During the spring and summer, freezing has taken place by pumping coolant into approximately 40 meter long pipes in the tunnel under Mårtensdal. The area consists of soft rock, and in order to be able to blast more safely, the rock is therefore frozen to at least -5 °C.
– We have reached about a third of the distance in one tunnel tube and have just started the other, says construction manager Mathias Tisjö Brinck.
This video shows the tunnel and the frozen mountain. Notice ice crystals on the tunnel walls:
It has been a bit of a slow start to this section as we had problems getting the shotcrete to stick to the frozen surface. Now, however, we have found a method that seems to work and hope that the rest will go on without problems.
Unattended train operation means, for example, that no driver’s cab is needed, but the passengers can see the tunnel during the trip. The solution already exists today in many places in the world, with Copenhagen being the closest.
– Exactly which trains we will buy in is not yet clear, but what we do know is that they will be customized according to our needs. Both in terms of number and furnishings, we are planning for the expected number of passengers by 2060. An advantage of unattended train operation is that it is easier to add more trains if necessary, says Johan Brantmark, director of the metro to Älvsjö.
Giant drill
The new yellow line between Fridhemsplan and Älvsjö will be separate from the rest of the metro system. That is why it can be built in a different way.
– The constructions that are underway now are all extensions, where we add some stations to existing lines. Then we need to build for the same type of train and the same signaling system. On the Yellow Line, we can do things differently – and then we must take advantage of the technological development that has taken place, says Johan Brantmark, director of the metro to Älvsjö.
In addition to unattended train operation, the metro to Älvsjö will be built using a different technique than the rest of the metro system – it will be bored with a large boring machine.
Smaller stations
The stations will also be smaller, with platforms 75 meters long instead of today’s 145 meters. This will allow us to run shorter trains with more frequent departures as it is more logical and causal and make the total travel time and waiting time shorter.
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.
The expansion of the metro system is well under way throughout the Stockholm area. Högdalen in south Stockholm is home to one of the most important construction sites for making the new metro system a reality – this is where the Högdalen Depot is being expanded to handle many of the new trains that will operate the new metro services.
Here, you can watch a video that better explains what a depot is, why this one is being expanded and how it is being done (In swedish).
“Once the metro expansion is finished, more trains will need to be cleaned, serviced and stored overnight. The depots are an incredibly important part of the ecosystem, and without them, we’d have empty tracks and stations because the trains would stop running. The depots are quite simply the heart of the metro system, ensuring that traffic can flow around the metro network,” says Henrik Unosson, project manager for the new metro system.
Come 2030, you can be 100 metres below ground at Sofia station, on your way from the lift from Stigbergsparken towards the platform and the metro services heading south. On this particular day, perhaps you have plenty of time, so you take a good look around. Perhaps you think, “What a great combination, that turquoise wall and gold-coloured letters,” while also wondering who came up with the idea.
“The artists have been involved in the process since back in 2015. They’ve worked closely with the architects and engineers to ensure that the artwork is an integral part of every station. The artwork in Stockholm’s metro system is important, and we’re proud to be continuing that tradition with the world’s longest art exhibition,” says Martin Hellgren, who is responsible for all the stations being built when extending the Blue Line south from Kungsträdgården.
Realistic for travellers
However, the new images do not simply show the artwork. They have been produced from the computer model used as the blueprint for the entire new metro system. They show the scenes as seen from eye level, and the colours and textures have been processed and reviewed by the artists.
“We use angles that are relevant and that provide future travellers with as realistic an image as possible. We also want to show things that we’re asked about, such as how the lifts at Gullmarsplan and Sofia will work,” says Martin.
Now, you’re sitting on a train heading south, that same afternoon in 2030. At Gullmarsplan, you lift your eyes from your screen – you’re soon at your destination – and see just how beautiful the station looks. “I wonder what that’s supposed to represent,” you think, but there’s no time to find out, it’s your stop, Slakthusområdet. Perhaps you look down now, remembering an article you read a long time ago. “The floor really does look like ice that someone’s skated across. I wonder who came up with that idea?”
Here, you can see all the new images and find out the names of both the artists and the artworks.
Sofia
Artists: Peter Johansson and Barbro Westling Name of the artwork: “Flux”
Sofia, mezzanine floorSofia, ticket hall and entrance.Sofia, mezzanine floorSofia, ticket hall and entrance.Sofia – the second deepest platform in the world.
