# Study on the \"Two-Zone\" Heights in Lower Slice Mining Under Thick Al Alluvium and Thin Bedrock
## 4 Height Prediction of 'Two Zones' of Overlying Strata in Lower-Layer Working Face
### Numerical Simulation of 'Two Zones' Height of Overlying Strata in Working Face
#### 4.3.1 Structural Characteristics of Post-Mining Overburden Failure in Upper Layer  
Simulation results indicate that the fracturing of overlying strata occurs when the upper-layer 11091 working face is mined to different positions, as illustrated in Figure 11.  
From the simulation results of the overburden failure structure in the strike direction of the upper-layer working face after mining, it can be seen that as the working face advances, the periodic collapse of the overlying strata gradually leads to the formation of a regular distribution of water-conducting fracture channels. When the working face  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Strata** & **Lithology** & **Thickness (m)** & **Density (kg/m\({}^{3}\))** & **Bulk (GPa)** & **Tensile (MPa)** & **Cohesion (MPa)** & **Angel of Internal Friction (\({}^{\circ}\))** \\ \hline Quaternary and neogene & Clay/gravel/fine sand/ sandy clay & 50 & 2300 & 3.2 & 1.6 & 0.7 & 33 \\  & Clay stratum & 5 & 2300 & 3.2 & 1.6 & 0.7 & 33 \\ \hline  & Sandy mudstone & 8.84 & 2550 & 3.7 & 1.6 & 2.7 & 32 \\  & Medium grain sandstone & 2.75 & 2680 & 4.5 & 2.2 & 3.5 & 38 \\  & Mudstone & 1.87 & 2350 & 2.8 & 1.2 & 1.4 & 30 \\  & Medium grain sandstone & 2.45 & 2680 & 4.5 & 2.2 & 3.5 & 38 \\  & Sandy mudstone & 4.71 & 2550 & 3.7 & 1.6 & 2.7 & 32 \\ Roof bedrock & Medium grain sandstone & 3.93 & 2680 & 4.5 & 2.2 & 3.9 & 38 \\  & Sandy mudstone & 8.81 & 2550 & 3.7 & 1.6 & 2.7 & 32 \\  & Medium grain sandstone & 12.31 & 2680 & 4.5 & 2.2 & 4.1 & 38 \\  & Sandy mudstone & 6.88 & 2550 & 3.7 & 1.6 & 2.7 & 32 \\  & Mudstone & 0.90 & 2350 & 2.8 & 1.2 & 1.4 & 30 \\ \hline  & upper layer & 3.5 & & & & & \\ Coal seam & lower layer & 2.5 (Bottom & 1330 & 1.9 & 0.6 & 0.5 & 24 \\  & lower layer & broken 0.3 m) & & & & & \\ \hline Floor strata & Sandy mudstone & 13 & 2550 & 3.7 & 1.6 & 2.7 & 32 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Formation thickness in simulation.  
Figure 10: Simulation of strata distribution.  
advances 80 m, the lower key stratum collapses and loses its stability. However, the key stratum retains some bearing capacity, preventing the overlying strata from collapsing. The sandy mudstone layer below the clay layer produces the phenomenon of separation, the clay layer above the bedrock is in the deflection deformation area, and there is no obvious water-conducting fracture structure. When the working face advances 120 m, the key stratum fractures periodically, resulting in a distinct fracture channel forming in the center of the goat. Despite the bending deformation of the sandy mudstone layer above the main roof, it remains intact due to the support provided by the upper key stratum. When the working face advances 180 m, the overlying strata, which have a thickness of 13.7 m above the coal seam, enter the caving zone. Additionally, there is no obvious water-conducting fracture structure present in the rock layer above the key layer.