Expansion of the Stockholm metro Case study of the passage of a fracture zone

Abstract: This paper describes the passage of the Birkavägen fracture zone during the excavation of an access tunnel for the expansion of the Stockholm metro to Nacka. The projects main challenges included requirements to limit ground water inflow and passing the fracture zone area, with a combination of low rock cover and densely fractured rock mass. The tunnel was excavated using drill and blast through foliated gneiss with granite inclusions. Through the drilling of probe holes, the rock cover could be estimated and the basis for design of the rock support could be verified. Pre-grouting works were conducted in two stages, with systematic drilling and grouting of a secondary fan. Post grouting was used to further seal the rock mass and limit local water inflow. Cooperation between the designer, contractor and client enabled a successful passage through the fracture zone.