Sickla
Artist: Anna Lerinder Name of artwork: “Utblick och riktning” (“View and Direction”)
Sickla – ticket hall, Sickla station house.
Järla
Artist: Cilla Ramnek Name of artwork: “Direction Home”
Järla platform.
Nacka
Artist: Thomas Karlsson Name of artwork: “Elefanten och Noshörningen” (“The Elephant and the Rhinoceros”)
Nacka – ticket hall at the corner of Skvaltans väg and Vikdalsbron.Nacka – ticket hall at the corner of Skvaltans väg and Vikdalsbron.Nacka – ticket hall in Jarlaberg.
Gullmarsplan
Artist: Jesper Nyrén Name of artwork: “Windows”
Gullmarsplan –mezzanine floor from which you take the escalator down to the platform.Gullmarsplan – way up at Mårtensdal.
Slakthusområdet
Artist: Helena Isoz Name of artwork: “Åkning” (“Trip”)
The Stockholm metro system is being complemented with eighteen stations and thirty kilometres of track.
In 2014, 16,000 Stockholmers voted on the colour of the new line between Odenplan and Arenastaden – and Yellow won. At the time, the plan was for a separate line with shuttle traffic, although this was changed in 2016. Instead, the line will continue south from Odenplan on the same tracks as the Green Line.
“Since then, we’ve discussed internally whether the Yellow Line should be part of the Green Line. Now, though, the Public Transport Administration, which is responsible for the actual metro services, has decided that this is a good time to make this change. On the one hand, we have a new line that needs a new colour, and on the other hand, the time is approaching to start running metro services to Arenastaden. So, it’s a good idea to decide this now,” says Niklas Bergman, head of FUT (Förvaltning för utbyggd tunnelbana), the administration responsible for the expansion of the metro system.
Accordingly, the future metro map is being changed. The Green Line will have a branch to Arenastaden, and the most recently decided line between Fridhemsplan and Älvsjö will be a Yellow Line.
“Since the metro to Älvsjö is a separate line, it’s good for it to have a colour of its own,” says Niklas.
“At the same time, it’s great that the colour that Stockholmers voted for almost ten years ago will remain on the metro map,” he ends.
The decision on the colour does not affect the routes or future services.
Jörgen Altin, senior project manager for the new metro system. Photo: Region Stockholm / NiLaAlLj, Wikipedia
The new political majority’s first budget this past autumn listed a number of possible metro services. On Tuesday 25 April, the Transport Committee within Region Stockholm passed a decision to initiate work on a needs analysis.
“The first thing we’ll do now is gather information. Building a metro service is a major investment, so we must build them in the right places. We’ll look at developments in the region, where people will live and work. We’re looking far into the future, all the way to the 2070s. All projects listed in the region’s budget will be examined, although we won’t be limited by them. Instead, we’ll also look for new opportunities,” says Jörgen Altin, senior project manager for the new metro system.
The draft list of ideas will be refined and prioritised in the autumn of 2023. Which routes the services could take and the impact they would have on travel and investment costs, for example. However, the are many pieces to the puzzle to lay before we can say exactly what will be built.
“At this stage, it’s not really a matter of bedrock quality and the like, those are more important later on. For now, it’s a matter of gathering knowledge about needs and trying different ideas, which we do together with other parties, such as the concerned municipalities and other departments within the region,” says Jörgen.
Right now, an 18x18 meter square can be seen in Jarlaberg. It marks the location of the future elevators that will take passengers up and down between the platform and the ticket hall. To the right, a glimpse of the elevators can be seen in the back.
– Our work is progressing steadily! We have excavated over 60 percent of the rock volume within the expansion in Nacka. During spring, we also completely finished the blasting to the east, says Erik Lindgren, Project Manager for the Blue line to Nacka.
At all end stations in the metro system, the trains need to be able to turn around. In Nacka the tracks will continue past the station to allow six trains to park over night. This eastern part of the tunnels is now completely finalized in terms of blasting.
Construction sites today – nice entrances in the future
Above ground, work continues with ticket halls at the future entrances.
