Barrett Byrd Associates

Bridges for bats, a canal diversion and a new steel river crossing designed to mimic concrete construction all feature on a short but challenging highway upgrade in Wales. Mike Walter reports

Single carriageway with central overtaking lanes is gradually being phased out in favour of dual two lane carriageway along 43km of the A465 between Abergavenny and Hirwaun in south Wales. A second, 5.5km phase of the upgrade is under way and although this is not likely to be the most complex stage, the design and build team of Nuttall and Gifford is still facing some challenging tasks.

The ‘Heads of the Valleys’ route is being enhanced through eight separate contracts, numbered in sequence from east to west. Nuttall and Gifford are currently working on section one. Some tricky engineering including cutting back rock slopes in environmentally sensitive landscape will follow in steeper sections further west. In section one, through the flood plain of the River Usk, the main emphasis is on widening and building structures in harmony with the local environment and its wildlife.

These include an existing concrete Usk crossing that will remain in place and is to be complimented by a new structure built alongside, identical in appearance, but different in design.

“We have been asked to design a bridge that mimics a post tensioned concrete deck structure that was built with different design standards in the 1960s,” says Gifford technical director Adrian Palmer. The concrete structure is around 60m long, is supported by two ‘V’ formation piers and has a slender form which reduces down to a depth of 700mm at mid span. Nuttall and Gifford have elected to use steel composite construction to achieve the same slenderness of the deck.

The new bridge, adds Palmer, “has been a huge challenge to design. We have had to over-engineer and add weight in parts of the new steel bridge to meet the slender design required. The crossing of the River Usk is a pleasant spot and local people told a public inquiry that they wanted to see an equally slender structure built alongside.”

Nuttall and Gifford are upgrading section one – the second phase to be tackled – of the Heads of the Valleys project, through a £23.8M target cost contract on behalf of the Welsh Assembly Government. Work began on site last June and the section one upgrade is due to be complete by the summer of 2007.

Overall, the 43km route is being upgraded with overtaking lanes removed to improve safety and capacity. The road passes beside the Brecon Beacons National Park and features three lane highway with steep gradients, often congested with cars caught behind heavy goods vehicles.

Alignment of the improvement to section one is largely designed to straighten out curves, with widening taking place to either side of the highway. Mostly in the Usk flood plain, the ground contains a variety of glacial tills, sands and gravels which extend down to depths of up to 26m before bedrock is reached.

Around one fifth of the existing pavement is to remain in place – overlaid with a fresh asphalt surface course – and traffic is being switched regularly from one side of the existing carriageway to the other to accommodate road construction. Vehicles currently make use of temporary ramps installed to accommodate changes in height between the old and developing sections of highway.

With the exception of the new steel Usk crossing, bridges which cross and carry the road are, in the main, to be demolished and replaced with larger structures. The Usk and surrounding area is a ‘Special Area of Conservation’ and as such, foundation piles for bridge piers can only be installed during short periods of around four weeks in the spring and autumn so as not to unduly disturb sea lamprey, otters and Atlantic Salmon.

Further to the west of the Usk bridge, a second waterway crossing takes the A465 over the Monmouthshire & Brecon Canal. The alignment of the new route would have made it very difficult for the narrow canal crossing to remain in place, so the project design team set about re-routing a 350m length of the waterway so that it would cross perpendicular to the road and achieve a more favourable sweep beneath the dual carriageway.

Nuttall’s design manager Ivor Barbrook says: “We had been advised by British Waterways that narrow boats often have difficulty negotiating the narrow approaches to the crossing and passing beneath the road, so the existing crossing is being replaced. We are widening the canal approaches to 8m in places and the new crossing will feature a box structure to be finished with a classic arch to mimic those used for traditional canal bridges.”

A section of canal either side of the road has been drained and a large rock hillside to the south of the road is being broken using heavy plant to allow for a the canal to be widened. Up to 80,000m3 of sandstone won from this part of the site is being crushed and graded using mobile plant for reuse in the foundation layers of the road upgrade.

Other major structures along the route include a new pedestrian underpass to replace an at-grade crossing and a new roundabout at the western end of the upgrade to accommodate a local road crossing and relieve acute traffic congestion.

Motorists, pedestrians and narrow boaters are not the only communities which are being considered on the upgrade to section one of the A465. A colony of Lesser Horseshoe bats that roost to the south of the route and fly across the road to find food have to be protected to ensure their safety and that of motorists.

“Bats tend to follow hedgerows when they fly and as we are changing the topography of the landscape by removing trees and hedges, we have had to introduce mitigating measures to ensure they fly where we want them to,” says Ivor Barbrook. These measures include creating or extending a series of ‘bat culverts’ beneath the roadway and installing two ‘bat bridges’ above it.

Each ‘bat bridge’ is constructed from wire rope mesh strung between steel supports and is designed to guide the bats into a channel that rises up above the road at a safe height of 6m. The mesh structures are being installed where bridges have been demolished and alongside which bats would normally fly. Fencing is being installed beside the road at strategic points to guide bats from the hedgerows into their designated crossings.

Adrian Palmer of Gifford adds: “Bats are a protected species and there is increasing provision of bat mitigation measures in modern highway schemes. There are examples of similar arrangements on the A303 road improvement in Wiltshire and much research has gone into seeing how effective these measures are.”

Fact Box:
A465 Heads of the Valley Dualling – Section One
Design & Build partners: Gifford and Nuttall
Client: Welsh Assembly Government
Client’s representative: Jacobs Babtie
Environmental consultant: TACP

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