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Swinging into action

Roads Selby bypass

After 60 years, the construction of a long awaited road scheme in the UK finally got under way last year.

NCE reports from the A63.

As long ago as 1938 Department of Transport officials declared the narrow streets of the Yorkshire town of Selby - the confluence point of several major roads - to be in need of a bypass. Since then traffic volumes passing through the town have risen to over 18,000 vehicles per day. Almost 10% are HGVs, creating almost continual gridlock.

'The number of cars and lorries is bad enough. But the old swing bridge which carries A63 traffic over the River Ouse on the eastern side of the town just adds to the problem, ' says Skanska engineering manager Keith Sleightholme.

'There is very little clearance below, so it has to be swung even to let small pleasure craft pass.

Every time, traffic is brought to a standstill for up to 10 minutes and the backlog can take some time to clear.'

Selby's case for a bypass is compelling and the £44M scheme was one of the first to be let under the government's new 10 year transport plan. Following a tendering process in spring 2001, which placed great emphasis on quality, the design and build contract was awarded to Skanska and its design consultant High-Point Rendel, in July last year.

If variety is the spice of life then Skanska has certainly got it on this project. When it opens in 2004 the 10km route will boast not just a swing bridge, but also a canal bridge, two rail crossings, four roundabouts, a piled embankment, an underpass and a cutting.

'The new single carriageway bypass will run to the south of Selby, from the A19 at Barlby on the eastern side of the town and rejoining the existing A63 on the western side at Thorpe Willoughby, ' says Highways Agency project manager Alan Begg.

'Spanning the River Ouse and its peat-rich flood plain without causing settlement to the new road and allowing free passage for river traffic was a major obstacle for the project.'

According to Begg, it was Skanska and High-Point Rendel's design to overcome this which helped secure the contract. The proposal included a 2.2km long piled embankment to take the new road from the A19 over the peaty ground to the bank of the River Ouse, from where a 100m long cable stayed swing bridge (see box, p24) will carry the traffic over the river.

On the western side of the river the ground improves and only requires surcharging before construction of the embankment can begin. Both embankments will incorporate flood relief culverts.

'Around 5,500 driven cast insitu piles are currently being installed by Skanska Cementation Foundations in preparation for the embankment construction, ' says Sleightholme. 'On average the piles - which are cast with an enlarged head - are being installed to a depth of 16m, founded in the underlying Sherwood Sandstone.'

Two 110t piling rigs, working on an 800mm thick piling platform formed from local quarry waste over a geotextile base, have been installing piles at a rate of 55 per day, allowing work on the embankment to begun.

'A thin sand layer is first placed over the pile head to protect the geotextile, ' explains Sleightholme. 'Then 800m wide strips of geogrid are placed across the width of the road followed by up to three wider layers of Huesker Fortrac geogrid placed along the length of the road.'

The embankment is formed from a compacted inner core of PFA from a local power station, protected by a facing layer of colliery waste known as minestone, also sourced locally. As the embankment is built, up to a maximum height of 9m, the geogrid will sag slightly, transferring the load on to the piles.

Along the whole route there is a flurry of activity. Excavation of the 700m long, 11m deep cutting at the western end is under way, and the outline of the roundabouts - offset from current road alignments to aid construction - are clearly discernible.

CFA piling for one of the two railway bridges has started and sheet pile walling for the bridge over the Selby Canal is complete.

All three bridges have been designed to require minimal closure of the canal and rail lines during construction. Skanska and High-Point Rendel worked closely with Railtrack to develop the designs, using wide spans to place the bridge abutments a safe distance from the line. A similar philosophy has also been adopted for the canal bridge.

'Deck installations are the only time we will need to have possession of the canal or railway line, and even then only for short periods, ' says Sleightholme. 'All three bridges are integral steel and concrete composites for minimal maintenance, with decks that are also quick to install.'

Pavement construction has been phased and surfacing contractor Hanson will begin work on the first sections towards the end of this year. The new road will be formed by a 250mm layer of cement bound base overlain by a 205mm binder course and topped with a 25mm layer of Hanson's Tuffgrip thin surfacing.

Landscaping is a major feature of the Selby scheme and more than 100,000 new trees will be planted along the new road. In one area where young oak trees had to be cleared, local people are being encouraged to plant an acorn at home for replanting in the cutting on completion in April 2004. Elsewhere, according to Sleightholme, there are also plans to create the appearance of a disused quarry to give a natural look.

People living around the town of Selby are keen to be involved in the project. A website has been set up to track the progress, and Skanska is also hosting visits for schools and other interested groups.

The bypass is not only good news for the town's traffic problem but is also attracting new business into the town. Negotiations are currently under way with a private investor to add another roundabout to the route to serve a new railhead.

INFOPLUS www. selbybypass.

Clearing congestion A 100m span swing bridge will be the most striking feature of Selby's new bypass and, given the flat topography of the area, will be seen for miles around. The bridge structure - being built by Cleveland Bridge - will use a slew ring bearing to swing the 1,200t, 55m navigation span away from the 45m fixed span on the western bank.

'We have designed the bridge to have a navigation clearance of 3.85m when closed so that most boats will be able to pass without opening it, ' says High-Point Rendel technical director Abdul Farooq.

'Six motors will move the bridge in and out of position in less than two minutes. Rapid opening, combined with the additional headroom, will reduce the potential for congestion on the new route.'

The bridge superstructure is formed from two steel plate girders joined to a series of plate girder cross beams and cantilevers at 4m centres, which support an orthotropic steel deck.

A central plane of cable stays attached at two points along both the fixed and swing spans of the bridge provide additional support to the deck.

The upper ends of the stays are attached to a steel box section tower, fixed to the deck structure, and located over in the centre line of the pivot pier. The bridge control cabin, which will be manned 24 hours a day, is mounted at the top of the steel tower to give the operator a good view of both road and river traffic.

Work on the piling for the bridge is already under way. A series of 50 tubular steel fender piles have been installed at 3m centres, both upstream and downstream of the bridge, to protect it from boat strikes.

Around the pivot pier the fender piles form part of the cofferdam which is being used to construct the foundation platform. Piles have also been installed to form emergency mooring on either side of the bridge.

'Construction of the actual bridge deck will take around eight months and is expected to begin late this year, ' says Farooq. 'The bridge units will be assembled parallel to the river on a temporary platform and slid into position. Once the mechanism is completed, commissioning trials can get under way but throughout construction there will be no obstruction to river traffic.'

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