Gothenburg is the modern European city, sitting on the southwest coast of Sweden. For hundreds of years it has acted as a key trading hub between Scandinavia and mainland Northern Europe.
The Gothenburg port remains the biggest in Scandinavia today and the city is also known for its extensive tram network. Gothenburg is connected by 190 km of tram line which has an annual ridership of 1.5 million passengers. However, there is a reason that Gothenburg is reliant on trams and that is because, unlike most European cities, Gothenburg doesn’t have a metro network.
The ground beneath Gothenburg is a nightmare for construction. The soft clay that Gothenburg sits on has made tunnelling beneath the city an impossibility, until now. New technology and better construction tools have made tunnelling beneath Gothenburg possible and with the existing city tram and road network being at capacity, Gothenburg is finally getting an underground train link to the centre of the city.
The West Link
Five years ago, construction on a new 8 km rail line called the West Link began. 6km of the line will run underground, with a major redevelopment of the city’s Central Station to accommodate the new line.
Gothenburg’s Central Station is currently a terminus station. The redevelopment involves building a new underground section of the station that will house 4 tracks for the West Link line. Tunnels will run from the station through the city, better connecting passengers to other parts of Gothenburg and further beyond.
Olaf Buerger is in charge of the redevelopment of Gothenburg’s Central Station explains the project, “What we’re doing here is building an underground station with two platforms and four tracks. The tunnelling will go through—or more, under—the city of Gothenburg. And this tunnel will give the opportunity for trains to go under Goldenberg and onto Stockholm or Malmo. So not anymore, the end station as it is today.”
However, the site of the Central Station redevelopment, the centre of the city, provides a few challenges. Firstly, the soft clay that the site sits on is already very densely built on top of, which will cause huge loading forces being exerted on the ground once excavation begins.
The second issue is the site is in between an active mainline rail service on one side and one of Sweden’s busiest roads on the other, leaving a small space to construct a major redevelopment.
The Tunnel
To construct the tunnel two 600m long sheet pile walls were put into the ground between which a cut and cover tunnel is being constructed. But the sheet pile walls would need additional support. So 12m wide lime cement columns were bored along the length of the tunnel. Olaf Buerger explains, “They put for stabilisation, lime cement columns even below the foundation level of the bottom slabs in order to have the cement lime structure as a passive support for the sheet pile walls.”
This method of ground stabilisation is popular in the Nordic countries, it is known as Dry Deep Mixing or DDM. To construct the columns, a high-speed drill advances a drill rod with radial mixing paddles near the bottom of the drill string into the ground. During penetration, the tool shears the soils to prepare them for mixing. After the tool reaches the design depth, the binder is pumped pneumatically through the drill steel to the tool, where it is mixed with the soil as the tool is withdrawn.
The dry soil mixing process constructs individual columns, rows of overlapping columns, or 100% mass stabilisation, all with a designed strength and stiffness. These buried lime-cement columns transform the soft clay into a much more rigid and stable material, making safe the excavation of the narrow cuts.
The Station
The challenge of excavating the area for the station was an even bigger challenge than the tunnel.
“The station here is 50 metre wide, and from the start we made some calculations in order to also use limestone concrete columns as passive support there, but as far as we knew at that time that hadn’t been done before. So we deemed the risk too high” said Olaf Buerger
They next considered using steel welded props at three levels across the 50m wide excavation all along the 400m length of the station. However to pull this off would require a huge amount of labour-intensive on site steel welding that wasn’t practical.
So instead, they turned to a new solution. 50m long, 29 tonne hydraulic props, supplied by Groundforce.
Sam Oldroyd is the European Sales manager for Groundforce, and he explained the props being used on the Central Station project. “So, we’ve got 41 props in total on the job. It’s one of the biggest projects that we’ve ever supplied. There’s a total of over two kilometres of tube on the job. The props themselves are 50 metres long. They weigh about 29 tonnes. They’re taking design loads of six and a half thousand kilonewtons. The best way of thinking about a hydraulic prop is that it’s basically a series of extensions. An extension is a tube, and they have flanges on each side. And then we bolt these extensions together. And then at the end of this, we have the hydraulic unit.”
It’s not just the 750 tonnes of weight that each of the hydraulic props can bear, Groundforce’s props come with additional benefits that help deliver a project of this scale more efficiently.
Modular Props
The excavation of the Central Station is not an exact rectangle, the width gets smaller towards one end of the station. This means the length of the props will need to change along the length of the project. But also given the length of the excavation the propping would be done in stages moving along the station.
