Barnstaple's Western Bypass features not only a five-span concrete cantilever structure but a temporary lift-bridge at the end of a causeway for site access. David Taylor reports
IT HAS BEEN a long time coming but at last the people of Barnstaple, North Devon, are to get their Western Bypass.
Known to locals simply as 'the downstream bridge' over the River Taw, the Barnstaple Western Bypass is the final element in the plan for an u rban relief road f irst mooted back in 1976.
In the 1980s, the Department of Transport's trunk road scheme saw the completion of the North Devon Link Road between Barnstaple and the M5 at Tiverton. Shortly afterwards the A39 southern bypass relieved Barnstaple of through traffic heading to the neighbouring town of Bideford.
But traffic travelling between locations north of Barnstaple and areas south of the River Taw and the Link Road/M5 corridor still had to pass through the town centre. In the summer, especially, the town is choked with traffic as holidaymakers head for the resorts of Woolacombe and Ilfracombe and the popular surfing beaches at Saunton, Croyde and Puttsborough.
In 2004, 23 years after the Western Bypass was first approved by central government, the client, Devon County Council, cleared the final hurdle and put the job out to tender. In November 2004 the main contract was awarded to Edmund Nuttall and work started the following January.
The £27.9 million project comprises 2.7 km of singlecarriageway road and a 300 m long in-situ reinforced concrete bridge across the River Taw. Nuttall beat three other bidders to the design and build contract largely on the merits of its proposal to construct a temporary causeway and lift-bridge to allow site traffic access along the entire length of the project.
'The council was concerned that lorry movements th rough Barnstaple should be kept to an absolute m inimum, ' says Nuttall project manager Steve Brackenbury. 'And we realised that if we built the causeway and a temporary br idge across the river we'd be able to bring material in from both north and south and effectively double the number of deliveries we could make. It also meant we could get from one end of the site to the other without having to cross the river on the old Long Bridge in the centre of town.' The River Taw at Barnstaple is tidal, with water levels fluctuating by as much as six or seven metres between tides. At low tide the water recedes to leave a huge expanse of mudflat and sandbar below the town with only a narrow navigable channel running hard up against the northern bank.
Consequently the causeway (consisting of about 25,000 cu m of stone imported from a quarry a mile outside the town) extends from the south bank across more than half the river's width to the main channel. The temporary steel lift-bridge, fabricated to Nuttall's own design, was built at a shipyard in Appledore on the River Torridge downstream from Bideford.
'Halcrow did a lot of flood modelling to test the effect of the causeway on tidal flows, ' says Mr Brackenbury. When you consider that the causeway almost dams the river at this point it is surprising how little impact it has on tidal flows.
Besides providing a temporary river crossing, the causeway also provides a dry working surface from which to construct the four concrete bridge piers. Steel sheet piles are driven through the causeway to create cofferdams, within which the concrete pad foundations are built. This removes the need to use percussive piling methods, which had been ruled out by the client because of their harmful effect on wildfowl in the estuary.
Although a neat solution, the causeway option was a highrisk st rategy, according to Mr Brackenbury. 'It relied upon us getting two consents, one from the Environment Agency and another, known as a FEPA [Food and Environment Protection Act 1985] consent, from Defra, ' he says. 'We knew it could take at least 18 weeks, but we couldn't afford much longer than that. Any significant delay would have a huge knock-on effect.' The contract was awarded in November 2004 and the team immediately sprang into action to get the applications in for the two essential consents. 'We were still stuffing envelopes on Christmas Eve, ' says Halcrow's assistant site representative, Sara Leach. The gamble paid off and consents were secured in May, by which time the contractor had been on site for five months.
The first element of the contract was to build the southern interchange with the A39 and establish access along the route of the bypass. Most of th is is across former agr icultu ral land and will require extensive excavation to level the road. Where the road enters the fringes of Barnstaple, above the railway station, the structure becomes more complicated, with a 14 m-deep cutting required through the side of Sticklepath Hill.
A high water table means drainage is critical, so a series of cut-off drains and transverse drains are being installed to de-water the site. Another problem is shrinkage of the clay strata as water is removed. The cont ractor has to phase the deposition of fill material, limiting it to 2 m per week, to give the clay time to drain and settle.
Much of this groundwater is channelled nor thwards to Sticklepath, where the road interchanges with local traffic at a large roundabout.
