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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without any interruption on-site . The sensor’s durability and insensitivity against electromagnetic interferences are essential with respect to the use in rail operation. This paper introduces the DFOS design and system realization inside concrete tunnel lining segments, currently being implemented at the Brenner Base Tunnel. The key aspects for using fiber optic sensors as well as the main monitoring objectives are discussed. Additionally, the sensor installation inside the concrete segment is described and the system implementation inside the tunnel regarding the overall monitoring setup during construction and operation is presented. Finally, the data acquisition and analysis in combination with the data transfer and the online visualization is introduced in order to provide a complete picture of the monitoring workflow. 2. Monitoring Objectives The DFOS implementation allows the condition monitoring of the machine-driven tunnel, with the aim of reducing risks and allowing optimizing maintenances works by monitoring during operation (Cordes et al. 2024). Machine driven tunnel structures within the BBT are typically constructed efficiently as single lining. For their expanded long service lifetime and the remaining uncertainties, a portfolio of construction measures as well as monitoring of the tunnel’s construction and operating status is required . Due to aforementioned uncertainties in the planning of deep tunnels, chord length measurements, geodetic displacement readings of discrete targets using total stations, laser scanning and fiber optic sensors in defined cross sections are used to monitor BBT segmental linings. While the first three are mainly focused on accompanying construction monitoring, the DFOS system is installed close to the intersection of rail tunnel and cross-passage as well as in tunnel stretches within geological fault zones. Deep-lying tunnels undergo high spatial variations in geomechanical loading. DFOS can enable continuous recordings of strain and temperature-induced strain changes in circumferential direction along the entire segmental lining ring with an appropriate accuracy for a very stiff, high strength concrete. The monitoring system in combination with an in-depth examination of the geological, geotechnical and hydrological conditions and extensive documentation of the entire construction of the exploratory tunnel (Cordes et al. 2024) provides a fundamental data base for the assessment of structural deformation processes during construction and operation. DFOS is intended to obtain information to pursue three main monitoring objectives: • Determination of the predefined construction measures for the opening of the segmental ring and the advance of the cross-passage • Proof of the load-bearing safety • Structural health monitoring during the operational phase without interruption 3. Fiber Optic Instrumentation of Concrete Tunnel Lining Segments The instrumentation of concrete tunnel lining segments can be performed outside of the tunnel apart from excavation. This procedure is beneficial in terms of installation as the sensing cables can be reliably attached without time-consuming inferences with the excavation works, which has not to be interrupted due to the sensor installation. The concrete segments are either pre-casted at a production hall, which is temporarily built on-site next to the tunnel access or are even manufactured apart from the construction site and delivered there subsequently. At the BBT, the tunnel segments are produced with classical reinforcement cages, where the sensing cable can be individually guided along the reinforcement bars (see Fig. 2). This enables the realization of strain sensing lines in circumferential as well as in tunnel drive direction and therefore, a complete coverage of the entire segment. Any supporting structure, usually known for steel fiber reinforced lining segments (see e.g. Soga 2015), is not required in this context. The strain sensing cable BRUsens V9 (Solifos AG 2024a) was evaluated and selected for the application. This cable type not only fulfills the BBT tender requirements regarding cable setup, tensile strength capabilities or operating strain range, but also fits well to practical aspects on-site. All layers of the cable are interlocking to guarantee a solid bond from the outer structured surface to the inner glass fiber core. In addition, one layer of temperature sensing cable