Application of Brillouin optical time domain reflectometry to dynamic monitoring of overburden deformation and failure caused by underground mining

Brillouin optical time domain reflectometry (BOTDR), a fully-distributed optical fiber sensing technology, was applied to monitoring the deformation behavior of overlying strata during underground mining. Three types of optical cables, Metal kieso sensing optical cable (MKS), Glass fiber reinforced sensing optical cable (GFRS) and 10 m interval fixed-point sensing optical cable (10 m IFS), were embedded vertically into two boreholes that are located 582 m and 1746 m from the open-off cut, respectively. The strain distribution characteristics of the cables and variations of the mechanical and hydraulic properties in the overburden were analyzed with consideration of the lithology and mining positions. The results show that MKS cable has better performance in strength and accuracy than the GFRS and 10 m IFS cables. The strata in front of the panel provided front bearing loads and were compressed in the vertical direction. As the panel passed the cables to approximately 90 m, the tensile stress increased, and the peak value moved up gradually. With the occurrence and development of the caved zone, the strains of the lower optical drill were negative and in the compression state. Throughout the monitoring process, the upper cable showed a process from compression to tension, while the lower cable showed a process from compression through tension to compression. In addition, the strain distribution of the cables corresponded well with the strata. The vertical compression of the strata appears to be inversely proportional to their Young's modulus. Tensile failure dominates in the overlying strata during the mining process. With the advancing of the panel, the tensile failure zone moved upward episodically, and the height of the fractured zone reached the maximum where the distance between the face and cable was approximately 90 or 100 m. Compared with the traditional monitoring methods, BOTDR monitoring provided more accurate data on the dynamic height of the fractured zone. The research results are of practical significance for monitoring overburden deformation under mining, and they are helpful to prevent or mitigate water inrush and surface ecological geological disasters.