## 4 Simulation Results of Fully Mechanized Mining with a Large Mining Height and the Analysis  
### Characteristics of the Failure of Coal Face at Different Dirt Bands  
In order to analyze the influence of the working conditions with different locations of dirt bands on the failure of the coal walls in fully mechanized mining with a large mining height, four different dirt bands were established. The dirt band of the thick coal seam in the Xinyuan Huoxi Coalfield is mudstone, and the dirt band is designed and simulated as follows: Condition a: without dirt band; Condition b: upper dirt band; Condition c: middle dirt band; Condition d: lower dirt band. Figure 5: Schematic diagram of the simulation of the hydraulic support. Figure 6: Curve of hydraulic support strength. A DEM numerical simulation was performed for coal wall failure under different dirt band conditions. It was found that the overburden migration and stress distribution were similar during advancement under different dirt band conditions, and that typical spalling occurred after advancement of 60 m. Therefore, the crack diagram, displacement cloud diagram, and force chain diagram under different dirt band conditions after advancement of 60 m were compared, and the comprehensive analysis is as follows:  
1. Condition a: When the coal seam was complete without a dirt band, the coal seam suffered wedge-shaped failure due to tensile and shear stress. During the initial mining, the stress endured by the coal wall did not change significantly. Cracks occurred, but these were not connected. No spalling was observed. With the advancement of the working face, the peak stress gradually transferred in the direction of advancement. Due to the movement of overburden, the middle and upper coal seam began to suffer wall failure. When the advancement distance was 60 m, the depth of the failure boundary was 1.79 m; the height of wedge-shaped failure in the failure zone was 2.23 m; and there were oblique cracks in the plastic zone. The simulation results are shown in Figure 7a;
2. Condition b: The dirt band was in the upper coal seam: During the initial mining, no cracks or spalling were observed. When the advancement distance reached 60 m, the depth of the failure boundary was 1.83 m; the height of the wedge-shaped failure in the failure zone was 0.87 m; and no failure occurred below the dirt band, and no oblique crack development was seen in the plastic zone. The simulation results are shown in Figure 7b;
3. Condition c: The dirt band was in the middle coal seam: During the initial mining, the stress endured by the coal wall did not change significantly. Some micro-cracks occurred, but these were not connected. No spalling was observed. With the advancement of the working face, the failure zone was limited between the dirt band and the roof. When the advancement distance reached 60 m, the depth of failure boundary point was 1.86 m; the height of the wedge-shaped failure in the failure zone was 2.15 m; and a small number of oblique cracks developed in the plastic zone but did not exceed the mudstone dirt band. The simulation results are shown in Figure 7c;
4. Condition d: The dirt band was in the lower coal seam: During the initial mining, the stress endured by the coal wall did not change significantly, and the internal tensile stress of coal wall was small. Cracks occurred, but these were not connected. No spalling was observed. When the advancement distance reached 60 m, the depth of failure boundary was 1.74 m; the height of wedge-shaped failure in the failure zone was 2.12 m; and oblique cracks developed in the plastic zone. The simulation results are shown in Figure 7d. Compared with occurrence without a dirt band, the changes in the position of the failure boundary points were small despite the different locations of the dirt band, which indicated that the dirt band significantly inhibited the crack development inside the coal seam.