This paper presents construction method and monitoring program for the stability for the existing Ekeberg tunnels in connection with the construction of the Follobanen tunnel - a new high-speed railway tunnel. When crossing the Ekeberg tunnels, the Follobanen tunnels are just few meters below. The construction of the Follobanen is approved with three conditions that (1) No negative effect to the stability of the Ekeberg tunnels, (2) The traffic flow in the Ekeberg tunnel must be maintained at all time, and (3) Any risk of instability in the existing tunnel must be detected beforehand to make necessary precaution actions.

In order to deal with the challenges, SINTEF has developed a comprehensive procedure, combining continuous rock stress measurements and displacement measurements with 2-D and 3-D numerical modelling. Continued rock stress measurements and rock stress change monitoring are actively used together with numerical model to monitor the stability situation in Ekeberg tunnel. This is to make sure that any risk of instability in the existing tunnel can be detected beforehand to make necessary precaution actions. This paper is extended version of the one presented in November 2017, in Norwegian conference - Rock Blasting Conference, Oslo, Norway.


This paper is an extended version of a paper published in Rock Blasting Conference in Oslo, Norway - a national conference (Trinh & Holmøy, 2017). Bane NOR (Norwegian National Rail Administration) has decided to construct the Follobanen Project - with new railway tunnels close to the existing Ekeberg road tunnels in Oslo area in Norway, connecting Oslo and Ski. The Ekeberg tunnels have diameter around 11.5 m, and the Follobanen tunnels about 9.5 m diameter. The Follobanen Project comprises a 22km long twin-tube-tunnel to be excavated mainly with tunnel boring machines (TBM), but also by drill & blast and drill & split. The drill & blast and drill & split tunnel section is in the first part of the Follobanen tunnels, near Oslo Central Station, and where the Follobanen tunnels go under the Ekeberg tunnels. The main construction phase commenced in 2015, and it is scheduled for completion in the end of 2021.

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