PSI - Issue 84

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

508

https://idrogeo.isprambiente.it/app/pir ) classifies the area as subject to moderate to high hydrogeological risk, confirming the presence of predisposing conditions for slope instability.

Fig. 1. (a) Viaduct overview; (b) longitudinal section crossing the draining wells, along the discharge pipe; (c) plan view of well locations.

The mitigation strategy adopted at the time focused primarily on groundwater control, through the installation of sub-horizontal drains and deep drainage wells, complemented by an articulated monitoring system including piezometers, inclinometers, and topographic targets installed on both the slope and the affected structural elements. These interventions allowed the stabilization of the slope–structure system and provided long-term information on its evolution. Within the current regulatory framework, the objective of the special inspection was not to reassess the need for a detailed structural verification, which had already been implicitly addressed following the original instability, but rather to evaluate the present-day condition of the structure and the surrounding slope several years after the implementation of the mitigation measures. In particular, the SI aimed to: (i) reconstruct and systematize the historical evolution of the instability and of the mitigation strategy through an in-depth review of existing documentation; (ii) verify, through expert inspection and available monitoring data, the presence or absence of new geomorphological or structural indicators of instability; (iii) assess the current state and effectiveness of the deep drainage system; and (iv) define a strategy for the long-term management of residual landslide risk in accordance with the Italian Guidelines, accounting for potential degradation or loss of performance of the mitigation measures. Accordingly, the special inspection consisted of a detailed review of previous investigations, monitoring records, and intervention reports, carried out in parallel with an expert field survey. The on-site inspection did not reveal new surface indicators of instability affecting either the slope or the structure. This qualitative evidence is consistent with monitoring results: the most recent inclinometer measurements, available up to September 2024, indicate a quiescent condition of the landslide at depths between approximately 14 m and 28 m below ground level, while piezometric data from the instruments still in operation show groundwater levels persistently below critical thresholds. At the present stage, the special inspection does not provide a definitive reassessment of structural safety but allows the definition of a technically sound and proportionate plan of further activities. In particular, the SI highlights the need to maintain and, where necessary, restore the functionality of the existing monitoring network, including piezometers and inclinometers in the vicinity of the previously affected pier. Where recovery of existing instruments is not feasible, the installation of new boreholes is envisaged. Additional planned activities include the installation of topographic targets and displacement transducers on the structural elements, as well as geophysical surveys aimed at refining the understanding of groundwater distribution within the slope.