Non-invasive tunnel convergence measurement based on distributed optical fiber strain sensing

This paper demonstrates the feasibility of tunnel convergence measurement using distributed optical fiber strain sensors, exploiting Rayleigh scattering and different fiber anchoring methods on the structure's circumference. The proposed innovative solution does not restrict the tunnel practicable section, withstands harsh environment and performs distributed measurement which can provide more information than standard methods. Here is presented an inverse analysis finite element based method to compute convergence from orthoradial strain measurement. Its performances are assessed by identifying which are the influencing parameters and how their uncertainty plays on the inverse analysis outcome. Finally its implementation as a test in a laboratory is reported, which proved the ability to determine geophysical convergence with 1 mm resolution, consistently with expected performances.