A66 Grade Separated Junction | Long Newton, Teesside

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



Reduce settlement on highways and motorways

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

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® Lightweight Expanded Clay Aggregate, 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® Lightweight Expanded Clay Aggregate which was completely encapsulated within a further geotextile membrane.

More than 5,000m³ of Leca® Lightweight Expanded Clay Aggregate 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.

A senior member of Amey’s design team explained: “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.”

The use of Leca® Lightweight Expanded Clay Aggregate, which has just one-fifth the density of conventional general fill, meant that the projected post construction settlements were within the required limit.

A further benefit of the use of Leca® Lightweight Expanded Clay Aggregate is that much reduced earth pressures are generated on the rear face of the abutment walls. This means that a significant saving in steel reinforcement can be made over that which would have been required using conventional fill material.

Three departures from Highways Agency standards were required in order to use Leca® Lightweight Expanded Clay Aggregate for the project. These were approval of the material for use as general fill; for its use as backfill to structures, and for the omission of a drainage layer which was not necessary due to the free-draining characteristics of the Leca® Lightweight Expanded Clay Aggregate.