Civil engineers rarely have the opportunity to tackle the same problem twice in quick succession. Hard lessons learnt while grappling with unexpected challenges are often forgotten soon afterwards. The Avonmouth Bridge strengthening project is different, however. Its twin steel box girder design means the site team can now apply the solutions developed with such pain on the first box girder to the second - with obvious and quantifiable cost and time benefits.
Structural engineer Hyder Consulting's resident engineer Allan Chambers says the real lesson from the first phase was the benefit of team working. 'The introduction of partnering and value engineering in 1997 changed the whole perspective.
'Originally the contractor faced lots of logistical restrictions on his method of working - but by the end of the first box we had developed a more flexible and practical approach.'
The first box was effectively complete by May this year. Much has changed since NCE first visited the site almost four years ago. Then, everyone involved was quietly confident that main contractor Costain would be able to complete the complex strengthening operation - and widen the M5 between Junctions 18 and 19 - by May 1998, at a cost of around £50M. This was based on the assumption that the box girders were in generally good condition, a reasonable assumption at the time given the pristine condition of the internal paintwork and the years of maintenance records (NCE 23 November 1995).
Reality turned out to be very different. Once the seven layers of lead- based paint had been blasted away a sorry catalogue of weld defects and laminar failures was revealed (NCE 22 January 1998). In the end the Agency had to agree to a revised completion date of March 2001, and total contract costs are now expected to be just over £125M. Direct costs of the strengthening proper will be around three times the original estimate.
But with the worst now almost certainly behind them, the site team has grounds for confidence in the revised plans for the next phase. 'In fact the second, 'south-bound', box is not quite a mirror image of the first,' says Agency project manager John Bourne.
'Because there's still a pedestrian walkway on this side the parapet loading is significantly lower, so strengthening of the cross girders and cantilevers is different. And of course there are significant detail differences in the strengthening work itself.'
Perhaps the most obvious design change is the use of insitu concrete to stiffen the bottom flange of the box girder in the approach viaducts at the piers. Instead of welding on dozens of small steel plates, strengthening subcontractor Kvaerner Cleveland Bridge will pour a 400mm thick layer of low shrinkage concrete up to 4m each side of the pier diaphragm.
Shear studs welded to the bottom flange will ensure structural continuity between the box girder and the concrete. A mix containing microsilica and shrinkage-reducing admixtures was specially developed by readymix supplier Tarmac Topmix.
The simplification and standardisation of strengthening plates developed on the north bound box will be carried over to its partner and developed even further, Chambers says. Kvaerner Cleveland Bridge project manager Andy McGhie adds: 'Last year we went through the drawings and surveys, standardising components where possible and ordering them well in advance.
'We also acquired an on site containerised shot blasting plant. Plates go through it immediately before they go into the bridge, which cuts down preparation time inside the box.'
Investing £800,000 in an extra set of sophisticated temporary props is now seen as a very wise decision by all involved. Longitudinal strengthening can only take place with the box girder propped at the one third span points (see box), and with the new props four workfaces can operate at the same time. According to Hyder deputy resident engineer Phil Bailey, the real breakthrough has been a better understanding of how much work can be done before propping.
'Detailed analysis has given us confidence in doing the transverse stiffening with the box unpropped,' he explains. 'And by using cherrypickers to weld on external angles to increase buckling resistance we've been able to install access gantries and bring in materials well in advance.'
Transferring operations from one box to the other was smoothed by a preliminary partial diversion of traffic back onto the north bound box in April. 'Remedial gangs' had been working inside the second box since January, repairing welds and smoothing the path for the main strengthening works.
In the end, says Bourne, there will be 'no one big change, just lots of little changes that add up to much better buildability'. Lessons will continue to be learnt over the next 15 months or so - but this time around the process is likely to be a lot less painful.