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Copenhagen’s new underground line took eight years to construct, cost billions and is a major part of the city’s strategy to be carbon-neutral by 2025. But this underground line from Arup is also a thing of beauty

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Words by Veronica Simpson

In November 2019, two long-awaited infrastructural and architectural events occurred in Copenhagen: one far above ground, the other far below. Bjarke Ingels Group (BIG) completed an artificial skiing, walking and climbing playground, Copenhill, wrapped around the roof of the city’s clean energy power station. And a new underground line opened, the Cityringen, or ‘city circle line’: 17 stations connected by 16km of twin-bore tunnels, all built beneath existing residential and commercial neighbourhoods.

Cityringen (M3) on the Copenhagen Metro map, including the city’s two existing lines (M1 and M2) and the planned M4 line

Perhaps surprisingly, it is the latter that seems to be getting the local Danes in a lather of excitement, judging by the crowds of families jostling for ‘front row’ seats on the Saturday morning I took my first Cityringen ride. Much like the city’s existing two metro lines, the driverless trains of the Cityringen have large windows at front and back, allowing travellers in the end carriages to see every nook and cranny, every contour of these brand new tunnels, as we rattle through them at an average rate of 40km per hour. It’s not unlike a fairground rollercoaster, thanks to the strategic undulations going up as you enter and down as you leave each station.

Children stand at the front of the driverless trains, looking out into the tunnel. Image credit: Coast / Rasmus Hjortshøj

These sudden inclines are practical: they aid natural deceleration and acceleration and reduce the train’s energy consumption. But in that carriage full of tots and parents, it felt like they had been designed purely for our enjoyment. As we lurched down, away from the platform, our momentum was emphasised by low-level illumination all along the 16km of tunnels — something that would be unthinkable in the ageing, dank and rodent-filled passageways of the London Underground (LU) or the New York Subway (NYS).

Perhaps the giddiness of these train-enamoured toddlers was infectious. I hopped off every time I came to a new station to admire both the universal and distinctive elements of their design. And what halls of splendour they are: decisively Scandinavian, stripped-back cavities, cleanly excised from the city’s limestone foundations.

The materials of interior tiles are chosen in relation to stations’ specific local landmarks. Image credit: Coast / Rasmus Hjortshøj

Apart from a few hills on the Jutland peninsula, the Danish landscape is largely devoid of undulation — its average height is only 31m above sea level — so you might expect caves to hold a special attraction. These vertiginous caverns are, on average, 20m deep, with some over 30m — but they are also sleek and civilized. Tiles of varying sizes, materials and hues wrap around each station’s interior, chosen in relation to specific local landmarks. At Marmorkirken, for example, famous for its 16th-century Marble Church, the station’s interior walls are lined with fossil-embedded, sand-toned limestone tiles.

The material palette includes natural stone floors along with ceramic, glass and stone wall panels. Image credit: Coast / Rasmus Hjortshøj

As to the universals: in every station, escalators and stairs are stacked sequentially above each other, positioned to illustrate clearly the route up or down from the lowest or highest point. Their tilted planes are lined with long, thin, LED strips, creating a dynamic, directional graphic pattern that indicates flow, while complementing the station’s own geometries. But there is also that rarity in most underground stations: daylight. It filters down through the large, doorless apertures that mark the stations’ exits and entrances and beams directly from the faceted skylights which punctuate the surface — usually the only marker for the station’s existence, above ground, along with a simple lift structure, and the distinctive Copenhagen ‘M’ for metro. As fingers of sunlight filter down to the platform, they augment the sense of these subterranean chasms as natural wonders.

Faceted skylights at ground level bring daylight down into the stations and double as smoke ventilation, opening if there is a fire. Image credit: Coast / Rasmus Hjortshøj

Designs for the Cityringen began in 2007, led by Arup’s Copenhagen office, which formed a joint venture for delivery of the DKK 21.3bn (£2.53bn) project with Danish engineering consultants Cowi and French engineers Systra, and the city’s underground operator Metroselskabet.

Neil Martini, project lead for Arup, and Kristian Winther, project architect, are more than happy to discuss this major contributor to the city’s growth and efficiency. Cityringen comprises two new lines, the M3 and the central part of the future M4 line (the north-east and south-west extensions of which will be completed in 2024), and is planned to reduce commuting time significantly, as well as diminish reliance on the car — a huge step towards the city achieving its goal of carbon neutrality by 2025. Winther has been with this project since the beginning. ‘I started as one of the conceptual architects. We had a team of five people and it grew to many hundreds,’ he says.

