On the £5.3bn high speed Channel Tunnel Rail Link it is tunnelling work under London that has caught the headlines. But tying CTRL into the UK's existing network is no mean task either. Nina Lovelace reports.
Not many single site jobs present such a mix of construction challenges as this phase of the Channel Tunnel Rail Link (CTRL): 600m of tunnels, five bridges, three viaducts, lowering a highway and a brand new concrete and aggregate batching plant. And few are in such a difficult location - smack bang in the centre of London.
But this is the challenge facing CTRL project manager Rail Link Engineering (RLE) and main contractor Kier/Edmund Nuttall joint venture on the £110M Contract 103. Some of the project's complex mix stems simply from laying a high speed line to St Pancras mainline station, where Eurostar trains will terminate once CTRL becomes operational in 2007.
CTRL will emerge from new tunnels at the London West Portal in the north-east corner of the site, before sweeping down into an extended St Pancras mainline station.
'Much of the complexity is there because we must provide an interchange between CTRL and the rest of the UK network, ' explains RLE contract manager Carl Devlin. The interchange element was added into the CTRL project by the government, keen to build in flexibility for domestic routes but also looking ahead to potential expansion of the high speed route at a later date.
To provide this, the construction team will build new chords between the CTRL and North London Line (NLL), with a view to Eurostars eventually accessing the West Coast Main Line (WCML) to Glasgow. New chords are being laid to link St Pancras to the East Coast Main Line (ECML) up to Edinburgh, and from the Midland Main Line (MML) to the NLL.
In total, 20km of new railway line is being be built, which will transform St Pancras from a station receiving four trains an hour to the city's largest transport hub, receiving up to 40 mainline trains an hour.
But to enable the new lines to cross over existing track, local roads and the Regent's Canal safely, a variety of new structures must be built cheek by jowl with live railways, highways, utilities and site traders.
'This job is about planning, managing interfaces and logistics, ' says Devlin.
The team must build three new rail bridges to carry the new chords over the ECML and MML, which bound the site to the east and west. The bridges' heights are dictated by the need to provide safe headroom over the live railways. These bridges set the level of the new chords, which over some of their length will be elevated on three new reinforced concrete viaducts.
In a 55 hour possession last Christmas the team launched two steel bridge superstructures, weighing 1,000t and 1,500t, over the MML. Work is also under way on the most easterly abutment of the NLL incline viaduct and on the western abutment of the new 72m span ECML truss-bridge. Once both bridge abutments are complete the 2,000t bridge will be launched from west to east, during an ECML line possession scheduled for Christmas this year.
The start date for construction of viaducts is on the programme's critical path because of complications caused by major site traders, including Castle Cement and Tarmac.
Much of the traders' sidings and aggregate and concrete plants lie over viaduct footprints, so before work could begin the team had to provide a major new aggregate and batching plant off-site to the west of the MML.
Over one of its elevated sections, the main incoming CTRL track would clash with a Victorian brick viaduct carrying a major London road, York Way, which crosses the site from east to north. To make way for the new rail line, part of the viaduct will be demolished in 2004, dropping York Way beneath the track over a short section.
York Way will also be lowered over the approach to the viaduct section. This will involve reducing the clearance of the York Way bridge over the NLL, requiring strengthening of the bridge foundations and the diversion of utilities passing across the site.
'We are boring minipiles and installing underpins to support the existing abutments, ' says Lang, adding that over 60% of the underpinning is now complete. The bridge itself will be replaced with a lower profile design. Work has also included soil nailing to strengthen an adjacent retaining wall.
Two replacement rail bridges are required toward the southern end of the site to carry the extra weight of the new high speed lines over Regent's Canal and Camley Street. MML trains are currently being run into St Pancras on the most easterly approach lines, allowing the team to demolish existing bridges and construct the western halves of the two new bridges. At the same time, enabling contractors on an adjacent CTRL contract are extending St Pancras' western station platforms. Soon the MML trains will be skewed onto the western lines to allow completion of the eastern bridge sections.
Surprisingly, the 600m of 6m diameter twin bore tunnel being built are nothing at all to do with the CTRL project, Devlin reveals. 'The tunnels running across the site into St Pancras are actually part of enabling works for Thameslink 2000, ' he says. Thameslink 2000 is a proposed project to extend the existing Thameslink rail service through London. CTRL engineers are building part of a new Thameslink station alongside St Pancras station as part of Contract C105.
When the government drew up the CTRL Act in 1996 it became obvious that it would be easier to carry out Thameslink tunnelling work in tandem with CTRL construction, rather than tunnel at a later date when the site had been developed.
Tunnelling the first bore will start in June 2003, says Lang.
When complete, the back end of the TBM will be brought back along the tunnel and attached to a second TBM shield, although boring the second tunnel will not start until 2004.