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Bridge & Scour Protection in Somerset using PA6 Woven Gabions

Gabions_Dunball_Maccaferri

Somerset County Council works with Dyer & Butler to repair the Dunball New Bridge, where the pillars were at risk due to the floods and the channel became constricted

Maccaferri supplied PA6 Woven Gabions as a solution to minimise the effect of the damaging turbulent flows, and prevent future hydraulic erosion. PA6 coated woven gabions provide up to a 120 year design life, the ultimate solution.

When the Sowy-King’s Sedgemoor drain (KSD) scheme was designed in the 1960’s, it was recommended that the original A38 bridge at Dunball (now the southbound carriageway to Bridgwater) would be replaced at a future date with a wider span to address a significant narrowing of the King’s Sedgemoor Drain at this location. This was not achieved and today there is a constricted channel of about 200 m in length, just upstream of Dunball sluice.

Dunball sluice is the only outlet for the King’s Sedgemoor drain (KSD) into the River Parrett and hence out to sea, when tidal conditions allow, or pumps are used. Therefore, it is important for the Somerset Levels and Moors that water should be able to flow through Dunball as smoothly as possible.

The damaging effects of this were felt during the exceptional floods of 2013/14 a short distance downstream, where scouring of the foundations occurred and the potential weakening of the pillars under Dunball New Bridge, which carries A38 traffic northbound.

To repair and avert possible future problems, work under Dunball Old Bridge was followed by a Somerset County Council scheme to repair and strengthen Dunball New Bridge. WSP/PB designed the solution which consisted of a double layer of 300 mm thick gabion baskets to protect the piers and the channel bed. The bridge bank was also reinstated with a gabion retaining wall structure.

PA6 coated double twist mesh gabions were chosen to provide a long-term solution that was also inherently flexible to accommodate any future potential differential settlement. The gabion units are already heavily galvanised, but the additional polymer coating is important in these aggressive hydraulic environments. The gabions are designed to protect the channel bed from the erosive turbulent water forces. The bridge structure has restricted headroom which will constrain the lifting and placement of riprap. The gabion units were supplied prefilled with rock as completed units ready to be lifted into the works, providing the added benefit of reducing the construction time.

The solution will minimise damaging turbulent flows encountered in the outlet channel to Dunball tidal sluice. This improvement was a project priority to avoid future erosion problems.

Download this case history to learn more about the project.

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