Surveying the Stockholm City Tunnel

The Stockholm City Tunnel construction project consists of the building of 6km of railway tracks under Stockholm’s central parts, and the separation of commuter train traffic from other railbound traffic.

Stockholm's new tunnels under the city
Stockholm's new tunnels under the city

Stockholm Central

Every day throughout the year, some 450,000 travellers journey to or from Scandinavia's biggest railway station, Stockholm Central. Long-distance trains and commuter trains meet here along with the underground system. If something goes wrong, it can lead to disastrous consequences. The trains needs to run smoothly and flawlessly, and tunnels and tracks need to be maintained and extended without any risks for the safety of the many thousands of passengers.

City Line - Citybanan

At the same time northern Europe's biggest infrastructure project is carried out – Stockholm City Line ("Citybanan"). The project consists of the building of 6km of railway tracks under Stockholm's central parts, and the separation of commuter train traffic from other railbound traffic.

Not unexpectedly the City Line has been described as "the most complicated infrastructure project planned in Sweden in modern time". The new railway tunnel makes it necessary to detonate large explosive discharges under vast infrastructure systems, such as the underground and traffic tunnels. On top of this, the blasts take place directly underneath sensitive and historic buildings in central Stockholm.

Therefore, extensive archaeological excavations have been carried out before the tunnel construction started. Among other items, a boat from the 18th Century has been found, as well as 19th Century objects such as chalk pipes, snuffboxes, coins and an antiquated porcelain doll head.

Crack measurements

Dr. Cecilia Montelius, a geologist with construction firm NCC, has been measuring the spread of cracks in the tunnels that have resulted from the blasts when constructing the City Line. She describes the challenges: "We don't have so much solid rock cover here above; that is, not so much mountain between the tunnel and the underground platform above. And we don't want the underground trains to fall down, of course!" It was therefore decided that NCC should visualise the geology of the mountain and adjacent grounds in 3D.

As the tunnel was blasted out at small intervals, cracks could conceivably radiate away from the tunnel wall. It was thus imperative to have complete control over the tunnel surfaces and the spread of the cracks all the time, in order for instance to be able to secure unstable rocks. Without this close control the safety of the train and underground passengers in nearby tunnels would eventually be jeopardised.

Robotic integrated camera

To solve this task, NCC made use of Topcon's Imaging Station (IS). In addition to the features of typical robotic total station, the IS also features two digital cameras, one with wide angle lens that is situated on top of the telescope and another camera which is coaxial. In addition, it has got a built-in scanning function – as in considerably more expensive laser scanners.

Mårten Korall is in charge of aftermarket support with Top Position in Sweden, which markets Topcon's products. He demonstrates Topcon's total station, which measures horizontal and vertical angles down to an accuracy of one second. The reflectorless distance meter has an exceptional reach for total stations – 2,000 meters (1.2 miles). Mårten tells more about Topcon's Imaging Station: "You would first describe a surface and say that "within this area I would like to take the measurements. And let's say that you would like the distance between the points to be set at five centimetres. And this you could do directly from the program here in the station or you could run it from a PC program with a WLAN connection." He continues: "Then I can tell the station to measure this mountain cross-section within this area and it will measure it reflectorlessly and come up with a 3D model of the surface."

Cecilia Montelius states it was the first time she had used a total station in this way. Additionally, the timing of the blasts was unusually critical: "The production pace at our site is high – we had something short period at the tunnel head to map the mountain for cracks and to scan the surface." After this short time there was simply no time to go back to the tunnel front and repeat the process if something had been neglected, since shotcrete would be sprayed over the tunnel walls directly afterwards. "It is therefore important to be sure that the result obtained is reliable, says Cecilia Montelius. And it was possible to check the results with this equipment directly at the tunnel head."

3D Photo image

With the help of the different scans and measurements NCC received a 3D model in Topcon's own software program ImageMaster showing any crack – a model that they then could use in evaluating the structural geology of the tunnel. And with the help of the photo documentation recorded, the IS could achieve a 3D photo image of the tunnel. As the blasts and the tunnel construction continue, and geologists and measurement technicians control the surfaces laid bare, the traffic to Stockholm city center continues day after day.

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