining conditions, high in-situ stress and high gas pressure acted on coal in the form of high vertical stress and high seepage force, causing coal failure, expansion of the plastic zone, and a significant increase in outburst risk. Furthermore, based on the analysis of mechanical properties of deep coal, the mechanical evolution process of coal and gas outbursts in four stages (preparation, initiation, development, and termination) was discussed, and the mechanical instability mechanism of deep outburst coal was revealed.