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The way ahead


Environmental and cost-saving benefits of geosynthetics have become increasingly recognised over the last decade. As part of this month's focus on geosynthetics, Ground Engineering takes a look at some recent contracts showing how geosynthetic materials can often offer engineers viable alternatives to conventional construction techniques.

Geocomposites will keep Eden green

Geosynthetics are playing a key role in the construction of two huge greenhouses for the Eden project in Cornwall.

THE £57M EDEN project near St Austell includes two huge domed hothouses built in a disused China Clay pit.

When Eden opens in spring 2001, these 'biomes' will be home to more than 10,000 exotic plant species and it is hoped the complex will become one of the world's leading botanical gardens.

The two biomes are made up of 13 intersecting geodesic domes covering 2.2ha, and their foundation consists of a continuous weaving 890m long ground beam.

Of the larger hothouse's 550m long foundation, only 100m is being built on bedrock. The rest, and all of the 340m foundation of the smaller dome, sits on soft fill reclaimed from the quarry.

To ensure the hothouses get as much sunlight as possible, they are being built part way up the steeply sloping northern side of the quarry. A bund between 14m and 20m high had to be constructed on the clay pit floor on the structure's southern edge before the foundation beam could be installed.

The ground beam is 2m wide and 1.4m thick and rests on a 2m wide base slab on top of the bund, anchored at centres between 2.5m and 5m with 10m long ground anchors. Foundation contractor was Dean & Dyball.

Rapid consolidation of the earthworks was essential to keep construction on schedule. But the bund material is variable clay fill from the quarry, so to ensure that consolidation would occur evenly and in the shortest possible time, geosynthetics had to be incorporated into earthworks design.

Consultant Anthony Hunt Associates chose Colbond Geosynthetics Enkadrain Wide supplied by MMG Civil Engineering Systems.

This consists of a drainage layer of looped polyamide filaments sandwiched between two geotextile filters, providing a flexible and resilient layer which has a 95% open area for water to pass through.

More than 170,000m2 of Enkadrain Wide has now been installed by contractor McAlpine Joint Venture Eden Project (a jv of Sir Robert McAlpine and Alfred McAlpine) at 1m intervals throughout the 95% compacted clay fill.

Mel Brash of MMG explained: 'The use of this high capacity longitudinal drainage geocomposite to replace sand or gravel drainage layers created much shorter and highly permeable drainage paths within the structure.'

Enkadrain Wide has also been installed between the fill and the faces of the quarry walls to provide a permanent near vertical drainage layer that will prevent a build-up of hydrostatic pressure and ensure long term stability.

Runoff will be channelled to a reservoir, where it will be treated and then used within the complex.

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