The underground tunnels are to be connected with the ticket halls above ground. In Jarlaberg, the future elevator shaft is taking shape (see picture) and in Järla and Sickla, work is underway with escalator shafts. After summer, work will begin with the escalator shaft outside Nacka Forum as well.
– Today we have large construction sites and it can be difficult to imagine the result. The stations will be completed gradually, says Erik Lindgren.
Installations from 2026
The current construction work – which largely consists of drilling, blasting and civil works – will be completed in 2026. After that, installations of escalators, lifts, tracks, electricity and various technical systems will take place. The station and all associated structures and construction work will be finalized both above and below ground. Traffic start in 2030.
Inside a warm, cosy cabin sits the bolt rig operator Kalle Pellinen. It takes him four minutes to drill a hole, fill it with cement and then insert the bolt. The only thing that can be heard is Kalle’s Spotify list.
“I have inserted bolts manually as well. It is heavy, hard on your back and shoulders and the cement splashes. This machine entails a significant difference, I can only see advantages,” says Kalle Pellinen.
So far, however, there is one experiment which is in progress just over 100 meters under Sofia church in Södermalm, the third consecutive experiment. This series of experiments is a collaboration between the Region Stockholm, the contractors constructing the tunnels and the companies manufacturing the machines.
New way of thinking
“We have learned a lot. It’s not just about bringing in a machine and starting to bolt, rather this entails a new way of thinking and planning,” says Martin Hellgren, Manager in charge of the expansion from Kungsträdgården to Nacka and to Söderort.
Bolting has been mechanised since long in the mining industry but the demands there are different.
“Our bolts should last at least 120 years. Within mining, the period is much shorter. We must make sure that the method is efficient as well as suitable for various types of rocks. Also, the bolts should last their entire life span,” says Martin Hellgren.
Significant difference for the industry
So far the results of the experiments look good. The three experiments will be fully evaluated by the end of the year.
“We want to be involved and lead the technological development in mechanised bolting. If we succeed, it can make a great difference to the industry,” concludes Martin Hellgren.
Experiment 2, in the tunnels under the Hammarby Canal, is a finalist in an annual health and safety competition for the construction industry. The competition will be concluded at lunchtime on Tuesday, March 28.
“We have an exciting period ahead of us. It is a very challenging and unique passage for the expansion of the metro,” says Peter Leiner, Project Manager.
The common method for blasting through the rock is to first seal the cracks in the rock with a cement mixture, a so-called pre-injection. This seals the rock and prevents too much water from leaking into the tunnel. This is followed by drilling and blasting the tunnel and finally reinforcing the rock.
But in one area in Mårtensdal, just to the north of Gullmarsplan, it won’t be as easy. Here the rail tunnels run through a so-called weakness zone, an area with a weaker rock type that makes the work significantly more complicated..
“The measurements we have taken show that theoretically there is around 2.1 metre rock above the tunnel roof but that is too little to construct as usual. And since the area lies at the end of the Stockholm ridge, the soil above the rock has high water transmission. This increases the risk of rock fall and of water entering the tunnel.
Unusual method
It has been known since long that the rock in the area is inferior, thus we have been able to develop and investigate different solutions..
“We looked at various alternative methods and found that the best alternative at this location is to freeze the rock.
The special boring machine in place in the drilling chamber. It weighs 38 tons. Photo: OHLA Sweden
The method involves freezing the rock and the surrounding earth so as to prevent water leakage and ensure that the area holds up better while the blasting is in progress. The method is unusual but has been used on a few occasions in Sweden when it was needed for one reason or the other, such as Hallandsåsen and the construction of the metro between T-Centralen and Gamla stan.
Process of freezing a rock
Step 1: Boring cooling pipes
Before starting the work, a so-called drilling chamber is built, which is an extra large space that is blasted out to accommodate a special drilling rig and other drilling equipment.
The first step is to drill holes for about 40-metre-long pipes that go straight into the rock. The holes are drilled around what will be the tunnel opening.
“The actual drilling will start in March and will go on for about 50 days,” says Peter Leiner.
Step 2: Freezing
The next step is to fill the pipes with a circulating refrigerant in order to bring down the temperature of the soil and rock to at least -5°C. A cooling equipment keeps the liquid cold the entire time. The work of constructing the tunnel starts only after the right temperature is reached throughout the route and at least 2 m outside the tunnel contour.
