PSI - Issue 84

Nicola Perilli et al. / Procedia Structural Integrity 84 (2026) 805–812

806

Keywords: Landslide Susceptibility; Slope Instability; Landslide parameters; Landslide-bridge interaction; Bridge risk assessment.

Introduction The Italian Guidelines for risk classification, safety assessment, and monitoring of existing bridges (LLG 2020) establish a multi-level risk assessment for structural, seismic, hydraulic, and landslide risks affecting bridges. Regarding landslide, bridges and their surrounding areas, undergo systematic visual inspections aimed at completing the Field Survey Form. Based on periodic inspection, inspectors evaluate primary parameters—slope activity (P A ), velocity (P V ), and magnitude (P M )—and secondary parameters ( Model reliability and Mitigation works ) to estimate the Class of Factor Susceptibility . This is then combined with Vulnerability and Exposure to ensure a structured CdA assessment (Tab.1). Field inspections often rely on subjective judgments for assigning primary parameters, while the evaluation of secondary parameters and consideration of potential landslides can introduce additional variability, thus complicating classification. Drawing on extensive empirical evidence—including inspections of over 250 bridges, re examination of a comparable number of cases, and detailed surveys of 95 bridges in areas dominated by engineered soils (Matera) and rock masses (Lucca and Pistoia) we found that logic flow for the estimation of the Factor of Susceptibility presents conceptual and operational ambiguities and uncertainties, which can compromise the reliability and reproducibility of CdA evaluations. For these reasons, to reduce ambiguities and uncertainties , this study proposes conceptual and operational refinements aimed at increasing reliability in the evaluation of the Factor of Susceptibility and improving the implementation of LLG 2020. In particular, to reinforce the accuracy, reproducibility, and operational clarity of CdA assignments for landslides threatening or affecting bridges, this study critically examined the conceptual validity of the following key terms: Susceptibility Class , Factor of Susceptibility , Slope Instability , potential landslide , proposing a coherent conceptual and operational redefinition. The study also examines the primary parameter criticality degree (P C ) and the secondary parameters Model uncertainty and Mitigation works (Tab. 1, 2), providing formal definitions and operational solutions for practical application. Table 1. Primary and secondary parameters used for the determination of the Factors of Susceptibility, Vulnerability and Exposure for the assessment of the CdA Landslide Risk. All Italian key terms are explained in the text. Factor Primary Parameters Secondary Parameters Susceptibility Activity state, Velocity, Magnitude Model uncertainty Mitigation works Vulnerability Type/robustness of the bridge Type foundation Extent of interference

Road alternatives; Type of crossed entity; Strategic importance of the structure

Ground elevation (TGM) Span width

Exposure

Table 2 N umerical values of the primary parameters Activity State, Magnitude, and Velocity of the Event (Adapted from LLG 2020)

Active Present at the time of survey or showing signs of ongoing movement

Inactive Not active for several seasonal cycles

Stabilized

Recognized landslide ( P A )

1

Potential landslide (PC)

2

Highly critical

Critical

Low critical

P A or PC

3

5

3

3

2 Overview of the Key Terms and Concepts: LLG 2020 vs. literature To understand the conceptual and procedural improvements proposed below, this section provides a synthesis of the LLG 2020 approach for assigning the Landslide Class of Attention (CdA). Subsequently, a very brief summary of the meaning attributed in the literature to some of the key terms used in landslide risk assessment is presented. 2.1 LLG 2020 Framework In LLG 2020, Susceptibility replaces Hazard , emphasizing spatial prediction due to uncertainty in event (time) probabilities. Slope instability (SI) is evaluated through primary parameters: P A , P V , P M , and Susceptibility or