ABSTRACT
A large-volume cave-in occurred in highway tunnel TU4, already fully excavated. The excavation of the collapsed section had been completed 7 months before in TU4 and 5 months in the adjacent TU3. Before the collapse, there had been no signs of possible ongoing criticality or instability; after their sudden first hint, in few minutes, the collapse developed with a 50 m high chimney up to the surface, forming a crater, of about 1800 m3, while the material accumulated in the tunnel had a volume of about 2600 m3 in bench. The performed preliminary investigations are reported, carried out to clarify the geological-geotechnical context and the possible causes of the collapse and to establish the best measures to be implemented to restore the tunnels. Hence, the design and the measures applied to re-excavate the TU4, to secure both tunnels and to prevent the occurrence of further collapses will be illustrated.
