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Cause for celebration

Bridges - Barnstaple bypass

The decision to build a temporary causeway is paying off for the design and build team at the Barnstaple Western bypass, as Margo Cole discovered.

It is a year since NCE visited the site of the Barnstaple Western bypass, a 2.7km long road scheme that, it is predicted, will cut traffic volumes by half in the north Devon town (NCE 28 July).

At that stage, construction had been under way for nine months.

The most striking feature of the project was the temporary causeway that contractor Nuttall had built across the River Taw to make it easier to build the project's main structure - a 408m long bridge spanning the river.

The benefits of the causeway are now apparent as construction of the five-span balanced cantilever bridge is well under way. 'I think we were the only contractor to go for a causeway, ' says project manager Steve Brackenbury. 'The alternative was a temporary jetty.'

There is a short stretch of jetty from the north shore to a temporary lift bridge that spans the river's navigation channel, which gives a good indication of the difficulties that could have resulted if the entire bridge had to be built from such a narrow platform.

'Can you imagine trying to turn articulated vehicles on such a narrow jetty?' says Brackenbury, as we watch a short queue of construction vehicles form.

'All we had to do was bring in 50,000t of muck and end tip it and we've created a massive area for storage and a working platform. It is 32m wide at the bottom.

'There is space to turn vehicles around and the cofferdams for the pier foundations were built from a dry causeway rather than in the water. The only risk was getting approval from the Environment Agency.' Chris Triggs, site representative for Nuttall's designer Halcrow, says: 'We carried out extensive remodelling of the river with the causeway in.'

This included studying how sand banks would develop up river of the causeway and how they would reduce once the temporary structure was removed - as well as the effect on the flow of the water and its quality. 'Obviously it does result in a much faster flow of water through the navigation channel, ' says Triggs.

According to Paul Fewtrell, Nuttall's deputy project manager, the main concern for the Environment Agency was that the causeway might make the Taw more likely to flood. 'Our original intention was to have the causeway at +6m AOD, ' he says. 'The Environment Agency asked us to drop that to +5m, so that if there was an exceptional high tide it would weir over the causeway.'

That situation occurred earlier this year when an onshore wind combined with heavy rain and a high tide. 'We knew that the conditions would result in the causeway being covered and we had measures in place for that, ' says Triggs.

Brackenbury says: 'The modelling during the tender period and after the contract award indicated that 6m would be okay and Barnstaple wouldn't flood. The Environment Agency's requirement to drop that to 5m was really belt and braces.'

The Nuttall/Halcrow design and build team changed Devon County Council's original design for the ve-span Taw bridge to make the hog steeper - particularly on the south side - resulting in lower embankments and a saving of half a million tonnes of imported ll.

Another design change was to found the bridge piers on mass concrete pad foundations rather than piles. The rock rises across the site from south to north, so piers ve and four (nearest the north shore) are founded on rock, while piers three and two are on gravels. Both abutments are piled.

Foundation construction was completed last year and all four piers are now in place. The first filI-section cast in situ concrete piers - measuring 6.8m in width and up to 1.5m thick - were built using conventional formwork, as were the box-section pier heads that sit on elastomeric bearings.

However, most of the deck is being built using the balanced cantilever method. Each of the bridge's spans is approximately 80m in length and is being built with nine cast in situ concrete segments either side of the four piers. Only the half spans from each abutment are being built with conventional formwork.

On the landward side of each pier Nuttall has installed a pair of temporary legs in the form of large steel tubes stressed down into the base of the pier and into the soft of the pier head, and braced back to the pier. 'They act as a tie as well as a prop, ' says Triggs. 'We pour on the landward side first and at that stage they act as a prop. Then, when the other side is cast, they act as a tie to hold down the deck.'

The hollow-deck sections vary in depth from 2.24m deep at centre span to 5.24m at the pier heads. The concrete mix also varies, with C40/50 concrete near the piers and C50/60 for the six sections at the centre spans. The sections are cast using an overslung gantry supplied by specialist NRS.

Nuttall is achieving a nine-day cycle for each pour, with the pours varying from 40m 3 to 80m 3. A strength of 35N/mm 2 is needed for the post tensioning, which is usually achieved about 24 hours after pouring. Every segment is post-tensioned, apart from the stitch sections at the span centres.

'Before we make the stitches we will jack the entire deck back 90mm to the south abutment to allow for shrinkage and creep, ' says Triggs. 'The bridge has a xed bearing at the north end, but the south abutment is where the expansion joint is.'

The first balanced cantilever deck section, consisting of nine segments either side of pier five, has just been completed and pier two casting is due to nish at the end of July. The structural components of the bridge will be completed by the end of this year, with the entire £27M bypass scheduled for completion in May 2007.

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