Distributed fiber optic sensors for tunnel monitoring: A state-of-the-art review

Distributed fiber optic sensors (DFOSs) possess the capability to measure strain and temperature variations over long distances, demonstrating outstanding potential for monitoring underground infrastructure. This study presents a state-of-the-art review of the DFOS applications for monitoring and assessing the deformation behavior of typical tunnel infrastructure, including bored tunnels, conventional tunnels, as well as immersed and cut-and-cover tunnels. DFOS systems based on Brillouin and Rayleigh scattering principles are both considered. When implementing DFOS monitoring, the fiber optic cable can be primarily installed along transverse and longitudinal directions to (1) measure distributed strains by continuously adhering the fiber to the structure's surface or embedding it in the lining, or (2) measure point displacements by spot-anchoring it on the lining surface. There are four critical aspects of DFOS monitoring, including proper selection of the sensing fiber, selection of the measuring principle for the specific application, design of an effective sensor layout, and establishment of robust field sensor instrumentation. These four issues are comprehensively discussed, and practical suggestions are provided for the implementation of DFOS in tunnel infrastructure monitoring.