PSI - Issue 84

Federico Foria et al. / Procedia Structural Integrity 84 (2026) 304–312

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1. Introduction In recent years, partly as a result of tragic events, the technical and scientific community has become increasingly focused on monitoring the behavior of existing bridges/viaducts and tunnels. This issue is complicated by the numerous factors involved in defining it, which concern not only the structures themselves (in geometric and structural terms, together with their age and state of maintenance) and their function (which includes average daily traffic), but also the physical context in which they are located. In fact, these structures may interact with slow-moving landslides, which can cause changes in the static scheme of the elevated part of bridges/viaducts or in the functionality of tunnel linings, associated with the onset and progressive development of damage (e.g., cracks); or with fast-moving landslides, whose impact with bridge/viaduct piers or with exposed parts of artificial tunnels can lead to unbearable impulsive actions. In this context, the promulgation of the Guidelines of the Higher Council of Public Works for risk classification and management, safety assessment and monitoring at first of existing bridges (HCPW, 2022a) and then of existing tunnels (HCPW, 2022b) has made it possible to define knowledge paths that require the application of a multi-level approach based on ordinal scales. Looking at an international level, other regulations exist. But they differ in contents among them and with the Italian Guidelines being the inspiring principles strongly conditioned by economic, social, environmental, cultural, legal, technical and political indicators. In this regard, an interesting comparative analysis of international regulations was recently provided by Leto et al. (2025) for those specifically concerning bridges and viaducts. Focusing primarily on European countries, the authors note that in France the so-called Detailed Inspection and Diagnostic Guide prioritize structural assessments but lacks a comprehensive strategy for other issues, in particular those related to hydraulic and geotechnical disciplines. Similarly, the United Kingdom emphasizes structural integrity. On the other hand, regulations of United States consider both structural safety and landslide risk through the National Bridge Inspection Standards (NBIS) (U.S. DOT–FHWA, 2022) and the Unstable Slope Management Program (USMP) (U.S. DOT–FHWA, 2005). As far as tunnels are concerned, numerous regulations and procedures at an international level specify the procedures and timeframes for the periodic inspection of existing tunnels. The methods for conducting these inspections are generally qualitative and usually much more detailed for lined tunnels compared to unlined ones. The European approach to hydrogeological risk evaluation for railway tunnels doesn’t rely on a single EU directive specific to hydrogeology, but rather on a risk management framework for transport infrastructure and methodologies developed by EU advisory bodies and international professional associations. The European Union Agency for Railways (ERA) focuses on safety and risk estimation across different transport modes, including rail. While the ERA does not define a standard hydrogeological risk procedure, the framework's principles require railway entities to address it through the steps that are usually followed in a standard risk analysis, e.g. hazard identification, vulnerability assessment, risk mitigation measures. Another example can be found in the standards developed in the United States and described in the Tunnel Operations Maintenance Inspection and Evaluation (TOMIE) (Bergeson and Ernst, 2015) Manual which provides a framework for a national tunnel inspection program in the United States. Its goal is to improve the safety and performance of highway tunnels through a uniform and consistent approach to operations, maintenance, inspection, and evaluation. The TOMIE Manual provides comprehensive guidance for maintaining highway tunnel safety and reliable service in the U.S., based on the National Tunnel Inspection Standards (NTIS) (U.S. DOT–FHWA, 2015). In this paper we propose the adoption of some quantitative indicators for the description of the rock mass supporting a more quantitative description of defects and a more objective definition of the condition state of unlined tunnels. thanks to an integrated approach developed and implemented by ETS and called “MIRET” and its MIRET-Tunnel AI software. 2. Integrated approach: MIRET The starting point of our work is the observation that the existing procedures essentially consider lined tunnels, while the assessment of the conditions of unlined tunnels is limited to a few qualitative considerations. We therefore