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An increasing number of visitors to Wigan has led the borough’s council to invest in a new £6.5m dual carriageway, opening in May 2007.

The Westwood Link Internal Access Road is being built to ease traffic congestion entering and leaving Wigan. “Threading the construction of a widened route through a historic city centre is always difficult,” says Brian Farrington, Technical Business Manager for Birse Civils Ltd (part of the Balfour Beatty Group). “But we encountered a specific construction bottleneck where the road crosses the Leeds/Liverpool canal.”

As part of the new road, the contractors had to rebuild the embankment and retaining walls supporting the Henhurst Bridge road, which runs over the canal and through the adjacent development area. Here, Tensar Technology provided a low environmental impact solution which overcame the associated ground problems quicker and less costly than conventional solutions.

The extensive site problems included: the soft, very low load-bearing ground; close proximity of buildings; underground services; traffic management; completion within a tightly specified time frame; potential cost of conventional sheet pile and reinforced concrete solutions; the environmental impact on the canal and the adjacent River Douglas; sourcing an environmentally sustainable solution and the impact on the local built environment.

Specified solution

Birse and their designer Faber Maunsell specified the Tensartech TW1 retaining wall solution as a value engineered alternative to the proposed sheet piled and reinforced concrete method. The Tensar option provided proven solutions to the above concerns, as well as being a very flexible system that could accommodate interfaces with underground services. In addition, it was some 50% less expensive than the original design detail, offered various aesthetic finishes to complement the surrounding buildings and provided a more sustainable procurement process.

“By selecting this particular Tensar developed construction method, Birse was able to comply with all the constraints and meet the concerns of the parties interested in the project, from Wigan Borough Council, through to the Environment Agency and British Waterways,” says Brian Farrington.

The flexibility of the Tensartech system accommodated the inevitable variations encountered in a complex project in constrained urban conditions. In addition, use of the aesthetically finished split face blocks ensured that the local built environment was enhanced, plus in other areas the “link” block was used to allow an external, masonary cladding to be added in order to visually fit in with existing structures.

“The invitation from Birse Civils to develop a specific design for the embankment retaining walls – affecting approximately 500m of the highway – was challenging but within the capabilities of Tensar technologies,“ says Craig Roberts, Area Civil Engineer for Tensar. “By being asked to contribute at a very early stage in the project, Tensar could make maximum impact to ensure the success of the construction by overcoming the numerous constraints and concerns presented by the project.”

Construction phase

In the construction of the embankment and walls, the contractor built up the layers with granular fill, reinforced with layers of Tensar uniaxial geogrids, which were secured to the TW1 block face with full strength polymer key connectors. The split face blocks were coloured to produce a specified sandstone effect to complement and blend with the surrounding buildings.

The flexibility of the system was demonstrated when Birse needed a temporary works solution to support the adjacent live road and redirect traffic in the constrained space. In order to minimise disruption to the public Tensar designed, supplied and installed a reinforced soil solution using steel mesh panels (SMPs) as a temporary facing, within just two days. The panels were securely fastened to the Tensar geogrids using a bodkin connection, providing a structure stable enough to support the redirected traffic.

Low environmental impact

The Tensartech TW1 wall system combines the cost effectiveness of reinforced soil retaining walls and bridge abutments, with the aesthetic appeal of an attractive block finish, designed to meet the exact requirements of the local environment.

The system – which has BBA fitness for purpose certification for roads and bridges over a 120 year life – was put forward for this particular project as it seemed the ideal solution for the highway retaining walls. It also eliminated the need for sheet piling and full height reinforced concrete facings which are costly and more difficult to plan around, especially where there is the added complication of underground services.

In addition it has low environmental impact through its ability to utilise low grade granular or locally won fill, where appropriate, and so met Wigan Borough Council’s sustainability requirements.

Project background

Wigan is historically an industrial city, with long established links with the mining and engineering industries and transport for these via canal. The city centre layout reflects the way it grew to meet the needs of 19th century industry – rather than through being planned from the start. In the early 21st century its needs have changed and Wigan is now a considerable tourist attraction. To make the centre a more congenial environment for residents, workers and visitors – and open up potential industrial and manufacturing sites – Wigan Borough Council is implementing many measures, one of which is improving the traffic flow.

The contract to improve the inner relief road and facilitate the eventual link via the A5225 to the M6 was awarded to Birse Civils Ltd in 2003, following approvals of their plans for the project. Tensar has been involved in the design side since December 2004 and in construction phases from May 2005 through to the end of 2006

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