## 4 Simulation Analysis on Stability of the Withdrawal Space in the Final Mining Stage
### Study on the Stability of the Withdrawal Space under Different Stopping Coal Caving Distances  
A reasonable stopping coal caving distance is related to the stable migration of overlying strata in the final mining face and the withdrawal space. In this section, it is proposed to analyze the migration characteristics and stress evolution law of overburden structures in withdrawal space under different stopping coal caving distances of 0 m, 10 m, 20 m, 30 m, 40 m, and 50 m. For the convenience of expression, \(L_{\text{C}}\) is used to represent the stopping coal caving distance. 4.2.1 Migration Characteristics of the Overlying Rock Structure in the Final Mining Withdrawal Space  
The migration characteristics of the overlying strata structure in the withdrawal space under different stopping coal caving distances are shown in Figure 8. From Figure 8, it can be seen that with the increase in the distance of stopping coal caving, the number and range of key blocks undertaken by top coal caving increase, and the rotation angle of the key blocks slows down. 1. When \(L_{\text{C}}\) = 0 m (Figure 8a), due to the large mining space, there is a large range of suspended roof area under B, C\({}_{1}\) compacts the gangue, and the vertical drop between B and C\({}_{1}\) is large. Only when B rotates and sinks at a large angle can it be hinged with C\({}_{1}\). At this time, the dynamic and static load stress generated by B rotation is concentrated on the overburden rock in the withdrawal space due to the large rotation angle of B, and the extrusion deformation in the withdrawal space is serious, resulting in a crushing accident. 2. When \(L_{\text{C}}\) = 10 m and 20 m (Figure 8b,c), part of the goaf below B is filled with unplaced top coal. The rotation angle of B is slower than that of not stopping coal caving. There are still some suspended roof areas below B and the rotation of B is blocked. The vertical drop between B and C\({}_{1}\) increases, especially when \(L_{\text{C}}\) = 20 m. The large drop  
Figure 7: Numerical analysis model of extra-thick comprehensive caving working face. The dark blue hinge support in the diagram represents the velocity and displacement of the left, right and lower boundaries of the constraint model; the light green down arrow indicates that a uniform load of 12.0 MPa is applied to the model. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Rock Stratum** & **Bulk Modulus (GPa)** & **Shear Modulus (GPa)** & **Cohesion (MPa)** & **Angle of** & **Tensile Strength (\(\mathbf{MPa}\))** \\ \hline Fine sandstone & 10.51 & 6.92 & 2.36 & 43.4 & 0.37 \\ Sandy mudstone & 2.41 & 1.73 & 1.19 & 32.4 & 0.03 \\ Grit stone & 12.34 & 8.13 & 2.65 & 44.9 & 0.49 \\ Sandy mudstone & 2.41 & 1.73 & 1.19 & 32.4 & 0.03 \\ Mudstone & 2.68 & 2.12 & 0.95 & 26.4 & 0.09 \\ Grit stone & 10.11 & 7.59 & 2.55 & 44.6 & 0.45 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: _Cont._means that the interaction force between C1 and B is limited. Most of the dynamic and static loads generated by B rotation act on the stopping coal pillar and the stopping coal caving filling area within a limited range. The overburden rock in the withdrawal space is still subjected to concentrated load. Especially when \(L_{\text{C}}=10\) m, the extrusion deformation in the withdrawal space is serious, and there is a risk of support crushing in the support area of the hydraulic support. 3. When \(L_{\text{C}}=30\) m and 40 m (Figure 8d,e), the goafs of B and C1 are filled with unplaced top coal. B and C1 are buffered and supported by the uncaving top coal, and the rotation angle of B is greatly slowed down. During the rotation and subsidence of B, it is supported by the full range of top coal caving, stopping coal pillars, and C1. At the same time, C1 is affected by the rear C2 interaction. Therefore, at this time, the key blocks are interlocked and interacted to form a stable hinged structure.