Transformation method of Φ-OTDR optical fiber strain and tunnel liner strain and its application in tunnel safety monitoring

A distributed optical fiber stress and strain monitoring system (DSS) is developed based on Φ-OTDR sensing in this study. To apply the system to tunnel monitoring, dynamic and static tests of concrete beams are proposed to calibrate the relationship between Φ-OTDR strain and structural strain. The development trend of Φ-OTDR strain accumulation over time under the influence of the external environment is εfiber, t=(0.023 ± 0.001)·t+(0.1425 ± 1.1795) through static tests. Combined with dynamic tests, the transformation relationship between Φ-OTDR strain and structural strain is εstruc = 1.263·εfiber-0.029·t + 0.708. The accuracy of the transformation method and the feasibility of Φ-OTDR fiber in structural strain monitoring are verified by tunnel field monitoring test. Finally, a tunnel safety warning platform is established based on Φ-OTDR technology. The calculation program and safety threshold library of monitoring indexes are implanted into the platform, making the system automatically judge and alarm the tunnel safety, thus ensuring the tunnel construction safety intelligently.