# Study on the \"Two-Zone\" Heights in Lower Slice Mining Under Thick Al Alluvium and Thin Bedrock
## 5 The Actual Measurement of the Height of the Two Zones' of Overburden Rock
### Analysis of borehole Peeping Results  
A total of four observation holes were measured during the field observation, with two observation holes of the caving zone and two observation holes of the water-conducting fractured zone observed. The borehole peep results of the 1# observation hole in the caving zone and the 4# observation hole in the water flowing fractured zone are shown in Figure 16.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Hole Number** & **Direction/\({}^{c}\)** & **Dip Angle/\({}^{c}\)** & **Hole Depth/m** & **Hole Structure** & **Remark** \\ \hline
1\# & 129 & +44 & 40 & & \\
2\# & 126 & +44 & 96 & Diameter of hole \(\Phi\) 113 mm & Whole hole coring \\
3\# & 123 & +48 & 37 & First-stage casing \(\Phi\) 108 mm & \\
4\# & 120 & +47 & 91 & Length 21 m & \\
5\# & 20 & +47 & 91 & End hole \(\Phi\) 94 mm & Whole hole coring \\
6\# & 350 & +47 & 91 & & \\ \hline \hline \end{tabular}
\end{table}
Table 5: Exploration drilling parameters.  
Figure 15: Statistical curve of drilling fluid leakage.  
The borehole peep length of the 1\(\#\) observation hole is m to 21.0 m-23.74 m. The transverse fractures of rock strata are wider, most being interlayer fractures. Longitudinal fractures are less frequent, less, and the fracture distribution is more uniform. It is considered that the rock strata within the range of 23.74 m (corresponding vertical height is 15.26 m) are located in the caving zone. In the 23.74 m deep hole, within a 40 m range at the bottom, there are several transverse cracks in the rock layer. The longitudinal cracks are more pronounced, and the distribution of cracks is relatively uniform. It is believed that the roof rock layer primarily exhibits tensile longitudinal damage and is situated in the fracture zone as it moves further away from the coal seam roof, with the mining influence gradually weakening.  
According to the results of borehole peeping collected from the 4\(\#\) borehole, it can be seen that the borehole has a significant inclination angle. The end of the casing (21 m long casing, with a vertical height of 14.32 m) is less than 1 m from the predicted caving zone height of 14.74 m. The drilling range from 21 m to 90.78 m shows obvious rock fragmentation, as well as vertical and horizontal fracture development. It is speculated that the height of the caving zone is less than 21 m (with a vertical height of 14.32 m). In the range of 21 m to 81.51 m, oblique fractures are developed and evenly distributed in the borehole. In the range of 81.51 m to 90.78 m, the cracks in the hole no longer develop, the hole wall remains intact, and the permeability of the rock layer is low. The observation results closely match the leakage observation results. Therefore, it is concluded that the height of the water-conducting fracture zone is 81.51 m (with a vertical height of 55.59 m).  
Through field observations of two boreholes for caving zone height and two boreholes for water-conducting fracture zone height, the leakage observations were mutually confirmed, resulting in a more reliable determination of the development height of the caving zone and water-conducting fracture zone. The statistical findings are presented in Table 6. The measured results of the height of the 'two zones' in the 11092 working face are as follows: the caving zone is 15.26 m and the water-conducting fracture zone is 55.59 m, with a corresponding caving ratio of 2.63 and a fracture ratio of 9.58.  
Figure 16: Observation hole peep results. (a) Peeping results of 1\(\#\) observation hole; (b) peeping results of 4\(\#\) observation hole.