Stockholm Northern Link
The Northern Link project (Norra Länken) is a SEK 14-billion (USD 2 billion) scheme to build a 4-km, mostly underground expressway to connect European routes E20 and E4 with the Värtan-Frihamnen port in the northeast of Stockholm. The Värtan-Freeport is Sweden’s most important port for passenger traffic to Finland, the Baltic States and Russia. The Northern Link, which will become the new eastern extremity of the Swedish part of the E20, will relieve local roads of thousands of heavy vehicles per day, routing them through tunnels instead. Local residents, cyclists and pedestrians will benefit greatly from the new link while inner-city traffic congestion will be reduced significantly. Importantly, the Northern Link will, at the same time, become the north-northeast quadrant and third major link in the prospective Stockholm Inner Orbital Expressway.
Challenging and difficult tunnel construction
Together with the twin, parallel main tunnels and various inlets and exits, there is a total of 11 km of tunnels in the Northern Link project, of which 9 km are in rock. Over one million m3 of rock is to be blasted, much of it with extreme caution. Five tunnels will pass either directly under or very close to sensitive and expensive instruments in the Physics Center at AlbaNova University. Six tunnels will cross a just few meters under or over existing tunnels in the Stockholm Metro, the city’s underground railway network. Disturbance of researchers in the Physics Center must be kept to a minimum, while the safety of passengers in the metro must not be compromised. At certain places, there is no rock cover and conventional drill-and-blast cannot be carried out without prior reinforcement of the rock – by means of wire mesh and shotcrete, for instance. Together these circumstances present both construction managers and contractors with enormous challenges. Moreover, project economy requires blasting to be carried out every day in five or six tunnels. In order to cause the least possible disturbance, these blasts should take place simultaneously. Such demands can be met through a combination of knowledge, IT support and advanced mathematics.
Nitro Consult’s role in the project
In co-operation with consulting engineers SWECO, our Design and Planning Department has been involved in ground investigations, design issues and the drafting of construction documents for the rock tunnels at Bellevue, Värtan and Frescati. Our vibration group has also conducted a series of test blasts in order to obtain basic data about vibration transmission through the rock, which contractors were then able to use during the bidding stage.
During the construction phase, Nitro Consult has construction managers on six of the contracts. We are also responsible for measuring vibration, noise and structure-born sound in all contracts, and for surveying about 200 objects. These controls are being carried out to enable engineers to assess, once the vibration-generating works are complete, whether the construction of the Northern Link has caused any damage.
Vibration measurements are taken on buildings, equipment and the underground railways. Approximately 100 measuring points are active daily. All measured values are reported to a server via GSM link. To check that the guide-values for structure-borne sound indoors and noise levels outdoors are not exceeded, sound-level measurements are taken 24 hours a day. The client and contractors can read these measurements continuously via the Nitro Consult NCVIB Web portal. Every morning the contractors post the day’s scheduled blasting times in the NCVIB portal. This enables researchers in the AlbaNova University Center and the Royal Institute of Technology (KTH) to plan their activities accordingly.
To prevent blast-generated damage to vibration-sensitive equipment, we have also installed comprehensive vibration isolation solutions, mainly in the AlbaNova University Center. Research work there is carried out in an environment where nano-precision is needed. Thanks to the isolation measures put in place, research activities have been able to proceed normally during the construction period.
Nitro Consult also serves as technical adviser to the Swedish Road Administration (Vägverket) in connection with questions from third parties relating to blasting and vibration. This is done through information meetings held about once a quarter.