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Take a load off Selby

GEOSYNTHETICS

A 1.8km long geogrid reinforced load transfer platform is solving the problem of embankment construction over soft river deposits on the long-awaited Selby Bypass.

Max Soudain reports.

Abypass for Selby was first proposed in 1929, not to relieve traffic congestion but to avoid the toll bridge across the River Ouse in the centre of the north Yorkshire town.

Although there is no longer a toll, the swing bridge still causes problems for motorists. Two major roads - the north-south A19 and the east-west A63 - converge with the smaller A1041 in the town and traffic from all three has to cross the ageing bridge.

To make matters worse, clearance beneath the bridge is so low that it has to be opened to let even small pleasure craft by, a process that takes at least 10 minutes.

With 18,000 vehicles passing through Selby every day, a tenth ofthem lorries, the town is fast approaching total gridlock.

Now, after years of false starts, route changes and campaigning by residents, local businesses and politicians, Selby is finally getting its bypass. The 10km single carriageway will run from the A63 near Thorpe Willoughby to the south west, around the town's southern edge, and join the A19 near Barlby to the east. It is hoped the new road will divert 40% of traffic from the town centre.

The £42.6M scheme is one of the first to be let under the government's 10-year transport plan.

The design and build contract was awarded to Skanska and its designer High-Point Rendel in July 2001.

Most of the route runs across low-lying agricultural land. It includes five new bridges because the bypass crosses over a road, the Selby Canal, the Selby to Doncaster and the Selby to Hull railway lines and the River Ouse.

There is also a 13m deep cutting at the western end.

The route is underlain by Sherwood Sandstone, a weakly cemented rock that forms the founding layer for all the piled foundations. It lies between 1m and 2m down at the western end, increasing to between 16m and 18m at the eastern end.

The sandstone is overlain by between 1.5m and 3m of sand.

Above this to the east of the canal bridge are glacial laminated clays, which thicken eastwards to a maximum of 16m, thinning again to about 5m at the eastern end. There are two distinct clay layers, separated by 1.5m of sand.

At eastern end of the route, the clays are overlain with alluvial deposits associated with the River Ouse floodplain. These are up to 9m thick and include a lower layer of very clayey peat and very peaty clay up to 6m thick in the last 2km of the bypass.

The peat is highly compressible with a high creep potential and is typically overlain with a soft to firm sandy clay.

Groundwater is typically less than 1m below ground level in the upper alluvial deposits to the east of the route and perched ground water is identified about 2m below ground level above the laminated clays. To the west, groundwater in the sandstone is typically between 3m and 5m below ground level. However, the piezometric level in the sandstone reduces to up to 6m below ground level in the east because of local abstraction from the aquifer.

Because about 3.5km of the road will be on embankments from 3m to 9.5m high, the potential for significant settlement in the glacial and alluvial deposits gave cause for concern, explains Hugo Wood, geotechnical engineer for High-Point Rendel.

The River Ouse floodplain presented the biggest geotechnical challenge. Embankments more than 9m high are needed to raise the road to the level of the centrepiece of the bypass, a new 95m long swing bridge over the river.

The bridge will be high enough to allow all but the biggest boats to pass. If it has to be opened, the process will take two minutes. It has a 55m navigation span, 40m back span and a 30m fixed span on the northern bank. The navigation span is supported on a pivot pier on the south bank, a landing pier on the north bank and an abutment wall at the end of the back span.

The pivot pier is founded on 18 vertical and ten 4:1 raked, 610mm diameter tubular steel driven piles, while the south abutment sits on five vertical and seven 4:1 raked, 660mm diameter piles.

Both groups are founded in the Sherwood Sandstone.

The landing pier and north abutment are supported on 750mm diameter CFA piles founded in the sandstone.

Thick peaty deposits on the Ouse floodplain meant piles were needed to control settlement and maintain stability of the 1.8km long embankment stretching to the A19 roundabout at the end of the bypass.

A platform of geogrids is used to transfer the embankment loads onto the piles, which are founded in the Sherwood Sandstone. This also allowed the pile spacing to be increased from the original design.

Cementation Foundations Skanska installed 5,000, 370mm and 425mm diameter driven cast insitu reinforced concrete piles with 900mm diameter cast insitu heads. Piles are between 2.65m and 3.2m apart, depending on embankment height, their heads formed flush with the top of the piling platform - a 45kN woven biaxial geotextile supplied by Huesker Synthetic, covered with 650mm layer of stone fill.

A 100mm thick layer of sand fill was placed above the pile caps, followed by a transverse layer of 800mm wide geogrid strips. The amount of transverse reinforcement depends on the height of the embankment, says Graham Horgan, an applications engineer at Huesker Synthetic.

'Layers of either Fortrac R1200 grid [a strength of 1200kN/m], or Fortrac 1600 grid [a strength of 1600kN/m] or a combination of the two are used, ' he says. For example, 4m high embankments have one layer of R1600, while the highest (9.3m) have two.

Longitudinal reinforcement is the same throughout: 5m wide strips of Fortrac R400 (400kN/m) placed directly on the transverse layer(s) and covered with 500mm of Class 6D fill. 'The piles and the grid form a composite structure, ' explains Horgan.

Embankment fill is lightweight pulverised fuel ash (PFA) to minimise loads on the piles. A lap length is left at each end of the transverse layers which are anchored beyond the outermost piles in Class 6D fill.

West of the river, embankments rise as high as 9.5m for the various bridge structures but their behaviour is governed largely by the laminated clay, typically a firm silty clay with laminations of silt. Time for 95% primary consolidation of the clay was estimated to be 100 weeks where it is thickest and hold periods were enforced before construction of the road pavement. However, in places where post-construction settlement was anticipated to be unacceptable, surcharging has been used.

At the eastern end of the bypass, the A19 roundabout was also going to be piled but further investigation revealed that the alluvial deposits were thinner here and surcharging could be used.

Basal reinforcement ensures stability during construction and surcharging, with up to 1.5m of surcharge fill placed. To ensure adequate drainage of the peat where it is confined between alluvial clay layers, about 800 vertical band drains were installed from a drainage blanket through the upper alluvial layers to the intermediate glacial sand. These should mean 95% of primary consolidation occurs during surcharging.

Sections of the embankments are heavily instrumented to monitor stability during and immediately after construction and for long-term monitoring using settlement plates, inclinometers and pneumatic piezometers.

The Oakney Road bridge, Middle Lane railway bridge and Magazine railway bridge are all single span structures supported on reinforced concrete abutments on 900mm diameter CFA piles.

Embankments immediately behind the abutments are all supported on a combination of up to 450mm diameter bored and CFA piles to prevent differential settlement between the bridges and earth structures.

When the bypass opens at end of 2003, it should have positive effects beyond Selby's traffic problem. The area has been devastated by past and recent closures of the coal mines that once made it prosperous. Locals hope freer flowing traffic will bring with it new opportunities.

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