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Up and over for new north route

ROADS:Doncaster bridge - Doncaster is finally going ahead with a long awaited viaduct to relieve its traffic congestion - one of the biggest city road schemes in the north. Adrian Greeman visited the site.

Ablessing in disguise is how the burghers of Doncaster should perhaps see the decade and a half of delays to their city centre traffic scheme.

Philosophical Yorkshiremen will be glad that their new 620m long road viaduct will be much more elegant and cost effective as a result.

But when the Department of Transport withdrew funding from the scheme in 1995, client the metropolitan borough council may not have felt so positive.

The viaduct plans had been on the drawing board since 1987, after all, and the scheme was ready to go. The news was no better in 1998 when a private finance scheme was abandoned in turn, mainly because conventional funding was finally allocated to the project.

But the delays did give the council a chance to refine its designs by putting the scheme out to a design and build contract. Tenderers were asked to bid on a base scheme and propose an alternative. The result had to help ease congested traffic flows in the city centre, and reconnect the traffic-isolated market district and St George's church to the main town. It was also required to help open up the old canalside waterfront for future development.

Contractor Amec Capital Projects, working with main consultant Mott McDonald and steel structures specialist Robert Benaim Associates, was able to offer a refined and cost effective solution. Key elements were removal of an old multi-storey carpark, retained in the reference design, repositioning of a roundabout link to make some space, and an economically engineered solution for the viaduct itself, which provides a new second road link through the town to the north.

Traffic congestion in the city occurs largely because there is currently only one bridge - the 'severely substandard' North Bridge - taking 66,000 vehicles a day across the East Coast Main Line railway, the River Don and the South Yorskhire Navigation Canal.

The new viaduct will leap over these obstacles and allow the North Bridge to be replaced. As part of the scheme its multiple spans will be rebuilt with a 60m span across the railway and a shorter 40m span. The new structure will then carry public transport traffic only as part of a new traffic circulation system.

The new 15 span viaduct with two 7.3m wide carriageways is the centrepiece of the scheme and was subject to a number of brainstorming sessions during the 1999 tender period before the final solution evolved, says Amec's northern operations project director Philip Girling.

Everything was considered from single and twin box concrete segmental construction to a steel ladder, a K-brace and multibeam with a haunch deck.

'We tried very hard to put ourselves in the client's position and second guess what was wanted, ' says Girling. The £46M project was let in March last year.

The chosen option for the viaduct is a twin steel girder structure with linking cross girders at 3.75m centres. Superstructure work for this was undertaken primarily by Benaim with Mott carrying out the earthworks and foundations and the general design for the rest of the project. 'But it was done as an integral team process with everyone working from the same office, ' emphasises Girling.

Omnia planking is laid across the steel girders which sit on a Compribond sealing strip. An in situ deck is then poured which acts compositely with the steel frame underneath. Maximum span is 47.3m The advantage of the steel ladder beam structure, with its two 2.45m deep side girders, is that it allows crossheads to be eliminated at the pier positions, says Robert Benaim project engineer Colin Edmonds. 'And column numbers can be kept to a minimum, just one each side at each pier position.'

The substructure was kept even more economical by using a single pile foundation for the columns. As a result these have to be fairly hefty with diameters up to 2m to allow them to take lateral as well as vertical loads.

Piles are bored through 'a couple of metres of made ground and then some 4m of alluvium over about a 10m layer of terrace gravels, ' says Edmond. Below that is Sherwood sandstone into which the piles are keyed for about 4m giving an overall length of around 20m.

'Up against the railway, however, we have used a more straightforward arrangement of 750mm multiple piles and a pilecap, ' says Amec project manager Alex Watts. The reason was not structural but to allow use of a tripod pile rig rather than the bigger machines used for the other piles. The tripod rigs are considered less likely to topple over, a safety consideration imposed by Railtrack, concerned about main line trains.

Superstructure steelwork has been fabricated by Watson Steel.

Once again Girling emphasises the team side of the project saying that Watson, and also precast concrete maker Buchan, which made the parapet units, were part of the team during the design stage. 'It is amazing what savings you can achieve if the fabricators talk to the designers, ' he says.

Steel deck sections were made up on the ground, which was easier said than done because space is tight around the site. But room was created by arranging alternative parking for vehicles using a busy council depot close to the site.

Sections were lifted in by crane, the largest a 262t lift carried out by a 1,000t Gottwald AK680-2, with a 195t counterweight ballast. That was over the railway in a night possession.

Night possessions will also be crucial for the replacement of the multiple span North Bridge.

Amec has started strengthening the foundations with mini-pile work around two columns of an existing centre pier. Later these will be encased within a beefedup pier which will support one end of a single 72.9m span across all the lines of the railway, currently interrupted by intermediate piers. A second 35.6m span will complete the link over the canal. The new deck will have continuous I section main girders linked with transverse girders. But that comes later.

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