Residual strength assessment of urban tunnels degraded due to environmental surroundings

In recent years, many tunnels have started showing worrying signs of degradation, causing major concerns regarding their structural safety. At the same time, the price of repairs and rehabilitation works have risen significantly. For these reasons, the study of tunnel durability is starting to get a lot of interest and great progress has been made towards tunnel modelling, in order to perform residual strength assessment. In this study, focus is directed towards the modelling of reinforced concrete tunnels degraded due to their environmental surroundings. In that regard, two tunnels have been studied: the Milan Metro Line 2 and the Leopold II tunnel. The objective was to determine the reduction of the original safety factor, to be able to assess the impact of degradation. Therefore, the main sources of reinforced concrete degradation and their consequences have been studied. Based on the deterioration observed for each tunnel, a collapse analysis was performed in the finite element program ADINA. Deterministic and probabilistic approaches have been implemented for a thorough interpretation of the results. The results of these analysis show an important impact on the residual strength. In both applications, the resistance of the most critical tunnel cross sections seems significantly more affected compared to other parts of the structure. Nevertheless, our study suggests that the consequences of environmental degradation are not critical with regard to structure failure, as the values of the resulting safety factors remain very large. These observations tend towards the conclusion that the two tunnels will presumably exceed their service limit state before being able to reach structural collapse. But further residual strength assessments need to be executed in order to corroborate the outcome of the present study.