# 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  
### 'Two Zones' Height Detection Techniques for Overburden Rock  
The height of the 'two zones' of overlaying stratum development in the working face is measured using the drilling peep method and the height observation method for underground water-conducting fractured zones. The downhole water-conducting fracture zone height observation method accurately locates the height of the water-conducting fracture zone by drilling into the underground water-conducting fracture zone and injecting water, then observing the leakage in each section of the borehole. The downhole water-conducting fracture zone height observation instrument consists of three parts, a double-ended water plugging device, a connecting pipeline, and a console, as shown in Figure 13. The double-ended water plugging device consists of two swelling capsules and water injection probe tubes, each controlled by the swelling console and the water injection console, respectively. The swelling console, swelling pipeline, and the two capsules of the double-ended water plugging device are interconnected to create a pressure control system for the swelling and contraction of the capsules. Similarly, the water injection console, water injection pipeline, and water injection probe tube of the double-ended water plugging device are linked to establish a water injection observation system for monitoring the water conductivity of the rock formation.  
The drilling peep method involves taking photos, peeps, and images of the borehole using a borehole imaging device to analyze the extent of borehole fracture [27]. The hardware system of the mine drilling imaging device consists of the host, probe, depth encoder, mine cable, wireframe, push rod, and more. During testing, the probe with a built-in high-definition camera is inserted deep into the borehole to collect image data of the borehole wall. The host computer stitches together the images of the borehole wall collected by the probe into a borehole wall expansion map based on the depth frame by frame, and records the three-dimensional coordinate angle information of the corresponding depth.  
Figure 12: Strike overburden structure after lower slice mining. (**a**) Working face advancing 80 m; (**b**) working face advancing 120 m; (**c**) working face advancing 180 m.  
By conducting borehole wall imaging, video recording, capturing images of key parts, and measuring borehole trajectory, the device enables the observation and quantitative analysis of the degree of separation cracks in the rock strata.  
A total of six observation boreholes were arranged, of which 1# borehole and 3# borehole were observation holes in the roof cavity zone, 2# borehole and 4# borehole were observation holes in the water-conducting fracture zone, and 5# borehole was the comparison hole in the original rock area. The boreholes were positioned 125 m from the lower crossheading of the 11092 working face. The vertical depth of the collapse zone height exploration borehole was 25 m, and the vertical depth of the guide crack zone height exploration borehole was 61.3 m. The borehole layout and parameters are illustrated in Figure 14 and Table 5. After completing the 11092 working face, the observation period lasted for about 3 months. Previous studies have indicated the height of the 'two zones' within 1 to 2 months after the working face is completed, and the maximum-stability period is at least 6 to 9 months [28]. Consequently, the 'two zones' of the overlying rock in the 11092 working face reached the maximum height.  
Figure 14: Observation borehole layout.  
Figure 13: Observation method for the height of the water-conducting fractured zone.