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Sealed for safety

ENVIRONMENTAL GEOTECHNICS CONTRACTS

A NEW FOUNDATION system can prevent gas migration into houses built on brownfield sites.

Housedeck II, the latest version of Abbey Pynford's house foundation system, is a continuous insitu reinforced piled concrete slab with a stainless steel edge beam.

Its monolithic design reduces the risk of shrinkage cracks and joint grouting is not needed.

Up to half of all sites in the UK have had some former development and, according to Ciria, contamination affects 360,000ha.

Government pressure for greater use of brownfield sites means developers are having to deal with contaminated land issues, including hazardous soil gases such as methane and carbon dioxide.

More than half of the claims settled by the National HouseBuilding Council are related to geotechnical problems.

In the past, the preferred method for dealing with hazardous gases has been to create a ventilated space under the floor slab and to expel the gases before they can enter the living space.

A 1997 DETR Partners in Technology report (Passive venting of soil gases beneath buildings - guide for design) concluded that for buildings with small plan areas, such as domestic housing, the most efficient way of providing sub-floor ventilation was the open void with vents to the external and internal wall.

Often precast concrete beam and block suspended floors are used. However, this type of construction is not gas-tight and so time and money must be spent grouting all gaps and cracks in the structure before installing a gas-resistant membrane.

Housedeck II avoids this problem and its design also removes the need for trenches, minimising excavation and spoil removal.

This is particularly important on contaminated sites where off-site spoil disposal may be an issue, especially with the introduction of the Landfill Tax.

Three systems are available:

one with a void and anti-heave precautions; one with a void and membrane as appropriate for contaminated sites; and one without a void where only settlement or subsidence is expected.

Abbey Pynford has been researching, testing and developing Housedeck II for three years.

Commercial director of the firm's house foundations division, Nick Hodder, said: 'Several medium sized housebuilding firms have already embraced the concept and the system now has a track record of dealing with complex brownfield sites in the housing sector, as well as light industrial/commercial use which minimises the risk to the developer.'

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