“We estimate the freezing process alone to take about 120 days. During this period we cannot do much more than control the temperature.”
Step 3: Rock engineering and concrete lining
Then the actual construction phase can begin. The rock is then blasted out with extra small rounds of shots. The tunnel is temporarily reinforced with sprayed concrete in several steps.
After the rock has been blasted and reinforced with sprayed concrete, a permanent reinforcement is formed and cast, a so called concrete lining.
Throughout the construction, the rock is kept frozen. The cooling aggregate can be turned off only when the entire tunnel is blasted out and the concrete lining is in place.
“We perform a precise control over the thawing of the terrain and rock so as to not damage the surrounding or affect it adversely. The work of freezing the rock and building an about 40-metre-long passage is estimated to go on until the summer of 2024.
Facts: Freezing of rock at Mårtensdal
• The area through which the tunnels run consist of graphite.
• The entire route that is to be frozen down is 39 m long.
• The area is to be frozen down to at least -5°C before blasting the tunnel.
• It is estimated that it takes 50 days to drill holes for cooling pipes.
• It is estimated that from the time the cooling starts it takes 120 days before the entire area reaches -5°.
• The tunnel work is estimated to be completed by around June 2024.
Svenska kraftnät's new tunnel for the Anneberg-Skanstull electricity connection is being built with the help of a tunnel boring machine. Photo: Tomas Ärlemo Photography
“We see several advantages. For instance, a tunnel boring machine has a lesser impact on the surrounding, with respect to how far the vibrations diffuse and the amount of water that leaks in during the construction. This is particularly good as we have some really tricky passages to get through. We know that there is a zone with weak rock where we will work in below Mälaren and Årstafältet consists of clay,” says Johan Brantmark, Manager for the Älvsjö metro.
The technique is as follows: a large drill is penetrated through the rock while building a watertight concrete lining. The tunnel is bored from the southern end of the new line to the northern end. In this way the work tunnels are not required either at all the new stations, as is the case for the routes currently under construction.
“Both the methods have their advantages and disadvantages but our compiled assessment today is that the tunnel boring machine is the best for the route between Älvsjö and Fridhemsplan. There is less groundwater leakage than with the drill-and-blast method, and the risk implied in the time-consuming grouting and reinforcement work is reduced,” says Johan Brantmark.
However, even if there is an emphasis on tunnel boring machines, it does not mean that the extension between Fridhemsplan and Älvsjö will be completely blast-free.
“No, the stations will be blasted out using the same technology as the other new metro lines,” says Johan Brantmark.
Each new metro route has its own conditions and various construction methods were investigated even for the routes currently under construction.
“We looked at the possibility of using the tunnel boring machine even during the planning phase of the current project This was before we had received the terms from the environmental judgements with stringent requirements for water leakage. We have this as well as other experience with respect to the tunnel boring machine technique which we will now be using for the new line,” says Stefan Persson, Deputy Head of Administration Manager for the new metro.
At present, all planning is based on the fact that the metro to Älvsjö will be built using the tunnel boring machine.
“During the year, we will continue to work on the planning. For example, we will continue to examine the terrain and the bedrock, assimilate the technical and financial analyses, continue the work on project design and the work on permits and production planning,” says Johan Brantmark.
The final decision on the tunneling method will be made by the end of the year.
Drilling and blasting work is currently being performed at a total of 61 underground fronts, in various directions. This concerns the building of track tunnels where the trains will run, but also service tunnels and connecting cross tunnels.
A front is the point at which the machines are about to penetrate the rock, in order to first seal the area around the tunnel and then drill holes for the explosives. After the blast, the front will have moved forwards a few more metres.
The new platform space at Gullmarsplan. Photo: Region Stockholm.
Where the most – and least – blasting work is taking place
One of the areas where construction is taking place on the most fronts at the same time is eastern Södermalm, where tunnels are being drilled and blasted at 8 fronts towards both the north and the south. The southbound tracks branch off towards Söderort and Nacka, which means that five tunnels are being worked on at the same time (track tunnels and a service tunnel).
In Barkarby, there are currently only three active tunnel fronts. This is because the space that has been excavated almost reaches the whole way between Akalla and Barkarby. Some blasting work remains to be done in some of the existing tunnels at Akalla station. This work will continue throughout 2023.
