ABSTRACT
In Stockholm, large investments have been made to improve and expand transport capacity in the city. The new blue metro line will improve the connections between the city centre and the eastern and southern parts of Stockholm. All stations are constructed underground in bedrock. In Nacka, the station is located below a highway and the location of the escalator shaft is lacking sufficient rock cover, meaning that part of the highway would have required excavation by cut-and-cover. This would cause unacceptable disturbances to the traffic and instead it was decided to strengthen the soil above the tunnel roof with JET-grouting and to install an “umbrella” of steel pipes above and parallel to the tunnel roof, often referred to as “spiling”. The tunnel was then excavated in short rounds below the steel pipes and immediately followed by a reinforcement and rock bolts and fibre reinforced shotcrete. Finally, a permanent concrete lining was installed. The paper will present the design principles for the combination of JET-grouting, the steel pipe umbrella and the temporary lining together with restrictions on the tunnelling sequences. Experiences from the construction of the escalator shaft will also be shown.
