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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Keywords: Smart Passive Anchor; Distributed Fiber Optic (DFOS); Smart Monitoring; Soil Stabilization; Strain Monitoring.

1. Introduction Distributed Fiber Optic Sensing (DFOS) represents a technology with notable advantages and accompanying challenges pertinent to its application in monitoring contexts. One important benefit is its ability to provide real-time and distributed monitoring over large areas, making it useful for a wide range of application in Civil Engineering. DFOS exhibits rapid responsiveness in detecting and precisely locating disturbances along the fiber optic cable, encompassing deformations attributable to external forces or fluctuations in temperature. Noteworthy advantages include its cost-effectiveness relative to conventional monitoring systems, particularly when necessitating high spatial density of measurements, its capability to survey extensive lengths of cable from a singular measuring position, immunity to electromagnetic interference, robust durability, heightened measurement sensitivity, and compatibility with both existing and nascent structural configurations. Nonetheless, accompanying these advantages are considerations of certain drawbacks. Foremost among them is the required expertise in installation and maintenance, requiring specialized knowledge that may pose challenges in certain organizational contexts. Due to emerges in recent decades as a promising solution for structural health monitoring, particularly in detecting strain deformations and temperature anomaly, DFOS technology has begun to be widely used inside various element such as piles (Sun et al. 2020), bridges (Siwowski et al. 2021), tunnels (Monsberger et al. 2021), levees (Fabbian et al. 2024; Schenato et al. 2022), dam (Brezzi et al. 2023), building (Hehr et al. 2020) and landslides (Zhu et al. 2021). Focusing on landslides topic, the stabilization of natural or artificial slopes susceptible to collapse via the implementation of structural reinforcements represents a conventional remedial strategy aimed at enhancing shear strength and mitigating sliding movements and associated hazards along the slip surface. Among the myriad solutions for structural reinforcements, composite anchors emerge as notably versatile alternatives (Bisson et al., 2016; Brezzi et al., 2021). These anchors, comprised of 3 to 6-meter-long hollow carbon steel threaded rods, are self-drilling passive sub-horizontal reinforcements that are introduced into the soil using a self-drilling technique and interconnected with coupling nuts to achieve the desired installation depth (Fig. 1). The external threading of the rods facilitates rapid coupling with nuts and enhances the interface between the rod, cement, and soil to develop high frictional lateral resistance. Subsequently, one or more harmonic steel strands can be inserted and cemented inside the rods to bolster mechanical robustness against the pulling forces exerted by the surrounding soil mass. Additionally, the anchor head is affixed to a bearing plate, typically a precast concrete disk, via a nut. The bearing plate is tasked with distributing the resisting force exerted by the anchorage over a larger volume of soil, at the surface. Although such anchorages are passive, meaning no pretension is applied to the tendons, when the bearing plate is positioned and subsequent tightening of the fixing nut is carried out, small tensions are often generated in the first meters of the bar itself. These tensions are negligible for the structural design of the anchorage and tend to decrease as the anchorage begins to perform its stabilizing action. When the unstable slope experiences displacement, soil movement induces shear stress along the soil-cement interface and the cement-threaded bar, thereby fostering a synergistic reinforcing effect and triggering a traction stress state in the bar, leading to its extraction by the stable deeper soil (pull-out mechanism). At that point, the external plate aids in distributing the pulling force exerted on the bar head over a larger surface area. The primary operational mechanisms of composite anchors revolve around the coupling effect of the bar to the grout/soil and the enhanced robustness conferred by the integration of strands into the bar. Initial attempts to evaluate the performance of composite anchors over time using electrical strain gauges yielded unsatisfactory results due to the poor spatial resolution of the measurements. Subsequently, a preliminary application of high-resolution distributed fiber optic sensors (OFDR) was proposed as a proof-of-concept (Cola et al., 2019). However, this approach was limited by the reduced distance range of the employed technique and necessitated the installation of dedicated fiber cables on the anchors. Other experiences, however, have yielded promising preliminary results. For instance, this technology was employed within a test field (Brezzi et al., 2022; Brezzi et al., 2023), where a small and under-dimensioned number of reinforcements were installed within a roto-translative movement to bring the reinforcements to working limit conditions and interpret their behavior in reduced time frames. In this paper, a new case study is proposed. Here, the anchors are interrogated with a Brillouin-based distributed optical fiber sensor system, representing one of the first comprehensive and efficacious implementation of a smart soil anchor system for anchors applied on landslide