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Bridge threatened after biggest ever main burst

CONSULTANT MOUCHEL was this week investigating the stability of a major London road bridge after nearly 26,000m3 of water from a burst 1.5m diameter main threatened to lift the deck off its bearings.

Engineers feared that the 46m span, voided, post-tensioned concrete Abbey Road bridge over the A406 North Circular - one of London's busiest trunk roads - would float clear after the flood completely filled the 6m deep underpass below.

There were also concerns that the massive volume of water, caused by what is thought to be Thames Water's biggest water main failure, would soften clay under the bridge abutments' strip footings or that debris might damage or silt up the bearings.

The A406, which carries 9,000 vehicles an hour during rush hour, was closed just after midnight on Sunday morning after the trunk main burst. The road is not expected to re-open before today, even if Mouchel gives the bridge a clean bill of health.

Highways Agency route manager Any Siva said engineers from Mouchel and contractor Prismo Doyle, who are responsible for trunk road maintenance in Agency area 21, would excavate the front of the retaining walls to compare ground with previous borehole information to check for settlement or softening.

He explained that there was also concern that subgrade material had been washed away and of embankments being softened.

The steel trunk main, built in 1952, carries water from the Ashford Water Treatment Works in West London to Thames Water's Fortis Green Pumping House in Cricklewood, North London. It is the largest water main other than London's ring main.

Early reports suggest that the failure was caused by corrosion on a disused emptying connection that was completely underground. It is believed the corroded plate on top of the connection opened 'like a hatch' sending a fountain of water 3m into the air.

Thames said it has not ruled out the possibility that an unexpected ground movement or that heavy traffic may have triggered the burst. Research and development engineers from Thames Water have taken the failed section away for inspection.

It took 10 hours to isolate an 8km stretch of the main. The north end was shut first to stop water back-draining from its reservoir source. A diversion was then put in to a nearby 1.2m main to keep customers' supplies going.

With help from fire-fighters, nearly 48 hours were spent pumping out the flooded junction. Up to 35 pumps with a peak capacity of around 1000m3/hour had to be brought from as far as Birmingham and Bristol. Water was pumped into the nearby River Brent.

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