ENGINEERS REMAIN confident that 'the world's heaviest lift of a single structure' can still go ahead before the end of this month, despite a forced postponement this week.
Glasgow's troubled M8 Kingston bridge was due to have its 50,000t main span jacked 15mm clear of its supports to replace weak piers and bearings next week. But problems with sticking jack valves forced specialist contractor VSL to delay the work at the last minute.
The £9M operation, involving a network of 128 hydraulic jacks, is already two years late after concerns over safety guarantees forced a redesign of computer software (NCE 22 October 1998).
Ian Telford, project manager for Glasgow City Council, agent for client the Scottish Executive explained: 'Lifting tolerance between adjacent jacks is just 2mm and, with an already weakened structure, we could not afford to impose additional forces on the deck. Factory and site tests of the original jacking system gave us concerns over tolerance controls so we have changed the jack layout and redesigned its computer control.'
The three span crossing over the River Clyde has suffered a catalogue of problems since completion in 1970, including a 300mm mid span sag and a leaning river pier with a slipped rocker bearing on top of it. The aim of the seven-month lift is to allow original piers to be replaced by already partially complete new supports and bearings.
Jacking specialist VSL will slowly raise the 143m long deck over a 35- hour weekend bridge closure. Forces throughout the structure will be monitored continuously by a bank of 180 instruments.
Over a second weekend closure, the span will be pushed 30mm southward along the bridge using a separate set of two dozen horizontal jacks to correct a displacement caused by the leaning pier.
The same monitoring regime will record all bridge loads until next May while existing weakened piers are slid sideways away from the bridge. Already constructed side sections of the new river piers will carry the deck, while the 2m gap between them - created by the removal of the old supports - is infilled with concrete (NCE 5 February 1998).
An integral deck post-tensioning operation, originally scheduled to take place while the bridge was raised, was completed out of sequence last month. But this will not reduce overall delays and it will be December 2000 before main contractor Balfour Beatty Construction finishes what was to be a 19- month contract.
Cost has more than doubled to £31.5M. The extra £17.5M - attributed entirely to the jacking delay - will be paid by the Scottish Executive through Balfour Beatty's cost reimbursable New Engineering Contract.