# Research on the Roof Failure Law of Downward Mining of Gently Inclined Coal Seams at Close Range
## 2 Overview of the Mining Area
### Numerical Simulation Analysis of the Evolution Law of Overlying Rock Fractures
The separation layer at 105 m above the top of Coal Seam 1 has developed further, but the development is relatively slow. The height of the water-conducting fracture zone remains at 89 m above the top of Coal Seam 1, and the collapse zone has developed to a height of 27 m. Therefore, after extraction of Coal Seam 2, the water-conducting fracture zone and collapse zone co-develop with those of Coal Seam 1. The final development height of the water-conducting fracture zone is 89 m above the top of Coal Seam 1, and the collapse zone reaches a height of 27 m above Coal Seam 1. After the goaf and overlying strata of Coal Seams 1 and 2 have stabilized, Coal Seam 4 is mined. As shown in Figure 8, when the working face advances to 20 m, the immediate roof experiences its initial collapse, with a collapse height of approximately 2 m and an overhang length of about 6 m. When the working face advances to 60 m, the main roof fractures and undergoes large-scale collapse, causing minimal disturbance to the goafs and overlying strata of the upper seams. At this point, the collapse height is 6 m, and the water-conducting fracture zone extends through the goaf of Coal Seam 2. When the working face reaches 100 m, the strata fracture again, and the collapse height extends through the Coal Seam 2 goaf, reaching 34 m above the top of Coal Seam 1. At 180 m, as the mining length of the lower coal seam increases, the influence on the overlying strata expands, and the water-conducting fractures develop upward to 101 m above the top of Coal Seam 1. This is located below the separation layer but does not penetrate it. The fractures in the overlying strata within the fracture zone are mainly horizontal, and the fractures in the upper part of the goaf gradually close. When the working face advances to 240 m, the collapse zone develops to a height of 40 m above the top of Coal Seam 1, and the water-conducting fracture zone reaches 112.6 m above the top of Coal Seam 1, penetrating the separation layer. After this point, the disturbance of the lower seam mining on the overlying strata decreases, and the collapse zone and water-conducting fracture zone no longer develop. The fractures exhibit an \"M\"-shaped distribution. At 320 m, mining has reached the stopping line. The height of the collapse zone remains at 40 m above the top of Coal Seam 1, and the height of the water-conducting fracture zone remains at 112.6 m. Therefore, after the extraction of Coal Seam 4, the water-conducting fracture zone co-develops with those of Coal Seams 1 and 2, reaching a final height of 112.6 m above the top of Coal Seam 1, and the collapse zone reaches a height of 40 m above Coal Seam 1. In summary, during the downward mining process of the gently inclined coal seam group, multiple reactivations of the overlying strata occur. Under the conditions of \"goaf-working face\" and \"goaf-goaf-working face\", the fractures in the overlying strata undergo six dynamic cyclic stages: generation, expansion, closure, regeneration, penetration, and reclosure. After the extraction of Coal Seam 1, the height of the water-conducting fracture zone is 36 m, and the height of the collapse zone is 8.4 m. After the extraction of Coal Seam 2, the water-conducting fracture zone and collapse zone combine with the roof damage caused by the extraction of Coal Seam 1, forming a joint damage zone. The height of the water-conducting fracture zone is 89 m above the top of Coal Seam 1, and the collapse zone is 27 m above the top of Coal Seam 1. After the extraction of Coal Seam 4, the  
Figure 7: Numerical simulation evolution process of Coal Seam 2 mining. water-conducting fracture zone and collapse zone, along with the roof damage caused by the extraction of Coal Seams 1 and 2, form a joint damage zone again. The height of the water-conducting fracture zone is 112.6 m above the top of Coal Seam 1, and the height of the collapse zone is 36.5 m above the top of Coal Seam 1.