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Isle solve the problem

A life-threatening gap in Britain's trunk road network will be closed by the new Swale viaduct. Mark Hansford reports from the Isle of Sheppey.

Getting to and from the Isle of Sheppey in Kent is a tiresome business.

Traffic levels of 20,000 vehicles a day significantly exceed Highways Agency guidelines for a single carriageway road.

And that is before the bottleneck that is the 1960s Kingsferry lift bridge is taken into account. Since marine traffic on The Swale - the stretch of water between Kent and the island - has right of way, chronic delays can often build up on the bridge approaches.

The bridge is raised 6-12 times a day, with the road closed for anything from 7-20 minutes.

'And that's assuming it doesn't get stuck up, ' says Highways Agency project sponsor director Peter Evans.

But the bridge often does get stuck in its lifted position, raising a far more serious concern - the emergency services are based on the mainland and they argue that the unreliability of the bridge puts the lives of Sheppey's 35,000 residents at risk.

Fortunately, this will soon change. Work is racing ahead on the construction of a new high level fixed link crossing of The Swale, as part of a £104M project to create a fully dualled link from the M2 at Stockbury to Sheerness port on the island.

The A249 Stockbury (M2) to Sheerness project is a design build finance and operate (DBFO) contract which the Highways Agency has awarded to Sheppey Route Ltd, a company established by Carillion Private Finance.

The £104M figure includes overlaying the existing dual carriageway with a thin lownoise wearing course, operation and maintenance of the whole route by Sheppey Route until 2034, and the £80M construction of the 5.6km long A249 Iwade Bypass to Queenborough Improvement.

The contract was awarded in February and new build work began in June. Scheduled completion date is 19 May 2006, with the existing A249 being overlaid next summer.

Carillion will operate as design and construct contractor on the new build - as well as operations and maintenance contractor - but has brought in a host of specialist designers and sub-contractors (see box).

'Being contractors we like to contract out risk, ' says Carillion project director John Franklin. 'Plus sub-contractors bring expertise to the job.

'We also like single point responsibility, so [lead designer] Capita Symonds appointed Cass Hayward as viaduct designer. As it happens Cass Hayward and Fairfield Mabey are both based in Chepstow, which will help in developing buildability, ' he adds.

Attention will naturally focus on the 1,250m new Swale Viaduct. A suspension or cable stay bridge was ruled out because of the potential to disrupt wintering birds so the low profile crossing will rise to a height of 34.7m above The Swale at a gradient of 6%. This will provide a 29m clearance for shipping above high water level and also give the required clearance over the existing approach to the bridge on the south side and the railway on the north.

The viaduct will be made up of 19 spans, varying in length from 43m at the ends to 93m in the centre. Its 18 piers (13 on land, five in the river) will range from 7m to 29m high. Each will taper from 4m diameter at the base to 3m diameter at the top. The two piers either side of the navigable channel will have 30m by 15m mass concrete caissons - it is calculated that ships would run aground long before striking the bridge.

Work on the substructure is getting under way with piling for the 18 piers and two abutments. Alongside this two temporary jetties are being built out into The Swale to build the five water-based piers. A 75m wide navigable channel will be maintained at all times.

In total 5,000 linear metres of bored piles will be installed, each between 900mm and 1,200mm in diameter. All but two piers will need 12 piles, bringing the total number of piles close to 240.

Work on the superstructure begins this month with casting of the first reinforced concrete columns.

Here there has been a design change. The columns are circular in cross section but have 1.5m deep 'notch outs' at the tops to carry temporary steelwork to be used in construction of the deck.

'These (the notches) were originally intended to be temporary but we then started scratching our heads for ways to fill them in without coming up against CDM and working from height problems, ' says Franklin. 'So we decided to leave them as they stand.

'I think it's nicer anyway, ' he adds. 'It's more of a structural statement.'

The piers will all be installed by April next year, with the final sliding bearings added by mid-May. But things will really start to hot up around February when the deck is constructed through a series of launches, lifts and pours.

The deck comprises four steel girders at 5.5m centres with a 350mm deep, cast insitu reinforced concrete slab. The girders vary in depth between 1.8m at supports to 3.2m at midspan.

Cross beams at 3.5m centres will support OMNIA precast concrete permanent formwork planks, and cross bracings will be installed on the middle span.

The total weight of steel used will be 10,000t. Parapets will be precast reinforced concrete.

The deck will be fixed at midspan, so that it articulates at either end. Up to 300mm of movement can be accommodated at each abutment.

The superstructure steel will be launched from three locations, with two happening concurrently. Launch one will send the south section (spans 2 to 7) northwards. These should take until the end of July. At the same time launch two will send the central section (spans 9 to 14) south over The Swale. This will take a month longer.

The final launch will send spans 15 to 17 south over the railway line. This will then take three weeks in August.

All four beams will be launched simultaneously, supported by cables played out from an A-frame erected on the launch pier. Upstands sitting in the notch outs at the column heads will receive the beams and a vertical roller will keep the bottom flange on line.

With the launches complete spans one, eight and 15-17 will be lifted in by 500t crawler cranes.

These sections cannot be launched because of changes in curvature. This work will be completed by Christmas 2005.

Once the steel for each span is in position Omnia planks will be installed, reinforcement positioned and the insitu concrete poured. This work will begin in August and should be complete by March 2006. At this point precast parapets will be installed and the bridge completed.

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