PSI - Issue 84

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

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Furthermore, the special inspection identifies the opportunity to develop simplified numerical analyses, such as limit equilibrium models, to explore potential critical scenarios associated with a progressive reduction in drainage efficiency. These analyses are intended to support the definition of specific alert thresholds for groundwater levels and displacements, providing an operational basis for early warning and for determining whether more detailed investigations or a renewed Level 4 verification may become necessary in the future, should changes in monitoring trends or boundary conditions indicate an increase in landslide-related risk. 3.2. Case B – Special inspection for rapid landslide hazards at the toe of a rock slope The second case study (Fig. 2) concerns a viaduct built in the 1980s in north-eastern Italy, located at the toe of a steep rock slope affected by rapid instability phenomena, and illustrates the role of special inspections in contexts dominated by rapid and impulsive landslide hazards. The slope is extensively classified in the IFFI landslides inventory as subject to rockfalls, toppling, and rapid flows (Fig. 2b), and is assigned high to very high hazard levels by the Hydrogeological Basin Plan, indicating a critical landslide setting. In contrast to Case A, where the instability mechanism is slow but has led to rapid structural instability and primarily governed by groundwater conditions, this site is characterized by impulsive, high-energy events, for which the main source of uncertainty lies in the identification of potential detachment zones, runout paths, and possible interactions with the viaduct and its exposed elements. Historical evidence confirms the recurrent nature and severity of rapid instability phenomena affecting the slope. In 2003, a flood event characterized by intense solid transport caused the undermining of an existing rockfall protection wall along the motorway embankment and the erosion of part of the abutment embankment. This event required the implementation of several protection measures, including the construction of rigid rockfall barriers to protect the drainage ditch, the installation of a double row of reinforced earth structures adjacent to the viaduct abutment, and surface erosion control works involving slope reprofiling, topsoil placement, and the installation of three-dimensional erosion control mats. Over the years, additional stabilization and protection interventions were progressively implemented along the slope, including the removal of unstable rock portions, local rock stabilization works, retaining structures, a section of artificial tunnel, rockfall barriers, and cable-based retention systems. Despite these measures, residual hazard remains, as demonstrated by a documented event in 2012, when a rock block approximately 30 × 30 × 40 cm in size overcame three rows of rockfall nets and reached the central median after bouncing near a lay-by. This event highlights the intrinsic difficulty of fully controlling rapid gravitational processes in steep rock slopes and reinforces the need for a robust and up-to-date assessment of potential landslide–infrastructure interaction. Within the current regulatory framework, the objective of the special inspection is not to reassess a single predefined instability mechanism, but rather to address a complex and spatially distributed hazard scenario involving multiple potential sources and kinematic processes. In particular, the SI aims to: (i) identify and characterize potential detachment zones associated with debris-flow initiation and unstable rock masses capable of triggering toppling or rockfall events; (ii) assess the spatial distribution, typology, and current condition of the existing protection works installed along the slope; and (iii) evaluate whether the present configuration of instability mechanisms and protection measures may give rise to direct or indirect interactions with the viaduct or with exposed structural and functional components. Accordingly, the special inspection builds upon an in-depth census phase and an expert field survey, during which available documentation, geomorphological evidence, and the existing protection systems are critically reviewed. However, given the extent, steepness, and partial inaccessibility of the slope, expert inspection alone is insufficient to reliably identify source areas and kinematic indicators of rapid instability phenomena. For this reason, the SI explicitly includes the execution of high-resolution remote sensing surveys, such as terrestrial or airborne LiDAR and photogrammetric surveys, as a core component of the investigation strategy. These surveys are intended to support a detailed morpho-structural analysis of the rock face and of debris-flow channels, enabling the identification of detachment niches, over-steepened sectors, discontinuity sets, and accumulation areas. The integration of remote sensing data with field observations provides the geometric basis for subsequent kinematic analyses, runout assessments, and impact evaluations. At the same time, the systematic mapping