## 4 Numerical Simulation
### Numerical Analysis of Charge Structure
#### 4.1.3 Numerical Results
The results indicate that when the interval length is 1.0 m, the damage to the rock mass near the interface is most severe, suggesting that the blasting energy transmitted to the layering interface is greater, thereby making the mixed coal and rock blasting extraction difficult to achieve. As the stemming length increases, the blasting energy that acts on and damages the rock mass around the borehole increases, while the energy transmitted to the interface decreases, resulting in a gradual decrease in damage at the measurement points. When the interval length is 2.0 m, the distribution of coal damage values near the interface becomes more reasonable. However, when the interval length increases to 3.0 m, the blasting energy transmitted to the interface is insufficient to cause effective rock mass damage, and the rock damage value decreases significantly, resulting in greater fragmentation unevenness in the coal. (2) Coal bench damage effect  
The damage to coal and rock under different charge structures was assessed based on simulation results with a rock bench spacing of 2.0 m to evaluate the rationality of the coal bench charge structure. Figures 12 and 13 present the blasting damage contour maps and damage curves for the different charge structures. Figure 12a indicates that the rock mass damage near the coal-rock interface is significant when the stemming length of the coal seam is 2.0 m, with the combined effect of blasting and reflection stress waves causing extensive coal damage and complete fracturing. Figure 12b demonstrates that when the coal fill length increases to 2.5 m, the tensile stress intensity at the interface is reduced, and the rock mass blasting damage decreases. Figure 12c shows the blasting damage when the fill length is 3.0 m, where the tensile failure of the rock mass near the interface further decreases, and the crack connectivity improves. In addition, the damage in the interval section increases, and crack propagation becomes more uniform. When the stemming length increases to 3.5 m, as depicted in Figure 12d, the damage range of the coal near the interface does not effectively propagate, but the damage in the interval section between the two charge segments of the coal seam increases. Figure 12: Coal and rock mixed blasting damage with different charge structures (2). Figure 13 illustrates the coal damage curves at each measurement point under varying stemming lengths of charge structures. The results indicate that the damage at the measurement points gradually decreases as the stemming length increases. When the stemming length is 2.0 m, the rock mass damage near the interface is more severe, indicating that the blasting energy applied to the coal seam interface is greater, which hinders the mixed coal-rock blasting extraction. As the stemming length increases in Scheme 2 and Scheme 3, the damage degree gradually decreases, the intensity of blasting energy is moderate, and the damage value distribution becomes more reasonable. When the stemming length reaches 3.5 m, the rock mass damage value is lower, the blasting energy is insufficient to break the upper part of the coal, and the expected fragmentation effect cannot be achieved. The simulation results demonstrate that the charge structure is rational when the rock bench spacing is 2.0 m and the stemming length of the charge structure in the coal seam ranges between 2.5 and 3.0 m. The blasting energy intensity transmitted to the interface by the rock and coal layer explosions is moderate, the damage cracks in the rock mass are well developed, and the fragmentation effect near the interface is favorable.