## 3 Numerical Analysis of Land Subsidence Caused by Repeated Mining in Multiple Coal Seams
### Result Analysis  
The vertical displacement of overlying rock stratum after working conditions 1 and 2 is shown in Figure 4a,b. The ground vertical displacement is shown in Figure 5, and the ground vertical displacement increment caused by mining on working faces 1408 and 5106 is shown in Figure 6. As can be seen from Figure 4a, the vertical displacement of the ground presented obvious asymmetry due to the coal seam dip angle, and the advance influence range on the left side of the 1408 mined-out area was larger than the advance influence range on the right side of the shallow 1110 mined-out area. Under working condition 1, the maximum vertical displacement of overlying rock appeared near the mined-out area of the 1404 working face. The maximum vertical displacement of ground appeared above the mined-out area of the 1404 working face and 1406 working face, and the displacement amount was \(-\)350 to \(-\)400 mm. The range of surface settlement was obviously larger  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Lithology** & **Thickness** & **Density** & **Bulk** & **Shear** & **Cohesion** & **Internal** & **Tensile** \\
**(m)** & **(kg\(\cdot\)m\({}^{-3}\))** & **Modulus** & **Modulus** & **(MPa)** & **Article** & **Single (\({}^{\circ}\))** & **Strength(MPa)** \\ \hline A-horizon & 38 & 2000 & 3.1 & 2.2 & 1.1 & 15 & 0.18 \\ Sandstone & 128 & 2850 & 7.6 & 5.2 & 5.5 & 38 & 6.8 \\ Medium fine sandstone & 52 & 2650 & 5.6 & 5.0 & 5.6 & 38 & 6.7 \\ Kern stone & 32 & 2850 & 5.3 & 4.5 & 6.5 & 36 & 4.9 \\ The first layer of coal & 3.5 & 1350 & 2.6 & 1.5 & 2.0 & 22 & 1.4 \\ Mudstone & 3 & 2543 & 5.3 & 4.1 & 2.9 & 32 & 2.0 \\ Post stone & 24.5 & 2800 & 6.6 & 4.0 & 4.5 & 35 & 5.1 \\ Kern stone & 11 & 2750 & 7.5 & 5.5 & 5.0 & 36 & 6.0 \\ Medium coarse sandstone & 17.5 & 2850 & 6.3 & 4.5 & 5.8 & 37 & 4.9 \\ Fine sandstone & 16 & 2800 & 8.6 & 6.0 & 19.8 & 42 & 10.0 \\ Sandy mudstone & 3 & 2600 & 5.6 & 3.8 & 7.9 & 40 & 2.9 \\ The fifth layer of coal & 8.5 & 1350 & 4.6 & 2.8 & 7.5 & 39 & 2.0 \\ Mudstone & 2.5 & 2500 & 4.3 & 2.1 & 4.5 & 33 & 1.8 \\ Medium coarse sandstone & 26 & 2750 & 9.6 & 5.8 & 9.8 & 40 & 5.0 \\ \hline \end{tabular}
\end{table}
Table 1: Lithology and mechanical parameters.  
Figure 2: Schematic diagram of the mined-out area distribution.  
than the mining scale of underground working face. As can be seen from Figure 4b, the maximum vertical displacement of the overlying rock appeared near the mined-out area of the 1406 working face under working condition 2. The maximum vertical displacement of the ground appeared above the mined-out area of the 1406 working face, the displacement amount was \(-550\) to \(-600\) mm, and the range of ground settlement was further increased. It can be seen from Figure 6 that mining in the 1408 and 5106 working faces resulted in two obvious settlement funnels on the ground. The maximum displacement in the center of the settlement funnels was \(-180\) and \(-211.8\) mm, respectively.  
Figure 4: Vertical displacement of the overlying rock under different working conditions: (**a**) working condition 1; (**b**) working condition 2.  
Figure 3: Arrangement of the measuring line.