# Study on the Evolution Law of Overlying Rock Collapse Induced by Mining Based on BOTDR
## 5 Vertical Optical Fiber Strain Monitoring Analysis  
### Overlying Rock Collapse Height and Optical Fiber Strain Analysis  
During coal seam mining, the advancement of the working face leads to overlying rock collapse, fracture expansion, and bending deformation, forming a typical \"three-zone\" structure. Optical fiber sensing technology enables real-time monitoring of rock strain dynamics, allowing for the characterization of overlying rock failure extent and evolution. As shown in Figure 12, the strain evolution patterns of vertical optical fibers S1, S2, and S3  
Figure 11: Fiber displacement and displacement curve: (**a**) H1 fiber, (**b**) H2 fiber. at different advancing distances provide insight into the relationship between overlying rock collapse height and vertical fiber strain. As shown in Figure 13, at different advancing distances, the strain data of S1 exhibit distinct stage-based variations. As the working face advances, significant changes occur in the strain behavior of vertical optical fibers S1, S2, and S3. Taking S1 as an example, the following patterns are observed:  
Figure 12: Characterization of optical fiber strain in 3 stages of working face advancing: (**a**) fiber optic strain near the working face, (**b**) fiber optic strain within the working face, (**c**) fiber optic strain farther from the working face. At 0-40 m Advancement: The working face has not yet reached the position of S1, and the rock mass remains in its initial stress state, resulting in no significant strain variations. At 40-60 m Advancement: Due to front abutment pressure, S1 exhibits compressive strain, which increases as the working face advances. This suggests that the rock mass undergoes intensified compressive deformation in this zone. At 60-140 m Advancement: The compressive strain transitions into tensile strain, and the strain curve exhibits a distinct \"stepped\" pattern. First step (0-10 m): Corresponding to the caving zone, where the step height aligns with the caving height, and strain values increase with advancing distance. Second step (10-40 m): Corresponding to the fracture  
Figure 13: Optical fiber strain curve at different distances of working face advancing: (**a**) optical fiber S1, (**b**) optical fiber S2, (**c**) optical fiber S3. zone, where strain increases with the expansion of fractures, and the step height grows as the fracture zone extends. At 140-240 m Advancement: The strain curve further refines into three stepped regions, corresponding to the caving zone, fracture zone, and bending subsidence zone. First step (0-10 m): The strain stabilizes at 870 \(\mu\)e, indicating a fully developed caving zone where compressive strain dominates. Third step (40-60 m): Corresponding to the bending subsidence zone, where strain values gradually decrease, indicating that the rock maintains continuity and integrity under bending deformation. These findings suggest that optical fiber strain monitoring effectively captures the progressive failure process of the overlying rock, providing crucial insights into overlying rock collapse mechanisms in coal seam mining. Based on the strain monitoring results of optical fiber S1, the overlying rock in the stope can be divided into three distinct zones, each exhibiting significantly different deformation and strain characteristics. Caving Zone: This is the area most directly affected by mining. Optical fiber strain data show intense fluctuations with alternating positive and negative changes, indicating that the rock mass undergoes severe collapse effects, including rotation, compression, and impact of falling rock fragments. The high rate of strain variation suggests that rapid and large-scale rock movement occurs within a short period. Fracture Zone: Located above the caving zone, this area exhibits more stable strain values, predominantly characterized by increasing tensile strain. This indicates that the fractured rock mass retains structural integrity, controlled by tensile stress, leading to gradual fracture expansion without immediate collapse.