Distributed fibre optic sensing and novel data processing method for tunnel circumferential deformation – A case study of an ageing tunnel at CERN

The European Organization for Nuclear Research (CERN) is one of the largest nuclear research centres in the world, operating the most powerful particle accelerator housed in a massive 80 km-long underground tunnel network. Over the decades, regular site inspection has observed the ongoing development of structural defects, such as widened cracks, water infiltration and misalignment of the particle accelerator beamline caused by differential tunnel floor settlements. Such ongoing tunnel deterioration necessitates long-term field monitoring and assessment of the continuous deformation behaviour of the tunnel lining. Recently, distributed fibre optic sensing (DFOS) has emerged as a promising tool for monitoring civil structures by providing spatially continuous strain measurements using fibre optic cables. Conventionally, a fibre optic cable can be continuously bonded onto a structure with a flat surface for continuous strain measurement (e.g., pile foundations, retaining walls). Nevertheless, to monitor tunnel circumferential movement, fibre optic cables usually can only be deployed at some discrete fixing points around a curved tunnel surface. The raw DFOS data obtained by this fixing-point method cannot be directly processed into continuous strain measurements due to the discrete nature of the installation. This poses a challenge for accurately analysing tunnel structural performance. To address the limitations of the fixing-point method, this paper presents an optimized data processing method for analysing DFOS strain measurements of curved circumferential tunnel behaviour at the CERN tunnel. The proposed method enables the determination of the actual stepwise strain profile between fixing points and its conversion into tunnel convergence by integrating along the path. The converted DFOS tunnel displacements offer a more realistic and intuitive representation of the tunnel deformation mode than the DFOS strain profile alone. Additionally, the results of tunnel displacement demonstrate agreement with convergence measurements obtained by the conventional total station method, indicating the enhanced capability of DFOS for existing tunnel monitoring and assessment. The DFOS method has the potential to provide highly accurate deformation measurements, with a much higher resolution than the typical measurement accuracy, for example, by total stations.