Number of fronts right now:
Here you can see how many tunnel fronts are being built at each location, and you can also click through to the progress map, where you can see how much has been blasted out of each section:
Region Stockholm has reviewed the greenhouse gas emissions stemming from the construction of access tunnels for the new metro. All of these contracts have achieved their climate goals with much room to spare.
Various measures to reduce emissions
On average, the construction projects have reduced their emissions by 32 percent. All in all, they have saved more than 6,000 tonnes of greenhouse gases. This is equivalent to the total annual emissions of several hundred Swedish households.
“Thanks to a variety of measures, emissions have been kept down,” says sustainability strategist Olivia Engstrand.
Olivia Engstrand, sustainability strategist. Photo: Region Stockholm
These include the use of electricity and diesel from renewable sources and concrete and steel with smaller carbon footprints, fewer and shorter transport runs, and one project that housed staff facilities in existing buildings rather than portable cabins.
Two access tunnel construction projects, one in Hagalund in the north and one at Hammarby fabriksväg in the south, were completed by the same contractor. By closely coordinating the two projects, the contractor managed, among other things, to reduce their carbon emissions.
“The contracts in Hagalund and at Hammarby fabriksväg have saved both resources and emissions by working together. For example, they’ve coordinated their material transport runs from factories, shared machinery with each other and used surplus materials in the projects to work efficiently and avoid unnecessary waste,” Engstrand explains.
Reaching goals by establishing requirements
The construction sector is responsible for a sizeable share of Sweden’s greenhouse gas emissions. The new Stockholm metro expansion is no exception. The project tries to reduce its unavoidable environmental impact, including by means of an overall climate goal. Proactive measures are to reduce this climate impact by 25 percent compared to taking no measures. According to the latest forecast, so far the metro construction project has cut its climate impact by about 30 percent.
“As a public service organisation, we must help manage our natural resources and there’s much we can do. It’s important that we establish requirements for our procured planners, contractors and operators,” says Engstrand.
Among the requirements that FUT, the department responsible for the expanded metro, has established for the access tunnels is that the contractors shall take a proactive approach to preventing and reducing unnecessary resource use and reducing climate impact. They are to organise workshops, prioritise improvement measures, establish goals, perform calculations and report results. Now that the construction work has been completed, FUT has compiled all the figures.
“We’re satisfied with the results and with all the experience gained from the various climate-saving measures. Our systematic sustainability work and the requirements we established provide everyone with lessons learned – not only our contractors, but also the entire industry. This is challenging work, even for us. So, as a client, we have to be on the ball at all times,” says Engstrand.
Drilling and blasting for the new metro are currently under way in many parts of Stockholm. Moreover, a completely new line is being planned. All in all, there will be 18 new stations and 30 kilometres of new tracks by the mid-2030s. You can see what the metro map of the future will look like here. Region Stockholm has a special department responsible for the expanded metro, known as FUT for short.
“Our job is to ensure that the decision to build the expanded metro is implemented. However, by building all the terminus stations in a way that makes it technically possible to extend the lines, we’re future-proofing the expansion,” says Stefan Persson, head of the department responsible for the new metro.
Stefan Persson, head of department. Photo: Region Stockholm
Maximum benefit
One example can be found in Nacka, where the expanded metro will run to Nacka Forum. However, it will be possible to extend the tracks further east, should such a decision be made.
“The metro expansion is closely linked to urban development. The metro system offers high capacity but is expensive to build. Simply put, it has to serve areas where there are enough people to provide the maximum possible benefit,” Persson explains.
Ability to connect?
Another prerequisite that is often mentioned is the ability to connect the existing lines 10 and 11 by extending the metro to Barkarby all the way to Hjulsta instead.
“Technically, this won’t be a problem. We’re ensuring this by laying the tracks in Barkarby to facilitate such an extension. However, such a decision would first require looking into how to run services on a connected line so that they work with the rest of the system,” Persson explains.
Technically, all the new terminus stations will have the option of further expansion in the future. And even the completely separate line between Fridhemsplan and Älvsjö will have this option.
“Of course, while it’s a good idea to consider all this already during the planning stage, it’s also important to remember that just because something is technically possible doesn’t necessarily mean that it’s a smart investment for passengers, at least not at this point in time. However, at a bare minimum, our designs and constructions shouldn’t limit future city planners,” says Persson.
