Rehabilitation of the Tunnel Kosman During the Construction of the Smokovac - Mateševo Highway Section in Montenegro

During the construction of the first section of the Montenegrin motorway from Smokovac to Mateševo, over 35 km of traffic tunnels were excavated. Such a volume of tunnel construction faced numerous challenges, with the primary task being to ensure the stability of the excavation.The most significant instability of the tunnel excavation was registered in right tube of the Kosman tunnel. During the underground excavation, in the portal zone, there was an intensive inflow of groundwater, which led to the collapse of the rock mass with the opening of a wide channel towards the surface. The Kosman tunnel was formed in soft rocks represented by tectonically altered flysch sediments, siltstones, marls and sandstones. At the moment of collapse, the rock mass was characterized as an excavation of the worst category, and all safety measures were applied according to the main design. Work was suspended and additional measures were implemented to secure the tunnel excavation. The paper analyzes the conditions that led to the collapse of the rock mass, as well as the applied remediation measures. It is estimated that the main reason for the instability was the intensive inflow of groundwater into the tunnel excavation.