Citytunneln, Malmö: geotechnical hazards and opportunities

Abstract

Penetrating limestone in Malmö, Sweden, the underground works at Citytunneln comprise two parallel 7·8 m internal diameter railway tunnels, 6 km long, excavated with tunnel-boring machines. Triangeln Station, halfway along the tunnels, is a 30 m wide cavern with 15 m soil/rock cover and a row of pillars for central support. Malmö C Station at the northern end is an open cut-and-cover structure. There are two contracts, one for the tunnels and cavern (Lot E201) and another for Malmö C Station (Lot E101). Differing risk management techniques maintained an appropriate balance in the risk distribution. Groundwater lowering close to the harbour and the stability of adjacent historic buildings were the main hazards in E101. The design was based on the client's geotechnical interpretative report (GIR), and encountered deviations were compensated within a unit rate contract (E101). By contrast, the tunnelling risks in E201 were handled in a design–build contract (Lot E201), with the contractor responsible for the GIR. The geotechnical risk management was based on contractual geotechnical reference conditions. The observational method was used successfully to mitigate geotechnical hazards as well as to exploit opportunities.