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Flow stopper Repairs to Glasgow's problem-ridden Kingston bridge have ground to a halt with the latest contract running at least a year late. David Hayward explains why. Pictures by Jim Mackintosh Pho

As dawn breaks over Britain's busiest motorway bridge, a lorry laden with chemicals crashes into a minibus. Its occupants are trapped as dangerous liquid spills over all 10 traffic lanes.

Kingston bridge, sweeping the M8 over the River Clyde near central Glasgow, is immediately closed.

The first wave of some 155,000 vehicles/day battles for space on diversion routes only able to squeeze in less than half the morning peak. By 8.30am, there is total gridlock on the motorway and a wide swathe of surrounding urban streets, plunging much of the city into transport chaos.

The region's industry and commerce are instantly denied the local and inter-urban mobility normally taken for granted on Scotland's most important strategic stretch of road. By the end of the day that chaos has cost at least £500,000 in traffic disruption alone.

Fortunately this is a bill yet to be incurred; for the lorry crash was only a nightmare paper scenario. It formed the focus of a major desk exercise last month at which teams of engineers, police and emergency services personnel were tested on their action plans.

'The direct and long term consequences of suddenly loosing the use of Kingston bridge would spell economic disaster to the whole of Scotland,' says Ian Telford, one of Glasgow City Council's engineers attending the exercise. 'Businesses would close and companies relocate.'

Telford's concern over bridge closure for whatever reason is acute. For he is also the man who, for the last six years, has project managed a full time team of engineers overseeing complex and expensive repairs to this severely weakened bridge. And this overriding need to keep the traffic flowing is now a major factor behind the latest repair problem - one that has halted all site work triggering at least a year's delay.

You name it, Kingston's got it. The list of structural defects on this 28 year old concrete box girder crossing is long.

Mid-span sag; inadequate bridge articulation and prestress; tilting weak piers and a displaced main span leaning on - and damaging - approach viaducts. Repairs could rumble on until 2010 at a cost of at least £40M (see box).

But all is not well as the Glasgow City Council team, working as agent for owner the Scottish Office, approaches the most difficult task yet. Right now the site should be a hive of activity with the 55,000t deck suspended on a network of 190 jacks. Weakened piers should be being slid out of the way as traffic speeds over the raised slab. The already partially built double thickness replacement supports would then be completed.

Instead the site is near deserted, with the contractor for this £14M repair stage, Balfour Beatty Construction, fielding only a skeleton staff. Its original 18 month contract is delayed at least a further year while engineers struggle to convince themselves that this world first jacking operation is 100% fail safe.

The problem centres on ensuring that the £2M state of the art jacking network can cope with two conflicting demands. Not only does it face arguably the most onerous positioning and pressure tolerances anywhere, but it must also accommodate every conceivable movement

the vast suspended deck can throw at it during six months of continuous traffic loading.

Only during the 17-hour lifting operation - now pencilled in for a weekend in September exactly a year late - can traffic be diverted off the bridge. Jacking subcontractor VSL's main bank of 128 large 1,000t hydraulic lifting jacks must always be within 2mm height of each other, with just 5mm level difference allowed over the full 143m deck. Equally exacting load differentials ensure that the already weakened slab receives only known stresses throughout its suspension.

But the very operation of lifting and holding the span clear of its bearings transfers even more onerous safety tolerances on to the VSL crew. 'The jacks have to act as replacement bearings responding to live loads in exactly the same way as the bridge would,' explains Balfour Beatty agent Shaun Nesbitt. 'They must be able to twist, tilt and allow sliding.'

And if that was not enough of a challenge, the deck will be bombarded with one-off additional forces as its prestress is increased and span pushed southward. This will redress both the 300mm mid span sag and its 165mm tilt to the north.

Every deck shudder and concrete crack will be monitored continuously, with the structure's position plotted to 0.1mm. Readings from over 150 strain and movement gauges will be downloaded every 15 seconds to computers in jack control centres either end of the crossing.

The need for such elaborate precautions is primarily to guarantee the safety of the structure and its ever-present motorists. But close behind is the fear that any eccentric loading, forcing the sudden removal of traffic, could result in the motorway facing gridlock within an hour.

'Our current difficulty is getting the separate bridge and jacking mechanisms to interact with each other so that controls automatically stay within operating limits,' says Telford. 'At present we cannot prove that the system is fail safe.' Because the main span is already pressed hard against its northern approaches, summer time lifting between May and August - when the deck could expand 30mm - is banned. The next 'cold weather' window starts in September and engineers are optimistic the control system redesign will be complete by then.

At contract award stage in spring 1996 it was hoped all work would be complete by last December. But even before repairs started, both sides realised the challenge had been underestimated and now claim to be working as a united team to solve the safety problems.

Latest completion date is put at September 1999 - a full 21 months behind original expectations, although contractually only a year late.

Because of this initial uncertainty, the work was awarded through the New Engineering Contract as cost reimbursable. This, say engineers, removes confrontation and cash claims with the client presumably picking up all the several million pound overspend.

'Safety is paramount and it will cost what it costs,' says a senior Scottish Office source. 'We will pay to identify the way forward and we are not withholding any payments (to the contractor) connected with this delay.'

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