## 5 Discussion
### Theoretical Prediction of the Impact of Changes in Working Face Length on Support Strength  
The collapse height of the roof rock layer under different working face lengths can be determined, and therefore, the mining pressure induced during the unloading period of the working face can be obtained as  
\[q_{2}=\frac{\gamma\bullet\text{L}_{g}}{2\text{f}} \tag{2}\]  
where \(q_{2}\) is the induced mining pressure in the normal working face, kPa, and \(\gamma\) is the rock bulk density, \(\text{kN}/\text{m}^{3}\). There are many coal seams with thicknesses greater than 6.3 m and a geological reserve of 1351.99 Mt in the Shendong mining area. The current problem faced by the Shendong mining area is what kind of coal mining method to adopt to achieve scientific, economic, safe, and efficient mining and to maximize the recovery rate of the coal resources. In general, there are three main mining methods for thick coal seams: layered mining, top coal caving mining, and full-height mining. The following work analyses and demonstrates the advantages and disadvantages of these three methods and their applicability to the Shendong mining area from different perspectives. Analysis of the Applicability, Advantages, and Disadvantages of Layered Mining in Extrathick Coal Seams  
(1) Layered mining is technically feasible  
For the mining of thick coal seams, layered mining was mainly adopted in the 1970s and 1980s. Many experts and scholars have conducted comprehensive research on the technical issues in layered mining of thick coal seams, making it a mature and feasible technology. When mining thick coal seams in layers, metal mesh must be laid during the mining process of the first layer to form a false roof and facilitate the mining of the lower layer. The hanging mesh method for long-wall layered mining of thick coal seams was studied for different roof types, and good results were achieved via trial. A study was conducted on the movement of overlying strata in the layered mining of ultrathick coal seams. Due to the increase in total mining height, the movement of rock layers in the fractured zone is different from that in the curved subsidence zone. The rock layers in the curved subsidence zone still follow the sinking trend of masonry beams, while the rock layers in the fractured zone move in a stepped manner. The horizontal distance of the steps is suitable for the pressure step distance in the mining area. An experiment using a cemented roof to mine thick coal seams layer-by-layer showed that after the underground solidification period of 3 months, the time for the cemented roof material to reach its maximum strength is 28 days. Thus, it is best to use cover-type supports at the mining face. (2) Layered mining has poor applicability in terms of safety, efficiency, production, and economy For the 7 m thick coal seam in the Shendong mining area, if layered mining technology is adopted, the impact on the high production and efficiency of the mine and the economic efficiency of coal seam mining mainly includes the following aspects. Large amount of excavation work and maintenance costs for layered mining tunnels The 7 m thick coal seam adopts a layered mining method, which requires at least two layers of mining. Therefore, the amount of excavation work for the tunnel needs to be doubled. Moreover, the roof of the lower layer roadway is prone to fragmentation due to the compaction of the collapsed gangue in the upper layer, resulting in an increase in maintenance costs for the roadway. The increasing number of moves in the layered mining face makes it difficult to achieve high productivity and efficiency. For the 7 m thick coal seam, layered mining requires at least two layers of mining, so the layered mining face needs to be moved at least twice, and frequent movement of the face will lead to a waste of time and difficulty in achieving high production and efficiency.