PSI - Issue 84

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

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landslide–structure interaction. Conversely, Level 4 assessments require significant economic and technical resources and cannot be systematically applied to all potentially exposed structures. The effectiveness of the overall framework therefore depends on the ability of SIs to provide a focused and technically sound increase in knowledge, supporting informed decisions on subsequent assessment steps. The present work builds on the combined experience of ASPI, TECNE S.p.A., and the University of Padua (UNIPD) in the development and application of SI plans for landslide risk affecting existing bridges, with the objective of defining an operational and efficient approach for managing this intermediate assessment phase. To this end, a case based approach is adopted, in which selected application cases are used to investigate specific and recurrent issues encountered during the implementation of special inspections. The cases are intentionally chosen to represent different interaction mechanisms, levels of prior knowledge, and decision-making needs. The objective is to move from the analysis of individual situations to the identification of general operational criteria, supporting the definition of standardized yet flexible procedures for the application of SIs within large bridge inventories. Within this framework, special inspections are not intended to consist merely of repeated site visits conducted by personnel with geological or geotechnical expertise. Rather, they represent a targeted and goal-oriented assessment phase aimed at refining the understanding of potential landslide–bridge interaction when preliminary evaluations indicate elevated hazard levels. Depending on the specific scenario and on the amount and quality of available prior knowledge, SIs may include a critical review of existing documentation, expert field surveys, and, where necessary, preliminary investigations and dedicated monitoring systems on both the structure and the surrounding slope. Special inspections are grounded in a structured analysis of available background information, carried out in parallel with an expert inspection by a geologist and a geotechnical engineer, aimed at identifying geomorphological and structural indicators attributable to landslide activity. When this knowledge-building phase does not allow a reliable interpretation of the interaction mechanisms, targeted investigations and monitoring activities may be implemented. In order to preserve the efficiency of the framework, such activities are strictly goal-oriented and focused on reducing uncertainty related to landslide processes and their potential effects on the structure. The outcome of a special inspection may therefore lead either to the identification of cases requiring detailed Level 4 verifications or to the exclusion of significant landslide-related effects, allowing the Landslide Class of Attention to be updated and more demanding analyses to be postponed, provided that no signs of ongoing instability are observed. 3. Operational validation based on case studies 3.1. Case A – Bridge affected by a known landslide with mitigation measures The first case study (Fig. 1) refers to a viaduct located along the Italian motorway network in north-western Italy, originally built in the 1960s. The structure, with a total length of 216 m and five spans crossing a stream, was affected in the early 2000s by a significant landslide event triggered by an abrupt rise in groundwater level following exceptionally intense rainfall. The instability involved one abutment and one pier, inducing within a few hours displacements of approximately 15 cm in the longitudinal direction and 7 cm in the transverse direction at the pier level, together with a deck misalignment of about 1 cm. Although the event predates the adoption of the current Italian Guidelines, the magnitude and rapidity of the observed structural displacements would, according to present criteria, directly justify an accurate verification. Following the landslide event, extensive investigations and structural re-assessments were carried out, leading to the identification of the triggering mechanism as a rapid increase in pore water pressures within the slope. Based on as-built documentation, the affected abutment is founded on a shallow footing resting on natural ground through a lean concrete layer, whereas the adjacent pier is supported by a large-diameter deep foundation shaft. From a geological perspective, the subsurface profile consists of superficial fill material, gravel and cobble layers embedded in a sandy–silty matrix, underlain by the Monte Antola limestone formation, locally weathered in its upper portion. Despite the occurrence of the instability, the landslide is not mapped in the IFFI landslides inventory ( https://idrogeo.isprambiente.it/app/iffi ), while the Hydrogeological Basin Plan (PAI,