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Laying it on the line

M60 widening - The North West's biggest road project is more than 60% complete.Bernadette Redfern reports from the M60.

The thing with the M60 is that it has just as many junctions as the M25, but it's only a quarter of the length.

If you think that 110,000 vehicles are using this stretch every day, and some junctions are only 1.5km apart, you can understand why it's congested, ' says Highways Agency project leader Graham Dakin.

So to increase capacity on one of Manchester's busiest stretches of motorway, the Agency has commissioned an Amec/Alfred McAlpine joint venture to design and build a £102M widening between junctions 5 and 8. Designer is Halcrow and consultant Mouchel Parkman is acting as adviser to the Agency.

'It is not a simple widening job though, ' says joint venture construction director Rob Coupe. 'The widening is asymmetrical because of adjacent developments and land availability, and the levels vary along the route.' The result is a complex scheme to open out a 7.4km length of motorway from three lanes - which would narrow to just two at junctions 5, 7 and 8 - to four lanes, with additional collector and distributor roads. But predictably, the Agency was determined there should be no deterioration in lane availability just because a major construction project was going on.

'It was key that however the work went ahead, we keep the same number of lanes open.

There are speed restrictions in place (40mph), but the philosophy is that the new road is built first and the traffic is then switched over to it, ' says Dakin.

Building the new road means constructing 16 new structures to carry the additional lanes alongside the existing carriageways. It also means the demolition of 10 existing bridges to make way for new, improved structures. 'Eight of these have already been taken down, but the joint venture came up with a new design that will save the existing structure at junction 5, ' says Mouchel Parkman assistant supervisor Alan Gill.

Amec/Alfred McAlpine senior civil engineer Anthony Burgess explains: 'We put forward an alternative proposal to retain the existing structures, which would ultimately carry traffic in one direction. The traffic in the other direction would then be accommodated by constructing new structures alongside the original ones. This proposal was clearly a more sustainable and safety orientated solution, ' he says.

The existing structure consists of two decks, each carried on a single row of leaf piers. The two cantilever deck sections then join in the centre (see diagram, above). Not only does this concrete structure have to be realigned with the new road bridge, it also has to be lifted by up to 1.8m, because the Highways Agency wants the Princess Parkway running beneath it to become a designated route for high loads travelling into Manchester or out to the nearby M56 and M6 motorways.

'The solution that we have devised is technically complicated and still under development, but it involves jacking up the existing structure and rotating it to fit the new geo metry, ' says Burgess.

The team will sit the jacks on the existing piers and progressively lift and prop up the structure until it reaches the required maximum of 1.8m.

Before the decks can be jacked, the concrete seam that links them will have to be broken out, 'probably with hydrodemolition', says Burgess.

Lifting and propping will then begin in earnest in October. Once the decks are at the new alignment, a new lap of reinforcement will be tied in and an insitu concrete pour will ensure that the two single decks become one unit.

The biggest concern the team has during the operation is controlling the jacking operation.

'Because it is post-tensioned, we have to lift the deck as a plate without introducing any torsion into it. We will be monitoring the loading carefully to ensure the load distribution goes as expected, ' says Burgess.

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