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Jam dodgers

Barnstaple bypass - A temporary causeway over the River Taw is helping Nuttall avoid a traffic clogged town centre during a huge muckshift. Jackie Whitelaw reports from Barnstaple.

Anyone heading for the north Devon beaches this summer will know the town of Barnstaple is an obstacle to be avoided, even if it means a lengthy detour through switchbacking minor roads.

If you are travelling on the A361 North Devon link road from the M5, or the A39 from Bude and Bideford, and want to get to Croyde or Lynton you have to go through Barnstaple, along with all local traffic. The one way system takes you over the River Taw twice. The detour and heavy traffic can add an hour to your journey.

So locals and visitors are enormously enthusiastic about Devon County Council's new Barnstaple Western Bypass, now under construction by the design and build team of Nuttall and Halcrow.

The 2.7km, £27M road has been almost 25 years in the planning. The single carriageway scheme curves round the town on the banks of the Taw, and crosses the river on a 408m long, five span, insitu cast concrete, balanced cantilever box girder bridge.

At £10,000 per metre, Barnstaple bypass is far from cheap. Its cost is explained by the technical complexities involved. In addition to the Taw crossing there is a three span 60m viaduct that carries the road over the disused Barnstaple to Bideford rail line, and requires piled foundations in a buried river valley. There is also a significant box culvert that carries a stream and otter path, two concrete underpasses, a bat tunnel and a huge 14m deep cutting. And add to that two new junctions connecting the bypass to existing roads and a significant roundabout in the middle.

Even so, Nuttall's tender was 25% lower than its nearest rival.

'We did an awful lot of value engineering in the tender period, ' says project manager Steve Brackenbury.

One bit of value engineering had been available to all tenderers however. Devon County Council had a very clear idea of what it wanted built.

The bridge has to provide a minimum navigation height for boats on the Taw. A normal deck gradient of 1%-2% would have dictated that the road either side of the bridge approached on high embankments. To reduce the embankment height, and therefore the volume of fill that had to be imported to site, the council agreed to increase the hog on the bridge to 5%.

'We had originally planned to allow overtaking on the bridge which required a flatter profile for the sight lines, but opted instead for a single white line down the middle and no overtaking so the profile could change, ' says the council's site representative Pete Smith.

That saved 250,000m 3 of material and countless lorry trips, but still left 500,000m 3 of fill to be delivered. To keep the trucks off Barnstaple's already over-congested roads, Nuttall proposed constructing a temporary causeway over the Taw, with a temporary lift bridge spanning the navigation channel.

This would enable the bypass itself to be used as an access road.

'The council had put a very stringent restriction on lorry movements in the town. They were only allowing 10 an hour.

When you are trying to move 500,000m 3 it makes you think, ' says Brackenbury.

The causeway is under construction at the moment, stretching out from the north bank of the Taw. At full height it will be 5m high. On Monday the lift bridge was craned into position.

The causeway has also made construction of the bridge's pier footings much simpler.

Cofferdams within which its pad foundations will be built are incorporated into the causeway rather than being out in the water.

Pad footings enable Nuttall to limit the amount of piling, which the client has banned between October and April because it would disturb over wintering birds.

'And nor could it take place between April and October because of migratory and nesting birds, ' says Halcrow principal engineer Philip Paterson. 'There are a few windows however, ' he adds wryly.

The job started on site in January and is due to complete in May 2007, when 70% of the traffic currently travelling through Barnstaple centre will be able to avoid it.

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