Segment misalignment, joint opening, and segment rotation monitoring for tunnels using distributed optical fiber sensing

Distributed optical fiber sensing (DOFS) has been adopted in various applications due to its spatial continuity, good precision, and versatile adaptability. Currently, its use in monitoring the longitudinal deformations for tunnels remains limited. This study proposes an instrumentation method based on a “W”-shaped fiber layout configuration, and the associated analytical solutions for capturing different tunnel deformation modes (segment misalignment, joint opening, and segment rotation) are derived. The effectiveness of the proposed scheme is evaluated by laboratory experiments and others’ research. Results demonstrate that the theoretical model can accurately calculate coupled tunnel deformations. Given the operational strain range within 12,000 με for optical fibers, a pre-strain of 6,000 με is recommended during fiber installation to capture both tension and compression. Segment misalignment can cause small axial fiber strain (opening strain), when the segment misalignment displacement is within ± 0.004·H (H is the tunnel segment height). Notably, true misalignment requires correction by subtracting the opening error strain from the measured value. Optimal fiber layout with b/H = 0.6 (b is the vertical projection of the inclined fiber) is suggested to enable the measurements of segment misalignment up to 0.004·H and joint opening up to 0.006·L (L is the tunnel segment length).