Retrofitting 150 bearings on the 1.4km long, 60,000t Thelwall Viaduct requires the sort of precision usually associated with brain surgery.
Carrying out this £52M cranial craftsmanship are consultant Atkins and contractor John Martin.
Thelwall Viaduct carries the M6 motorway over the Manchester ship canal and River Mersey near Warrington. Built in 1963, the viaduct is made up from riveted steel plate girders and steel crossbeams.
The current project began in October 2003 and involves replacing all roller bearings with spherical slider bearings and strengthening the deck steelwork (News last week).
Engineers are now halfway through the bearing replacement and are working round the clock to ensure the spring 2005 finish date is met.
'Replacing a roller bearing isn't an easy operation, ' says Highways Agency project team leader David Brindle. Quite an understatement given that the engineers are working to tolerances of less than a tenth of a millimetre.
Each crossbeam running between the longitudinal steel plate girders of the viaduct rests on a complex set up consisting of four roller bearings and two pairs of rocker bearings. These sit on a concrete pierhead.
Hydraulic jacks are used to lift the beam by just 3mm to allow the old cylindrical roller bearing to be removed and a new sliding spherical bearing to be installed.
'Installing the first bearing is relatively easy, but when you get to the fourth one you have to balance the load more carefully, ' says Brindle.
If one bearing is 0.2mm above the others, it will receive between 50t to 100t more load, explains Atkins project manager Jeff Sharpe.
Atkins is monitoring the viaduct's movement to ensure the structure is behaving as predicted during and after jacking.
'Thelwall's problems began when the viaduct was refurbished in 1996 (see box). There were cost considerations which influenced the form of the structure and whether to include provision for replacing bearings, ' says Brindle.
Since then Thelwall has suffered 18 months of emergency repairs, starting in July 2002. Then, a routine inspection revealed that one of the main pier-deck cylindrical roller bearings had split in two leaving the deck beam resting on a knife-edge (NCE 25 July 2002).
The following months saw 11 more bearings which showed signs of failing in the short to medium term being replaced.
The consequences of failure could have been significant.
'The integrity of the viaduct is reliant on the connection between the deck and concrete piers, ' says Brindle.
So in April last year the Highways Agency made the decision to replace all the roller bearings and to close five lanes of the six lane motorway for a further 18 months (NCE 3 April 2003).
Investigations revealed that the stainless steel cylindrical roller bearings had suffered intergranular corrosion. This is believed to be a result of stress cracks which developed during the production of the hardened stainless steel.
Two rocker bearings have also been removed for testing, with results back so far showing no signs of deterioration.
'We're still trying to determine why the cylindrical bearing failed, so we went for the next best bearing type which fulfilled all the design requirements, ' says Brindle The cylindrical roller bearings are being replaced by sliding spherical bearings. These consist of a stainless steel hemisphere with a polished aluminium surface sliding inside a steel dish casing lined with PTFE. A 40mm thick stainless steel plate sits on top of the hemisphere, separated by another PTFE lining. This plate connects the bearing to packing plates tight up against the bridge beam.
Consultant Atkins has rationalised 150 different bearing stresses to 20 different sliding spherical bearing designs.
The Agency has also put in place provisions for future structural maintenance.
Permanent stiffeners will remain, ready to receive the jacking box units.