Geologists are carefully studying the rock dawning as the tunnels are blasted into being. Their tunnel mappings are important for how the work will proceed.
“The tunnels are blasted in stages of up to twenty meters. After that, we'll do a mapping before the tunnelling can continue. A mapping takes about an hour and is a collaboration between us, the construction management, and the contractor working on the site,” says geologist Björn Ensterö.
Early in the planning of the subway a forecast of the mountain was made, but the mountain can still surprise.
“We document and characterize all mountains. We draw in and photograph what rocks are found in walls and ceilings, drawing cracks and groups of cracks and what properties they have. It determines how the mountain is to be handled and reinforced.”
So what kind of rock is the new metro being built in?
“Granite can be found almost everywhere. Amongst them Stockholm granite, which is a light grey or slightly red variant. It is massive and has high density. It is a good mountain to build tunnels in, which we are always happy to find, says geologist Jussara Lourenço.
Gneiss, which is common as well, contains undulating patterns and layers. That’s good as well, although there are often cracks between the layers.
However, all mountains aren’t good mountains for tunnelling. The geologists were less happy when they found the rock episyenite in Barkarby.”
“It’s porous and has low density. It’s enough to tap it lightly for it to break, so there we have to reinforce the mountain significantly. We hope that we don’t come across it in more places,” says Jussara Lourenço.
Other types of rock that are good for tunnelling are dioritis and gabbro.
“Generally the Stockholm area has good bedrock for tunnels.”
Mountains contain more or less cracks. Some cracks contain clay while others carry water and need to be sealed. The picture shows a sawn-off section of rock at the mouth of the work tunnel in Sickla in Nacka.
Sealing cracks and water emanation
The density of the rock is largely dependent on the amount of cracks and crack systems. Some cracks are filled with mud, while others contain water, and then the tunnel must be sealed.
More cracks mean more work due to having to reinforce the rock with concrete and bolts.
“Selective bolting can be chosen if the rock is good – in that case bolts are drilled into the rock in specific places only. If the mountain is less dense, bolts have to be attached systematically, which means more bolts in special patterns,” says Sofie Eskilander.
The worst type of rock is crush zones, where the mountain has been crushed together and stretched out making it brittle.
“That's where the mountain crumbles apart completely. We usually talk about granular rock (sockerbitsberg).
Crush zones are found, for example, at faults in Gullmarsplan and Södermalm.
Important for the future
The work of geologists is important not only in the construction of tunnels, but also for the future.
“The tunnels are covered in concrete and sometimes also in materials that redirect leaking water. In those cases it's our documentation they have to adhere to when the tunnels are to be maintained. If something needs fixing in a certain place, it should be possible to read about what the mountain looks like right there.
The new metro has about 15 geologists today, but there will be more, according to Jussara Lourenço.
“We will work on more tunnel sections at the same time and eventually also work in shifts.”