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Case study: High Wycombe, Bucks

Soil Washing

Second Site, the property management arm of UK energy company National Grid, has the task of cleaning up some of the country's most contaminated sites - former gas works. In an earlier incarnation, the company was client for the UK's fi rst soil washing job at Basford Gasworks in 1998. Now it is pushing ahead with its second soil washing scheme, in central High Wycombe, Buckinghamshire.

Potential for contamination to the local water supply made the High Wycombe site a priority. 'Although there was no immediate risk, long term it was an environmental liability. Sooner or later they would have had to clean this up, ' says contractor Morrison's remediation manager Steve Lilly.

The 1.44ha site is on a hillside, with old tar and ammonia tanks located at the bottom of the slope. These sit below the water table, posing the danger that toxins could leach into groundwater. This is where the main excavation and remediation effort is being focused.

Environmental onsultant Konnex designed the £3M environmental clean up which is being managed by Morrison, with specialist contractor DEC providing the soil washing plant.

The top 2m of clean made ground has been removed and stockpiled for use as backfill.

Lower layers, down to putty chalk bedrock, are a combination of made ground and a gravel-clay mix contaminated with coal tars and ammonia. This is being separated into three grades of waste and treated.

Of the 37,000m 3 of spoil removed, 8,000m 3 is being washed and 80% of that will be reused as backfill. However, 13,000m 3 of the material is so heavily contaminated there has been no option but to send it to landfill.

A cellular excavation system (see diagram) has allowed Morrison to prevent any mobilisation of contaminants.

Sheet pile cofferdams are installed and ground water within each cell is pumped down, ensuring that any water flow while excavation is carried out will be inward only.

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