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Bypass toes the lime

HIGHWAY MANAGEMENT - Polegate bypass: Innovative use of lime cement stabilisation is cutting costs on the Polegate bypass. Alan Sparks reports from East Sussex.

Polegate in East Sussex will soon be rid of the heavy traffic that growls through its charming town centre.

The £17.2M bypass now under construction will take away the hazards of being Eastbourne's main transport artery.

'Strong support came from the local residents, as the project will reduce traffic through the town's narrow streets from 17,000 vehicles a day to around 4,000, ' explains Highways Agency project sponsor Graham Link.

The project consists of a new 2.9km dual carriageway and a further 1km widening of existing roads. Also included are four bridges, two retaining walls, an underpass and four roundabouts, one of which is elevated to allow a possible westward extension.

Sweeping through a greenfield site, the project is being constructed under standard Agency conditions of contract, with a post contract partnering agreement. WS Atkins is the Agency's representative on site and principal contractor is Miller Civil Engineering Services.

Miller's designer Owen Williams suffered a setback when its offices in nearby Lewes were flooded during the worst of the winter weather. Despite this, the construction team is confident that the design and build lump sum contract is still set to meet its completion date of May next year.

A key player has been earthworks subcontractor McArdle Stabilisation. The firm worked with Miller and Owen Williams to develop the lime cement stabilisation technique being used on the new road.

Lime cement stabilised subbase is not new. But the Agency believes this is the first time the technique has been employed on a UK trunk road as a direct replacement for granular subbase.

Lime cement works out cheaper, and there is no time penalty.

The technique was considered ideal for this job as the ground conditions consist of uniform Weald Clay, and a mains water supply was available on site.

Initially, the clay is trimmed to the top of the eventual subbase level. The top 300mm is then pulverised with one of McArdle's fleet of Wirtgen recyclers, and 2% lime added, resulting in a consistent material with a constant water content.

This is then rolled, sealed and left for 24 hours to allow the lime addition to 'mellow'. The Wirtgen then makes a second pass adding 6% cement and enough water to reach the desired moisture content.

The treated area is then graded and compacted. It must be sealed within two hours using a bitumen-based emulsion to produce the 290mm thick subbase.

Continual testing and quality control have been undertaken as the contractors faced a steep learning curve.

'Cracking was a problem early on until the importance of the moisture content was appreciated. Since then we've had no problems at all, ' says McArdle Stabilisation director John Thompson.

Tight specification requires quality workmanship, with moisture content a key factor.

'Unlike granular sub-base, you only get one shot at meeting the prescribed tolerances - heightening the control required at each stage, ' explaines McArdle site engineer Brian Heron.

As the technique was unfamiliar, it was decided to avoid traffic crossing the sub-base until the next stage was under way. This increased the amount of traffic management required.

However considerable savings have been made. 'By using this lime cement stabilisation we have calculated that 3,850 vehicle movements have been saved, ' says Miller project manager Gary Phoenix.

'If you can use the materials already here on site, large transport savings can be made.

When the forthcoming primary aggregate tax comes in, this will be even more true.'

As well as dealing with an innovative construction technique, the team is making great efforts to ensure local rights of way are maintained throughout the work. It is also trying to keep disturbance from the new road to a minimum both during construction and when the road is opened to traffic.

A 170m long contiguous pile retaining wall is being built close to local houses. The reinforced concrete piles are 1.2m in diameter and range in length from 18m to 26m. The wall will be faced with a sound absorbing blockwork, further reducing impact on local residents.

An additional pedestrian crossing supported off this wall is being installed to access local farms.

'The first concern raised by locals at the initial consultation was ensuring their popular egg supply from the farm be maintained, ' reflects Phoenix.

Keeping in with the locals

Polegate's A27 bypass was the first scheme approved under the then Department for the Environment, Transport and the Regions' Targeted Programme for Improvement (TPI).

The Labour government brought in TPI to reassess the criteria for road building, adopting a more structured approach towards the environmental and economic effects of any proposed project both locally and nationally.

Archaeologists at Polegate discovered post medieval dwellings and a 17th century brick kiln. These are thought to be linked to nearby Otham Court, which dates back to the 13th century.

More recent inhabitants include great crested newts which emerged from hibernation and had to be relocated locally by Highways Agency ecologists.

Bat roosts were also blocked off during construction, with substitute boxes erected away from the site.

Local badgers are catered for with the inclusion of tunnels.

Peanuts are used to tempt the protected animals into using the tunnels.

More than 100,000 trees and shrubs are to be planted along the route, minimising the impact on the local environment.

Community links have been maintained throughout the job by regular meetings with local groups and councillors.

Technical details

£17,2M contract value 4km of dual carriageway 500,000m 3muckshift 170,000m 3from local borrow pit 55,000m 2of lime-cement stabilised sub-base (only on HA portion of works) 89,000m 2of surfacing 14.4km of drainage 3400m 3of structural concrete

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