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Weber - Saint-Gobain

A66 - Long Newton

Consulting engineers faced with the challenge of limiting settlement beneath a £12 million embankment and bridge over the A66 dual carriageway at Long Newton, near Darlington, specified Leca® lightweight aggregate.

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Deep glacial deposits, including some soft lake sediments, meant that the loadings of the structure over the A66 had to be strictly limited. While the bridge was piled, a geotechnical solution to reduce settlement was required for the approach embankments which carry the road, allowing villagers easier and safer access to the A66.

Following an analysis by geotechnical specialists based at the company's Lewes office, consulting engineers Amey developed a solution involving the use of pulverised fuel ash (PFA) for the majority of the approach embankments and a wedge of Leca®, placed in layers, behind the bridge abutments.

In order to accelerate consolidation settlement beneath the approach embankments, band drains were installed, together with a drainage blanket and geotextile separating membranes. Near the bridge bunds of PFA and general fill along with a geotextile membrane were gradually built along the flanks of the embankment in order to contain the Leca® which was completely encapsulated within a further geotextile membrane.

More than 5,000m³ of Leca® was placed during the construction phase. The client, the Highways Agency, imposed an onerous limit of 20mm post-construction settlement adjacent to the bridge in order to avoid differential settlement between the embankment and the bridge.

Explains a senior member of Amey's design team: "Due to construction deadlines we were not able to use general fill and wait for it to settle. The ground conditions also meant that we had to limit loadings as much as possible."