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Joined up thinking

Tensar International geogrids played a major part in the construction of a road bridge over a railway line at Locking Castle, Weston-super-Mare.

The single span bridge will link two new housing developments on either side of the railway line.

A 20m thick alluvium layer underlies the site and so foundations for the two S-shape approach embankments and the abutments were piled. The original plan was to build a reinforced concrete raft over the piles beneath the abutments but instead main contractor Dean & Dyball commissioned Tensar to provide a design and supply package for the load transfer platform as well as for the reinforced soil bridge abutments and approach walls.

The load transfer platform is a 900mm thick layer of granular fill reinforced with three layers of Tensar biaxial geogrid. The geogrid forms compressive zones in the fill which transfer the vertical loads on to the piles, dramatically reducing the vertical load acting between them.

A Tensar Basetex high-strength geotextile was used as a lateral restraint to control the lateral forces coming from the walls, which meant the piles did not have to be reinforced against horizontal forces.

For the retaining walls, the Tensar TWI link wall system was chosen. The drylaid modular block retaining wall system comprises concrete blocks, geogrid and connection, allowing a fascia of masonry or brickwork to be tied to the structure.

The system, which was recently awarded a British Board of Agrement roads and bridges certificate, has a 120-year design life.

A total of 1200m 2of walls were built at Locking Castle.Tensar says the system is flexible enough to cope with changes in the wall layout and the S-shape of the embankments. And because the walls are dry-laid, the extreme weather at the end of last year did not affect construction, which was finished within the original programme.

Tensar biaxial geogrid was also used to reinforce the temporary site access roads to the bridge construction site because of very poor ground conditions.

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