2015 | ONGOING
The Follo Line project in Norway consists of a new high-speed dual-track, 22 kilometres long, that will connect Oslo central station and a new hub for public transport in the city of Ski.
The new line was designed to decongest the traffic in the south-eastern outskirts of Oslo, and to integrate the public mobility of the small towns around the capital: when it finally starts operating, commuters will be able to halve their average travelling time from about 22 minutes to about 11 minutes.
The contract involved the construction of around 64 kilometres of new tracks, which run through two twin tunnels. It is the longest railway tunnel ever excavated in Scandinavia.
Follo Line is one of the most important high- speed railway projects in the world because of its technical complexity. It is also one of the first in Northern European history to feature a double tunnel created simultaneously with four Tunnel Boring Machines (TBMs), commonly known as “moles”. TBMs are powerful machines capable of excavating tunnels in a mechanized way and in complete safety. The machine ends with a metallic cylinder, known as a shield, on which is positioned a rotating wheel equipped with cutters.
In line with tradition, the moles were given the names of women: Queen Euphemia, in honour of Queen Euphemia, wife of Håkon V of Norway, famous for her cultural interests; Queen Ellisiv, like the wife of King Harald III of Norway; Anna, who gets her name from a famous 1950s merchant from Kloppa; and Magda, after Magda Flåtestad, who ferried passengers from one side to the other of Lake Gjersjøen so that they could reach the church of Svartskog.
In this building site a TBM with a dual shield, measuring 9.96 metres in diameter, was chosen for the excavation, instead of the drill and blast method, which is traditionally used in Norway, and requires the controlled use of explosives. Indeed, TBMs had not been used in Norway for over twenty-five years owing to the type of rock, considered too hard and abrasive for the machine’s cutters.
The mechanized excavation with the TBMs also allowed for problems on the surface to be avoided, such as noise, vibration, and possible damage to the aquifer.
An innovative solution was used for the access of the TBMs. The project comprised the construction of a single access site in the rural area of Åsland, about midway between Oslo and Ski. From there four moles were launched, two in each direction, which excavated at the same time and at a close distance for long stretches. The underground assembly tunnels, 20 metres high, 50 wide and over 60 long, are the biggest ever made in Norway. It is a record-breaking building site.
In Åsland itself a low-impact base camp was built, with over 500 homes for the workers, offices, a gym, and a canteen. A factory to manufacture the segments needed to line the tunnels was built as well. These can thus be transported while avoiding traffic outside the building site. The tunnel was lined with over 20,000 rings, comprising over 135,000 segments. A typical characteristic of the Ghella building sites, whenever possible, is the reuse of the excavated rock for the concrete that is put back into the building site. This means saving energy and the natural resources available.
On 11 September 2018 two of the four moles, Queen Eufemia and Queen Ellisiv, made the tunnel breakthrough simultaneously, ahead of schedule; the other two TBMs, Magda and Anna, made a spectacular double breakthrough as well, in Ski, on 26 February 2019, bringing the mechanized excavation to an end.
Overall, the excavation by the four TBMs lasted 100 months, involving over 750 workers and 100 engineers and technical staff from twenty-five different countries.