In Scandinavia, long rail tunnels have hitherto been drilled and blasted rather than bored through the extremely hard Norwegian rock. However, on the Follo Line Project four tunnel boring machines (TBM) by Herrenknecht worked simultaneously to excavate the main part of two 20km railway tunnels between Oslo and Ski.

The final double breakthrough of the two Herrenknecht tunnel boring machines ‘Anna' and ‘Magda' (Double Shield TBM, Ø 9,900mm) in the Norwegian Ski completed the excavation of both 20km-long tunnel tubes of the Follo Line railway.

The first two Herrenknecht TBM ‘Queen Eufemia' and ‘Queen Ellisiv' had already reached their destination in opposite Oslo in September 2018.

The successful completion of the project represents an important milestone for tunnel boring machines in a country that has so far been relying primarily on conventional drill and blast tunnelling.

"If we want to stay fit for the future, we can't just say ‘we'll do it the way we've always done it'," said Anne Kathrine Kalager, project manager for Bane NOR. "We must always keep looking for the best methods - and as it turns out, in this project TBM have decisive advantages."

With drill and blast it would have taken a total of seven construction sites in the metropolitan area, some of them difficult for trucks to access, which would have placed a great burden on traffic and residents. In contrast, the TBM solution gets by with only one central point of attack. Two 900m long access tunnels connect the large Åsland jobsite on the surface with two underground caverns. From there, two machines each bore south and north respectively to create the two tunnel tubes.

To master the extremely hard rock with up to 300MPa compressive strength, Double Shield TBM by Herrenknecht were employed. The technically sophisticated tunnel boring machines combine the functional principles of Gripper and Single Shield TBM in one machine. They can simultaneously bore and line the freshly bored tunnel with rings made of reinforced concrete segments - 140,000 of these segments were placed during the Follo Line Project.

The 19in diameter disc cutters on the machines, which were made of special steel with each weighing up to 372kg, were pressed against the extremely abrasive rock with up to 32t of pressure. "The wear of the disc cutters was enormous," stresses Matthias Flora, a specialist in hard rock at Herrenknecht. Up to 50 of Herrenknechts cutter experts in Schwanau refurbished a total of around 8,000 worn disc cutters for reuse for the four machines in Norway over the entire project period. "In close cooperation with our customers, we feel our way ever closer to the optimal parameters for the disc cutters," Flora explains about the special project challenges.

"Even the tiniest improvement to the discs counts. If optimal cutter management allows us to complete just one more tunnel ring per day and machine, this ultimately adds up to put us two to three months ahead of the planned project time," stated Fernando Vara, project director of the Acciona Ghella Joint Venture.

Thanks to regular maintenance and excellent teamwork, consistently high advance rates and peak performances of up to 15m per week were achieved in Norway. Such advance rates can only be achieved with particularly strong and robust machines. Each of the four cutterheads weighing 265t was driven by 13 engines with a drive power of 4,550kW each, which corresponds to 475hp each.