PSI - Issue 84

Lorenzo Brezzi et al. / Procedia Structural Integrity 84 (2026) 505–512

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of existing protection works allows the evaluation of their spatial continuity, state of preservation, and potential vulnerabilities under extreme loading conditions. At the present stage, the outcome of the special inspection consists in the definition of a structured investigation and analysis plan aimed at reducing uncertainty related to rapid landslide hazards and their potential interaction with the viaduct. This includes the prioritization of critical slope sectors, the identification of scenarios with possible impact on the structure or its ancillary elements, and the definition of criteria for evaluating the adequacy of existing protection measures. The results of the planned activities will provide the technical basis for determining whether further detailed analyses, targeted mitigation upgrades, or specific monitoring strategies are required in accordance with the Italian Guidelines, to ensure an appropriate level of safety for the infrastructure and its users.

Fig. 2. (a) Viaduct overview; (b) landslide phenomena mapped in IFFI , highlighting debris flows (in green) and the entire area affected by rockfalls and toppling (in red).

3.3. Case C – Special inspection triggered by inventory-based landslide mapping and limited prior evidence The third case study concerns a viaduct located in central Italy, partially intersecting an area mapped in the IFFI landslides inventory as a quiescent rotational–translational slide. Consistently, the Hydrogeological Basin Plan assigns a high landslide hazard level (P3), corresponding to a high-risk level (R3), to the same area. The viaduct, built in the 1960s, has a total length of approximately 1.3 km and consists of 29 spans of 42 m each; both abutments and piers are found on piles. Unlike the previous case studies, this site is characterized by the absence of a documented history of landslide-induced damage or clear evidence of ongoing instability affecting the structure. The activation of the special inspection was therefore not driven by findings from ordinary inspections or by known instability events, but solely by the presence of a mapped landslide intersecting the infrastructure. This configuration represents a frequent operational scenario within large-scale risk assessment frameworks, where inventory-based information must be translated into site-specific decisions under conditions of limited prior knowledge. Between 2016 and 2023, two geotechnical investigation campaigns were carried out to characterize the subsurface conditions along the viaduct alignment. These included continuous core boreholes with sampling, execution of SPTs, installation of piezometers and inclinometers, and geophysical surveys based on MASW tests. All these investigations were distributed along the overall extension of the viaduct and were not specifically designed to investigate the landslide mapped in the IFFI inventory. As a consequence, none of the investigations were performed within the area identified as potentially unstable, and no direct information is available on the geological and geotechnical profile of the landslide body. From a geological perspective, the site can therefore be divided into two main sectors, with the mapped landslide affecting the southern sector characterized by a thicker cover of superficial deposits overlying a clay formation. Within the framework of the special inspection, an expert site survey was carried out to verify the presence of geomorphological indicators associated with the landslide mapped in IFFI. The inspection identified changes in slope gradient, locally alternating with terraces and mild counter-slopes, as well as inclined trees and shallow fractures in the upslope area of the viaduct. These features, partially masked by agricultural reworking, are consistent with a slow-moving and intermittently active instability. Shallow incisions observed near the landslide crown may represent