The need to adapt to other underground or built-in facilities is common when building in Stockholm. Good dialogue and early cooperation are therefore important.
When the Bypass tunnels at Akalla were blasted into being, the Swedish Transport Administration adapted the work to the upcoming metro:
“We have had good communication throughout, and the Swedish Transport Administration blasted the bottom of the tunnels very carefully on this particular route, so that the mountain remained as intact as possible. We will also make blasts with care right here. In part so as not to damage the mountain, but also to not even risk damaging anything in the Bypass tunnel, says the new metro project leader Martin Brantberger.
That means shorter bursts that take the tunnel forward shorter distances at a time.
And we're not blasting the entire volume of the tunnel. In parts of the roof we use a method called rock fracturing, which is done without explosives, damaging the rock less.
A lot to be considered
The distance between The Bypass and the Blue Line to Barkarby, the so-called rock overburden, is at least five meters, but it is enough, according to Martin Brantberger.
“We could go closer, too. What we'd have to do in that instance would be to reinforce the mountain further or replace the mountain with concrete altogether.”
The reason that the Blue Line does not go deeper under the Bypass is due to the fact that it should connect to Akalla station 500 meters away.
“We can't have too steep tunnels, so this is as deep as we can go right here.”
Blasts this close to a tunnel where there is interior work going on right now means that there are also work environment aspects to be considered.
“Those working on the Bypass are affected if we blast too close to them inside the mountain. The vibrations could make objects crash to the ground, thereby creating potential danger if someone is on a ladder or unprepared at the time. That's why we have set blast schedules and good communication when we’re as close as we get to each other.