Escalators are lined with LED strips that create a dynamic, directional graphic pattern indicating flow. Image credit: Coast / Rasmus Hjortshøj

Underground train systems are fairly new to Copenhagen. The first two lines, complementing an existing overground rail network, only opened in 2003: Metro 1, which connects the west and south to the centre, and Metro 2, which goes west to east and links to the airport. The Cityringen is far bigger, both in scale (there are only 12 stations each on the first two lines) and complexity, as all the stations are underground, unlike Metros 1 and 2.

‘We are excavating in the inner city of Copenhagen, as well as bridging the outlying areas,’ says Martini. ‘In all cases, we are building infrastructure in a dense urban environment.’ Says Winther: ‘A lot of time and energy was spent on trying to make it integrated and minimal impact. Sometimes as an architect or engineer, it’s more difficult to do a minimal solution than a maximal one.’

The stations are placed in different kinds of urban spaces. Nørrebros Runddel sits in one of the older churchyards in Copenhagen. Image credit: Coast / Rasmus Hjortshøj

For example, says Martini: ‘The main stair and lift shaft are always together. We were thinking: what could the beacon be for the station? In the end, we decided it was having the lift tower above ground: where that is, that’s where the station is. It takes you directly to the platforms. There’s also the skylight on top of the stations. You can see where the station is. You can actually see into the station, and when you’re in there you get daylight.’

The bright red stations indicate where people can transfer to above-ground trains. Image credit: Coast / Rasmus Hjortshøj

I comment on how rare and welcome that daylight is. Martini says: ‘Right from the outset, we wanted to get daylight into the station. It also comes from this architectural aspiration to have one big open space, so you can see where you’re going to and where you’re coming from; not making people walk around corners or endless, peculiar-shaped tunnels. And then as you leave the station, the skylight makes a big difference: you can see where you are, you can tell if it’s night or day, what the weather’s doing. The skylights also work as smoke ventilation; they open if there is a fire.

The vertiginous stations are, on average, 20m deep, with some over 30m

There’s no mechanical ventilation in the station, and that lack of additional equipment helps the station to be more compact.’ Conceptual design was informed by community consultation, says Winther: ‘We did a lot of stakeholder workshops, to ask people what they liked about the existing Metro and what they didn’t like. They said they would like the stations to be more distinct, one from the other. And there was a real desire to have it better integrated with the city, so it would reflect the neighbourhoods. It was also important to continue the design language from the first Metro lines [which Arup didn’t design]. That’s why we came up with having tiles that reflect different neighbourhoods, but also clarifying navigation: the very bright red stations refer to where you can transfer to normal trains or commuter trains, which are red in Denmark. It’s about storytelling.’

Neil Martini describes the ‘architectural aspiration to have one big open space, so you can see where you’re going to and where you’re coming from’. Image credit: Coast / Rasmus Hjortshøj

To build in a city centre, while keeping the city operational, is no mean feat, as Martini outlines: ‘In city centre sites like Marmorkirken, you have a very limited surface footprint to be able to insert a station. Your engineering solutions have to be very well considered, minimised wherever possible.’ It took a lot of complex construction planning, he explains, and strong dialogue with the planners. What’s more, parts of the city are very old: ‘The buildings are very delicate and there are a lot of sensitive structures, which you have to pay great attention to, making sure you don’t have an impact on those heritage assets,’ he says. ‘A lot of buildings in Copenhagen are founded on wooden piles, and the groundwater in the city is really high. You have to locally de-water the area so you can excavate out the soil. There was a ground water planning system we put in place, heavily controlled in order not to jeopardise those buildings.’

Østerport connects to multiple above-ground rail lines. Image credit: Coast / Rasmus Hjortshøj

Copenhagen, says Winther, is less challenging for tunnelling than the varied terrain of London, where the delayed Crossrail construction soldiers on at a cost of many extra millions. ‘Crossrail is doing some stuff that’s cutting edge,’ he says. ‘But ground conditions are not really similar in London. In Copenhagen, having seen it done so recently was very informative. When you get down to the limestone layers, they are really good for tunnelling. The whole design of the system has to maximise the amount of tunnel that’s in limestone rather than softer upper ground.’ If there was one particularly difficult station it was at Gammel Strand, which is by a historically important canal. ‘We had to keep the canal open while excavating,’ says Martini. ‘That took a lot of planning.’

