Distributed strain monitoring of tunnels
Distributed strain monitoring of structures using optical fiber sensing is a brand new technique which has opened new possibilities in tunnel survey. Cementys was mandated to install its SensoluxTM® sensor for two different structures: the brand new extension of the Parisian metro line 12 and the 35 years old Fréjus tunnel, the longest road tunnel in Europe, between France and Italy. The cable-sensor was set up on sensitive areas during important operations: a fire-access during tunneling for RATP and underneath a fresh air gallery before the excavation for the SFTRF. The optical sensor is based on Raman (respectively Brillouin) backscattering to measure temperature (respectively deformation).
The cable-sensor detects the deformation over its whole length, without any blind spot, with a spatial resolution of 0.5m, a precision of ±5µdef and a range of several dozens of kilometers.
The small diameter of the sensor (2mm) makes it easy to integrate into tunnels by gluing the optical cable in grooves carved on the concrete. The optical cable is thus protected for long-term monitoring, without any maintenance of the measuring system and with no deviation of the sensor. Moreover, the installation presents a low level of intrusiveness because the cable is integrated into the concrete.
Any circumferential deformation of the structure is transmitted to the cable rings, cohesive to the concrete, and detected by the interrogators.
The first measure is considered as the baseline and the following measures are compared with the first one to track relative motions. Consequently, the initial stress of the cable due to gluing does not impact the measures.
The sensor measures the deformation (compression or expansion) of the concrete to detect local damages like cracks or global motion such as convergence phenomenon. Data is retrieved from a long distance with the optical fiber for real-time monitoring of the tunnel thanks to optical technologies.