# Research on the Outburst-Rockburst Coupling Disaster Law Based on True Triaxial Unloading Tests
## 4 Numerical Simulation of Outburst-Rockburst Coupling Instability Disasters
### Numerical Model Construction  
Given the limitations of the monitoring data in the previous physical test, it was required to further obtain the characteristic information of the crushed coal body in the outburst. PFC3D (numerical simulation software) can be employed to effectively characterize the dynamic evolution process of cracks and the temporal and spatial transportation characteristics of particles in the disaster process. Hence, the outburst-rockburst coupling instability disaster was simulated with the aid of PFC3D. As shown in Figure 8a, the numerical model of the coal body was established with the consideration of the size effect and computational efficiency. The particle size of the model ranged from 0.035 m to 0.070 m, with 41,398 particles in total. The stress environment of the coal body was restored by the wall element in PFC [34], in which the X-axis direction was the horizontal tectonic stress loading direction, the Y-axis direction was the tunneling direction, and the Z-axis direction was the vertical stress loading direction. Figure 7: The schematic diagram of engineering background simulation of outburst-rockburst coupling disasters. A parallel-bond contact model was adopted to restore the contact and bond between the coal bodies [35; 36]. Additionally, the meso parameters of the particles corresponding to the macro-compressive strength of the coal bodies were obtained by the trial-and-error method (These are shown in Tables 1 and 2). Meanwhile, a fluid-solid coupling calculation module (compute fluid dynamics [CFD]) was preset in PFC. CFD can be utilized to basically restore the fluid pressure gradient and further settle the force between fluids and particles by settling the flow direction and velocity of fluids based on the porosity and permeability coefficient between the particles, thus realizing the coupling between particles and fluids. Therefore, as shown in Figure 8, in this paper, a 26 \(\times\) 26 fluid grid is constructed around the coal body with the CFD module to restore the gas in the coal body, and the parameters of the CFD model are shown in Table 3.  
seen from Figure 9a, the gas pressure decreased gradually from the deep coal wall to the outburst opening in the original equilibrium state. When the coal body was subjected to instantaneous disturbance loading, there was no significant fluctuation in the gas pressure in the deep coal body. However, the crack distribution diagram revealed that some tensile cracks and shear cracks were formed in disorder in the coal body. This indicated that the number and size of the cracks in the coal body were small under the action of instantaneous disturbance loading, which exerted little influence on the seepage of gas fluids and thus did not cause a locally high gas pressure. With the outburst opening as the center of the coal wall, several concentric hemispherical gas pressure uniform distribution spaces with an increasing radius were formed. This may be attributed to the development of cracks around the outburst in the coal body under the joint action of instantaneous disturbance loading and gas. Under the influence of pressure gradients, gas flowed along the new crack to the free space outside the outburst opening, which caused this gas pressure distribution pattern. When the coal and gas outburst occurred in the coal body, the deep gas pressure still decreased gradually. However, the fluid monitoring results demonstrated that the gas flow fluctuated slightly at this moment compared with the steady state, and some fluids deviated from the normal flow direction. Combined with the crack development, it was speculated that the crack caused by the disturbance further expanded under the joint action of crustal stress and gas in the deep coal body, thus providing a channel for gas seepage and changing the gas flow direction. However, the gas fluid velocity was almost unchanged, which indicated that the crack was small and the gas seepage resistance was still large.