Citybanan starts complex sinking operation 01 May 2013
Peter Kenyon, TunnelTalk
Immersion is under way of the first of three 100m-long elements for the immersed tunnel section of the new US$2.4 billion 6km Citybanan rail line in Stockholm.
Construction of the 300m immersed concrete tunnel by the Züblin (Germany)/ Pihl (Denmark) JV is the most technically challenging section of a new alignment that will double north-south rail capacity through the heart of the Swedish capital once it is complete in 2017. The JV was awarded the US$194.7 million immersed tunnel contract by Trafikverket, the Swedish National Rail Administration, in 2008, with construction getting under way the following year.
On Tuesday (30 April, 2013) barges floated the first element into position in the waters of Riddarfjärden, between Riddarholmen and Söder Mälarstrand, before the immersion process began. Nicole Jakobs, Section Engineer for Marine Operations, told TunnelTalk from Stockholm: "The very narrow construction site and a lot of other difficult circumstances have made this project very interesting and very challenging."
Second element immersion 23 Jul 2013
Peter Kenyon, TunnelTalk
Immersion is due to take place today (23 July) in Stockholm of the second of three 100m-long elements for the 300m water crossing of the new Citybanan rail line.
Second element ready for immersion

Second element ready for immersion

The Riddarfjärden waterway was closed to boat traffic to allow the complex operation at the northern end of the alignment to take place. The southern element was lowered into position in April, while lowering of the central element that will complete the tunnel is set to take place in the next few weeks.
Section Engineer Nicole Jakobs told TunnelTalk that the element will connect with the jointhouse on the Riddarholmen side. The jointhouse, a concrete structure measuring 9.5m long, 25m wide and 12.5m high, has been constructed in a deep pit where the rock tunnel emerges at the riverbank. Two rubber gaskets, each measuring 65ft (19.8m), will be used to seal the joint and allow the tunnel some movement capacity when water temperature fluctuates.
To make fitting of the central, and final, section easier, the northern element will be pushed 1m into the jointhouse and then drawn out to complete the connection.
  • Complex underwater connections of the 300m immersed tunnel

    Complex underwater connections of the 300m immersed tunnel

  • 6km Citbanan tunnel alignment

    6km Citbanan tunnel alignment

"Due to space constraints it was not possible to build a dry dock for element construction in Stockholm itself so we had three prefabricated steel shells constructed in Talinn (Estonia) barged across the sea to the work site through a very narrow lock."
Constructing the 'joint house' at the northern end

Constructing the 'joint house' at the northern end

The entry lock into Stockholm was too narrow for the enormous shells to pass through so they had to be mounted on top of the barge instead. Once the shells arrived at the work site concrete casting of the three elements, each measuring 100m x 20m x 10m, was able to take place in situ inside the floating steel shell.
Construction of the immersed tunnel is further complicated by the soft sub-soil of the sea bed, which rules out immersion of the tunnel elements directly on to the sea floor. Instead an underwater 'bridge' has been constructed with the elements laying on four support structures.
The first section to be immersed is at the southern (Sodermalm) side of the Riddarfjärden, where a watertight pit was constructed close to the shore and under an existing roadway that had to be diverted around the jobsite using a temporary pontoon. This was to facilitate excavation of a connection point, 17m below the water level, with the main tunnel alignment (Fig 1). With the pit flooded, braces supporting the side walls were lifted and the retaining concrete wall at the front was removed. The temporary pontoon was then moved back over the flooded pit to enable the element to be floated into place (Fig 2).
  • Fig 1. Pontoon bridge is moved back over the pit

    Fig 1. Pontoon bridge is moved back over the pit

  • Fig 2. Immersed element can be positioned

    Fig 2. Immersed element can be positioned

Later, a second element will be immersed at the Riddarholmen (northern) end of the tunnel, where it will connect via a cut-and-cover 'joint house', constructed below the water level behind concrete piles. The joint house has been cast and from there a concrete tunnel takes the new City Line into the bedrock for later connection with the drill+blast main tunnel. This whole connecting section, like the one on the southern shore, will be flooded to eventually form an underwater link with the main drill+blast tunnel once the elements are in place. Each element weighs 20,000 tonne, and will be anchored to piles drilled along the underwater alignment into rock below the sea bed.

Linking the immersed tunnel elements

"Finishing works including cut-and-cover finalisation and quay-line rebuilding will follow once all the elements are in place," said Jakobs.
Project scope has also included 6km of double track rail tunnels (approximately 12m wide x 7m high) excavated by drill+blast at depths of between 10m and 40m through Stockholm granite, as well as two new underground stations to serve central Stockholm: City and Odenplan. A parallel service tunnel has also being constructed, along with single track station approach tunnels.
Underground blasting below the city, completed under separate construction contracts, is complete for most of the 6km alignment. The Blue Line has previously been subject to planned closures to enable blasting to be completed, and to accommodate a connection between the City Line and the existing metro.
The new tunnel will effectively allow freight and through traffic to use the two existing overland lines to bypass the congested city centre. The tunnel, which will extend from the existing commuter station at Stockholm Södra, under Södermalm, Söderström, Riddarholmen, City and Vasastan, ends at Tomteboda. Once complete it will provide a dedicated track for commuter trains, with the new underground stations linked to existing subway networks. Currently all rail traffic passing through Stockholm has to share two overground tracks that were built in 1871. Today more than 550 trains/day pass through the city on tracks that were built when just 10 passed through.
Citybanan's geotechnical models are among the most complex ever run in Sweden. The new tunnel intersects existing subway in two locations and interchanges with existing service tunnels carrying traffic control, power supply, water, wastewater, telecoms and energy lines.


Citbanan scoops STUVA prize - TunnelTalk, December 2011

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