Threading the Channel Tunnel Rail Link route through the centre of Ashford in Kent has been a difficult task, says Rail Link Engineering (RLE) Contract 430 project manager Alasdair Cathcart.
'Ashford is all about planning, ' he says, explaining that 5.2km out of the 15.2km contract length runs through the town centre, and also has to tie into the existing Eurostar line.
The track alignment had to be designed to avoid the many constraints a built up area brings, as well as tying in to the existing Ashford International Station, while avoiding any tight curves to carry the 300km/hr trains.
Not surprisingly then, the eventual route through the town goes through a 1.6km cut and cover tunnel, then across a 1.5km viaduct.
Following a contract site start date of December 1998, the viaduct was the last part of construction to start because of the number of constraints RLE had to deal with, explains Cathcart.
These included building a new sidings depot for Connex South East as the viaduct route passed through the existing space.
Construction of the viaduct did not begin until March 2000.
However, RLE and its contractor Skanska used the time to do some value engineering on the design.
The viaduct's piers have to stand on piles because of poor ground conditions caused by the floodplain that underlies much of Ashford.
'Initially we had planned the piles all to be bored, in the order of 900-1,500mm diameter, 30m deep, ' explains Cathcart. 'But as 1999 was a dead time for viaduct construction because of the enabling works, we decided to look at the feasibility of changing 60% of the bored piles to continuous flight auger (CFA) piles.'
Bored piles were chosen to withstand braking forces from the Eurostar trains. However, it was discovered that this could be achieved by using bored piles for every fifth span, with the remainder replaced with 750mm diameter, 24m deep CFA piles.
'Although we had to use more of them, we saved time and money, ' says Cathcart, who estimates three months were saved.
Skanska project director John Hollaway adds: 'With CFA, you don't have to use bentonite. You can use less plant so it's more efficient.'
The viaduct also has to cross both the Great Stour and East Stour rivers. 'We had to agree with the Environment Agency to sequential working rather than concurrent when we constructed the cofferdams to sink the piles, ' says Cathcart. 'This was to ensure we weren't taking away any floodplain capacity.'
The viaduct was also due to cross two station car parks, the existing Ashford to Canterbury railway line and a footbridge.
'The pedestrian footbridge was originally built over the existing Eurostar line, ' he continues, explaining that the new viaduct runs parallel to the existing line, thereby cutting through the existing footpath route.
This meant the footbridge had to be diverted beneath the Eurostar line through a subway, which was jacked into place while the Eurostar service was not running.
'We calculated it would take 33 hours to do the work, ' says Cathcart, 'but closing the Eurostar costs £50,000 an hour.
'It seemed ridiculous to spend £1.7M in rail possessions to place a £300,000 subway.'
So, instead, the work was done during the only time window the team could find: between 11pm on Christmas Eve and 8am on Boxing Day 1999.
The final constraint was the most difficult, however, because some of the viaduct's piers were due to be built through a maze of Railtrack's electrical cables. 'We had to protect the cables for Railtrack's Integrated Electrical Control Centre (IECC), ' explains Cathcart. 'That services the whole south east network.'
A damaged cable would have serious consequences, so steps had to be taken to ensure cable protection. This involved handdigging, putting the cables in sleeves and ducts, and diversions where the piers were due to go.
It was a long, drawn out process, so the cabled area was last to be piled - although work on pile caps, piers and spans continued on the rest of the structure.
Cathcart says piling is now over 90% complete, as are 70% of the 84 bases and 168 piers. Piling should be completed by Easter and the entire viaduct finished by the end of the year.