PSI - Issue 64

Christoph M. Monsberger et al. / Procedia Structural Integrity 64 (2024) 1665–1672 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Fig. 5. Schematic representation of overall monitoring setup during construction (top) and operation (bottom)

The installed fiber optic sensing cables have been interrogated using the fTB 5020 from fibris Terre Systems GmbH (Germany). Based on the Brillouin optical frequency domain analysis (BOFDA) technique, this sensing unit enables distributed measurements up to 25 kilometers within a measurement time of several minutes per monitoring channel, a standard spatial resolution of 0.5 m and a strain repeatability of about 2 – 10 μm/m depending on the sensing fiber (fibris Terre Systems GmbH 2024). The interrogation unit is placed inside the DFOS equipment boxes, where the raw data is collected and transferred to external databases via FTP gateway. The raw measurement quantities (e.g. frequency or intensity of backscattered signal) must be converted to strain (and temperature) using appropriate calibration coefficients. Many manufactures do not specify own parameters and refer to literature values instead, which may cause errors of several percent (cf. Moser 2016 or Monsberger 2017). For that reason, an individual strain and temperature calibration is performed for each sensing cable production batch using unique calibration facilities at Graz University of Technology (Woschitz 2015) to derive individual coefficients for cables installed at BBT. The derived strain and temperature profiles for all monitoring sections can be accessed by an online visualization dashboard. The data is processed and visualized with respect to the tunnel’s cross -sectional geometry in circumferential direction (as depicted in Fig. 6) or in tunnel drive direction for all installed sensing layers. Moreover, the temporal strain and temperature progress can be analyzed for each location along the section to investigate the deformation behavior over time. Filtering options (e.g. moving average filter with different filter lengths) can be individually applied to minimize or even emphasize signal irregularities. The strain and temperature data can be exported in various formats with respect to client’s requirements to provide an optimal basis for further evaluations .