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Triumph of a bird- brained scheme

AIRPORTS: The trend-setting structure that frustrates Essex seagulls was an intriguing design challenge. Dave Parker reports

The Harris hawk is back in her mews, the pyrotechnic bird scarers have been locked into the security cabinets. From now on the busy landfill site at Ugley in Essex will rely entirely on a less active method of keeping seagulls away from both the refuse and the nearby flight path to Stansted Airport.

The Civil Aviation Authority has acknowledged that the UK's first permanent netting enclosure over a landfill site has achieved the result its designers intended. Nick Shutes, landfill operations director at site operator Hales Waste Control, says the CAA, which advises the local planning department, took a lot of convincing.

'For the first year of operations we had to have supplementary active deterrents like the hawk. We even had to erect one of those plastic strip doors at the entrance to the enclosure, just in case any seagulls tried to hitch a ride inside on the back of the refuse trucks.'

It was the growing influence of the CAA over landfill planning applications close to airports that prompted Hales' search for a really effective answer to the seagull problem. The site, a former sand and gravel pit, had been used for landfill until 1987, but when Hales applied to re-open it in 1996 the first reaction of the planning authorities was unwelcoming. Birdstrikes, particularly those involving large birds like seagulls, are an increasing problem for major airports. Conventional control measures such as pyrotechnics and hawks are not trusted by the CAA (see box).

'It's putrescent waste, mainly discarded foodstuffs, that attract the birds,' explains Shutes. 'But without planning permission to take this sort of waste the site would not have been economic.'

Inspiration came from the existence of a commercial temporary enclosure system, mainly intended for litter control on windy days. Litter control is one of Hales' priorities - there is housing close to one boundary of the site - but this particular system had limited headroom amid other drawbacks.

Hales therefore asked site design engineer Golder Associates to carry out feasibility studies on larger, more substantial enclosure systems. Golder called in structural engineer Robert Benaim & Partners to assist.

'We actually looked at six different enclosure schemes, including one held up by helium balloons,' Benaim director Brian Bell reports. 'Air- supported structures were also considered, as were truss and spaceframe structures, before we decided on a cable net system.'

Shutes adds: 'The problem with an air-supported structure, apart from being too expensive, was that the gases given off by the waste couldn't escape. And the cost of helium is terrifying.'

A tent-type design, with the supporting cables taken down to well spread- out ground anchors, was out of the question. Hales wanted to work up close to the boundary of the site. This meant perimeter towers subject to high overturning forces.

It was also obvious that headroom would have to be considerable - some modern refuse trucks need 8m of clearance with their bodies in the highest tipping position and refuse will eventually be 20m deep.

And economic design seemed to mean supporting the net with intermediate masts within the cell, keeping maximum spans down to 80m. These masts would have to be 28m tall to provide the necessary clearance.

A drawback of this method was that putting a mast within the cell meant the lower section would be progressively buried by the refuse, which would make moving the enclosure to subsequent cells somewhat impractical.

The answer was to design the masts in two 600mm diameter tubular sections, with a sacrificial lower section which would be left behind when the enclosure was moved. The upper section, 'the expensive bit', could then be re-used - seven times at least at Ugley.

Overturning forces on the 10m high steel lattice perimeter towers are countered by ground anchors under the pad foundations. An 85mm square polypropylene mesh was selected for the main covering This is available commercially in 10m widths with a 10mm rope down each edge.

Planning permission was finally granted in early 1997, subject to one month's trial operation without putrescent waste, to check the structure's integrity. Needham Market-based J Breheny won the contract. Earthmoving began in May 1997 and the first load of innocuous waste arrived in December.

Total investment by Hales to date is close to the £1M mark. Shutes says this is equivalent to £2/ton on waste entering the site. 'Essex County Council is so delighted by the system, especially the way it controls litter and nuisance, it would like to see it on all landfill sites,' he adds.

'I don't see this happening. But it is a very effective solution for sites close to airports.'

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