## 5 Field Measurement of Overburden Damage Height
### Detection Results and Analysis  
At present, the most common and accurate method for detecting the height of an overburden hydraulic fracture zone is the drill hole flushing fluid leakage method, which is used to determine the development height of an overburden hydraulic fracture zone by recording the leakage amount of flushing fluid and the change of water level in the drill hole during the drilling process. The observation of drilling flushing fluid leakage is carried out simultaneously with drilling, and the water level of the water source tank is measured when the flushing fluid forms a cycle, and the drilling depth is recorded. Thereafter, the water level of the water source tank and the hole depth of the borehole were recorded once for every 0.5 m of drilling, and the water level in the hole was recorded once for every 6 m  
Figure 11: Working face exploration drilling arrangement.  
of drilling until the end of the observation of interruption of flushing fluid circulation, and the observation of flushing fluid leakage from the borehole was schematically shown in Figure 12.  
From Figure 13a, it can be seen that the amount of flushing fluid leakage starts to change at a hole depth of 267 m, and suddenly increases and enters the top of the fracture zone. The drilling is drilled to 287 m for plugging, and continues top leakage drilling after the plugging is ineffective. The hole does not return to the slurry when drilling reaches 358.60 m, when all the flushing fluid is lost. There is no water in the hole after up-drilling, and there is an significant wind absorption phenomenon in the hole. The amount of flushing fluid leakage, water level change, and abnormal phenomenon in the hole were combined, and the rock layer at a depth of 266 m was judged to be the top boundary of the fissure zone, while the top boundary of the collapse zone was judged to be 358.60 m deep.  
From Figure 13b, it can be seen that when drilling to 262 m, the flushing fluid leakage was interrupted and gradually increased, and at 283 m deep, all flushing fluid was lost, and no water was returned after repeated water injection tests. In order to investigate the  
Figure 12: Field borehole observation.  
Figure 13: Field borehole observation. (**a**) #1 borehole leakage volume chart. (**b**) #2 borehole leakage volume chart.  
height of the collapse zone, we drilled to the hole depth of 382.90 m when all the flushing fluid was lost and decided that this was the top boundary of the collapse zone.  
According to the geological record of the core and the borehole observation, the position of the top interface of the fissure zone development is approximately 234.85 m in the depth of Hole No. 1, and the height of the water-conducting fissure zone development is 161.15 m, which is 17.9 times the mining thickness. The location of the top interface of the collapse zone is around 352 m deep in Hole No. 1, the height of the collapse zone is 39.85 m, and the collapse ratio is 4.4. The location of the top interface of the fracture zone development in Hole No. 2 is around 253.8 m deep, the height of the water-conducting fracture zone development is 170.76 m, which is 19 times of the mining thickness, and the location of the top interface of the collapse zone is around 225.24 m deep in Hole No. 2. The height of the collapse zone is 47.76 m, and the ratio of collapse to mining is 5.3; the development height of the No. 5 coal bending and sinking zone has reached the surface, and the surface is subject to uneven settlement to produce tensile fissures.