Isambard Kingdom Brunel's famous Royal Albert Bridge, which crosses the river Tamar, has been restored to its original design with the help of a timber support system and some tightly synchronised planning.
Adrian Greeman reports
A FINE penetrating rain driven by a cold wind off the English Channel made the tightly synchronised renovation work on the famous Royal Albert Bridge that much more difficult this winter. But despite the wet and earlier biting cold during a two-day Christmas possession, there are smiles all round. The historic job, just winding up, has gone well.
Mike Smith, design engineer and engineer supervising construction with client Network Rail's western region says meticulous planning, particularly by contractor Mowlem and its site agent Paddy Rosborough, has helped the overall £3 million operation go smoothly.
He points out that the job is not just unusual but unique. There is no other structure like Isambard Kingdom Brunel's combination of a chain suspension bridge and tubular steel compression arches. And there are few other jobs that involve removing modernisations, essentially to replace an original design feature.
However, for this Grade One listed landmark structure spanning the Tamar between Devon and Cornwall, a reversion to Brunel's conception for track support was considered best by Network Rail (formerly Railtrack) and consultant Faber Maunsell.
A heavy static load on the bridge was compounding the sway and vibration effects of live train loads. The possible fatigue effects on hangers and pin connections was potentially damaging for the 150-year-old bridge (Construction News, 28 November 2002).
The load has been lightened by removing the ballasted track installed at the turn of the 20th century and reverting to a timber support system, explains Mr Smith. Rather than sitting on conventional sleepers, the track will be bolted to longitudinal members, themselves supported on special chairs bolted to the bridge's cross beams.
That has meant a complex sequenced operation to remove the old track and trackbed, clean up the bridge structure, then drill and fix the chairs and timbers. There has been a total of 130 men, divided into small teams on site. Each team, dedicated to a different task, follows the one that goes before it. The teams have had to carefully synchronise their tasks to ensure everything is done on time.
'The main work has been done in four 50-hour possessions, ' explains Anna Cornish, deputy project manager with Network Rail who has overseen the operation for the client. The first of these took place last Easter when just over half of the first of two main spans was tackled. But remaining work waited for a second contract this winter, not least because the single track bridge is the only railway link to Cornwall and further blockades during the tourist season were unacceptable.
'We had some experiences during the first segment which were useful, ' says Mr Smith. 'For example, we found we could shave something off the width of the timber supports because a fitting for the railing needed less space than we thought.'
Slimmer timbers meant longer sections of 7.5 m could be used, instead of 6 m pieces, which cut down cycle times. 'The limitation is basically in the weight, ' explains Mr Smith 'since the road-rail work units can only carry so much. But the longer the single lengths, the better'. The timber used is the very dense and impermeable and therefore, hopefully, long lasting Ekki tropical hardwood, imported from Cameroon. Prepared in Holland it is then delivered to a supply point just up the line where it is cut to length.
Wood and other materials for the work on the bridge have all been brought in from the Devon side because the bridge is single track and, once the rails are removed, there is no means of supplying a second point of work. Old track, ballast and track bed timber is removed at the other end, however, to an access point at Saltash station just on the Cornish side.
The single direction for the work (from Devon to Cornwall) is one of the main constraints on the job, and means site logistics were crucial in doing the work. This, combined with the limited time spans for the main possessions, each one being just 55 hours, meant careful organisation was vital.
For site agent Paddy Rosborough, work had to begin months in advance. 'We planned the job down to quarter-of-an-hour segments, which is very precise for civil engineering, though it is more normal on railway work, ' he says. 'But with a track possession bookable only months or even years ahead, you cannot afford to overrun.'
A key element was to make sure that the various crew teams could do their job in the right timespan and that various tasks in the sequence did not get out of synchronisation. Drilling for the timber seating chairs had to keep pace with cleaning and preparation work on the bridge beams ahead, for example, and that could only happen after the railway team had taken up track, removed ballast and taken out the old sleepers. Then timbers had to go in before the rail crews could come back to reinstall the track.
Altogether there were 130 men working on round-the-clock shifts. Work has been done by Mowlem Civil Engineering and several subcontractors. Mowlem Railways has done all the railing work, for example, and steel chair elements were fabricated, supplied and fitted by Taylor & Son of Cardiff. Rosborough worked with each of these to build up an idea of how long the various task units would take.
'To get the precision it was necessary to go right down to first principles using basic times for various tasks and then build up the work time blocks from there'.
Rosborough planned on 45 per cent of the track being renewed in each of the first possessions and just 10 per cent in the last, leaving plenty of room for contingencies and other follow-on jobs. 'And that's why when you visited us on the third possession, we were all looking so relaxed, ' he says. 'It was a bit tighter on the first two.'
Planning details even included making sure the Devon side site canteen offered a Christmas dinner during the first of the three main possessions. As well as keeping spirits up during some severe weather, this also helped keep men on site who might otherwise have gone looking for a local pub and a Yuletide drink, observes Rosborough cannily.
Some work could be done ahead of time and Denholm Industrial Services, which supplied the scaffolding and safety crash deck beneath the bridge, was able to come in eight weeks before the main possessions to get on with its job. There were a number of short Saturday night possessions ahead of the main works too and currently one or two short possessions are being used to finish tidying up works.
All those involved feel they have had a unique experience working on such a famous structure. One site foreman for Mowlem, Trevor Millard, even delayed his autumn retirement date to be able to finish the job.
'His and Mowlem's enthusiasm for the job is one of the reasons that the second stage contract was awarded to them, ' says Smith. 'And we are delighted with the result.'