Major roadworks are rare on the A30 in mid-Devon. When they happen, drivers go nuts. Speed restrictions and lane closures put in place during construction of a grade separated junction at Merrymeet, just outside Okehampton, have provoked a flurry of complaints from disgruntled motorists.
'The A30 carries less traffic pro-rata than the M25 [around London], and suffers from far fewer roadworks, ' notes Peter Heron, Parsons Brinckerhoff customer care consultant for Highways Agency Area 1.
'But the Agency has received proportionately far more calls about delays resulting from the Merrymeet works.' Anxious to avoid disruption where it can, the Agency leapt at an opportunity to steer clear of lane closures when major work was required 20km down the A30 near Launceston, at Dunheved Bridge.
Dunheved Bridge spans the River Tamar. The twin deck, three span, post-tensioned, reinforced concrete structure was built in 1978, but only 25 years after construction was already showing signs of decrepitude.
'It has concrete cancer, ' states Agency contract supervisor John Litherland.
Cracks and patches of soft, rotten-looking concrete are easily visible on the structure's soffits.
The Highways Agency planned to protect the deck from the elements with a glass fibre reinforced plastic enclosure, so slowing the rate of deterioration and extending its life another 30-40 years.
Contractors were asked to tender for the £3.3M design and build job last summer. Bids were to be evaluated 70% on quality and 30% on price. Dean & Dyball won the contract. 'But we felt that building an expensive enclosure around an ultimately doomed structure was the wrong thing to do, ' says marketing manager Mike Shepherd.
We bid for the job as specied, but submitted an alternative proposal, worked up very quickly. We set out plans for complete deck replacement and costed the work at £3.6M.
As well as showing we could give the Agency a brand new bridge deck for only £300,000 more than it would cost to extend by a couple of decades the life of the existing deck, we showed we could do it without the need for long term trafc management.' Shepherd explains that to install the GRP enclosure and to waterproof the decks it would have been necessary to close the bridge a deck at a time, reducing trafc from dual two lane running to single lane contraow.
'The GRP enclosure was a temporary alleviating measure - nothing like as good as the solution put forward by Dean & Dyball, which we accepted, ' Litherland enthuses.
Dean & Dyball is therefore replacing Dunheved Bridge's 4,500t reinforced concrete decks with steel and concrete composite alternatives weighing in at a modest 1,500t apiece.
Plans were initially to construct a Bailey bridge alongside the existing westbound deck, allowing trafc to be diverted during demolition and reconstruction. The contractor approached consultant Benaim to review and develop its idea.
'We looked at the cost of hiring a Bailey bridge for the duration of the works and realised that it would be more effective to build a new deck beside the existing decks and then slide it into place. It does involve putting all trafc across a single deck for a fortnight, but it enables you to do the entire job for what it would have cost just to hire the Bailey bridge, ' says Benaim managing director Mark Raiss.
Dunheved Bridge's reinforced concrete leaf piers are in good condition and are being retained.
Temporary steel towers have been erected on cast insitu concrete caisson foundations beside each pier to support the new westbound deck. Once it has been completed this autumn, trafc will be diverted off the old westbound bridge deck and onto the new deck - the approach embankments have been widened either side of the bridge, allowing new temporary abutments to be constructed.
Eastbound trafc will then be diverted onto the original westbound deck, freeing the far side deck for demolition (see diagram). A new eastbound deck will then be built insitu.
With trafc restored to the new eastbound deck, the old westbound deck will be demolished. Barry Fletcher, Dean & Dyball project manager, says that he will be ready to slide the new deck into place in February or March next year, when trafc on the A30 is at its lowest.
The existing decks are cellular reinforced concrete structures, each with five post-tensioned longitudinal beams. Deck edges are cantilevered. The new decks are composed of four longitudinal beams. Those for the westbound deck were craned into place using a 50t crawler in May/June. Fletcher points out that side spans are far shorter than the central span, at 27m compared to 45m. To simplify lifting, beams were divided into three sections.
'Side spans came in as 35m long, 25t beams and cantilevered into the centre span to reduce weight of centre span lift to 17t, ' Fletcher says.
Connections were bolted.
Transverse beams link the longitudinal members, while concrete omnia planks provide the base for a cast-insitu reinforced concrete topping.
Fletcher downplays the slide.
'It's a standard procedure, done all the time for railway bridge replacements, ' he says matter of factly. Supporting beams atop the temporary towers will double as skid rails.
The way Dunheved Bridge copes with movement has been rethought. Existing decks are fixed at one end and 'have a big, clanging movement joint at the other', says Litherland.
Steel sliding bearings atop each pier allow for longitudinal movement. The new decks will be fixed at the piers, with semiarticulated abutments to cope with the anticipated +/-20 mm movement.