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Durweston flood arch replacement

This year, Dorset Highways will be undertaking major construction work to replace three flood arch structures on the A357 at Durweston, near Blandford.

Between each of the flood arches a series of embankments support the road, and together they form the causeway across the flood plain.

Recent inspections have identified that the flood arches are deteriorating at an increasing rate and should be replaced to improve the safety of road-users. The current brick arches will be replaced with around 30 pre-cast concrete units, each weighing 13 tonnes, which together will form the three new flood culverts.

Deterioration of the embankment slopes is also beginning to undermine the highway. These slopes will be re-graded during the project – with the footprint of the embankments widened and slope of the embankments slackened – to provide appropriate support for the road.

Dorset Highways will start building the site compounds and works access roads from Monday 18 March.

The arch replacement and embankment modifications will improve the safety of this section of A357 and lower ongoing maintenance costs for the council.

The half-a-million-pound scheme will be completed, with the compounds dismantled and access tracks removed, in September 2019.

What is the work?

Work consists of:

  • installing concrete foundations and scour protection for each of the new structures, on both sides of the highway
  • demolishing three bridge structures
  • building one bridge structure consisting of three adjoining pre-cast concrete culverts, with a total span of 11m
  • building one bridge structure consisting of two adjoining pre-cast concrete culverts, with a total span of 8m
  • building one bridge structure consisting of a single precast concrete culvert, with a total span of 3.6m
  • reprofiling 225m of embankment on both sides of the highway, these will be retained using gabion baskets
  • building new road surface, including providing 2m wide verge on both sides of the carriageway
  • installing new galvanised steel vehicle parapets for each of the new structures, and realigning 225m of existing iron rail fencing

The new precast reinforced concrete culvert units will be installed in the same position and on the same alignment as the existing arches. The height of the road and adjacent land will not change, but the footprint of the new structures will be wider than the existing ones so that new verges can be provided.

The embankment slopes will be re-vegetated with grass and the newly created verges will sown with a native grass/wildflower mix.

Will the road be closed?

For the majority of the work, there will be some short periods off-peak stop/go or temporary signals as required for deliveries and safe vehicle movements.

There will be a road closure on this part of the A357 from 22 June for four weeks – with a diversion route signposted along the A350.

The closure is needed as the road and flood arches will be demolished so the new culverts can be installed.

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