“The public spaces visible to passengers are really only a fraction of what’s being built,” Malin Harders explains.
All the new metro lines and stations will be built underground, and there is an entire system of tunnels and spaces needed to enable everything to work as it should.
“When we build the rail tunnels, we need space for more than just the trains. We also need space for various types of installations, such as for electricity and lighting, as well as for a bunch of concrete structures. This is why the tunnels are blasted larger than their final size.”
The space for the platform and rail tunnels at Barkarbystaden, February 2022. Photo: Region Stockholm.
Like an underground house
According to Harders, building a station is a bit like building a house, only below ground. The first construction phase involves blasting out large caverns. Then there are large numbers of concrete structures to be built, some of which are cast in situ while others are prefabricated. Stations usually have three levels: the platform where you wait for the trains, then the escalators to the mezzanine level, and from there the escalators up to ground level.
All stations need special rooms for technical installations.
A station is like an underground house spread over several floors. The red areas in the illustration are all the spaces being built for Hagastaden station. Click the image to enlarge it. Photo: Rundquist.
“The tunnels are also equipped with a range of installations and signalling systems that take up space. We also build walkways inside the tunnels. These provide access to service tunnels running parallel to almost all rail tunnels, which can be used to evacuate in an emergency.”
Malin Harders, project manager for the Yellow line to Arenastaden. Photo: Region Stockholm
Different types of tunnels
The service tunnels are smaller than the rail tunnels and are asphalted so that service vehicles can use them. This is a type of tunnel not found in the existing metro system but that is needed now due to new legal requirements.
The service tunnels to the left are slightly smaller than the rail tunnels to the right, each running parallel along almost the entire length of the rail tunnel.
To be able to move between the rail tunnels and the service tunnels, cross tunnels are also built at regular intervals along the entire length of the tunnels.
The stations also need escalator shafts. Moreover, alongside most escalators, there should also be space for an inclined lift.
Like a bicycle pump
Pressure relief shafts are being built at all stations as well. These are needed to equalise the air pressure on the platform.
“A metro train running through a tunnel acts a bit like a bicycle pump. The train is constantly pushing the air in front of it and that air has to go somewhere, otherwise you’d be blown over when standing on the platform. We also build ventilation shafts to route air up or down between the tunnels and ground level for ventilation,” Harders explains.
The support structure is comprised of so-called secant piles, which are overlapping piles cast in situ to form a watertight wall.
“This task involves several challenges. On the one hand, the bedrock slopes, and on the other hand, there are large boulders in the ground that take time to bore through. However, drilling and casting the secant pile wall is a necessary part of the construction of the lift shaft for Sofia station,” says Johanna Nilsson, the geo-construction manager for Stigbergsparken.
On the left are the steel pipes that are drilled down into the ground while extracting the loosened soil. Each pipe has teeth that allow it to be drilled into the ground. These need to be replaced at regular intervals due to wear. On the right is the screw used to extract the soil.
“This requires powerful machines. At the moment, we’re working with a Liebherr 36 that drives the pipes down and then drills while also extracting the soil with a screw,” Nilsson explains.
The machine, which weighs 116 tonnes, was shipped all the way from Germany to Stigbergsparken.
To ensure that the pile wall is anchored in the bedrock, the hole for each pile is pre-drilled with a different machine equipped with a rock drilling rig. The drill bit is lowered into the casing and drilling can begin.
To ensure that the pile wall is anchored in the bedrock, the hole for each pile is pre-drilled with a different machine equipped with a rock drilling rig. Pictured in the background on the right is the Liebherr 36 machine, while in the foreground is the Bauer BG28 machine, which is equipped with rock drilling equipment.
The first thing that is done is to drill holes for every other pile. Once the pile is ready for casting, it is filled with concrete. Every other pile is reinforced. This combination ensures a watertight and robust wall. While a pile is being cast, the steel pipe is lifted up and out ready for use on the next pile. Once the concrete in the hole has hardened, a stable cast-in-situ pile has been created.
Every other pile is reinforced. Pictured are the reinforcement cages used for the piles.
