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© Kim Høltermand

BUILDING BENEATH THE CITY–WHILE NOBODY ABOVE GROUND NOTICES

INFRASTRUCTURE

Meter by meter, the tunnel boring machines are driving the five-meter-wide tunnel forward beneath houses, parks, and watercourses. At street level, city life goes on almost undisturbed, thanks to the state-of-the-art tunneling technology from HOCHTIEF, which is being used in places such as the Scandinavian capitals of Copenhagen and Stockholm.

The lamps on the ceiling of Copenhagen’s Nordhavn subway station look like they were folded out of paper. That’s why Dr. Frank Abel, Head of Tunneling at HOCHTIEF Infrastructure, calls it an origami ceiling. The lamps shed a bright, diffuse light that’s easy on the eye in the underground station, which is scheduled to open in the beginning of 2020. This Nordic chic is totally in keeping with the brand-new district of Nordhavn, which is currently one of the biggest urban development projects in Scandinavia. Residential and business buildings are appearing in an area that used to be occupied by abandoned storage sheds. Not only does Denmark’s booming capital need attractive new residential areas—the United Nations organizations that are based in Copenhagen will also be housed in this neighborhood. Copenhagen is already the cycling capital of the world, and in recent years the city has built a completely new ring subway line in order to reduce automobile traffic further. HOCHTIEF has been building a branch line of this subway for the new district of Nordhavn since 2014. It consists of two tunnels that connect the area to the subway ring and the new stations Nordhavn and Orientkaj. “We worked with two tunnel boring machines using a shield-driving method,” Abel reports. Named “Alice” and “Frida,” the two machines were custom-made by the German manufacturer Herrenknecht. Operated by crews of about eight each, the machines advanced at a depth of up to 25 meters toward their destination, the Østerport ring line station.

BORING THROUGH GROUNDWATER 

Tunneling deep underneath Copenhagen is a challenging task, because Denmark’s capital is built on limestone with a high water content. “We are more or less boring through groundwater,” says HOCHTIEF engineer Lennart Staggenborg, who works at the site. Numerous vertical test bores were made in the area surrounding the construction site and water was pumped out. However, the water pumped up out of the ground can’t be simply poured into the sewage system—much less directly into the sea—without any compensatory groundwater measures. Most of the pumped-out water is, in fact, used to raise the groundwater back up to its original level. Moreover, some of the water is contaminated due to a former industrial site. Before the water is pumped back into the ground or sent into the sewage system, it is purified using sand filters directly at the construction site. Halfway along the tunnel, HOCHTIEF has dug a vertical supply shaft where any water that seeps into the tunnel in the future will be collected and pumped out. However, the shaft, which reaches 40 meters into the ground, will not only serve as a sump but also, more importantly, provide rapid access to the subway tunnels for emergency services such as the fire department. Another challenge was posed by the need to build the shaft in the middle of the Krauseparken residential area. “We had to build it in such a way that the noise stayed within limits and the local residents didn’t notice,” says Abel. Following an in-depth simulation, HOCHTIEF and the city agreed on the conditions before construction began. 

» WE HAD TO BUILD IT IN SUCH A WAY THAT THE LOCAL RESIDENTS DIDN’T NOTICE. «

FRANK ABEL, HEAD OF TUNNELING 

SENSORS MEASURE EVERYTHING 

Sensors were attached to the surrounding buildings in order to measure and monitor noise and vibrations. The city of Copenhagen also had access to this data at all times. “It’s also important to realize that the construction measures don’t directly benefit the local residents, so everything was viewed even more critically,” says Abel. Another challenge was found deep underground and arose from the fact that access to the two subway tunnels had to be provided. The difficulty is that every concrete tube is a sort of unified system whose stability declines when parts are removed in order to build a cross-tunnel, for example. To solve this problem, special anchors were set into the Krauseparken shaft. These anchors distribute the forces that act on the tunnel so that the tube as a whole remains stable. The pipes for extinguishing water and other support facilities are permanently installed in the tunnel anyway. In an emergency, powerful fans would blow fresh air from the stations into the tubes. An interesting feature of these three-meter wind machines is that they have a sophisticated sound insulation system so that the noise doesn’t bother passengers and local residents when the devices are tested every two months. Despite all of the challenges, no incidents occurred during the construction of the Nordhavn line, “We were even able to set a record. The HOCHTIEF team bored 56 meters of tunnel in just 24 hours—the best progress achieved during the entire construction of the Copenhagen subway,” says Abel. 

NEW PROJECTS BEING PLANNED

The subway line runs above ground after the Nordhavn station, from where it continues to the Orientkaj station. Although the line currently ends there, an extension cannot be ruled out if the northern harbor area is developed further. However, concrete preparations are under way for the next Copenhagen subway project. Under the technical supervision of HOCHTIEF, work will commence this year on extending Line 4 southward from Copenhagen’s central station. The extension will encompass two 4.4-kilometer tunnels and five subway stations. In Stockholm, meanwhile, HOCHTIEF in a joint venture with Implenia will begin to bore a tunnel for electrical cables early next year. The client is Svenska kraftnät, the Swedish National Grid. At a depth of 50 to 100 meters, the planned tunnel will run 13.4 kilometers from Anneberg north of the city through downtown Stockholm to the south. The work on the tunnel will be vastly different from that in Copenhagen. “That’s due to the ground,” says Abel. “We will bore mainly through granite in Stockholm.” As a result, the tunnel won’t require any lining. However, to prevent the overlying ground from getting drained, which would cause the groundwater level to drop, the tunnel boring machine will have to stop every 15 meters so that smaller test holes can be made in a fan-like spread. If the workers encounter excessive water, they will inject cement into the holes. The Land and Environmental Court has imposed specific requirements when it comes to noise. The requirements specify that the construction work have specific noise limitation during the day and depending on what day of the week it is. However, the tunnel boring machine can run around the clock on condition that the noise stays under the specified noise restriction requirements. The tunnel is scheduled to be completed in 2023.