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PLANES ON A SNAKE

GEOSYNTHETICS

With three planes and sweeping curves, the geometry of an embankment for Bodmin's A30 bypass required close attention to detail on site. Damon Schünmann reports.

Iron Bridge, which carries the Par to Newquay railway line over the A30 in Cornwall, is regularly hit by tall vehicles - just one reason why the single carriageway needed to be improved.

The A30 is Cornwall's main road link to the rest of the UK, and an 11.5km setion from Bodmin to Indian Queens is now being upgraded and widened in a £93M project (of which £60M is construction costs) for the Highways Agency.

'Goss Moor is famous for traffic jams and there's a low railway bridge [Iron Bridge] that gets hit at least once a month, ' says Tony De Jager, design manager for main contractor Alfred McAlpine. 'This new road will be all dual carriageway where the old one was single.'

McAlpine and consultant Scott Wilson realised that where the road crosses a valley at Holywell, changing the design from a viaduct to a high embankment would save millions of pounds.

This was partly because there were to be areas of cut at either end of the site that would provide suitable stone fill for the embankment.

A reinforced soil slope was chosen to minimise land take and reduce fill volumes. Huesker won the £90,000 geosynthetic contract to supply 50,000m2 of Fortrac geogrid ranging in strength from 35kN to 110kN, and HaTe 23.142 erosion mesh for the inside of the front faces.

Huesker applications engineer Niall Corney says: 'The structure, at 22m high, is without doubt the largest reinforced soil slope in the UK supporting a dual carriageway, and is arguably the highest reinforced slope in the country.'

The Holywell underpass embankment is in the centre of the A30 upgrade project and will incorporate a culvert to allow a local farmer to cross it. The embankment is being built with a 63° face to further reduce land take.

Construction is in two phases: the lower, 12m high tier will be separated from the 10m high tier above by a bench on which the underpass will sit. Site workers will build the second tier over and around this passage.

The fist tier will include a stream culvert and an underpass for wildlife, which includes otters, badgers, snakes, lizards and some protected species.

Because the embankment's shape is complex, getting it right required close attention to detail.

'We've got three planes here that make it hard to design for, ' says McAlpine senior site engineer John Speak. 'The main carriageway orientation is east-west, then the culvert and mammal tunnel go north-east to south-west.

'But the valley's natural sides are north-west to south-east and there's a 10m natural ground fall from one side of the embankment to the other, so the culvert is on a grade of about 8° grade. This means we have to account for all these planes making it difficult to tie in the primaries [primary geogrids].'

The embankment's face also features sweeping curves along its width, adding further complexity to the shape. 'It's a three-dimensional problem as we normally like geogrid reinforcement to go in horizontally, ' says Laurence Tomlin, an associate at consultant Peter Brett Associates which provided detailed construction drawings, based on a Huesker design, for subcontractor Kerbline to work to.

Kerbline's construction manager Andy Cotter says: 'You've got so many curves angles, it makes it more difficult than working in straight lines.

'The geotextile reinforcement detail down the bottom was difcult to fit around the concrete slab that protects the culvert pipework.

It looks like the prow of a ship, but we're pleased with it now it's finished.'

A stepped drainage blanket underlies the structure to prevent creating a slip plane on the valley face. And because the embankment crosses a valley, it is longer at the top than at the bottom.

Corney says: 'Reinforced soil slopes made of granular lls up to about 40° do not require a 'wrap-around' face since the soil is generally able to be formed at such angles and no long term erosion problems occur.

'However, when you are looking at much steeper faces you have to ensure that the fill material, especially the topsoil at the face, is retained.

'To do this the layers of geogrid buried horizontally within the reinforced soil block extend out from the fill and are brought up the front face and wrapped back over the top of the lift of compacted fill, then in turn buried by the next lift.'

This sequence is then repeated with the use of temporary shutters or formwork.

Kerbline is building the Bodmin embankment with a primary layer (longer tail lengths and stronger Fortrac grades) for every 1m lift, with a secondary layer midway between each primary.

The secondary layers have standard 3m tail length and are all Fortrac 35kN.

This means site workers form the face with wraparound lifts of 500mm, typically backlled with 250mm deep layers of ll compacted to 95% of maximum dry density.

McAlpine public liaison ofcer Alun Jones says: 'Because it's a design and build, early contractor involvement job, it has meant we have barely had to dip into our risk register for the project.

'We came on board three years ago and the most obvious benefit of early involvement was we could plan the environmental mitigation in advance.

'If we hadn't had this we would have had a six-month delay to cater for [environmental mitigation] at the start of the job, which would have put a couple of million on the price with the way oil and steel costs were going up at the time, ' Jones says.

Work on the Holywell embankment should complete by the end of this month while the A30 upgrade, which began in July 2005, is due to finish in July next year.

Geosynthetic installation was a challenge. 'It's a 3D problem - we normally like geogrid reinforcement to go in horizontally, ' says Laurence Tomlin of consultant Peter Brett Associates.

Conservation issues

The A30 upgrade passes through several environmentally sensitive areas, so local wildlife had to be catered for.

Main contractor Alfred McAlpine has been creating paths for animals to cross the new road, as well as relocating others.

'We have about 18 designated reptile areas with fencing used to relocate them. It has an inclined face so they can get out but not back in, ' says McAlpine public liaison ofcer, Alun Jones. 'We have also captured 361 adders, grass snakes and lizards.'

Scott Wilson environmental manager Alan Phillips says: 'The otter paths played hell with the reinforcement design and needed a lot of thinking.

'It led to quite a bit of mirth in the office as we tried to think like otters to see how they can migrate, even to the point we used a stuffed otter to see if it could get through the migration paths.

'But we decided we were better at thinking like reptiles.'

Jones adds that while one badger sett had to be closed, and 12 bat boxes were installed.

An archaeological study found traces of Cornish henges - at areas surrounded by a ditch or bank - and a 3500-year-old roundhouse.

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