## 4 Simulation Results of Fully Mechanized Mining with a Large Mining Height and the Analysis
### Failure Characteristics of Coal Walls Under the Roof-Cutting Scheme  
According to the simulation results of the original rock roof, it is believed that the rotation and caving of the key plates of thick hard roof are the main cause of the large-area spalling. In fully mechanized mining with a large mining height in Huoxi Coalfield, there was a large mining height, a large roof caving gap, large key blocks of masonry beam under the original rock roof conditions, and a cantilever-masonry beam structure. The violent pressure from the caving of cantilever beam and the rotation of the roof jointly caused the stress concentration of the coal seam.  
In order to eliminate the hazard of long and overweight key blocks to the coal wall stability in fully mechanized mining with a large mining height in Huoxi Coalfield, a roof-cutting scheme was proposed. The blasting roof cutting can reduce the impact energy of the roof, reduce the size of key blocks, accelerate the formation of masonry beams, and reduce the stress concentration of rotation, being a fundamental measure to prevent the coal wall failure. On the basis of numerical simulation results, and by reference to actual engineering, the height of roof cutting was designed to be 23 m, and the key stratum of limestone was cut with an interval of 10 m. The schematic diagram of the overburden migration after roof cutting is shown in Figure 9.  
Figure 8: Overburden migration of original rock roof.  
Figure (a)a reflects the movement of the overburden in the working face under the condition of roof cutting when the working face is advanced by 80 m. It can be seen that the pre-splitting by the blasting of the working face roof effectively controls the development of fractures within the designed range. As the working face advances, the key rock blocks formed on the roof subside in the shape of small steps. Due to the effect of roof cutting, both the subsidence and rotation of the roof are significantly reduced, and the roof is well supported by the hydraulic support. When the working face is advanced by 90 m (Figure (b)b), due to the relatively small roof pressure, a short cantilever is formed. Even with the presence of pre-splitting, it does not collapse in advance and cause the support to be crushed.  
As shown in Figure 10, by comparing the distribution of cracks in coal wall before and after roof cutting, it is concluded that after the cutting top, the initial rotation and caring of the roof occurred after mining advanced to 70 m. At this time, the spalling on the working face was the most serious, with a depth of 1.85 m at the failure boundary point and a height of 3.17 m at the wedge-shaped failure zone. Compared with the original rock roof conditions, the occurrence of large-area spalling at the working face was reduced, and no total coal wall failure occurred in the failure zone.  
The depth of the failure boundary point and the state of the failure zone before and after roof cutting were analyzed. The average depth of the failure boundary point before roof cutting was 1.41 m, and it was 0.77 m after the roof cutting, decreasing by 45.4%. The depth curve of the failure boundary point before and after roof cutting is shown in  
Figure 10: Numerical simulation result of roof cutting. In this figure, (1) results under original roof conditions; (2) results under roof-cutting conditions.  
Figure 9: Overburden migration after roof cutting.  
Figure 11a. The average height of the wedge-shaped spalling in the failure zone before roof cutting was 2.58 m, and it was 1.62 m after roof cutting, decreasing by 37.2%. The height curve of the failure zone before and after roof cutting is shown in Figure 11b. It can be seen that advance roof cutting can effectively control spalling under thick hard-roof conditions during fully mechanized mining with a large mining height. This control effect is consistent with on-site research, detection and analysis.  
Mechanism of Spalling in Fully Mechanized Mining with a Large Mining Height and the Control Schemes