Feasibility study on distributed optical fiber monitoring of coal roadway roof

Based on the traditional roof "point" mode monitoring, BOTDR was applied to the distributed monitoring of the roof settlement displacement of the coal roadway along the trough in coal face. Firstly, through the theoretical analysis of BOTDR monitoring principle and the deformation state of coal roadway roof, it was proposed to use rockbolt (anchor cable) to carry out optical fiber fixed-point layout in the roof of coal roadway. According to the optical fiber axial strain and deformation measured by BOTDR between adjacent fixed points, the mathematical expression of optical fiber strain quantifying the roof settlement displacement in coal roadway was derived. Then, the indoor tests of fiber-optic fixed-point stretching and fiber-optic section stretching were carried out to simulate the roof settlement. In the tests, the errors between the displacement represented by the fiber-optic strain and the fiber-optic stretching distance were 0.690% and 1.545% respectively, the stretching distance and the displacement represented by the fiber-optic strain are basically the same. Finally, BOTDR field monitoring test of coal roadway roof in air inlet roadway during the mining process of coal face was carried out. In the field test, there showed two roof settlement areas characterized by the optical fiber strain. The maximum settlement displacement of the tow areas were 64.09 mm and 135.63 mm respectively. Compared with the underground cross observation method, the maximum error of the roof deformation range is 5.60%, the maximum error of roof settlement displacement is 4.96%, the absolute error of vertical roof displacement is less than 4 mm, which showed the feasibility of BOTDR in quantitatively characterizing the roof settlement range and displacement.