## 6 Discussion
### Separation Layer Analysis Based on the Micro-Strain of DOFS  
By analyzing the micro-strain data obtained by the DOFS inside the LC1 borehole, it can be found (as shown in Figure 9) that, as the working face increasingly advances past the LC1 borehole, the deformation inside the overburden continuously develops from the bottom up. Under the influence of mining, the DOFS is broken at a depth of 475.5 m (according to 186.1 m above the roof of the coal seam), and the corresponding position is the lower part of KS4. When the working face advances past the LC1 borehole by 46.3 m, the deformation height of DOFS undergoes the first jumping change, and the deformation height develops to a depth of 453.7 m (according to 207.9 m above the roof of the coal seam). The corresponding deformation stratum is the contact surface between KS4 and the adjacent siltstone stratum just above it. When the working face advances past the LC1 borehole by 52.6-64.3 m, the deformation height of DOFS undergoes the second jumping change, and the deformation height develops to the depth of 390.4 m (according to 271.2 m above the roof of the coal seam). The corresponding deformation stratum is the contact surface between KS5 and the adjacent medium sandstone just above it. When the working face advances past the LC1 borehole by 101 m, the deformation height of DOFS undergoes the third jumping change, and the deformation height develops to the depth of 351.3 m (according to 310.3 m above the roof of the coal seam). The corresponding deformed layer is the joint plane of thicker medium sandstone and coarse conglomerate in the Lo-ho Formation. After that, the working face continues to advance away from the LC1 borehole, and the breakpoint of DOFS remains unchanged, indicating that the influence of mining on the deformation of the rear overlying strata is gradually weakened. Figure 9: The breakpoint depth of DOFS in LC1 borehole. The No. 6 drainage hole was also arranged in the middle of the working face with the same inclined section as the LC1 borehole. The strain breakage curve of DOFS inside the LC1 borehole was analyzed by contrast with the curve of water level change observed in the No. 6 drainage hole (as shown in Figure 10). The final depth of the No. 6 drainage hole is 645 m, about 14 m away from the roof of the coal seam, and its bottom is in the caving zone. In theory, when the working face advances past the drainage hole, a significant water level drop would appear in the hole. As can be known from the curve correspondence (Section I) in Figure 10, when the working face advances past the LC1 borehole and No. 6 drainage hole by 0-52.6 m, the overburden deformation inside the LC1 borehole was 475.5 m deep (according to 186.1 m above the roof of the coal seam) and remained unchanged. By contrast, the water level depth of the No. 6 drainage hole remained roughly stable at 335 m, without any significant change. The reason for the water level not dropping was probably due to the completion of drainage hole construction in advance, mudstone argillation, and mud precipitation, which led to the blockage of the lower part of the drainage hole. Meanwhile, the water-conducting fracture zone was not connected with the blocked section under the influence of overburden mining, thus causing the delay of water level drop. According to the curve correspondence (Section II) in Figure 10, when the working face advanced past the LC1 borehole and No. 6 drainage hole by 52.6-64.3 m, the deformation inside the overlying strata moved upward to the depth of 453.7 m in LC1 borehole (according to 207.9 m above the roof of coal seam) and the water level dropped significantly. In this case, it can be concluded that the deformation was mainly vertical through fracture and the deformation height was within the water-conducting fracture zone. Figure 10: The correspondence between the strata movement inside LC1 borehole and the water level inside No. 6 drainage hole.