Photos: Look under ground in Hökarängen
A new tunnel is being built between Hökarängen and Högdalsdepån so that the depot can receive trains from both the Green Line to Farsta and to Hagsätra which in the future will be part of the Blue Line. Join us under ground to see how it goes.
At Örbyleden in Hökarängen in southern Stockholm is the the entrance that is used to bring machines and workers down into the tunnel, and take out the blasted rock. The new tunnel will be 2.5 kilometers long and is planned to be completed by the end of 2024.
Groundwater is a major concern when building a tunnel. To seal off water leakage, a technique called grouting is used. When grouting, long holes are first drilled diagonally outwards in the rock in front of the tunnel front. For each grouting session, about twenty holes of 24 meters long are drilled. Then cement is injected into the boreholes. The cement finds its way into cracks and cavities in the rock and prevents groundwater from leaking into the tunnel. This process can take up to ten hours.
In the yellow machine, a so-called grouting platform, the cement used in grouting is mixed. The white bags contain dry cement that will soon be mixed.
When the grouting is complete, it is time to drill holes for blasting. A computerized drilling rig drills up to 250 holes into the rock, with precise length and position settings. It usually takes between five and seven hours to drill all the holes.
Before blasting, workers load all the holes with igniters and explosives by hand. They use a lifting crane to reach the higher holes.Each blast advances the tunnel by five to ten meters.
After blasting, all the blasted rock must be unloaded. After each blasting, workers unload 1200 to 1600 tons of rock. This is a quieter job that can be done at night.
Underground, it is hard to keep track of the blasting direction. That’s why the measurement technician plays a vital role. They measure the tunnel direction and ensure that it follows the right course and shape.
Sometimes some rocks are not fully blasted away. In that case, workers mark the protruding rocks with spray paint. Then they do a small extra blasting before resuming the work as planned.
Then the tunnel needs to be secured. First, workers remove loose stones by scrapping. Then they spray concrete on the roof and walls to prevent new stones from falling down. Finally, they drill long bolts into the rock wall to stabilize it. Then they repeat the whole process from grouting.
The Högdal depot is being expanded
When the metro system in Stockholm gets longer, more trains are needed. The Högdals depot needs to be expanded to accommodate the new trains. A new track tunnel is also being built to connect the depot and the metro to Farsta, so that trains can enter and exit via both Högdalen and Hökarängen. The entire work on the expanded depot in Högdalen will be completed in 2026.