Passive activity

A brownfield site in West Drayton, Middlesex, needed treatment during the second phase of a residential development or Acton Housing Association. The site was required or 70 housing units, including semis, terraced houses, flats and an area of sheltered accommodation. The first phase of the development was constructed on an uncontaminated site on the other side of the River Fays, where 50 units were installed for a cost of £3m.

The brownfield site had previously been used for the production of paints, resulting in a cocktail of organics, inorganics and heavy metals polluting soils and groundwater. Following a consultation period with the Environment Agency and the Local Health Officer, a soil mixing approach was considered to be the most cost-effective option for remediating the site.

UK first
"This was the first time that the technique had been used commercially in the UK to treat organic contaminants," maintains Dr Chris Evans, development engineer for May Gurney's Piling Division. "In heavily contaminated locations, such as under the original paint storage and transfer areas, a series of overlapping treatment columns were installed using a specially adapted auger plug and piling rig. A conventional continuous flight auger piling rig rotates a modified mixing auger into the ground to treatment depth. The auger is then reversed and withdrawn as the cement-based slurry is pumped from nozzles at the base of the auger. The action of the auger allows the cement-base slurry, incorporating specially designed organophilic clay additives, to be mixed with the soils to produce homogenous columns, chemical fixing and encapsulating pollutants."

Active containment
An in-ground active containment system to prevent groundwater pollution migration was also installed. This consisted of a low permeability passive barrier, constructed to divert groundwater flow away from the adjacent river and to funnel water towards a more permeable pillard clay active section. This section acts like a micro-chemical seive, removing pollution from the groundwater as it flows through.



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