Tunnelling under London has captured most of the attention on the Channel Tunnel Rail Link, but tying into the existing network is no mean task, says Nina Lovelace.
Not many single site jobs present such a mix of construction challenges as the US$172M Contract 103 of the Channel Tunnel Rail Link (CTRL): 600m of tunnels, five bridges, three viaducts, lowering a highway and a 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 in laying a high speed line to St Pancras mainline station, where Eurostar trains will terminate once CTRL becomes operational in 2007.
'Much of the complexity is 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.
More chords will link St Pancras to the East Coast Main Line (ECML) to Edinburgh, and from the Midland Main Line (MML) to the NLL. This 20km of new rail line will transform St Pancras to the capital'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 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 rail bridges to carry the 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 chords, which over some of their length will be elevated on three 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 72m span ECML truss-bridge. Once both are complete the 2,000t bridge will be launched from west to east, during an ECML line possession at Christmas this year.
The start date for viaduct construction is on the programme's critical path because of complications caused by major site traders, including Castle Cement and Tarmac. With their sidings and aggregate and concrete plants lying over viaduct footprints, before work could begin the team had to provide a new aggregate and batching plant offsite.
To avoid a clash on one of its elevated sections, part of a Victorian viaduct carrying the major York Way will be demolished in 2004, dropping the road 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 foundations and the diversion of utilities.
'We are boring minipiles and installing underpins to support the abutments, ' says Lang. 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 high speed lines over Regent's Canal and Camley Street. MML trains are currently being run into St Pancras on the easterly approach lines, allowing the team to demolish existing bridges and construct the western halves of the two new ones. At the same time, enabling contractors on an adjacent CTRL contract are extending St Pancras' western station platforms. MML trains will then be skewed on to the western lines to complete the eastern bridge sections.
Surprisingly, the 600m of 6m diameter twin bore tunnel running across the site is nothing to do with the CTRL project, Devlin reveals. 'It is actually part of enabling works for Thameslink 2000, ' he says, to extend the suburban rail service.
When drawing up the CTRL Act in 1996 it was clear that it would be easier to carry out Thameslink tunnelling in tandem with CTRL, rather than after the site had been developed. A new Thameslink station alongside St Pancras station is being built as part of Contract C105.
INFOPLUS www. ctrl. co. uk
On the new high-speed TGV line between Antwerp and Rotterdam, carefully targeted environmental protection works are being carried out to minimise the acoustic nuisance of rail traffic. Methods include overhead noise barriers and noise protection walls.
Near Peerdsbos, Belgium, the solution is a 3,500m long cut and cover tunnel. A highly streamlined formwork concept - and a specially designed formwork carrier - have allowed the tunnel walls to be cast insitu very economically.
Sections of wall measuring 9m by 12m are being cast in a two day cycle on either side of the tunnel, using Doka's Top 50 largearea formwork. By reinforcing the formwork with an extra walinglevel of vertical steel walings WU 240, it was possible to reduce the number of form-tie points in the large-area formwork elements so as to achieve the uniform tie-point pattern stipulated by the client.
Given the length of the structure and the consequent high number of repeat uses of the formwork, the most efficient solution was to use a mechanised formwork carrier. A rail-mounted carrier designed and supplied by Doka Belgium, provided the rapid progress of work that the contractor, a joint venture of TV AB-MD-Heijmans was looking for.
Using two carriers, it has been possible to cast one section of wall every day, on alternate sides of the tunnel.
Completion of the tunnel is scheduled for July 2003.
On another contract, a combination of four carriers and Doka's Framax framed formwork is also doing sterling service on a 7,900m long noise protection wall between Brasschaat/Schoten/ Brecht. The height of the wall varies between 3.15m and 6.15m and the 12.5m long Framax gangs are stacked vertically to the desired height working on a twoday cycle. Just nine months has been allowed for completion of the acoustic wall, but the deadline seems likely to be met without problems. The contractor is TV AB-Heijmans.
The TGV line is scheduled for completion in 2006.