Abstract
Just south of Oslo Central Station the new high-speed Follo Line railway tunnels pass beneath the existing Ekeberg road tunnels. This paper presents the construction methods, numerical model, and monitoring program used to assess the stability of the E6 road tunnels during the excavation of the Follo Line tunnels only a few metres below. The construction of the Follo Line was approved subject to three conditions: (1) There should be no negative effect on the stability of the Ekeberg tunnels, (2) The traffic flow in the Ekeberg tunnels must be maintained at all times and (3) Any risk of instability in the existing tunnels must be detected beforehand so that necessary precautionary action could be taken in good time. In order to deal with the challenges, SINTEF has carried out different investigations (including rock stress measurements), established a comprehensive numerical model, and a thorough monitoring program for analyses. The activities were combined in a so-called "rock mechanic tool", and the tool has been used successfully in this project, operating smoothly to provide adequate information for excavation planning and decision making.