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Norra Länken 11

Our assignment

Skanska’s assignment consists of several different parts – a rock tunnel including a ventilation tunnel, a concrete tunnel, two operational spaces, a pump station and road work. The client is Vägverket Stockholm.

Handling a project of this size is a challenge in and of itself. We are also working in Stockholm’s most heavily trafficked intersection where over 100,000 vehicles pass through each day. Traffic must continue to flow as usual even as we need to take Karolinska Hospital’s operations into consideration as well as all of those who live and visit the area. Moreover, we are operating in a very crowded space with numerous restrictions.

Consequently, planning is critical. Right from the outset we involved a number of key individuals who created the long-term plan and developed safe and efficient solutions. That work resulted in us now anticipating being able to complete the project 18 months ahead of schedule.

Also worth mentioning is the fact that Skanska’s large size as an international company has been utilized for the first time on our home turf with Skanska Slovakia serving as a subcontractor for the tunnel work.

Construction process

Rock tunnel

The rock tunnel runs beneath Karolinska Hospital and has therefore been dubbed the Karolinska tunnel. The tunnel also runs below the Eugenia tunnel, with only 70 cm between the two tunnels. In other words, there is no margin of error.

Work on the rock tunnel began in 2008 and was completed in 2009. A total of 102 blasts were made. Before each blast, between 100 and 250 holes were made in the rock. The holes were between two and six meters long and were filled with blasting agents.

Planning and caution were of the utmost importance during this phase. No hole could be drilled crooked, no hole could be overfilled and no holes could be blasted simultaneously.

When it was time to blast, traffic was shut down for a couple of minutes. Once the dust settled, several hundred tons of blasted rock had to be transported out with the help of trucks.

The tunnel rock was then secured using bolts and covered in sprayed concrete so that no stones would loosen and fall down.

The entire Karolinska tunnel was then covered with rubber cloth to prevent water from leaking into the tunnel. This is a new method that we developed during work on Södra länken. An additional 10 cm of sprayed concrete was then applied over the rubber cloth, protecting the cloth and helping it to withstand future ice and explosion loads. The ceiling is painted in a brighter color to guide motorists.

The blasting was planned in consultation with Karolinska Hospital since the tunnel runs directly beneath sensitive equipment at the hospital, including an MRI camera. We therefore blasted more cautiously than normal and informed the hospital several days in advance, sending both text messages and e-mails a half hour before blasting commenced.

In addition, 30 measuring devices were placed to measure vibrations after each blast. The goal of the rock blasting team was that we would not have a single vibration violation, and that is something we managed to accomplish.

The hospital even had a special telephone hotline to the blasting managers that they could use to cancel a planned blast if they received a sick or injured person that required immediate scanning.

Concrete tunnel

When the rock tunnel was completed, we started work on the concrete tunnel. For that, we relied on our own engineers, Skanska Teknik, to do the projecting work and to create the building structure.

With a concrete tunnel we are not working underground but instead creating an open pit. Rock walls are reinforced with netting, rock bolts and sprayed concrete so that we can work safely. From there we proceed to build a concrete tunnel that looks like a 10 x 10 meter rectangle. Once the tunnel is fully cast the entire surface will be covered with topsoil and Äppelparken above will be restored.

Work on NL11, at Norrtull, began in 2008 and we anticipate that the project will be completed by December 2010.

Project in figures

700 meter long rock tunnel
245 meter long concrete tunnel
50,000 m3 of rock blasted away above ground
50,000 m3 of rock blasted away underground
12,000 m3of concrete