## 3 Results
### Analyzing Force Chain and Deformation due to Overburden
#### 3.2.2 Analyzing Movement of Overburden and Surface  
(1) Overburden movement  
Coal seam excavation causes the displacement, deformation, and destruction of the overlying strata. Figure 20 shows a map of the displacement field of the overlying strata during the advance of the working face, viz. horizontal and vertical displacements respectively. The color variation of the numerical model indicates the constant change of the magnitude of the displacement of overburden, and the same color indicates the same magnitude of the displacement in the region. Figure 19: Structural model of force chain arch. Figure 20: Horizontal and vertical displacements caused by mining. When the working face had advanced 48 m, the main roof broke for the first time. Because of the small excavation space, the change in displacement of the rock layer was small, and the upper part of the panel appeared to bend and sink. A slight separation phenomenon was observed, and the rock layer surrounding the panel moved to the panel due to the action of self-weight and horizontal stress. When the working face had advanced 56 m, the first collapse of the main roof occurred because the excavation space became bigger, the phenomenon of some layer separation appeared above the panel, and the surface is cracked by extrusion. When the working face had advanced 128 m, the separation from the top of the panel was significant because the force chain arch developed to the surface, and the surface load layer occupied a large area of subsidence. The horizontal displacement of the overlying rock above the panel was opposite to that of the loess layer because the surface loess layer was affected by the sliding of the slope downhill, as the overlying rock layer filled the panel with the advance of the working face. When the working face had advanced 144 m, because the load of the overlying rock layer exceeded its own bearing limit, fracture instability occurred, and the period of the basic roof occurred to press. At this time, the upper rock layer of the panel collapsed over a large area. In the vertical direction, the farther the rock layer was from the coal seam, the greater its sinking value was, because after, the main roof occurred the period of pressure, which lead to the increase of the surface subsidence. (2) Surface movement  
Figure 21 shows the curve of the surface subsidence of the coal seam during the mining process. With the expansion of the mining area, the subsidence and area of influence of the surface increased gradually. With the advancement of the working face, the subsidence curve assumed a \"V\" shape, which was not symmetrical at the center of the panel. The maximum subsidence inclined to the upward slope, and the curve of surface subsidence as a whole showed a trend of the basin edge being small and the basin center being large. When the working face had advanced 48 m, the first half of the surface observation line followed the law of the general subsidence curve. The maximum subsidence was located at point 4, at 0.595 m. The second half of the observation line did not accord with the law of the general subsidence curve because, following the first break of the main roof, the rock layer in the area was filled with the panel under weight and horizontal stress, and exhibited the trend of moving toward the panel. As the working face continued to advance, the maximum subsidence point shifted to the center of the panel. When the working face had advanced 128 m, the top of the panel collapsed to the surface, and the vault of the arch structure developed to it. The overlying rock layer of the coal seam lost its support, and the subsidence of the surface increased dramatically. Four mutations occurred in the surface subsidence curve in the mining process of the working face due to the steplike subsidence of the surface after the periodic weighting of the main roof. With each periodic weighting, the surface added a new  
Figure 21: Subsidence curve.