Shani Wallis, TunnelTalk

TBM excavation with a precast concrete segmental lining is specified over drill+blast methods for majority excavation of the new twin-tube, 20km-long railway tunnel between Oslo and Ski in Norway. The decision by the National Rail Administration, Jernbaneverket, paves the way for national and international bidding of an EPC (engineer, procure, construct) contract to build Norway's longest railway tunnel to date.

TBM excavation specified for 20km Oslo-Ski rail connection

The announcement in favour of TBM excavation combines the social, environmental, time and cost advantages that mechanical excavation can offer over drill+blast. The new 22km-long double track Follo Line between Oslo Central Station and the public transport hub at the town of Ski runs 20km through twin tunnels beneath the commuter suburbs and rural towns south east of Oslo. The issues of construction traffic, noise, vibration and protection of the ground water table were major concerns. TBM excavation confines construction to one mid-point access adit at a secluded rural site at Åsland. From here the specification is to use four TBMs, two each working in opposite directions to complete drives of up to 10km each south to the tunnel portals at Langhus and northwards to Bækkelaget.

With the decision to specify TBM excavation and a precast concrete lining now made, the project schedule established by Jernbaneverket is to have preparatory works and the prequalification process for the main contract begin in 2013, with award of the EPC contract and start of main works in 2014. Services are programmed to open in 2019.

Table 1. Facts about The Follo Line Project 22km of new double track line between Norway's capital Oslo and the public transport centre of Ski 19.5km in twin tube tunnels with cross-passages every 500m Construction phase scheduled to commence in 2014 Preparatory work to start in 2013 Completion scheduled in 2019 Designed for 250km/h The project facilitates a potential high-speed link between Norway and the Continent via Sweden 1.1 million residents in the Oslo region with a 30% increase in population and jobs anticipated by 2025 An estimated 150,000 passengers daily are anticipated

Complex underground connection of the Follo Line (red) to the Oslo Central Station

In announcing the method of construction, Harald V Nikolaisen, Director of Infrastructure Construction at the Norwegian National Rail Administration, said that suppliers and collaborating parties need to start preparing themselves for prequalification and the tendering process. He invited interested parties to a project meeting planned for Tuesday, November 27, 2012. The international gathering is aimed at attracting potential main contractors experienced in major multi-billion rail infrastructure works. These will compete for an envisaged five major EPC contracts including the TBM tunnel excavation works and the system wide operating services.

"We look forward to being the contracting authority for the project," said Nikolaisen. "It is important to develop capacity in the supplier market in order to ensure the implementation of future rail expansion projects and we will be able to achieve this innovation by positive interaction between national and international suppliers."

A specified precast concrete segmental lining

Rock TBMs and precast concrete lining
The decision in favour of TBM excavation is taken after extensive study, geological survey and expert consideration of the technical, logistical and environmental issues. An earlier decision by the client ruled out design of a single-tube, double-track tunnel of 118m² as this would have required either escape adits to the surface at regular intervals, or a parallel 25m² service tunnel with escape connections every 1,000m. Single-track parallel tunnels of 70m² with cross passages at 500m intervals fulfil the operating safety requirements. Drill+blast or TBM excavation was appropriate for single-track twin-tube tunnels and the final decision might have been left to the successful EPC construction group. After considering the pros and cons of each, the client has specified TBM excavation.

TBMs of more than 10m diameter will be needed to create the minimum 9.1m i.d. of tunnels designed for 250km/hr high-speed train services. A bolted and gasketed precast concrete segmental lining is also specified as part of the TBM excavation operation to meet strict specifications for the control of ground water inflows. Using single or double shield TBMs and erecting the segmental lining as excavation progresses also adds programme advantages to the TBM method for the long drives.

Several options were studied

The one central working site eliminates up to five additional access adits that would have been needed to divide drill+blast of the long running tunnels into as many as eight headings of about 2km each and complete excavation within the same planned TBM excavation programme.

Drill+blast will be used on the project for approximately a third of the rock excavation required. This includes excavation of the central working access adit of up to 1,000m long, depending on final design alignment, and the emergency cross-passages between the two TBM drives. Drill+blast is also likely to be used for excavation of the final 2.8km of the twin running tunnels on the approaches to the Oslo Central Station. The alignment through the Ekeberg Hill passes beside existing road tunnels and other underground infrastructure and final decisions about the most appropriate method of excavation are under consideration.

The most significant factors to influence the choice of TBM as the primary method of construction, and as explained in a statement by Jernbaneverket, are the tunnel's length, the character of the rock, and a limitation of the number of access adits which could have resulted in noise and inconvenience in the surrounding areas. Design life expectancy was another noted advantage. According to the statement, "it is now an internationally accepted norm that tunnel constructions, such as those on the new Follo Line, are designed for a life expectancy of 100 years or more. The industrialised process that the TBM method enables, is a reliable guarantee of high quality work."

The selection of TBM excavation for the new tunnel reignites an expertise that Norway dominated in the 1980s, primarily in the field of long TBM tunnel excavations for hydroelectric projects. Through alliance building with international contractors, it is expected that the use of TBM technology on the Follo Line will facilitate an upgrading of TBM excavation skills within the Norwegian tunnel construction sector. Nikolaisen emphasised that this "will strengthen the competiveness of Norwegian contractors within the tunnel construction sector both at home and abroad."

References

Direct twin-tube route for Oslo-Ski railway - TunnelTalk, October 2010
Outreach for Norway's major rail tunnel project - TunnelTalk, March 2011
International bid for Norway rail project - TunnelTalk, December 2008

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