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The relief of Taunton

CONTRACTS

ONE OF the big culprits for congestion on the main entry roads to Taunton in Somerset has been a level crossing where traffic clogged at the barrier for about 25% of the day.

Somerset County Council commissioned Atkins for the design and construction of a bridge, part of which needed 8m high embankments to carry the A3065 over the Paddington to Penzance railway line.

The ground features soft alluvium and sits on a flood plain with three distinct layers: 5m of alluvial clay over 6m of alluvial gravel above a thin layer of reworked Mercia Mudstone.

One of the main areas of concern was the compressibility and strength of the alluvial clay. Atkins realised the embankment could not be built in one lift without causing bearing capacity failure through the soft alluvium, so the team devised a two-stage construction process with 4m lifts.

Pennine and Cofra UK built a series of band drains along the site to speed up the consolidation of the soft clay. These were on a grid ranging from 1m-1.6m, according to the underlying thickness of the clay.

Buried concrete slabs and old foundations on the northern side required a more substantial Cofra piling rig to puncture the material.

Standard industry procedures predicted 170mm settlement in the area of maximum fill. However, given the difficult nature of the ground and as a precautionary measure, Atkins monitored this by placing 12 settlement plates at about 40m intervals along the embankment.

Adjacent to these plates, the team installed vibrating wire piezometers to measure pore pressures as the embankment load was applied and their dissipation to confirm consolidation settlement. But a lack of saturation and layers of sand causing excess pore pressure to dissipate faster than could be measured, so the piezometers did not respond as predicted.

At the end of the first construction stage, measured settlements had already reached 188mm, indicating that total settlements were likely to significantly exceed predictions.

Atkins immediately re-evaluated its initial findings to ensure full consolidation settlement took place. Now the second stage of filling has been completed, site workers will finish the surfacing and drainage.

The project began late last year and the bridge should open to traffic later this month.

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