Stability Assessment of an Ancient Roman Heritage Tunnel: The Crypta Neapolitana

The Crypta Neapolitana is a historic tunnel attributed to Lucio Cocceio Aucto, a Roman architect of the first century AD. It runs within the current urban area of the city of Naples and crosses the Posillipo hill, that separates the Gulf of Pozzuoli from that of Naples. Since about one century the tunnel has been not used for its original purpose, due to the creation of other larger road and rail tunnels nearby. After being abandoned, several disruptions occurred along its route and the Crypta is not accessible anymore to the public. The tunnel runs across different pyroclastic formations: the deeper layer is a weakly cemented old tuff, with poor mechanical properties, that can be considered a transition material between an uncemented pyroclastic silty sand (pozzolana) and a tuff, with some specific sections in which cohesion is close to zero. To check the current static conditions of the tunnel, an in-situ investigation was conducted and elastic-perfectly plastic 2D analyses were performed with the FE code Plaxis. The main results are briefly summarised in this paper. They indicate that most times the critical instability mechanism is the onset of tension cracks at the sidewalls and the subsequent collapse of blocks, consistently with the evidence of the field survey. Therefore, before the historic tunnel is re-opened to the public, a diffused reinforcement is required.