('It's actually rectangular, so we call it the 'squareabout', ' says Mr Brackenbury. ) The level of this junct ion is designed to allow for futu re rail construction, and to allow this a new viaduct is being constructed to provide pedestrian and cycle access to the station and into the town centre.
At Sticklepath Fields the groundwater will be collected in a series of ponds and wetlands, which have the dual purpose of harbouring local wildlife and storing the water. Because of the high tidal f lows, water cannot be drained into the River Taw at high tides so it will be stored here and allowed to drain into the river at low tide.
The bridge itself is a five-span in-situ concrete balanced cantilever structure over 400 m long.
Nuttall's bid took advantage of the client's relaxation of highway alignment requirements to lower the height of the approach embankments and raise the curvature of the deck.
'This reduced the visual impact of the bridge, but mainly it meant we needed much less f ill for the approaches, ' says Mr Brackenbury. In fact, this proposal reduced the amount of fill by 250,000 cu m ? about half the specified amount.
Construction of the four concrete piers is now well advanced and work star ts on the br idge deck in October. Locals are bound to welcome the sight of their downstream bridge finally taking shape; in the late 1980s a survey recorded 92 per cent support for the scheme locally and that enthusiasm has not waned.
Although this is already a popular scheme, the const ruct ion team has spared no effor t to keep the community informed of progress, producing a regular monthly newsletter and delivering talks to local schools, the Rotary Club and other community organisations. Interest in the project is certainly keen: 'We've have about 30,000 hits on our website since May and over 10,000 people visited our webcam in June alone, ' says Mr Brackenbury.
No effort spared for otters, plovers, badgers and bats
THE BARNSTAPLE Western Bypass scheme secured planning perm ission in November 1999, but there were strings attached. English Nature and the Environment Agency demanded the preparation of an Ecological Monitoring and Mitigation Scheme to cover the periods up to the start of construction, construction of the works and five years following completion of the works.
The client had already started an environmental study of the scheme and, by the time the project got the go-ahead, had spent about £750,000 on environmental monitoring.
But even as late as 2001, objections by North Devon Friends of the Earth looked set to scupper the scheme.
'The objections were all about the site's status as a site of special scientific interest and whether it should be redesignated an Special Protected Area , which is an even higher level of protection, ' says Devon County Council site agent Pete Smith.
The Taw estuary is an important wintering ground for birds, in particular the golden plover. But following the closure of nearby Yelland Power Station in the 1980s the ambient water temperature in the Taw estuary fell and the plovers moved elsewhere.
Despite this, FoE took its fight all the way to the House of Lords, which ruled in favour of the scheme in December 2003.
'The site is still a SSSI and could still become an SPA one day, ' says Mr Smith.
Certainly, no effort has been spared to safeguard the area's wildlife habitat.
All the fencing on the scheme is designed to protect badgers and otters, preventing them from straying onto the road and guiding them to safety.
A 2.5 m diameter bat tunnel under the Sticklepath viaduct will provide access for 10 species of bat that visit the area from roosts up to 5 km away.
And a large area of salt marsh, excavated on the southern bank of the Taw to make way for construction of the temporary causeway and bridge abutments, is being stored and irrigated elsewhere on site to be reinstated when the bridge is finished.
THE PROCUREMENT ROUTE
IN EARLY 2002, the government decided the Barnstaple Bypass scheme should be assessed for its suitability as a PFI project.
'There seemed to be a preconceived idea that everybody wanted to go the PFI route so we had to prepare a business case, ' says Devon County Council site agent Pete Smith. 'But DCC didn't want PFI and our business case demonstrated that PFI was likely to cost more in the long run.' At £27 million, the 2.7 km-long scheme is hardly cheap but was nevertheless too small to make sense as a PFI project. The client proved its case and chose to let the scheme as a design and build job.
Bids were assessed on a 60:40 price: quality ratio. Although there was little to choose between the bidders on quality, Nuttall's bid came in 25 per cent lower than its nearest rivals, giving it an unassailable aggregate score.
The client was clear that this project should be undertaken on a partnered basis and Nuttall's low price was largely the product of value-engineered solutions stemming from close involvement with designer Halcrow and key subcontractors. Adoption of the causeway, reduction in fill requirements and a resulting contract programme of 105 weeks ? 25 weeks early ? clinched the deal for the Nuttall team.
Client: Devon County Council
Main contractor: Edmund Nuttall
Design engineer: Halcrow
Piling: Bachy Soletanche
Steelwork: Fairfield Mabey
Formwork: NRS AS (Norway)
Contract dates: Start March 2005, complete summer 2007