Abstract: Internal deformation data of surrounding rock serve as a critical basis for evaluating the stability of surrounding rock in deep underground engineering. Based on the principles of ultra-weak fiber Bragg grating (UWFBG) and time division multiplexing sensing, an externally anchored UWFBG strain sensing optical cable was developed. With a sensing point spacing of 1 m, the proposed optical cable enables quasi-distributed measurement of surrounding rock deformation. Calibration tests were conducted to investigate the sensing characteristics, error indices, and long-term stability of the proposed strain sensing optical cable. Subsequently, the optical cable was embedded into cement mortar specimens with varying strength grades to carry out three-point bending tests. These tests were designed to explore the strain transfer effect of the optical cable within cement mortar media exhibiting different mechanical properties. On this basis, a correction coefficient for strain transfer efficiency that takes the uniaxial compressive strength of the host material into account was proposed. Furthermore, drawing upon the deep roadway engineering project at Pingmei No. 4 Coal Mine, field tests were conducted on the strain sensing optical cable to validate its reliability and accuracy under complex geological conditions. The research results demonstrate that the strain sensing optical cable exhibits excellent linearity, repeatability and long term stability, with a maximum measuring range of 30 000×10−6. After introducing the proposed correction coefficient, the measurement accuracy of the strain sensing optical cable is significantly improved.

Key words: ultra-weak fiber Bragg grating, strain sensing optical cable, transfer effect, deformation monitoring, roadway