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THE NETHERIDGE SWING BRIDGE

ROADS AND BRIDGES

THE 40 m-long steel twin box Netheridge swing bridge and Two Mile Bend section of the Gloucester to Sharpness Canal are key to the success of the scheme.

'It is the final missing link in the delivery of the bypass. A new road junction and swing bridge that will open up the whole project, ' says Nigel Edgeworth, Gloucestershire County Council's project manager.

A new section of the canal has been dug to iron out the bend and help designer Halcrow ensure a minimum clearance from mean canal level to bridge soffit of 4.7 m. This ensures that only exceptional boat traffic will force bridge operators British Waterways to close the bypass itself, helping limit the impact on road traffic.

The section has been dug to a depth of 4.9 m but, where the canal narrows as it passes under the bridge, main contractor Norwest Holst excavated to a depth of 5.5 m to allow for the boats' tendency to 'squat'. As a boat passes th rough it pushes huge volumes of water in front of it, lifting the bow and forcing the stern down, hence the need for the extra depth.

With the Netheridge section accounting for some £13 million of the total construction costs, mostly met by the County Council but with some topping-up from the South West Regional Development Agency, the site team were keen to see any time and money saving ideas.

'We were going to treat the fill excavated from the bottom of the abandoned section of canal with lime before reusing it but we found that just by drying it out the silt classified as non-hazardous suitable fill, ' says Mr Edgeworth.

One problem for the project team has been the existence of a 40-year-old 33 kV electricity cable running straight through the centre of one of the three earthwork cells being built to site the new Bristol Road and bypass junction immediately to the south of the bridge.

Initial plans had been to protect the cable and work over it but fears over its integrity have forced it to be completely decommissioned.

'It is in the central cell sitting just below the surface, it should be fully decommissioned by the end of September, ' Mr Edgeworth says.