## 3 Numerical Simulation of Full-Columnar Overburden  
Numerical simulation was carried out with reference to the strata column in Figure 2. In order to establish the model better, in addition to processing key strata in the column according to actual lithology, other rock layers were simplified as unified soft rock due to the excessive amount. Finally, the 99 layers were simplified into 43 layers, including surface soil layer, key strata, soft rock, coal seam, and floor. 3 DEC software was used for numerical simulation, which is based on the discrete element method to describe the discrete medium and perform mechanical calculation. The finite difference method was used to simulate the discontinuous mechanical behavior of the contact surface. The length and width of the 3 DEC model are 1600 m and 800 m, respectively, and the height is consistent with the actual depth of coal seam. The upper boundary of the model corresponds to the ground surface and belongs to the free boundary; the bottom and surrounding boundary of the model are fixed. The whole model itself is subject to vertical gravity. The strike length of the simulated working face is 1000 m, with 300 m coal pillars at each end. The inclined width of the working face is determined according to each scheme, and the boundary coal pillar of the same width is also left at both ends. The lithological mechanical parameters in the model are assigned with reference to the measured results in Table 1.  
Under the condition that the mining width is fixed at 300 m, three models with different mining heights, namely 3 m, 4 m, and 5 m, were established. Under the condition that the mining height is fixed at 5 m, three models with different mining widths, namely, 250 m, 300 m, and 350 m, were established, respectively. Through the above five simulation schemes, the evolution law of the internal separation layer and fracture of overlying strata movement under different mining heights and widths was simulated and studied. The specific model parameters are shown in Table 2.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Lithology**} & **Uniaxial** & **Uniaxial** & **Uniaxial** & **Elastic** & & & \\  & **Compressive** & **Tensile** & **Shear** & **Modulus** & **Ratios** & **Cohesion** & **Angle of** \\  & **Strength** & **Strength** & **(Mpa)** & **(Mpa)** & **(Gpa)** & **Ratio** & **(Mpa)** & **Friction (\({}^{\circ}\))** \\ \hline Fine sandstone & 23.97 & 1.5 & 6.28 & 3.8 & 0.26 & 2.58 & 45.32 \\ Medium sandstone & 19.23 & 1.17 & 4.79 & 3.61 & 0.3 & 2.07 & 41.97 \\ Silstone & 38.5 & 4.11 & 10.69 & 5.3 & 0.16 & 6.32 & 34.7 \\ Sandy mudstone & 39.31 & 5.03 & 11.47 & 5.34 & 0.18 & * & * \\ Mudstone & 31.73 & 4.14 & 5.85 & 4.07 & - & - & - \\ \hline \hline \end{tabular} Note: “-” denotes the lack of specimens of corresponding specifications; “-” denotes data exception.  
\end{table}
Table 1: The mechanical parameters of major lithologies in overlying strata.  
Figure 3: Tests of rock mechanical parameters: (**a**) Compression test; (**b**) Tensile test; (**c**) Shear test; (**d**) Poisson’s ratio test.  
In order to master the strata movement law of each model under the influence of mining, a vertical observation borehole (NLC) was arranged in the middle of the working face. A measuring point was arranged on each key stratum to monitor the stress and displacement data during mining. During the entire mining stage, the stress and displacement results of the mining overburden were monitored within the whole process of mining.