PSI - Issue 84

Federico Foria et al. / Procedia Structural Integrity 84 (2026) 296–303

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illustrated in Table 1. The approach used adopts the term “susceptibility” instead of “hazard”, referring only to spatial prediction and neglecting temporal prediction, given the inherent difficulties in defining the probability of an event occurring. To assess susceptibility, a scoring system is used that assigns numerical values to three main parameters: the state of activity for existing landslides or the degree of criticality for potential landslides, the maximum velocity and the expected magnitude. These values are added together to define the slope instability class, which is then combined with secondary parameters related to the reliability of the assessment and the presence of mitigation measures. Vulnerability, on the other hand, depends on the static layout, the light, the number of spans, the material and the foundations of the structure, which allow the structural and foundation type to be estimated, then corrected using a parameter linked to the extent of interference between the landslide and the structure. Finally, the level of exposure is based on traffic data (e.g., the average daily traffic, ADT) for the road network in question and the average span of the structure, associated with secondary parameters relating to the presence of alternative roads, the type of entity crossed by the structure and its strategic importance for seismic emergencies. The combination of susceptibility, vulnerability and exposure levels allows the Landslide Attention Class of the structure to be determined. Table 1. Primary and secondary parameters for determining susceptibility, vulnerability and exposure factors associated with landslide risk (HCPW, 2022a). Primary parameters Primary parameters Susceptibility Slope instability (magnitude, velocity state of activity) Model uncertainty, mitigation measures The method for determining the Geological Attention Class associated with landslide risk, primarily applied to tunnels with low overburden and in the portal areas, is based on the evaluation and scoring of three main input parameters: Geological Susceptibility, Geological Vulnerability and Geological Exposure. For landslide risk, the term "Susceptibility" is used instead of "Hazard" to exclude the assessment of the event's timing, which is generally difficult to evaluate. Susceptibility depends on slope instability, which represents a primary input parameter of the analysis. It is assessed by the state of activity (active, suspended, quiescent, inactive, stabilized), the volumetric magnitude, and the expected velocity. These factors are combined to get an Instability Index. The presence of landslides along the tunnel alignment can be assessed by exploiting existing landslide archives (generally at regional scale) and focused on the study area by conducting ad hoc geomorphological studies. Satellite interferometry (InSAR) can be helpful in assessing the landslide state of activity as well as data from ground-based monitoring systems if available. It has to be noted that in case of presence of active landslides require a higher-level accurate assessment, bypassing the classification. The presence of Mitigation Measures (secondary parameter) such as slope stabilization works or monitoring systems can lower the susceptibility class. Vulnerability is mainly determined by the relationship between the tunnel and morphological conditions, specifically the overburden thickness and the level of available knowledge. Overburden levels considered are deep (> 50 m), medium (20 to 50 m), and low (< 20 m). Exposure is based on average daily traffic, tunnel length, heavy vehicles, maximum design speed, alternative routes, and interference with buildings/infrastructure. It has to be noted that the methodology proposed by the Italian Guidelines is suited on road tunnels rather than on railway ones and therefore requires adaptation to the latter. In the case study proposed below the exposure was assumed to range from medium-low to medium-high as a first approximation. The Geological Attention Class is determined by combining Susceptibility, Vulnerability, and Exposure using the general risk matrix presented in Figure 1. Vulnerability Bridge type/strength and foundation type Extent of interference Exposure ADT level and bridge span Road alternatives, type of entity affected, strategic importance of the artwork.