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Bridges: Road to recovery on the Isle of Grain

Drivers heading for the Isle of Grain’s container port and LNG terminal are taken aback by the number of fading flower bouquets tied to lampposts on the road across this exposed former marshland on the Kent coast.

Danger spot

The A228 is dangerous.

Recently introduced camera enforcement hopes to cut speeding on this notorious section of road, but Medway Council also wants to upgrade the road to take out dangerous curves and bends.

The biggest of the current projects being built under this strategy is the Stoke Bridge, which will lift the road across a single track freight line to the port. The 250m long curving five span bridge will remove an obstructive level crossing, a couple of partial blind curve approaches and an adverse camber.

The live railway has been less of a problem than might be thought for main contractor Birse Civils during construction, which began last August and will finish next year.

There is a train only every hour or two and Network Rail is a little more relaxed than it would be for a busy passenger route. “And trains don’t run at weekends so it is fairly easy to get possessions if needed,” says Birse site manager Paul Hinkly.

The complexities of the job lie more in the area’s difficult marshy ground and its exposed, location.

“There are a lot of major services to avoid or move, including a Government Pipeline and Storage System line for aviation fuel and big gas pipelines”

Birse site manager Paul Hinkly

Environmental preservation has also been important, with parts of the area designated as a Site of Special Scientific Interest and also as a special protection zones for some species, including water voles.

“And there are a lot of major services to avoid or move, including a Government Pipeline and Storage System line for aviation fuel and big gas pipelines,” says Hinkly.

Piling is needed for both the main bridge structure, which has five 50m spans, and for some of the embankments that bring the road to bridge height and are up to 4.5m high.

“The ground here is about 1m of made ground with 6-7m of estuarine alluvium underneath. Below that is either London Clay or Lambeth Group,” says Peter Rutty, geotechnical designer at Mott MacDonald, which has designed the new bridge.

Harsh conditions

The soft alluvium requires 20m deep bored piles for the piers. Piling has also been selected for embankments over 2m high, with a load transfer platform above individual pile caps at 2.5m centres. For the bridge structure 900mm piles are being used, while the embankments need 11m piles of 450mm and 350mm diameter through the soft ­material.

“We chose to use CFA [continuous flight auger] because bored piles would have needed casing down through 11m of the alluvium,” explains Hinkly.

Piling was completed before the very cold Christmas by Balfour Beatty Ground Engineering, and the concreting for the piers by March this year, along with pilecap work for the embankments. The weather was terrible, says Hinkly, first with the extreme cold and then heavy rain and winds “which blow right across this exposed landscape”.

Keeping the trains on the track

The bridge structure continued with installation of steel girders in March for the composite deck. Mabey Bridge fabricated the beams, which needed a 1,000t crane for installation during a number of possessions.

On those, a GRP permanent formwork was installed for the deck concrete, all of which was done “without any need to stop trains”, says Hinkly.

“As long as you can satisfy Network Rail, it is possible to work during normal hours”

Birse site manager Paul Hinkly

“As long as you can satisfy Network Rail that there will be no leakage of grout, and that the concrete booms are not within a collapse radius for the rail line, it is possible to work during normal hours.”

For the cantilevering parapets Birse’s concrete subcontractor, Kent firm Ainsworth Civil Engineering, worked with formwork specialist Doka from fairly early on in the project, using a tailored bridge formwork system called Paratop. For this project they were made in advance at Doka’s Sheffield depot and brought to site as 6m long units, which could just be lifted into place.

The units attach to the bridge girders using a quick system of clips and brackets rather than bolts, and cantilever outwards. Ainsworth coordinated with Mabey, so that any necessary connectors were welded onto girders in clean conditions at the factory.

Knitted fabrics

Meanwhile, work was under way on the embankments. These were kept entirely separate from the main structure, with a 100mm gap between the abutments and the bridge itself “which avoids transferring lateral load”, according to Rutty.

For the higher piled sections a 600mm transfer platform was formed above the 900mm square pilecaps using a Tensar geofabric, Basetex 1000, and granular fill. The knitted manufacture of the fabric keeps the main fibres straight along its length for higher strength.

Specialist ground contractor Cofra laid the fabric, which is unrolled firstly across the line of the embankment, underneath the transfer fill material, and then longitudinally in a layer embedded 300mm up. Cofra’s excavator driver needed some dexterity to handle the heavy fabric rolls, which are mounted on a beam to unroll.

Cofra also installed a series of wick drains up to 9m long for settlement on lower parts of the embankment.

The embankment face for the 100mm gap at the end was made using a reinforced earth structure using a Tensar system of split face concrete blocks and geogrid.

Work on the western end was underway this month with the eastern section waiting service diversions. Full bridge completion will be next year.

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