PSI - Issue 84

Antonio Fiorentino et al. / Procedia Structural Integrity 84 (2026) 757–764

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landslide, erosional, or alluvial phenomena, or within zones classified as high hydrogeological hazard; (b) evidence of possible interference is observed between slope instability and structural components (piers, abutments, foundations) or the functional context of the structure (accessibility, serviceability, protection works); (c) historical documentation or inspection evidence indicates recurrence of events over time (e.g., rockfalls, debris or earth flows) within the interference area or immediately adjacent areas, including presumed or potential instability not formally inventoried. If the Special Inspection highlights particularly critical conditions, such as distress attributable to slope instability, direct interference with foundations or load-bearing elements, or credible scenarios involving rapid kinematic processes with potentially severe consequences, the assessment pathway shall be anticipated and the structure shall be directly referred to Level 4, bypassing Levels 2 and 3. This option is especially relevant for rapid phenomena (e.g., rockfalls and flows) and when observed anomalies cannot reasonably be decoupled from slope-instability processes. Special Inspections shall provide, as minimum outputs: (i) a documented description of instability conditions and of actual or potential interaction mechanisms; (ii) a reasoned assessment of the need to proceed to Level 4 verification, including justification of the decision pathway; and (iii) operational indications for subsequent actions (e.g., urgent territorial measures, temporary risk-mitigation interventions, operational restrictions, or monitoring requirements). In this sense, Special Inspections act as a decision-support procedure ensuring a traceable, technically justified transition and serving as a prerequisite for Level 4 assessment. A central element of the MARIE methodology is the definition, during Special Inspections, of preliminary geological and geotechnical models, conceived as uncertainty-reduction tools and as the technical basis for subsequent phases. These models, placed at the core of the decision-making process, enable: (a) discrimination of cases in which landslide–bridge interaction is effectively relevant, including presumed or potential landslides; (b) reduction of application-related discretionary judgments; and (c) correct planning of subsequent investigation, monitoring, and modelling activities. The preliminary models are developed through a graded sequence of activities calibrated case by-case, aimed at supporting more accurate knowledge-building while minimizing investigation costs. Activities include documentary reconstruction, photogeological analyses, satellite interferometric data (EGMS) (Cernuto et al. 2024; Vitaletti et al. 2026), geomorphological and geomechanical surveys, LiDAR/TLS acquisitions, and rapid geophysical and geotechnical investigations. This approach supports a structured and cautious management of uncertainty, particularly where landslide phenomena are not formally inventoried but can be recognized through morphological evidence and predisposing conditions. Within the MARIE methodological framework, soil slips are assessed at the territorial scale using physically based models such as SLIP (Montrasio et al. 2023), driven by historical rainfall time series and realistic adverse scenarios. The analysis combines morphological filtering of potentially unstable areas with physically based modelling to evaluate landslide–infrastructure interaction. In this context, Special Inspections assume a different meaning and are required only in specific cases, mainly to support model application through targeted morphological or topographic investigations (e.g., for the definition of an accurate Digital Terrain Model). 3.3. Knowledge project Based on the outcomes of the Special Inspections and the preliminary models, the Knowledge Project for the Level 4 accurate safety assessment is conceived as an organic, purpose-designed process aimed at building a coherent knowledge framework of the slope–bridge system through detailed geological and geotechnical models (and, where required, geotechnical–seismic models), in accordance with the current regulatory framework (NTC 2018 and CSLLPP Circular No. 7/2019). The Knowledge Project first requires the definition of the significant geomorphological domain and the reconstruction of the geological–structural, lithological, and hydrogeological setting of the slope, with the objective of identifying existing or potential failure mechanisms and kinematics, predisposing and triggering factors, and possible modes of interaction with the structure. In cases where the geological model of the slope proves insufficient for a proper definition of the hazard, particularly when interactions among hydrological processes acting at wider scales are involved (Larcher et al., 2024), catchment-scale models can be adopted, such as the SMART-SED model (Corti et al., 2023; Gatti et al. 2023). Where known or suspected instabilities are present, the framework shall be consolidated through a 3D reconstruction of the phenomenon (plans and cross sections at an appropriate number and scale), integrating surface surveys and coordinated subsurface investigations;