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The Follo Line project is a new 22 kilometers long, double-track rail line, currently under construction by Acciona Ghella Joint Venture between Oslo Central and the new Ski Station.
The line comprises a long, twin-tube rail tunnel – the first of its kind in Norway and the longest tunnel within the Nordic region.
Once complete it will be the nation’s largest infrastructure project to date – accommodating forty trains at speeds of up to 250 km/h and reducing journey time between its currently bottle-necked connecting stations by fifty percent.
Until recently, all railway tunnels in Norway were excavated using conventional drill & blast technology; but the size and geophysical properties of the Follo Line tunnel require a sophisticated combination of both conventional and mechanized excavation methods.
Ahead of the Australian Tunnelling Conference we spoke with Karin Baeppler – Head of Business Development, Geotechnics & Consulting for Herrenknecht (the project’s TBM supplier) – who gave us some insights into the project’s unique features and challenges.
What is unique about the Follo Line tunnel?
First of all, it’s geophysical properties are quite unique and challenging. It is made up of mostly Precambrian gneisses with banding and lenses of amphibolite and pegmatite, giving it a strong and abrasive rock mass.
Combined with its significant diameter, length and risk of high water pressures, it requires state-of-the art tunneling solutions.
As such, it is one of the first tunnels in the country to be excavated by tunnel boring machine (TBM).
The bored section is 18.5 km in length, but some drill & blast and drill & split techniques will also be utilized in conjunction with boring.
What are some of the project criteria?
The tunnel is expected to have a long service life (up to 100 years), optimal train handling and minimal maintenance requirements.
It must be impermeable, safe, and have a minimal impact on the local environment both during construction and in operation.
Why were TBMs chosen for most of the excavation work?
To carry out the project with drill & blast – as quickly as it could be done with mechanized tunnelling – a total of seven job-sites would have been needed in the metropolitan area; some of them difficult for trucks to access.
So drill & blast tunnelling would have meant a huge burden on the traffic and the many residents living near the job-sites.
In contrast, the TBM solution gets by with only one central point of attack.
Two 900 meter long access tunnels connect the large Åsland job-site on the surface with two underground caverns.
From there, two machines each bore south and north respectively to create the two tunnel tubes.
The caverns initially served as assembly sites for the four TBMs and are now used as logistical nodes for the drive.
The large job-site is linked directly to the E6 highway. There are no nearby residents to be disturbed by the years of construction work.
What is significant about the use of TBMs in this project?
It is the first time Norway has used TBMs for a project of this size and nature. As a result, we can expect to see more international collaboration, skill development and innovation in this space in the future.
Presenting at the Australian Tunnelling Conference, Ms Baeppler will talk in further depth about the challenges encountered during the tunnel excavation process and the features and methods used to overcome them.