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UP TO SPEED

Three new bridges, the alteration of another and extension of cuttings and embankments are all part of the project to upgrade 10km of the A1 in West Yorkshire to motorway standard.

Atkins Geotechnics in Leeds is undertaking the geotechnical design for the Highways Agency's Bramham to Wetherby Early Contractor Involvement project.

This work, together with the provision of a new local access road, ensure plenty of variety for people like senior geotechnical engineer Richard Deakin.

'We are providing the geotechnical and foundation preliminary design for the three new bridges and working closely with the structures design team from Atkins Highways & Transportation, ' he says.

Two of the new bridges - Walton Road and Wetherby Grange - are being designed as integral structures. 'There is a current drive towards integral bridges, ' Deakin explains.

'Obviously the bridge deck expands and contracts with temperature variation. As the total deck expansion can be somewhere between 20mm and 40mm, the structures have to be designed to cope with these changes.

'As part of the abutment foundations we are using steel H piles which are strong enough to support the structure, yet flexible enough to accommodate the deflection without being damaged.'

These two structures will carry small local roads across the A1, but the River Wharfe Bridge is much larger. Two existing bridges over the River Wharfe independently support the north and southbound carriageways.

These will be joined together to form one structure for the northbound carriageway and a new bridge will be built alongside for the southbound carriageway.

'The new bridge will be founded on spread footings on Magnesian Limestone, ' Deakin says.

'Dissolution features have been identified within a zone of the limestone surrounding the river. Some of the rock has been dissolved due to localised water flow, leaving small voids, which have resulted in a reduction of the rock's bearing capacity.

'As a result, we may have to consider grouting works to strengthen the rock, which will avoid the need for piling.'

The fourth structure is the extension of a pedestrian walkway passing underneath the A1 in a box culvert.

'We are proposing to widen the main carriageway at this point so will need to extend the existing box culvert underneath it, ' Deakin says.

'Differential settlement is our biggest challenge on this part of the scheme.'

The problem facing engineers is that the 4m high embankment needed to extend the box culvert is considerably heavier than the concrete structure itself.

This means the ground beneath the embankment will consolidate and settle to a lower level than that beneath the culvert - something the engineers have to take into account.

'To avoid potential problems with differential settlement we are considering the use of stone columns beneath the embankment, ' Deakin explains. 'These will both minimise the amount and accelerate the rate of settlement beneath the embankment by providing additional drainage paths.

'With the use of stone columns or sand drains, settlement that would normally take four years can be accelerated to be complete within the construction period.'

The same strategy is being applied to differential settlement issues on an approach road to the Walton Road Bridge. The design proposes building the new part of the embankment on a variety of compressible materials.

'When we construct the new embankment we will deal with differential settlement using a combination of stone columns, sand drains and surcharging, ' Deakin says.

The new local access road designed by Atkins is a relatively straightforward shallow cut and fill operation, but Deakin says it illustrates the sustainable approach at the forefront of the company's thinking.

'We need a careful balance of cut and fill. In the earthworks strategy with Balfour Beatty, we need to make sure we make optimal use of materials on-site to ensure a sustainable approach - it's all about recycling.'

The project is a big one for Atkins. It draws on the skills of numerous disciplines as well as the geotechnics division from Atkins Environment.

Other Environment personnel - ecologists, landscape designers, air and noise staff and environmental impact assessors - are involved along with the highways, structures and transport planning teams from Atkins Highways & Transportation Division.

Atkins is also working closely with main contractor Balfour Beatty to ensure buildability is considered at this early stage of the project lifecycle.

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