## 3 Results and Discussion
### Influence of Gas Seepage on Outburst-Coal Breaking  
The occurrence of coal and gas outbursts is intrinsically related to in situ stress, coal's physico-mechanical properties, gas pressure, and geological structures. Structural instability and mechanical failure of coal constitute prerequisites for outburst initiation, primarily driven by stress redistribution due to mining disturbances, where stored gas within coal seams further contributes [3; 4; 11]. Post-failure, coal undergoes progressive fragmentation under residual energy until ejected from the working face, resulting in the characteristic ultrafine coal particles (size \(<\) 0.075 mm) observed in outburst events [2; 9; 65]. Current theories identify two dominant energy sources for outbursts: gas expansion energy (free and adsorbed gas release) and elastic strain energy stored in the coal matrix. Notably, it is verified that the elastic energy from the stress of overlying strata alone can hardly pulverize coal to sub-75 \(\upmu\)m particles. This necessitates gas in driving coal fragmentation during outbursts [49; 50; 51; 65]. Gas participates in the outburst-coal breaking through two primary means: gas adsorption and gas migration (encompassing desorption, diffusion, and seepage). It has been evidenced that gas adsorption, especially long-term adsorption, significantly degrades coal's mechanical strength by altering its pore structure and surface energy. This degradation facilitates pore-crack system propagation and promotes coal fragmentation, which is conducive to outbursts initiation [4]. A positive feedback effect would be established in the gas desorption-diffusion process, where gas desorption-diffusion makes coal breaking into more tiny particles, increases the surface area, and further accelerates gas desorption and diffusion in turn. This feedback effect plays a positive role in outburst-coal breaking [16]. While gas seepage behaviors as the pressure-driven directional flow through the coal's pore-crack network, in which a counterforce on pore surface, termed seepage force, arises from seepage resistance and gas-pore interactions, proportional to the gas pressure gradient as follows:  
\[F_{seepage}=\gamma_{g}\cdot\frac{\partial p}{\partial x} \tag{10}\]  
where \(F_{seepage}\) represents the force induced by gas seepage, \(\gamma_{g}\) is the bulk density of gas, \(\partial p/\partial x\) is the gas pressure gradient. The experimental results demonstrated that briquette coal exhibited 1-2 orders of magnitude higher permeability than raw coal. Elevated axial and confining pressures compress the coal matrix, reducing pore connectivity and permeability, and higher forming pressures also diminish permeability due to enhanced particle cementation. Notably, the Klinkenberg effect at low pressures facilitates gas seepage by reducing flow resistance in micropores. That means, when coal and gas outbursts occur, mining-induced stress redistribution destabilizes the coal matrix, triggering abrupt unloading and structural failure. This abrupt unloading and failure accelerate gas seepage, which in turn promotes further coal fragmentation through energy transfer and pore pressure transients. These interactions establish a self-reinforcing cycle between gas seepage and coal fragmentation, as illustrated in Figure 8.  
Specifically, when coal and gas outbursts are triggered by mining disturbances, abrupt stress unloading and coal matrix destabilization initiate coal fragmentation and accelerate gas seepage. The resulting propagation of pore-crack networks enhances gas seepage, meanwhile promoting gas desorption and diffusion. In this process, elevated coal permeability allows for more gas participating in seepage, inducing more stored gas to quickly and heavily desorb and diffuse. Since the expansion work performed by the gas desorbing and diffusing from coal is linear with the cumulative amount of diffused gas [16], the gas expansion energy therefore will be amplified.