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Forth Bridge works under way

MARINE WORKS started in October on the 1.18km e180M Upper Forth (estuary) Crossing at Kincardine near Edinburgh in Scotland.

Piling contractor Seacore has started drilling 20 marine piles into the estuary's bed after a marine ground investigation found the soils to be a layer of alluvium and glacial deposits followed by layers of silt, sandstones, mudstone and occasional coal seams.

'The site investigations show a highly complex geological situation both within the alluvial glacial deposits above the rock and within the upper levels of the rock, ' said John Cathro, deputy engineering manager for the main contractor, a Morgan Vinci joint venture. 'It's certainly given the designers something to think about.'

Seacore has started drilling reinforced concrete piles from 3m to 3.8m diameter for the 25 bridge piers in the seabed. A steel casing is sunk into the seabed using a jack up rig which effectively creates a coffer dam within the casing. The piles are being drilled to depths of between 15m and 30m below the sea bed.

Steel cages are then dropped in and the piles cast in-situ.

Works will later be carried out to drill 19 piles to support four piers over a protected salt marsh on the south side of the bridge. The piling will be carried out from a 200m long steel piled tubular access bridge to cut disturbance to the marsh, which is a protected site.

A total of 23 piles are required to support the bridge abutments. Here, site workers will drill 750mm diameter CFA piles into competent rock.

The team will also create foundations for the casting area of the 'bridge deck push launch' on the northern embankment where pre-cast bridge segments are fed out into the Forth.

Due to the poor ground conditions, the casting area has been anchored using 120, 750mm diameter piles.

Piling work is due to finish in spring 2007.

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