The beauty of Ugley tip

Ugley might be considered an apt name for a town that has a landfill tip as its main claim to fame. The small Essex town might be saddled with the name, but if there can ever be such a thing as a good-looking landfill tip, the Ugley site must come as close as you can get.

There are no flocks of seagulls circling overhead or scavenging their way through the domestic refuse. Neither has the wind strewn bits of paper and polythene around the site or transported them to become eyesores tangled in the surrounding trees and bushes.

The reason for the absence of birds and litter is down to an enormous tented structure that completely encloses the area where the waste is being tipped. The reason for the netting, which has been dubbed the 'Millennium Fruit Cage' by structural engineer Robert Benaim and Associates, is that Stansted airport is only about 5km away.

Fears about bird strikes led the Civil Aviation Authority to request that planning permission to dump refuse be conditional on landfill operator Hales installing some form of deterrent to large flocking birds like gulls and crows. The fruit cage works a treat.

Ranged around the perimeter of the tip area are a number of galvanised steel towers - not too unlike those depicted in World War Two prisoner-of-war camp films, but without the guards. In the central area of the tip, there are tubular steelwork masts planted in large concrete bases. The towers and masts are positioned to create grids approximately 80m x 60m.

Steel cables, 38 mm in diameter, supplied by ropemaker Bridon, run between the tops of the towers and masts and are used to support the netting at heights up to 30m above the tip. The netting is attached to large concrete blocks at 10m centres which act like tent pegs, to kept the 85mm x 85mm mesh size plastic nets anchored to the ground around the perimeter of the tipping zone.

Benaim director Brian Bell recalls that: "Six different designs were kicked around, including an air-supported structure, before we settled on the tented structure."

Bell says the air-supported idea was soon dismissed. "It would have been dangerous to work inside the enclosure. I would hate to think of the level of the exhaust fumes inside the dome."

Also the installation and running costs plus the limited size of the air-dome would have severely dented the economics. The other options were dismissed for equally compelling reasons such as impracticality or just straightforward economics.

The netting solution allows Hales to enclose a cell that can take 100,000m3 of refuse.

The Essex site has been divided up to provide 14 cells. Hales' landfill manager Richard Hill estimates that it will take 12 years to completely fill the 10m-20m deep pit created by sand and gravel extraction.

On average, Ugley receives 60 truckloads of refuse per day, roughly 500 tonnes.

As each cell is filled, the netting is moved on to the next cell to provide a new enclosure. This job, for main contractor Breheny, is being undertaken by cable supplier Bridon which in turn has appointed abseiling contractor CAN to do the aerial trapeze work.

Although the netting enclosure is 'open' to the elements, it has nevertheless been designed to withstand high wind and snow loadings.

For example, wind-blown debris could be trapped against the netting to create an impervious barrier and thereby induce structural loading on to the cables and the towers.

Therefore the structure has been designed to withstand debris being trapped around the bottom 3m of the netting and subjected to wind speeds of up to 42m per second.

Likewise, consideration had to be given to snow and ice that could build up on the netting and which could cause a spectacular structural collapse. The innovative solution chosen by Benaim was to design the plastic clips that hold the netting in place to act as a "structural fuse."

These clips snap if the snow loading gets too high and thereby allows the netting to fall to the ground without causing damage to the towers or cables. The other solution was "to have a cherrypicker on hand and send people up to shake the netting," quips Bell.

The effectiveness of the structure to deter gulls was apparent almost from day-one says Hill. "You would get one or two fly over but, as soon as they see the netting and that there is no way in, they would disappear."

Although the netting has proved a success, Hales still has to keep another deterrent on site for seven days per week at a cost of £2,000 per week. This is a Harris hawk which usually given a flight in the evening time but the only prey it gets to find are rats and mice.

Both Hales and Benaim are a little cagey about revealing the cost of the enclosure. All Hales will say is that the total investment at Ugley is around £1 million, which also includes roads and other infrastructure works.