Stockholm awards cable tunnel project 07 Feb 2019

Excavation of a 14km cable tunnel at up to 100m below Stockholm is awarded by the Swedish national grid company Svenska Kraftnat to a joint venture of Hochtief and Implenia Valued at about €90 million (US$102 million), Hochtief has the lead technical role in the 50:50 JV which will use a gripper TBM to bore the 5m diameter Anneberg-Skanstull cable tunnel (Fig 1).

Fig 1. Route of the cable tunnel under the city centre

The shield will bore through typical Scandinavian hard rock comprising metagreywacke, granite and gneiss. The tunnel is designed to remain largely unlined, except for some spot bolting and shotcreting, explained JV Project Director, Christian Zimmermann of Hochtief, but with the vertical alignment below groundwater level, at up to 100m deep, the contract requires significant pre-grouting ahead of the TBM.

Briefing TunnelTalk on the task, Zimmermann added that the TBM will be stopped every 15m to complete the rounds of systematic grouting. The main challenge, he said, will be to achieve maximum utilisation of the TBM while undertaking the pre-grouting tasks within available working time due to noise and vibration limits while working beneath the city.

Contract agreement and signing in Stockholm

Contract works also include construction of six ventilation shafts with associated service tunnels to be excavated by drill+blast. Four 4m-diameter shafts will be excavated by raise boring and the other two of about 40m2 in cross-section will be drill+blast shaft sinking operations, said Zimmermann.

The cable tunnel is due to be completed in 2024 and will form a key part of Svenska Kraftnat’s City Link project to improve electricity supplies in the wider region around the capital.

Recently the two contractor, again in JV and this time with Implenia in the technical lead role, were awarded Lot VE41 on the Munich second main rail line in Germany. The €400 million (US$455 million) contract is for excavation of a large box structure with the 210m long platforms at a depth of about 40m, for the new Marienhof Station and connecting tunnels directly behind the Munich City Hall. Work is due to start by mid-year 2019 and be complete by 2026.