Comparison of different fibre-optic strain sensing methods in tunnel linings (TS experiment)

Understanding the ground response and induced strains in the tunnel lining is needed for choosing a construction method and designing the lining in a way that complies with operational and long-term safety requirements of a deep geological repository. The TS experiment (Testing different tunnelling Supports in sandy facies during extension of Gallery 18) at the Mont Terri rock laboratory was initiated to test different tunnel support systems in the sandy facies of the Swiss host rock for a deep geological repository (Opalinus Clay). This experiment has two objectives: within the scope of construction technology, to analyse and understand the effect of support type, gallery orientation and facies on ground response; within the scope of monitoring, to compare different fibre-optic strain sensing methods for different tunnel linings in clay rocks. Conventional monitoring approaches apply displacement measurements of geodetic targets at the inner surface of tunnel linings using total stations. In addition, electrical strain gauges can be installed on or inside the tunnel lining to provide continuous in-situ measurements. However, the number of measurement points of both techniques is limited. Fibre-optic (FO) strain sensing is an innovative monitoring approach that offers several advantages and is being used increasingly often in tunnel monitoring (e.g., Monsberger & Lienhart 2021). The advantages of FO strain sensing include the small size of the sensor, which is the FO cable itself, and distributed measurements along the entire sensing fibre resulting in many measurement points with a high spatial resolution. Also, specifically for monitoring a deep geological repository FO strain sensing was identified as a suitable monitoring technique (AITEMIN et al. 2013). Several experiments and studies were already conducted to investigate the performance under repository-like conditions (e.g., Piccolo et al. 2023). In the TS experiment FO strain sensing is applied for the first time in Opalinus Clay in tunnel linings. We use different strain sensing technologies for a comprehensive comparison regarding total strain, qualitative patterns of distributed strain, spatial resolution, overall functionality and effects particular to each sensing technology. Our results and conclusions could serve as a recommendation for the instrumentation of similar structures or similar experiments not only in Opalinus Clay but also in other rock types.