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As part of the £8 million Blackbrook Diversion scheme for the A58 near St Helens, planners routed the new 1.3 km single carriageway through an environmentally sensitive area with very poor and diverse ground conditions.

To help stabilise the ground and minimise differential settlement, Birse Civils used Tensar geogrids as low impact solutions to reinforce the road sub-base in the approaches to the box culverts required for nine small watercourses in this area; as well as in constructing the walls and abutments using the Tensartech retaining wall system.

“The complexities of the site required careful planning, not least because it runs through one of the most environmentally important areas in St Helens, with a local nature reserve, site of special scientific interest and semi-natural ancient woodland nearby,” says Tensar Area Civil Engineer, Craig Roberts.

Part of the problem was mine workings which required extensive grouting and capping. The project also called for St Helens Council to implement a coherent environmental masterplan for the whole area, including re-introduced ponds and wetland areas. This formed a Sustainable Urban Drainage System which alleviated the flood risk from road run-off.

Project co-ordinator Dave Whalley of Birse Civils commented: “A particular benefit of using Tensar geogrid solutions for the sub-base around the watercourses and the walls and embankments, was that we could build up the approaches and embankments over this very soft ground, quickly and without additional civils work, such as piling. We were also able to strip the topsoil, and build up the sub-base and embankment with the geogrids and imported granular fill, which allowed a period of settlement while we worked on the rest of the project. We then returned to complete the road over the sub-base.

“Looking at sustainable technologies is also part of our procurement policy, and Tensar solutions offered excellent benefits in terms of cost and time savings compared to conventional road building techniques.”

For the highway approaches to the culverts, Tensar geogrids were used to reinforce the sub-base layer, reducing its thickness by nearly 30%. To support the concrete box culverts – which carry the watercourses below the highway – the contractors constructed wingwalls to the embankments using the Tensartech TW1 Walling system, which has been BBA certified for a 120 year design life specifically for highways.

The wall faces were constructed from TW1 split face wall blocks, dry laid along the water course.and special connectors fasten the blocks to the uniaxial geogrids which are used with local fill to build the 1.5 metre high embankments at either side of the culvert and roadway.

Site clearance commenced in February 2006 and upon completion (scheduled for May 2007) the Blackbrook Diversion scheme will not only enhance the local environment for the residents, but also re-establish and protect the local ecosystem for a sustainable future.

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