The metro is being extended to both the north and the south – and from several directions at once. Work on the Blue line to Barkarby was begun first, and this is also the stretch with the most progress to report. All that remains to be done here is tunnel work at Akalla station and the blasting of the last 150 metres under the Barkarby shopping area. Following this, work will continue on transforming the caverns and bare tunnels into the new metro.
As for the metro to Arenastaden, a fair bit of blasting work remains to be done. The new station in Hagastaden is the focus of the most intense work at the moment. Here, the tunnel is nearing Vasastan on its way to connect to the existing tunnels at Odenplan.
The largest part of the expansion is the Blue line from Kungsträdgården to Nacka, Södermalm and Söderort. Here, work is conducted from eight access tunnels at the same time. The access tunnels lead down to the underground rail tunnels and allow excavated rock, machinery and personnel to be transported between the surface and the underground construction sites.
On this stretch, the tunnel is nearing Ladugårdsviken, Saltsjön and Hammarby Canal. Here, the tunnels will be built into the bedrock under the water. Read more about building under water.
A lesser known part of the expansion is the depot in Högdalen, which needs to be expanded along with the metro. Here, work will soon be completed on the access tunnel in Hökarängen, which is needed to build an underground train yard and a new rail tunnel between the depot and Farstagren station on the Green line.
Click on the picture (in Swedish) to take a closer look.
See where we’re currently blasting
You can see the progress made on each active construction site on a map. These maps are updated once a month and allow you to see roughly where work is currently under way.
Blue line to Barkarby: 2026 Metro to Arenastaden: 2028 Blue line to Nacka, Södermalm and Söderort: 2030 Expanded depot in Högdalen: 2026 Metro to Älvsjö: Currently being planned. When it will be in service is yet to be decided.
About 250 unique concrete structures will be built along the four-kilometre stretch from Akalla to Barkarby. These include tunnel walls, utility buildings, pump pits, station buildings, escalator shafts, ticket halls and ventilation towers above ground.
“The production principle is the same for all structures,” says Christer Vestergren, construction manager for concrete and steel structures.
Christer Vestergren.
The wooden formwork is built, the rebar is put in place and the concrete is poured. After three or four days, the concrete has hardened and the formwork can be removed.
“We don’t pour concrete where doors, gates or pipes are to be installed. In such places we create cavities, voids inside the formwork.”
A concrete wall with openings for wiring and doors, for example.A ventilation tower for the new metro has already been erected at the old airfield in Barkarby.
40 concrete works under way
The rough concrete surfaces might not look that great, but they will rarely be visible in the finished metro. However, concrete provides the foundations for the escalators, ticket halls and platforms to be built later on.
While the last tunnels are being blasted for the Blue Line to Barkarby, some 40 concrete works are also under way. The next phase will then begin: kitting out the stations and tunnels with, for example, power, tracks, escalators, lifts, floors and other surfaces.
“But we won’t be finished before then. There will be some overlap in the work. That’s how the metro extension has been planned,” says Christer.
Getting all the pieces of the puzzle in place requires effective coordination, planning and logistics, which is a challenge when working in a tunnel under the ground. “In a tunnel, you always have a roof overhead. When building above ground, there’s usually a tall crane on hand to help move the heavy rebar. Getting the construction materials in place takes a lot longer in the tunnels.”
The longest underwater tunnel is the one from Kungsträdgården to Södermalm and Sofia station. At its deepest, it will be about 90 metres below the surface of the water. Naturally, working under a large body of water increases the risks during construction.
Saltsjön in cross-section. This is how the metro will run under Ladugårdslandsviken, Saltsjön and Hammarby Canal.
Familiar with the geology
“Much of our work is about making sure we know the risks and how to manage them. When building under this much water, the risk of large amounts of water entering the tunnels increases simply because it’s under such pressure. This is particularly true if the bedrock between the lake or sea bed and the tunnel is of poorer quality. This is why we have to be familiar with the geology, we need to know the structure and behaviour of the bedrock, how much water it lets through,” says Göran Leverud, project manager for the tunnel that will be built from Kungsträdgården under Saltsjön to Sofia station.
The tunnels are constructed the same way as all other new metro tunnels in Stockholm – by drilling and blasting the tunnel out of the bedrock. In all tunnels, any cracks in the rockface are sealed with a cement mixture before blasting, which stabilises the bedrock and prevents water from leaking in.
