Full-Length Tunnel Structural Monitoring

Abstract

New and existing tunnels can present structural risks related to surrounding geotechnical and hydrological conditions as well as unknowns related to design assumptions and construction materials. Such risks can materialize through the apparition of abnormal deformations, cracks, water ingress or, in the worse cases, collapse. The localization of such events or their precursor signs is a-priori unknown, so traditional instrumentation in chosen cross-sections is ineffective for damage detection and localization. Regular visual inspection is more effective in terms of detection probability, but is typically limited in terms of temporal intervals between visits. Additionally, tunnels are often difficult to inspect since the access is restricted due to operational reasons. If such structural risks have been recognized in the design phase or have been identified by inspection, installing a distributed fiber optic sensing system allows a permanent monitoring of the tunnel over its whole length. Sensing cables are typically installed longitudinally along the tunnel length at different positions around the section and provide detection and localization or abnormal deformations and settlements, formation or development of cracks and unusual temperatures. This contribution presents the application of distributed optical fiber sensing to the permanent monitoring of a highway, a railway and a penstock tunnel. For each project we provide information about the system design, installation and monitoring results.