# Study on the Evolution Law of Overlying Rock Collapse Induced by Mining Based on BOTDR
## 4 Analysis of Horizontal Optical Fiber Strain Monitoring in Slope
### Evolution Law of Horizontal Optical Fiber Strain in Rock Strata  
As shown in the figure below, an excavation was performed on a similar simulation model, and the strain curves of horizontal optical fibers were monitored at different ad  
Figure 9: Theoretical tensile distance and strain calculated displacement curve: (**a**) fixed-point optical fiber tensile displacement curve, (**b**) single collapse optical fiber tensile displacement curve. vancing distances. To facilitate observation, measurements were taken at working face advancements of 115 m, 155 m, 195 m, and 240 m. Optical fibers H1 and H2 were installed 10 m and 30 m above the roof of the 72-coal seam, respectively. As shown in Figure 10a, the boundary coal pillars on both sides measure 30 m each. Before excavation, the overlying rock strain remains stable, with an average optical fiber strain value of 12 \(\mu\)e, indicating that the rock mass is in a relatively intact and uniform stress environment, unaffected by significant disturbances. The failure of the rock mass is primarily caused by tensile stress. When the internal tensile stress reaches the ultimate tensile strength of the rock mass, fractures begin to form. At the fracture location, the optical fiber experiences a sudden tensile stress surge, leading to a sharp increase in strain. As the overlying rock collapses and gradually compacts, the optical fiber primarily endures the gravitational load of rock fragments and relatively stable tensile stress, causing the strain value to level off. This phenomenon indicates that the optical fiber undergoes tensile deformation due to rock layer collapse and experiences varying degrees of tension within fractured rock zones. As shown in Figure 10b, at 115 m and 155 m of working face advancement, the strain in optical fibers H1 and H2 remains relatively low, indicating that the roof has not yet experienced large-scale failure. The strain curves exhibit a relatively smooth double-peak pattern, with slightly higher strain values at both ends and lower values in the middle, forming a'saddle-shaped' distribution. Experimental images confirm that, at this stage, the rock mass mainly develops localized fractures and small-scale collapses without significant  
Figure 10: Optical fiber strain curves at different advancing distances: (**a**) Collapse schematic. Horizontal optical fibers H1 (10 m above the 72-coal seam), horizontal optical fibers H2 (30 m above the 72-coal seam). (**b**) H1, H2 horizontal fiber strain curves. structural instability. The left peak is primarily concentrated within the 0-50 m range from the boundary coal pillar, indicating that this area is significantly affected by stress concentration, leading to active rock failure. Meanwhile, the right peak shifts forward as the working face advances, suggesting that the rock failure zone continues to expand. As the working face advances to 195 m and 240 m, the peak strain values increase significantly, as shown in Figure 10b. Particularly at 240 m, the peak optical fiber strain exceeds 2000 \(\upmu\epsilon\), indicating that the roof has entered a stage of intense failure. At this stage, rock fractures on both sides of the working face become evident, with sudden strain fluctuations, while the strain within the goat remains relatively stable due to compaction by overlying rock. The double-peak phenomenon persists, with the left peak position remaining unchanged and the right peak continuing to shift forward, confirming the progressive expansion of the failure zone. At 195 m advancement, strain peaks increase significantly, particularly in the left-side strain region of H1 and H2, where local strain values approach 1000 \(\upmu\epsilon\). Experimental images demonstrate that the collapse zone expands, fractures penetrate deeper, rock block sizes increase, and the severity of failure intensifies. The right peak in the strain curve moves forward, indicating that as the working face advances, the range of roof failure expands accordingly.