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Drilling starts for Upper Forth Crossing

CHALLENGING marine ground engineering works started last month on the 1.18km long Upper Forth Crossing at Kincardine near Edinburgh. Piling for 25 piers on the £120M A876 road crossing started after a large drilling rig was delivered to the site on a barge at the start of October.

The B50 piling rig operated by subcontractor Seacore has started drilling 20 marine piles into the seabed 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.

'A major fault traverses the bridge in a north west, south east direction, ' said John Cathro, engineering manager at contracting joint venture Morgan Vinci engineering manager, John Cathro.

'The rock strata is generally thin sandstones and mudstone bands with a few coal seams above and mixed with sandstone layers.

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. It's certainly given the designers something to think about.'

Under the solution, Seacore has started drilling reinforced concrete piles from 3m to 3.8m diameter for the 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 between 15m and 30m below the sea bed. Steel cages are then dropped in and the piles cast in situ.

'The depths of the piles depend on the rock levels and how reliable the mudstones are, ' said Cathro.

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 site of special scientic interest.

'Work in the salt marsh is highly sensitive and requires constant liaison with Scottish Natural Heritage, ' said Cathro.

On the north side of the river, extra care had to be taken to ensure that piling works for the first pier did not interfere with an existing sea wall. Here 19, 750mm diameter CFA piles will be used. 'Any breaches in that wall would flood the whole area, ' said Cathro.

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

The team, which also includes bridge designer Benaim, will also create foundations for the casting area of the 'bridge deck push launch' on the northern embankment where precast 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. Further piling will also be required around the surrounding area to take the embankment loading.

Piling is due to finish next spring.

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