# Stability Control of Multilayer Roof Strata in the Large Mining Height Gob-Side Entry: A Case Study
## 2 Engineering Background
### Roof Failure Monitoring  
During coal mining of the 22301 working face, borehole inspection methods were adopted to investigate the structural characteristics of the roof rock layers, as well as the internal fractures and delamination forms within the surrounding rock 80 m ahead of the 22301 head entry. Five monitoring stations were set up at 10 m, 25 m, 40 m, 60 m and 80 m ahead of the working face along the head entry. The vertical borehole with a parameter of \(\phi\)32 \(\times\) 10,000 mm was drilled vertically into the roof of head entry, 2.25 m away from the entry side. A ZKXG30 intrinsically safe mining borehole camera was used for inspection. Based on the roof information collected from monitoring stations, data analysis was performed and the distribution patterns of the fracture density inside the boreholes were obtained. A planar borehole expansion diagram was generated and proportionally drawn using AutoCAD. By analyzing the fracture evolution patterns, the distribution of rock layers and fracture characteristics ahead of the working face can be inferred. The fracture distribution diagram for monitoring station #5 is shown in Figure 3. Roof delamination is mainly concentrated within 1.2 m at the top side of the surrounding rock, and the delamination extends from shallow to deeper areas. The circumferential fracture density range is larger than the vertical fracture range. Shallow fractures in the surrounding rock are predominantly circumferential, while deeper fractures are mainly vertical. Figure 3: Fracture distribution for monitoring station #5. The fracture evolution characteristics in the roof rock layers for monitoring stations #1 to #5 are shown in Figure 4. As the vertical depth away from the roof surface increases, the integrity of the rock mass improves, transitioning from a fractured state to an intact one, with a corresponding reduction in both circumferential and axial fracture densities. In the range of 1.4 m inside the roof of head entry, the rock mass is in a fragmented and loose state, with significant delamination. Between 1.4 and 2.6 m, the density of circumferential and axial fractures is highly developed, and the roof integrity is relatively poor. From 2.6 to 6.3 m, the roof rock layers become more intact. While in the range of 6.3 to 7.2 m, the fracture development degree is denser due to the presence of a thin coal seam. Above the range of 7.2 m, the rock mass exhibits good integrity, which is illustrated in Figure 4a. As the distance away from the working face increases, the distribution area and density of the fractured and fissured zones in the roof rock layers decrease. Within 1.1 m of the roof in the head entry, the rock mass is in a fragmented and soft state with significant delamination. In the range of 1.1 to 1.7 m, the circumferential and axial fracture density is highly developed, leading to poor rock mass integrity. From 1.7 to 6.5 m, the roof is relatively intact with only a few fractures. However, in the range of 6.5 to 7.1 m, the fracture density increases significantly due to the presence of a thin coal seam. Beyond 7.1 m, the rock mass shows good integrity with low fracture density and overall structural soundness, as shown in Figure 4b. As the distance is further away from the working face, the fractured zone in the roof significantly diminishes, with delamination occurring only in a small area near the surface. The deeper rock mass is relatively intact, containing few fractures, as depicted in Figure 4c-e.  
Figure 4: Crack characteristics in the roof rock layers measured by five monitoring stations. By analyzing the crack characteristics of roof rock layers at the five monitoring stations, the fracture distribution in the range of 10 m to 80 m ahead of the working face is illustrated in Figure 5. Crack density and the crashed area exhibit a decreasing trend as the distance away from the working face increases. In the areas influenced by mining activity, the degree of fracture development and the extent of the crashed area are significantly higher.