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Stansted strip

Airports - Resurfacing the runway of one of Britain's busiest airports calls for frenzied overnight working. Jon Young reports from Stansted.

A typical 737 weighs 50t and hits a runway at 240km/h when it lands. This combination of speed and weight means that a runway like Stansted's, which handles 200,000 ights a year, gets quite a pounding.

Keeping the 3km strip of asphalt in full working order is a challenge for maintenance contractors as a long term shut down of the runway is out of the question, given the volume of traffic using the airport.

As a result, contractor Amec is resurfacing it in increments over 12 months. It is laying 30,000t of extra-tough Marshall Asphalt so the runway can take another 15 years of pounding.

The resurfacing process is similar to that used in motorway resurfacing, says airport operator BAA's airside general manager David Powel. But the challenges of working on a live runway are different.

After 18 months of planning and liaising with air traffic control and airlines, the £30M project was split into three phases.

Phases one and three are at each end of the runway, while phase two is the central section.

In February, Amec's Pavement & Infrastructure Team, working with client BAA, started phases one and two.

Phase one involves closing the 800m western end of the runway for three nights a week to allow resurfacing to take place while aircraft use the remaining 1,900m.

This and phase three are the most stressful for all concerned as aircraft must land or take off on a shortened runway while resurfacing takes place.

A safety buffer of 300m in the live runway provides some security from planes over running or touching down early.

Phase two is being resurfaced at the weekends when the runway will the shut on Saturday and Sunday nights between (midnight and 6am).

At midnight on Mondays, Tuesdays and Wednesdays the lights are killed before they and radar guidance systems are recongured to the shortened runway length, says Powel.

This has to take place in a 15 minute window as aircraft will be held in the sky before they can land on the shortened runway.

As soon as Amec gets the all clear, 84 vehicles carrying or towing 134 items of plant rush onto the runway to begin a six hour race to resurface a 60m long section of the 45m wide runway and 7.5m wide shoulders.

Once on site the contractor must quickly assemble oodlighting before two planers can begin stripping a 25mm deep section across the runway, cutting partly into the previous night's work.

They then turn through 90° and planes east to west along the 60m length of runway to be tackled that night.

As soon as the rst lengthways strip is planed a bitumen tacking is applied and the new asphalt surface poured and rolled - rst with a steel wheeled roller, then with a hydraulic one, and then with the steel one again.

The hydraulic roller is important to getting all the air out of the asphalt. Its wheels knead the material like dough, explains BAA project director David Hannay.

Marshall sphalt, hich more expensive than traditional asphalts, is used on the runway because of its strength. 'It behaves much more like a concrete than an asphalt, ' adds Hannay.

Pouring the 60m stretch must be completed by 5am so it has an hour to cool from 180°C to the ambient air temperature before the runway reopens.

By this time the asphalt will have gained enough strength for it to take the massive force of an aircraft touching down.

Three nights later the team will return to the same section to cut grooves at 25mm intervals into the new surface.

These will improve the skid resistance of the surface and reduce the risk of aquaplaning in poor weather.

Alongside the resurfacing work, a massive lighting replacement project is also being carried out.

Approximately 1,000 lights are being replaced, including side lights that line the runway shoulders every 60m, centre lights 15m apart and touchdown lights spanning the runway every 60m for the first 900m at either end.

Precast concrete plugs to house the runway lights are set in resin in the runway rather than being cast insitu. The resin will set more quickly than concrete and will reach its design strength 6am if in place by 2.30am.

Once the night's work is complete, Hannay walks the length of the works site, inspecting for any foreign objects or small stones that could get sucked into an aeroplane's engine.

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