Direct twin-tube route for Oslo-Ski railway
Oct 2010
Shani Wallis, TunnelTalk
- Twin-tube running tunnels and a direct route with no intermediate stops are the options approved by Norway's Ministry of Transport for the new 24km high-speed railway link between Olso and Ski.
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Oslo-Ski direct (right) is chosen
- The decisions recommended by the Norway's National Rail Administration Jernbaneverket, overrule earlier considerations for an intermediate station stop at Kolbotn and options for single-tube, double-track tunnel alternatives.
- Despite a more expensive estimate, the twin-tube option for the 19km long tunnelled section was recommended and approved "because this solution provides better safety and maintenance conditions," explained Anne Kathrine Kalager, Jernbaneverket's Project Manager for the Follo Line. "When we made the decision, we had mainly safety and quality in mind but total life cycle costs were also a topic. Studies showed that life cycle costs were almost equal between a single, double track tube and twin tubes, even though the initial investment costs differ."
- The decision also brings drill+blast and TBM excavation of the long tunnels into head to head competition. Options for a minimum 118m2 single-tube, double-track tunnel were considered favourable for drill+blast excavation only but would have required either escape adits to the surface at regular intervals or a parallel 25m2 service tunnel with escape connections every 1,000m. Parallel single-track tunnels of 70m2, with cross-passages at 500m intervals, can be excavated by TBM or drill+blast and the advantages and cost benefits of both are under concentrated study by the design teams for possible specification of the preferred method.
- A single-tube, double-track bored tunnel would require a TBM of 13-14m o.d. and was said to be nonviable. Penetration rates of disc cutters into the rock and the advance of such a large TBMs through the hard Precambrian gneiss with dikes and layers of amphibolite and metadolerite were calculated as being too slow to be feasible.
- A direct route without an intermediate stop had plans for further national and international high-speed lines in mind.
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Fig 1. Twin tubes it is
- "High-speed rail is a flag-ship of the current government," said Kalager, "and as the inner section of a possible international link to Sweden, planning for the future was a prime consideration when making this Oslo-Ski decision."
- Including an intermediate stop would provide little extra time benefit for commuters to Oslo and would have added five or so minutes to an 11-minute direct high-speed service. The advantages of the new line for the existing line also outweighed the benefits of an intermediate station. There are 12 stations on the existing line and it takes about 22 minutes to travel between Oslo and Ski. The new high-speed will more than double rail capacity between the two cities and moving express passenger services and slower freight traffic to the Follo Line will allow more trains to be added to the existing double-track service."
- With the new line running almost entirely underground - linked to the stations either end by sections of shallow cut-and-cover and at-grade alignment - the route draws on Norway's extensive tunnelling experience and provides the utmost concession to social and environmental concerns. The route runs through a densely populated commuter area south-east of Oslo and construction traffic, noise, dust, vibration and ground water control are major concerns for construction plans and excavation methods.
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National rail improvement programme
The Follo Line is Norway's first dedicated high-speed rail project and includes construction of the country's largest and longest rail tunnel to date. Other projects in the country's planned rail improvement programme include extensive tunnel construction. Two are in the Oslo region. -
- Upgrade of the Vestfold Line, southwest of Oslo, is split into several projects. One of the largest is a new 14km double track section that will replace a current 15km single-track service from Holm to Nykirke in the northern part of Vestfold. This new section includes a 12.3km rock tunnel and a new underground station through and under the town of Holmestrand. Construction started in August 2010 and will be completed in 2015.
- On the Line's Farriseidet-Porsgrunn section, a new alignment of double track railway will replace the current track, which is outdated and has many tight curves. The new route of 23km includes seven rock tunnels of between 110m and 4,700m long for a total of 14.5km, and will reduce travel time between Larvik and Porsgunn from 34 to 12 minutes. Construction is due to start 2012 and be completed in 2016. Once completed the new works will reduce journey times from Tønsberg to Oslo to an hour, compared to the current 90 minutes, and will save an hour on journeys between Skien and Oslo.
- A 60km upgrade of the Dovre Line from single to double track between the towns Eidsvoll and Hamar includes construction of four rock tunnels for a total of 7.8km. Construction of the first 17km long stage is due to start in 2012 and be completed by 2015.
- Further a field, the final 7km long Lysaker-Sandvika section of the new Asker Line to Drammen, which is due to open next year, includes a 5.5km long rock tunnel, and a 5km long shortcut, the Gevingåsen tunnel, is included on the Nordland Line.
- On the Bergen Line, planning has started for extending from single to double track between Bergen and the suburban town Arna, which will include the new 7km long Ulriken tunnel, and further plans for double track upgrade of the Østfold Line beyond Ski includes a new tunnel and underground station in the town of Voss.
- Norway's civil construction market is normally closed to outside contractors but opening the Follo high-speed line contract to international tender is a break with tradition that is expected to attract increased competition to cap its cost and maximise value for public money expenditure. The invitation for RFQs (requests for qualification to tender) will be advertised in the European trading zone journal, which is circulated to countries of the European Union as well as non-EU countries. "We are interested in receiving expressions of interest from any qualified tunnelling contractors," said Kalager.
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Drill+blast or TBM excavation
The TBM option is based on using double-shielded rock TBMs and to erect a precast concrete segmental lining as they progress. Four TBMs of more than 10m diameter would work in opposite directions from a single mid-point access adit towards the four portals with the backfilled rings of bolted and gasketed segments providing a one-pass watertight final lining built concurrent with excavation. Cycles of pre-excavation grouting would consolidate zones of weak rock as encountered and control ground water ingress as required. - Drill+blast would progress from five adits to limit advance in each heading to no more than 2,000m. Excavation from the tunnel portals is not permitted as they lie close to residential communities. Primary support of rockbolts and shotcrete would be installed as excavation progresses and a second pass would be required to install a waterproofing system for permanent ground water ingress control into the operating tunnel.
- "A final insitu concrete lining is not mandatory for the drill+blast option," said Arnulf Hansen, adviser to the Rail Administration, "but a new regulation that specifies a water and frost protection shell throughout new rail tunnels could result in a full waterproofing membrane system and a final insitu concrete lining being adopted for this long high-speed rail tunnel."
- Environmental advantages in conjunction with tighter ground water control measures and life-cycle maintenance cost benefits are said to favour the TBM option. A single work site and access adit for the TBM operation is located in a remote area shielded from urban and rural communities and with the possibility of blasting restrictions on a drill+blast operation, the one-pass excavation and final lining of the double-shield TBM approach has the potential to build the tunnel in a shorter construction period. A waterproofing membrane and insitu concrete final lining would also add time and cost to Norway's otherwise frequently unlined drill+blast options.
- With the route and its configuration now confirmed there is a collective urgency by the government to get the line built as fast as possible. "We are planning to complete the design and have contract documents ready for an international invitation to tender in about summer 2012," said Kalager. "We are working towards making a firm decision between either drill+blast or TBM excavation but if it is a close race between the two, both will go to tender for contractors to price the options and the final decision made from there."
- Construction of the new line with its long 19km tunnel alignment was estimated at NKr 11.9 billion (US$2 billion), in 2009 figures. That estimate was based on a single-tube, double-track tunnel and new estimates currently being developed are expected to rise with the choice of the twin tube option. Construction is programmed to start in 2013 with services due to begin in 2018.
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International bid for Norway rail project - TunnelTalk, Dec 2008
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