Sam Oldroyd explains how they first installed the props on the project, “We can get to pretty much any shape and size of excavation. The scope of this job was to start in what they call the funnel sections, which is where it tapers. And the idea was to supply the smallest props first, and they get progressively bigger until you get into the main trench. Now you could have done that with fabricated steel, but it’s worth having props: at different lengths, that’s quite tricky. The requirement of the job was to then take those props, and then jump them, it’s what we call leapfrogging into the main excavation. And that’s incredibly difficult to do with fabricated steel. Whereas for us, it’s really quite simple.”
A hydraulic prop could be put up while the excavation was taking place. Then additional support walls could be installed allowing for the prop to be taken down, moved along the station and reinstalled. This process of taking out, moving, and reinstalling the hydraulic props can be done very quickly, as Richard Dawson senior operation manager at Groundforce explains, “We can take these 50-odd metre struts out within about two hours. And we put them in in two hours. If you had to weld and tube, or similar sort of dam to prop, using the old method of welding and works, you’d be weeks if not a couple of weeks to do in and out. Whereas we can do one in a complete day or do two in a complete day.”
This process allowed for far fewer props to be used on site than would otherwise be necessary, and Groundforce was on hand at the start to provide training on moving the hydraulic props so the NCC team construction the station could use them as efficiently as possible.
“We had a guy from Groundforce on site who will help us put the tubes out in the right way, making the board connections and so on and then putting the jacks out in the right way. So that we had initially and once we proceeded, we were on our own, so it was quite easy to adapt how to use the system and then we took them again for the first replacement, as once we got them out, because that was a new moment for us, so we decided to get somebody from Groundforce. But it was very easy for us to adapt to using the hydraulic props.” Said Olaf Buerger.
Data Sensors
Along with the high load bearing capabilities and the modular design, the hydraulic props have another advantage over welded steel props. These high-tech props are fitted with multiple sensors that collect data on the forces and conditions that props are experiencing.
Sam Oldroyd describes the sensors that Groundfroce has installed, “So the way we actually do this, at the end of every prop is a pin arrangement that connects to the end plates. And we take this pin out and we replace that with what we call a load cell that’s connected out through a sender which goes out on the GPRS network. So that, it takes the actual load that’s actually running down the axis of the prop. Once it’s in position, you can then log on to our website, and you can log into the construction project, and you can see all the props being used on that project and you can see in real time what’s actually happening in terms of the loads.”
For the Gothenburg Central Station project every third prop in use is fitted out with this sensor, they can check to see what loads they are experiencing and even get warning if there is a change, Sam Oldroyd explains, “We can actually set up trigger levels. So that as it gets near a certain level, there is an alarm that goes off. And this just gives additional sort of security on the project.”
Another sensor on the prop monitors the temperature, as fluctuations can have a big impact on props as Richard Dawson explains, “So for example, in the winter, it’s quite cold in Gothenburg, we’ve had it up to minus 15, just recently. So, the props shrink, whereas in the summer, when we can get 20/30/40 degrees, we could see an increase of 200 tonnes potentially, in the hot climates from a night time to a daytime vision, which is good to monitor the critical buildings or infrastructure near the excavations, so you can amend the pressure that we need to.”
Collapsing walls in open cut excavation is a real threat on construction sites, with 35 people being killed by collapsing trench walls in the US alone in 2022. These sensors can give early warning for something going wrong and give the construction team enough time to make corrections.
Small corrections like painting the props black or having a water-cooling system can help control the temperature of the props and reduce the risk of failure, but this only works if you can monitor the performance of the props in real time.
Karla Tower
Groundforce’s hydraulic props are not just helping Gothenburg build and underground train line, it is also helping the city build up into the sky. The first skyscraper in the city is currently under construction and it will be the tallest building in Scandinavia once complete.
Karla Tower’s excavation is even bigger than at Central Station, at 60 m wide. Due to this the hydraulic props could be installed as struts running the length of the excavation, instead they installed raking props which sit at an angle and carry the load from the excavated wall into the floor.
“The main thing to be aware of, with raking props, we have to be very accurate in terms of the geometry. So that’s one of the key things that we need to establish, it’s the geometry and the connection details, how we’re connecting to the wall and beam and also how we’re going to connect to the base slab.” Said Sam Oldroyd.
Propping technology is continuing to develop. The team at Groundforce are already looking at potential benefits from adding more sensors, like an optics system to measure the deflection of the props. Their main focus is on finding ways to make their equipment even bigger and capable of shoring up even bigger loads. Richard Dawson says “Our biggest challenge is the one we’re working on now, a 1000 tonne prop.”
But for now these hydraulic props are helping the city of Gothenburg that it hasn’t been able to do in its 400 year history, build an underground metro and construct a skyscraper.