The 16th-century Marble Church at Marmorkirken has inspired the limestone tiles of the new station interior there. Image credit: Coast / Rasmus Hjortshøj

In terms of the passenger experience, the lighting scheme is no less impressive. Developed with Arup’s in-house lighting team, the concept has very few components: strips under the escalators, rooflights, and the uplights, the origami-folded ceiling that shines light in. ‘We had to put in higher lighting at platform level,’ says Winther, ‘but ensure it was right — that’s something you spend a lot of energy on, because you want it to be a pleasant light, you want to see people’s faces, but you don’t want it to be glaring. We did a lot of studies and simulations. We had an amazing scale model built, so you could adjust temperature and intensity. We could do photometric modelling. And we also put the cladding elements through that, to see how they would reflect the light in the right way.’

The material palette includes natural stone floors, and brick for the walls — several versions of a handmade-type brick — along with ceramic panels, glass panels and natural stone. ‘We really had to work around lots of strict fire requirements, and technical requirements,’ says Winther, ‘but we wanted materials to give a feeling of what kind of part of town you’re in, what station you’re in.’ Durability is also key: these stations are designed to be around for at least 100 years.

Where skylights don’t exist, uplights illuminate the origami-folded ceiling that disperses light throughout the station. Image credit: Coast / Rasmus Hjortshøj

The fact that time and money have not been wasted on unnecessary structures above ground seems a particularly egalitarian, Scandinavian gesture — that money has been spent instead on improving public realm, which was again part of the integrated JV strategy, but led by Cowi. Says Winther: ‘The landscaping is quite subtle in most places. Some of the stations have furniture, like benches, and greenery. One really big challenge with landscaping was to fit all the bikes in. There was a requirement for parking for hundreds of bikes in each station.’

Constructing the station at Gammel Strand was particularly challenging: the adjacent historic canal had to be kept open while excavating. Image credit: Coast / Rasmus Hjortshøj

The stations are placed in very different kinds of urban spaces, explains Martini. ‘Some in the city centre are next to classically beautiful historic places that were already there, but if you go further away, some of them were just streets before — streets that have now been turned into public spaces. There is a special one that’s placed in a churchyard, Nørrebros Runddel, one of the older churchyards in Copenhagen. There is a neoclassical little building that relates to the churchyard: we actually rolled the building away to the side while the station was constructed, and then it was rolled back again so it’s sitting on top of the station.’ The graveyard, he says, ‘was already being used more as a park than as a churchyard’. Some graves had to be removed, but Martini is keen to point out that the grave of Hans Christian Anderson was not disturbed in any way.

‘Right from the outset, we wanted to get daylight into the station,’ says Neil Martini. Image credit: Coast / Rasmus Hjortshøj

Arup and its engineering partners are gratified to see how well the Copenhageners have taken to their new Metro line. ‘It does feel very different from other cities’ metro systems, both in its architecture and its functionality,’ says Martini. ‘The catchphrase is: small trains, high frequency. The trains are much smaller than you would have on the LU or NYS, and far more regular, so you can move just as many people but with much higher frequency.’ Copenhagen’s public transport system is well used already, but where the Cityringen now makes a difference is in areas that were previously only connected by bus.

Despite the investments made over the last decade in cycle transport, bus and car traffic is still heavy at peak hours, which is why bikes have become the most popular and rapid way to move around. But cycling isn’t appropriate to everyone, nor relevant for all occasions — especially the harsh, Danish winters. Movia, the Danish public transport provider, expects 34 million passengers to switch from bus to the new metro line. The route of the trains has been carefully planned to bring the less popular neighbourhoods within easy access of the city centre. In total, 85% of all educational facilities, homes and offices located within the city’s inner neighbourhoods will be a maximum 10-minute walk from either a metro or overground rail station.

København H, the city’s central station. Cityringen’s route was planned to bring less popular neighbourhoods within easy access of the city centre. Image credit: Coast / Rasmus Hjortshøj

It’s too early to say what impact the Cityringen will have, says Winther: ‘What it has done is upgrade some areas that were pretty run down. And that’s partly thanks to the idea of the station being in a public space: you now have 17 new spaces in the city that are improving that place. And you can see that the neighbourhoods that used to be a bit out of focus, people are going there, more restaurants, cafes and shops are popping up. The life around those stations, around the north, now there’s lots of people walking on the street.’

But it’s a recent reaction to the new public space inside that tickles Winther most: ‘One of my colleagues took a photo of a transfer tunnel between two stations, which we had made into a big, open circular space. His photo shows a kindergarten having a lunch break, sitting up against a brick-tiled wall; I like that unexpected use of an underground space.’