## 5 In Situ Monitoring Result
### MPBXs Data  
When DOFS were used inside the borehole to observe the micro-strain of strata movement, several MPBXs were also arranged to monitor the strata movement in the mining process. The MPBX has a freely movable wire rope inside a loose cable. The anchor head at the bottom of the wire rope is coupled to the drilled rock by sealing, and the top of the wire rope is connected to the displacement meter at the orifice. When the anchor head at the bottom of the cable moves along with the stratum under the influence of mining, it moves together with the wire rope connected to it and then feeds the displacement data back to the displacement meter at the orifice. When designing the monitoring scheme, the monitoring anchor point should be set on the key stratum according to the discrimination result of the key strata location in the overlying strata, so as to monitor its movement.  
Six MPBXs were arranged inside the LC1 borehole. Since the monitoring equipment was installed on 24 July 2022, the displacement data of each MPBX has been automatically stored in the data acquisition instrument. A curve (see Figure 7) is generated from the displacement data obtained. The abscissa represents the relative distance between the LC1 borehole and the working face (the negative value indicates that the borehole is in front of the working face, and the positive value indicates the distance that the working face advances past the borehole). The ordinate represents the movement of MPBXs at different depths relative to the surface. The 1#MPBX was the deepest measuring point with a corresponding depth of 534 m, and other MPBXs were arranged upward in turn.  
Figure 6: The strain curves of DOFS: (**a**) the strain curves during 17–24 August 2022; (**b**) the strain curves during 31 August–25 September 2022.  
When the working face advanced past the LC1 borehole by 7.3 m, tensile deformation began to appear at the 4\(\#\)MPBX with a depth of 453 m. When the working face advanced past the LC1 borehole by 15.9 m, tensile deformation began to appear at the 5\(\#\)MPBX with a depth of 388 m. When the working face advanced past the LC1 borehole by 38.4 m, tensile deformation began to appear at the 2\(\#\)MPBX with a depth of 520 m. When the working face advanced past the LC1 borehole by 1\(\#\)MPBX with a depth of 534 m. When the working face advanced past the LC1 borehole by 58.1 m, tensile deformation began to appear at the 3\(\#\)MPBX with a depth of 492 m. At the 6\(\#\)MPBX, only slight changes were observed.  
The change velocity of each MPBX along with the daily advance distance was calculated, and the corresponding curve was obtained and shown in Figure 8. As can be seen from the changing curves, there exists a certain correlation between the displacement velocity of MPBXs inside the LC1 borehole and the advancing distance of the working face. In other words, the displacement velocity of the overlying strata undergoes several jumping changes with the increasing of the advancing distance. When the working face advanced past the borehole by 240 m, the displacement change velocity remained 0 although the working face still continued to advance at a certain rate, indicating that the overlying strata basically reached a stable state at this time.  
Figure 8: The changing curves of displacement velocity of MPBXs inside LC1 borehole.  
Figure 7: MPBXs displacement curves inside the borehole.