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A1 For Technique

Steel bridges are being positioned across the busy A1 at Blyth as part of a plan to remove roundabouts from the route.

Efforts to ease congestion on the busy A1 at Blyth are now in full swing, with a 30m long steel bridge being constructed over the dual carriageway, replacing a busy roundabout.

"There is quite often a lot of queuing and delays on this stretch and the works will alleviate this by allowing motorists on the A1 a clear run without roundabouts," explains Dave Abraham, construction manager for main contractor Interserve.

At the Blyth A1/A614 interchange near Retford, as with another five junctions, between there and Peterborough, the new bridge is being constructed to link new roundabouts either side of the dual carriageway.

To reduce disruption and speed up the work, the new bridge was fabricated on an adjacent site, at the same time as the abutments were being built. Once both were finished the bridge deck was transported as one large structure over a distance of 200m and jacked into its final position.

While the bridge abutments were being readied, steelwork contractor Fairfield Mabey was busy assembling the bridge steelwork on a temporary platform to the side of the new bridge position. The composite steel and concrete deck consists of six longitudinal steel beams, each 1.2m-deep arranged in three braced pairs forming a 30m-long, 13.5m wide structure. It was assembled 1m above grade and this also had the benefit of minimising working at height.

Once the entire bridge deck was assembled, along with the concrete decking and edge protection, the lifting, jacking and transportation process was set in motion, transferring the structure to its final position.

"Luckily there was space close to the bridge to construct the deck," comments Richard Selby, project manager for Fairfield-Mabey.

"Interserve didn't want to transport it too far in order to minimise time and any potential damage to the bridge."

Interserve employed heavy lifting specialist Mammoet to carry out this procedure.

The assembled deck weighed 550t and to move this huge section into place Mammoet used its self-propelled modular transporters (SPMTs).

These remote controlled multi-axle units transported the deck over levelled ground to its final position where it was jacked up and lowered onto the abutments.

In the end the whole transportation and jacking process was completed in a single night time operation. "The whole process went very well," comments Abraham. So well in fact that a similar operation will be carried out at the Colsterworth A1/A151 and Carpenters Lodge A1/B1081 junctions in April.

Colsterworth requires two steel bridges, one for the junction and one which will replace a troublesome A1 cross-over.

The other three steel bridges on the A1 are being erected in a more traditional manner.

Markham Moor, the A1/A57 junction, has been the most complex. The new steel bridge has been constructed in the middle of the existing roundabout, with live traffic circulating around it day and night.

Once complete, the A1 will run right through the Markham Moor roundabout with new junctions either side of a new over bridge. After the bridge abutments were constructed Fairfield Mabey brought the steelwork to site and erected braced beams in pairs.

"It's the location which makes this bridge so complex," says Abraham. "Getting the equipment and materials to site has been challenging.

But if the bridge was built with concrete we would have had twice as much traffic and materials going to site, adding to an already busy and at times congested junction," he says.

Keeping disruption to a minimum, with the least amount of closures and contra-flows, is key to the whole scheme. "Using steel and utilising the SMPTs has ensured we've meet this target," sums up Abraham.

All junctions are scheduled to be completed by October 2008.

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