Back in the seventeenth century the bridge at Kingston-upon-Thames was the 'second most important river crossing in England'. Downstream of Kingston the only other Thames crossing was London Bridge.
Even now the existing five span masonry structure dating from 1828 carries 40,000 vehicles a day, 2,000 an hour at peak periods, and is a vital cross- country and local link.
So when a 1993 appraisal by Travers Morgan (now Symonds) indicated that a 3t weight limit should be imposed on the crossing, the dilemma for the Royal Borough was excruciating.
The nearest alternative crossings - Richmond 7km downstream or Hampton 4km upstream - were already close to capacity, so diverting heavy traffic from Kingston over them was no realistic long-term alternative. Nor was single lane working, or propping. Strengthening to modern standards was the obvious best answer, albeit with practical and logistical problems attached - but what to do in the meantime?
'Luckily, there were no signs of really serious distress,' reports Symonds project manager Julian Counsell. 'There was some cracking, some missing bricks, quite a lot of water leakage but no hingeing or major deflections.
'So the final decision was to leave the bridge open to traffic up to 38t, while carefully monitoring its condition.'
Developing a strengthening method was far from straightforward. The structure is Grade II* listed and English Heritage was naturally keen that nothing should be done to change the external appearance radically. It seemed the only option that fulfilled all the structural and visual criteria would be to remove the existing roadway, excavate down to the arches and install reinforcing saddles - but that had the potential to cause massive traffic disruption.
Counsell says a detailed traffic analysis made it clear that long-term lane closures were just not on. Four lanes of traffic would have to be kept open at peak times. This meant two lanes at least would have to be diverted somewhere else to allow half the existing roadway at a time to be dug up and strengthening works carried out. 'The question was, where were these lanes to be,' Counsell adds.
There were only two real alternatives. Some, like English Heritage, favoured a temporary two-lane bridge upstream. Others pointed to the long- term advantages of a second, permanent upstream widening of the existing structure. Whatever the chosen solution, it had to recognise the very congested nature of the site, the restricted access and the need to maintain navigation through at least two spans at all times.
In the end it was the reduced environmental impact of the permanent widening plus the benefit of being able to add bus and cycle lanes to the crossing that tipped the balance, despite strong resistance from English Heritage. But the appearance of the widened bridge had to be virtually identical to the existing structure, which ruled out many options.
The final design which appeared in 1997 featured extensive re-use of the existing Portland stone cladding on the upstream elevation. Foundations would be cast insitu, concrete piles supporting concrete pile caps, piers concrete faced with Portland stone.
Precast concrete arch units linked together into a two-pinned arch would be composite with a lightweight concrete saddle (see box). Except at the arch crowns, where the saddle extends up to roadway level, foamed concrete infill was specified to reduce foundation loads even further. Total extra width worked out at 6.6m.
Gleeson was awarded the £7.5M contract in October 1997. Gleeson contracts manager Peter Martin says the initial problems on the contract stemmed from the poor access.
'There's none from the Kingston bank. On the Richmond side we could get in through a conservation area owned by Hampton Court Palace, but this meant finding new homes for the houseboats moored there.'
A fleet of barges, two mounting cranes up to 50RB size, was assembled. Cofferdams would be needed to construct the foundations in the 3m deep river, but only two could be set up at one time, to avoid restricting the boat traffic.
It was the 1914 concrete-filled steel cofferdams (see box) projecting upstream from the existing piers that posed the next challenge. The new piles had to pass through them into the riverbed, which meant coring clearance holes through nearly 3m of steel and concrete.
'It was a nightmare,' Martin reports. 'We tried everything from thermic lances to high pressure water jetting. In the end it was a low-tech combination of stitch drilling and jack hammers that worked best.'
Once the pile caps and piers had been cast, arch shell erection could begin. Temporary towers each side of a span supported each shell until the central joint had been completed. A 450mm thick saddle of lightweight Lytag concrete supplied ready mixed by Pioneer Concrete was the next key operation.
Nearly 18 months into the 139 week contract all the arch shells are in position and all saddles and spandrels have been poured. The upstream elevation is largely complete. Currently, the saddles are being waterproofed prior to the pouring of the foamed concrete fill, and service duct installation is beginning.
Traffic is due to be diverted onto the new section in July. Meanwhile, says Martin, dire warnings about the loss of trade that would be suffered by nearby pubs and restaurants have proved to be unfounded. 'We did our best. We even stopped noisy operations like piling during the lunch hour.
'But in fact I think we actually attracted custom to the area, there was so much local interest in what we were up to.'