## 3 Analysis of Fracture Evolution of High-Intensity Mining Overburden under Reservoir Dams
### Overlying Rock Layers and Dam Body Rock Transport Evolution Pattern  
Mining of the No. 5 coal working face in the No. 11 mining area of Wangwa coal mine caused transport damage and fracture dynamic evolution of the overlying rock layer, which posed a threat to the stability of the reservoir and the dam, thus affecting safe mining under the reservoir. Based on the mining damage theory [32; 33; 34; 35; 36] and through monitoring each observation point, the No. 5 coal face mining process overburden rock fall height, overburden subsidence, and surface subsidence curve were mapped out as shown in Figure 3.  
As the No. 5 coal working face advances continuously, the overlying rock layer experiences pulling and cracking, direct top initial collapse, basic top breaking, and ground surface sinking. When the working face advances to 85 m from the upper coal pillar dam body level distance, the delamination phenomenon is significant, the roof rock layer collapses along the coal wall behind the formation of articulation structure, the surface movement deformation is not significant, and the surface experiences weak sinking. At 118 m, the collapse zone height no longer continues to develop upward, the overlying rock fissures are mainly horizontal fissures, the roof plate occurs larger sinking, and there is fissure zone upward development. At 148 m, the top plate collapses in a large area, the upper part of the overlying rock forms an articulated structure, the original lower part of the cladding layer gradually closes, generating new overlying rock cladding layer, and the overlying rock layer sinks but still plays a bearing role. At 175 m, because the overlying rock layer is hard, the fractures develop to a certain extent and fractures appear, the overlying rock layer sinks so that the collapse height increases, and vertical fractures  
Figure 2: Physical similarity model of Wangwa Coal Mine. (**a**) Schematic drawing. (**b**) Physical drawing. are produced at the open-cutting eye. At 200 m, the mining area is compacted, the overall bending and sinking of the rock layers mainly appear but do not expand, the amount of surface sinking increases sharply, and the sinking amount reaches 4.6 m. At 223 m, the overlying rock fissures above the working face are mainly horizontal fissures, and the fissure zone no longer continues to develop upward. Advancing toward 260 m, the overlying rock collapses in a large area, and the articulation structure is formed on the left side; the overlying rock layers sink, and the original lower part of the fissures gradually close; Due to the thicker coal seam, the overall surface subsidence is larger, forming a \"concave\" basin. It is found that the surface subsidence near the left and right boundaries is smaller because the boundary effect causes the left and right boundaries as the overlying rocks do not fully collapse, and the surface subsidence is the largest. As shown in Figure 4, after the recovery of the working surface, the distribution of the overlying rock layer collapse zone and the fracture zone is obvious, and the overlying rock collapse occurs in a large area. The height of the collapse zone is 52.4 m, the hinged structure is formed on the left side, the coal wall collapse angle is 64\({}^{\circ}\), the overlying rock layer sinks, and the original lower part of the detachment gradually closes. The form of destruction is mainly the separation or cracking along the layer and the cracking or fracture of the layer vertically or obliquely. The maximum height of the collapse zone of the Wangwa coal mine is 52.4 m, and the maximum collapse ratio is 6.1, which is within the theoretical range. Figure 3: Overburden rock displacement curve. (**a**) Overlying rock collapse height. (**b**) Fissure zone development height. (**c**) Surface subsidence height. In the fracture zone, due to the influence of cyclic incoming pressure, the axial stretching of the overlying rock layer exceeds the limit it can withstand.