PSI - Issue 84

Lorenzo Di Taranto et al. / Procedia Structural Integrity 84 (2026) 1007–1014

1008

Keywords: Landslide; Viaduct; Monitoring; Inclinometers; Slope stability.

1. Introduction Slow-moving gravitational movements or slow-moving landslides are widespread phenomena within the recent uplifting area such as Italian Apennine. Indeed, from a geological perspective, Italy is a geologically young territory where many slopes have not yet reached equilibrium, even under gravity-induced loads. This is particularly evident in mountain chain areas or regions of recent tectonic uplift characterized by clayey or marly lithologies. Although these phenomena are characterized by very low displacement rates and are not always easily identifiable, their impact on infrastructure is not always simple to predict. Within the field of civil engineering, the interaction between these instability phenomena and linear transport infrastructures, such as bridges and viaducts, constitutes one of the most complex issues in landslide risk management (Barla, 2018; Gabrieli et al., 2024; Nappo et al., 2019). In this context, bridges and viaducts represent extremely vulnerable infrastructures: as rigid structures with limited resilience to soil-imposed deformations, they can undergo stress states and differential displacements over time that may compromise their functionality and safety. An emblematic case of collapse due to the interaction between slow gravitational movements and landslide phenomena, which critically stress the interposed structures, is the so-called 'Nutcracker' mechanism (D'Ambrosio et al., 2023; Simeone et al., 2024). This phenomenon occurs when slope movement—generally a Deep-Seated Gravitational Slope Deformation (DSGSD) or a slow-moving landslide involving one or both of the structure's supports - exerts axial thrust or compression on the bridge or viaduct. If the stress exceeds the structure's compressive or shear strength, a sudden failure (collapse) can be triggered, as occurred with the bridge over the Magra River in Caprigliola (MS) in 2020. The scientific understanding of DSGSDs and their associated risks has only consolidated in the last forty years, starting from the foundational works of Guerricchio and Melidoro (1981). Recent international literature (Lacroix et al., 2020; Pánek and Klimeš, 2016) confirms the systematic rather than sporadic nature of these phenomena, highlighting their pervasive global presence. This paper presents the analysis of a significant case study regarding the 'Santo Stefano' viaduct, located along State Highway 655 'Bradanica' in Southern Italy. The structure crosses a slope with a complex and steep morphology, primarily composed of Subapennine Clays with sandy interbeds, affected by deep deformation phenomena that necessitated recent deck repositioning due to extensive recorded structural displacements. The objective of the research is to deepen the interpretation of data collected through an integrated monitoring network, operational in various phases from 2013 to the present, focusing specifically on evidence from the most recent campaigns (2021 2024). Through differential analysis of LiDAR surveys, inclinometer investigations, and piezometric monitoring, the study highlights the presence of active sliding surfaces at significant depths (25-30 m) and documents a close correlation between rainfall events, groundwater circulation within thin sandy layers, and the acceleration of deformation phenomena. Therefore, this work aims to demonstrate the importance of overcoming the challenges associated with monitoring slow-moving phenomena, showing how a detailed understanding of hydro-mechanical interactions is essential for managing the safety of infrastructure interacting with deep-seated instabilities. 2. The sites investigated The study site concerns the 'Santo Stefano' (Figure 1a) viaduct, a strategic infrastructure located along State Highway (S.S.) 655 'Bradanica'. The geological setting (Figure 2) of the area is defined by the stratigraphic succession of the Bradanic Foredeep, characterized by the contact between the Subapennine Clays (Qca) and the calcareous quartzose sandy units (Qcs), locally overlain by terraced alluvial deposits (fl). The elevation profile of the slope begins at approximately 425 m a.s.l. and descends valleywards with highly heterogeneous gradients: the uppermost portion is characterized by steep slopes, estimated between 35% and 40%, while the central belt shows a gentler morphology with gradients of 15-20%. Finally, the terminal sector near the viaduct flattens out drastically, with values below 5%. The slope interacting with the structure exhibits a complex and markedly unstable morphology, defined by numerous slope breaks and structural discontinuities that give rise to secondary scarps converging towards a main kinematic mechanism. A fundamental integration for understanding the slope kinematics involves the presence of subsurface