PSI - Issue 64

Emilia Damiano et al. / Procedia Structural Integrity 64 (2024) 1628–1635 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction Catastrophic landslides are widespread across Europe, with a rising trend over the last two decades, especially in Italy (Haque et al., 2016), posing significant risks to infrastructure and communities. Conventional stabilization methods often fall short, particularly in managing complex gravity-driven processes, usually slow-moving phenomena involving great soil volume that can experience sudden acceleration and collapse (Carey et al., 2007). Understanding slope behavior and its response to external factors is very complex and, for this reason, monitoring remains an indispensable tool for risk reduction strategy. In this context, an upgrade of the traditionally most used and useful instrument for the observation of slope kinematics, the inclinometer, based on the use of Distributed Fiber Optic Sensors (DFOS) able to provide real-time, continuous spatial measurements over large areas, is under development to overcome the two main limits of the conventional device: the discrete point data obtained from the inclinometer (spatial resolution of 0.5m) that can hardly reflect the overall stability of the slope with the risk of missing critical points (Damiano et al. 2017), and the manual time-consuming data acquisition that cannot be employed for early-warning scopes. The use of DFOS for in situ strain monitoring in geotechnical fields is not new as pioneering work dates to the 1990s (Schreck et al., 1997). However, achieving a fully standardized new generation of DFOS-based inclinometers has not yet been reached (Acharya and Kogure, 2023). Several key challenges need to be addressed: • The durability of cables in harsh underground environments is a significant concern. While various coatings are used to enhance durability, they can also affect strain detection accuracy by causing interfacial slippage avoiding a full strain transfer between soil and the sensing fiber core (Wang et al., 2019). Selecting appropriate packaging materials is crucial for ensuring the effectiveness and longevity of sensing fibers (Brezzi et al., 2023); • Field installation procedures can impact DFOS performance. Current inclinometer setups involve gluing fiber cable along the outer surface of a vertical pipe or in grooves appositely made along it, requiring careful handling to avoid breakage during assembly, especially for long pipe configurations. The entire operation remains delicate and time-consuming (Choi et al., 2021); • Interpreting and validating strain data from DFOS-based inclinometers with other techniques (extensometers, inclinometers, numerical analysis, etc.) poses challenges. While they can detect overall deformation and major slip surfaces (Sun et al., 2016), quantitative estimation of deformation fields is often limited (Ye et al., 2022). Machine-learning approaches have been proposed to address this limitation (Zhang et al., 2020), but they require substantial training samples, leading to increased costs and limited applicability in certain locations. Regarding sensing techniques, Rayleigh and Brillouin scattering are widely used for monitoring slope deformations. Rayleigh offers high spatial resolution (up to a few centimeters) but limited extension (a few tens of meters), while Brillouin provides broader coverage with slightly lower resolution (tens of decimeters). Brillouin sensing technique remains prevalent due to its adaptability and extended measurement range. However, compensating for temperature variations is necessary due to the dual dependence of the Brillouin Frequency Shift (BFS) on temperature and strain. Starting from previous field experiences, revealing shortcomings in cable selection, installation procedures, and data processing, this study introduces a novel DFOS-Inclinometer that employs the Brillouin sensing technique, designed to address the described limitations and to enable more comprehensive analysis and monitoring of complex deep-seated landslides. These advancements aim to enhance accuracy and reliability in monitoring, thereby improving landslide risk mitigation and disaster management strategies. 2. The novel DFOS-based Inclinometer: setup and data interpretation for slow landslides The novel inclinometer is realized by equipping a standard inclinometer tube with four DFOS-strain transducers, namely the New Smart Hybrid Transducer (NSHT), and a sensor for independent temperature measurements