PSI - Issue 64

Lorenzo Brezzi et al. / Procedia Structural Integrity 64 (2024) 1589–1596 Author name / Structural Integrity Procedia 00 (2019) 000–000

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deep deformations and an evaluation of the effectiveness of mitigation measures for subsequent implementation. In this context, smart passive anchors instrumented with DFOS represent one of the few available solutions.

Fig. 2. Paradisi Landslide: a) Test site locations; b) Pre-intervention status in 2021; c) Status after reprofiling and drainage trenches in 2022.

2.2. Installation of Smart Composite Anchors Equipped with DFOS A total of 42 anchors were installed in a 5-row configuration, with each row set at an angle between 15° and 20° relative to the horizontal plane and spaced approximately 8 meters apart (Fig. 3a). These anchors were designed with a projected length of 30 meters, ensuring a minimum embedment depth of about 8 meters into stable bedrock. Constructed from a 76 mm hollow bar containing 4 internal tendons, the anchors were engineered to support horizontal structural loads from the landslide. Notably, 3 of these anchors (A1, A2, A3) were equipped with armored corrugated optical fiber cables (BRUSens® V9, Solifos) for strain measurement purposes. Approximately 70 meters of fiber cable were installed within each of these instrumented anchors, alongside the tendons, using a loop configuration to connect the fiber to the reading device in a double-end mode. For practicality, transmission-only fibers were used to establish connections between A1-A2 and A2-A3, thereby creating a single closed circuit for measurement, accessible near A1, located along a driveway. Furthermore, A1 was equipped with an additional cable for temperature compensation (BRUSens® DTS STL PA, Solifos), specifically designed to mitigate the effects of thermal variation on the optical core within the fiber. The installation procedure capitalized on the composite nature of the anchors, allowing for the integration of optical sensing fibers within the bar cavity alongside the tendons. Following the cementation of the cavity, a cohesive link between the bar and the fiber was established, transforming the system into an integrated mechanism for monitoring deformations and temperature variations along the bar. In contrast to alternative anchor designs (Monsberger and Lienhart, 2019), the versatility of the composite bars enabled in-situ instrumentation of optical fibers, facilitating their integration across extended bar lengths and enhancing their survivability during installation and operational phases. The arrangement of steel strands within the anchor structure provided the necessary support for fiber installation, which occurred in two sequential steps: initially, the hollow bars were installed using self-drilling techniques, with four harmonic steel strands inserted into the internal cavity of the bar to secure the optical fiber cables in place using specialized devices. A proper support was affixed to the base of the composite bar to support the cable loops and avoid excessive bending. Subsequently, the strands and fibers were encased within the rods using a specially formulated mortar injected through a central tube. A critical aspect of the installation process involved ensuring secure connections between the fiber sensor cables and the patch cords to the instrumentation. To address this challenge, IP67-rated enclosures were deployed to accommodate the terminals, as depicted in Fig. 3b. The cables were then interrogated using a Brillouin Optical Frequency Domain Analyzer (BOFDA) from FibrisTerre (Germany), featuring, thanks to the closed-loop configuration, a maximum sampling resolution of 5 cm and spatial resolution of 20 cm. Measurement accuracy for strain and temperature was reported as 2 με and 0.1°C, respectively.