## 3 Results and Discussion
### The Time Evolution Law of Vertical Displacement of Overlying Rock Strata  
During the coal-mining process, the overlying rock strata will be broken, subside, and become unstable. This will inevitably lead to the horizontal dislocation and the change in vertical displacement in the overlying rock strata [20]. To further analyze the movement state of the overlying rock strata during the mining of the working face of the shallow-buried soft coal seam, this paper mainly studies and analyzes the vertical displacement. Since the horizontal displacement is relatively small, it will not be analyzed here. When the working face is advanced to 60 m, as shown in Figure 7, in addition to the immediate roof, a small amount of displacement also begins to appear in the main roof. When the working face is advanced to 70 m, as shown in the figure, with the continuous advancement, the main roof reaches its ultimate span, and obvious displacement changes occur in the main roof above the entire goat. The overall displacement trend gradually decreases from the middle of the working face to both sides. The maximum subsidence value in the middle of the working face is 2.99 m. This indicates that the middle area of the overlying rock strata is greatly affected by the mining, but it has no impact on the rock strata above the key stratum. At this moment, the overall displacement field exhibits a \"rectangle-trapezoid\" configuration. The displacement of the overlying rock strata of the working face is symmetrically distributed along the middle. When the working face is advanced to 80 m, as shown in Figure 7, the displacement gradually expands upward. The displacement in the middle area of the overlying rock strata has expanded to the ground, and the overall subsidence of the upper rock strata is 0.67 m. The change is not obvious, but a certain trend has been shown. With the fracture of the key stratum, the overlying rock strata it bears will show obvious caving and subsidence phenomena. When the working face is advanced to 100 m, the key stratum has begun to cave and subside, the range of the displacement area in the middle of the overlying rock strata is expanding, and obvious settlement appears on the ground surface. Until the working face is advanced to 250 m and the mining is completed, the movement area of the overlying rock strata continues to expand along the advancing direction. Finally, in the vertical direction, there is a negative correlation between the height of the overlying  
Figure 7: Temporal evolution of displacement. rock strata and the subsidence amount; in the horizontal direction, the displacement gradually decreases from the area near the middle to both sides. Due to the different mechanical properties and the degrees of influence by mining of the overlying rock strata at different layers, the magnitude and distribution of their displacements will also be different. Therefore, the displacement of the upper overlying rock strata is relatively small, and the displacement of the lower overlying rock strata is relatively large, showing that the movement of the overlying rock strata has a certain stratification. The shape of the overall displacement field evolves from the \"rectangle-trapezoid\" (the formed rectangle has a length of 60.6 m and a width of 17 m; the trapezoid features a top base of 63.1 m, a bottom base of 80 m, and a height of 30.5 m) shape in the early stage to the \"hyperbola-like\" shape in the middle stage and finally evolves into a \"trapezoid\" shape (the formed trapezoid features a top base of 214.9 m, a bottom base of 250 m, and a height of 154 m). As the working face advances, the displacement of the overlying rock strata gradually increases. The middle area of the overlying rock strata is always the area most affected by mining, and the area with larger displacement will gradually expand, showing a certain time evolution trend. In addition, it can also be seen that from the initial stage of mining, the overlying rock strata may first generate small displacements in local areas.