Drilling ready for injection. Photo: Region Stockholm
Concrete tunnel under Saltsjön
Three tunnels will be built under Saltsjön – two for the tracks and one for service access. The service tunnel will be blasted first, and it will always be about 20 metres ahead of the other tunnels.
“This allows us to check whether the properties of the bedrock are as we expected before we blast the metro tunnels,” Göran Leverud explains.
In addition, under Saltsjön, a concrete tunnel will also be built inside each rock tunnel, a so-called lining, to ensure a safe, stable and dry tunnel.
Early in the planning process, studies were conducted to check whether it would be possible to instead build a so-called immersed tunnel, which is a tunnel built on top of the lake bed. However, this would have been considerably more expensive with greater impact on the environment.
One advantage of building in the bedrock under the water is that the surroundings are not as affected.
“Because we’re far from the nearest building, as well as deep under the lake bed and deep inside the bedrock, we don’t have to worry as much about the noise and vibrations caused by our work,” says Göran Leverud.
Are fish affected by the blasting?
“Animal life in the water above the tunnel construction won’t be noticeably affected. The pressure waves from the blasting won’t propagate into the water above our tunnels. The sound waves, however, can propagate in the water and be perceived as unpleasant if you’re in the water.”, says Göran Leverud.
Where is the metro being built under water?
Ladugårdslandsviken between Blasieholmen and Skeppsholmen
“You could say that we’ve already decided on the circulation, we know where the tracks will be. Now we need to decide on the best place for the depot, the heart that keeps the trains running,” says Johan Brantmark, head of the west department, which is responsible for building the metro line between Fridhemsplan and Älvsjö.
This autumn will see the start of a study on the best location, and there are many options on the table.
“We’re looking into whether the best solution is to expand an existing depot or to build a brand-new one. If we decide on a brand-new depot, it needs a good location close to the tracks, somewhere that’s easy for the trains to access,” says Henrik Unosson, project manager for the depot for the new metro line.
It’ll be several years before you even start construction, so why do you need to decide on the depot location so early?
“Finding a good location takes time, as does designing a depot. A depot needs to work well for many years, it’s a fundamental part of the system for keeping the metro trains running. We may need to make changes and minor adjustments to the new line. This is why it’s important to take our time looking at both the location and the design early and thoroughly,” says Henrik.
What’s happening under our feet? This is something many Stockholmers are asking themselves now that the metro is being expanded and the work can be both heard and felt in many places. In Under Your Feet (Under dina fötter in Swedish), now available on the Nya tunnelbanan Youtube and social media channels, you can follow the work on the expansion close up.
Watch the first episode:
“Building the new metro is a major investment that will affect many people living in Stockholm. This is a way for us to offer Stockholmers a sneak peek at how the new metro lines and stations are actually being built. We hope that this series will arouse interest and that many people will want to join us on the journey towards a new metro,” says Marianne Dunér, head of communication at the department responsible for the metro expansion.
Join us inside the drilling rig, 100 metres below ground. Photo: Region Stockholm.
In the first few episodes, viewers will be able to experience a drilling rig from the inside, hear about the challenges of building a depot, see how tunnels are measured to make sure they end up in the right place, and much more. You will also get to see how completely new metro lines are planned.
The Under Your Feet series will also offer viewers a glimpse of the planning work for the new Älvsjö line. Photo: Region Stockholm.
“There are an incredible number of aspects to the construction of the new metro, and it’s great to be able to show the breadth and complexity of the work in this way,” says Oskar Hannler, project manager for Under Your Feet.
You can now watch the first three episodes on the Nya tunnelbanan Youtube channel. Don’t forget to subscribe to the channel to avoid missing new episodes when they are released.
The procurements of all major rock contracts are now either complete or in progress. Those in progress are the Hammarby kanal, Slakthusområdet, Järla in Nacka, Södra Hagalund and the underground connection track between the depot in Högdalen and the tracks towards Farsta. All are expected to be completed in the spring of 2022.
– It’s clear that we’re on our way to a new stage. Focus next year is on procurement in construction, installations and BEST – everything that will make the tunnels to a metro, says procurement manager Iveta Sandström.
Iveta Sandström, procurement manager.
Approximately half of the total sum of procurements in kronor is completed. The charts below show what percentage is done, what is in progress and how many are left to do.
