A new commuter train tunnel under Stockholm City is currently being designed. The main portion of the tunnel is to be constructed in crystalline bedrock using drill and blast methods together with conventional support elements in the form of shotcrete and rock bolts. Standard rock reinforcement classes, which are dependent on excavation span and rock mass quality, will be used to determine the required reinforcement for the majority of the tunnel system. The rock mass was divided into four rock classes (A-D) and the tunnel into four categories for excavation span. Potential failure mechanisms were identified for the different combinations of rock mass quality and excavation span and appropriate analysis methods were selected for the various combinations. For all analyses, a range of material properties and initial stress conditions were used to account for natural variation in the input data. Preliminary standard reinforcement classes were determined by evaluating the results from all analyses and using the application of engineering judgment. Numerical analyses, including the preliminary reinforcement were then carried out for chosen combinations of rock types and tunnel sizes.
A new commuter train tunnel under Stockholm City, denoted the City Line, is currently being designed. The tunnel comprises a 6 km long double-track tunnel with two underground stations (Odenplan and City), service tunnel and exits to ground surface and existing subway tunnels. The main portion of the tunnel is planned to be constructed in crystalline bedrock using drill and blast methods, with conventional support elements in the form of shotcrete and rock bolts. The City Line connects to the existing rail network at Tomteboda in the north and to the existing station Stockholms Södra in the south, see Figure 1.
This paper presents the methodology used to design the standard rock reinforcement classes for the City Line. The standard reinforcement classes are to be used for the majority of the planned tunnel system. Standard reinforcement classes will not be used in areas with low rock cover (less than half of the tunnel width), crossing points with existing tunnels, tunnels with a span greater than 20 m, and poor rock quality.
The City Line is, with respect to civil works, divided into 5 different contracts: a) Odenplan/Vasa, including single- and double-track train tunnels, service tunnels and an underground station, b) Norrmalm, including single- and double-track train tunnels and service tunnel, c) Norrström, including single- and double-track train tunnels, service tunnels and an underground station d) Söderström, including a submerged concrete tunnel (not included in these analyses), and finally e) Södermalm, including single- and double-track train tunnels and service tunnels. The same design methodology was used for all contracts.
(Figure in full paper)
A comprehensive geotechnical site investigation was conductedtocharacterizethegeologyalongthe plannedtunnelalignmentforeachcontract. Project guidelinesformappingandcoreloggingwere establishedearlyinthedesignphasetoensure consistencyinthedescriptionandcharacterizationof therockmass.