# Research on the Roof Failure Law of Downward Mining of Gently Inclined Coal Seams at Close Range
## 2 Overview of the Mining Area
### On-Site Measurement Process and Result Analysis  
The elevation of the LD4-1 borehole opening is 1732.786 m. The Quaternary loose sand layer is drilled with a \(\Phi\)219 mm spiral drill or soil auger until the bedrock interface is reached. Then, a \(\Phi\)190 mm casing is lowered for water sealing, and cement is used to fix the casing in place. After the cement hardens, a \(\Phi\)133 mm diamond bit is used to drill to the final borehole depth, with clean water being used as the flushing fluid to observe the loss of circulation. After reaching the bedrock layer, the loss of circulation is observed for every meter drilled. When the borehole reaches a depth of 175 m, the consumption of flushing fluid suddenly increases, and the circulation stops. From 175 m onward, although no fluid returns, the water levels are detectable. Drilling continues to a depth of 179 m, but no water level is present in the borehole above this depth. Below 179 m, the loss of flushing fluid is severe, and after attempting leak sealing without success, drilling continues, despite the continued loss of circulation. During drilling, there is no return of slurry at the borehole opening, and all fluid is lost, with no water level being detected inside the borehole. The curve of flushing fluid loss with respect to borehole depth is shown in Figure 10. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \multirow{2}{*}{**Drilling Hole Number**} & \multirow{2}{*}{**Office Coordinate**} & **Office** & **1. Coal Base** & **2. Coal Base** & **4. Coal Base** & **Hole Depth** \\  & & **Elevation** & **Plate Elevation** & **Plate Elevation** & **Plate Elevation** & **Plate Elevation** & **Hole Depth** \\ \cline{2-8}  & **X** & **Y** & **H (m)** & **H (m)** & **H (m)** & **H (m)** & **H (m)** \\ \hline LD4-1 & 4,013,267.155 & 36,377,111.407 & 1732.786 & 1440.351 & 1423.447 & 1399.823 & 318.20 \\ \hline \end{tabular}
\end{table} 
Table 7: Drilling position information table. Figure 9: On-site construction drawing. As can be seen from Figure 10, the loss of flushing fluid between borehole depths of 105 m and 164 m shows little variation, remaining relatively stable. At a borehole depth of 165 m, the loss of flushing fluid begins to change, showing an upward trend. When drilling reaches 175 m, the loss of flushing fluid increases suddenly, and circulation is interrupted. At a depth of 179 m, all the flushing fluid is lost. After multiple water injection tests, no fluid returns, and it is initially determined that the top of a fracture zone has been reached at this location. The vertical distance from the top of Coal Seam 4 to the borehole opening is 330.3 m, and the vertical distance from the top of Coal Seam 1 to the borehole opening is 289.5 m. Based on the above analysis, the water-conducting fracture zone extends to 110.5 m above the top of Coal Seam 1. During the drilling process of the LD4-1 borehole, bit loss began to occur at a depth of 242 m. Below this depth, bit loss became frequent, and issues such as stuck drills and increased tool vibrations occurred. There was a noticeable airflow suction phenomenon in the borehole, and core recovery was limited and very fragmented. Therefore, it is determined that the top of the collapse zone in the LD4-1 borehole is at a depth of 242 m. The collapse zone extends to 47.5 m above the top of Coal Seam 1. A total of two optical imaging logging operations were conducted during this construction, as shown in Figure 11. The maximum logging depth was 318 m. Using the in-hole intelligent panoramic imaging logging technique, 360\({}^{\circ}\) digital images of the borehole walls were created. By observing these images, the development of fractures within the rock layers was identified, allowing the determination of the development locations of the upper boundary of the water-conducting fracture zone and the upper boundary of the collapse zone. Based on the analysis of the logging images, the borehole wall from the bottom of the casing to a depth of 174.9 m in the LD4-1 borehole is smooth, with no fractures detected.