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A dynamic solution

Foundations : Dynamic compaction

A combination of compaction techniques has been used to develop an old landfill site in south London.

Land is scarce in many developing urban areas, so landfill sites offer an obvious opportunity for future expansion. However, this is not always straightforward.

Costly technical and environmental issues need to be resolved. Organic matter in various stages of decay - often generating gases and producing dangerous leachates - is a common concern. And tips have often been filled to above the surrounding ground, requiring extensive levelling to allow for development. That was just the case at distribution and services firm ProLogis' 22,250m 2Croydon warehouse site.

Tipping stopped 10 years ago, leaving a plateau 7m to 8m above ground level - too high for the proposed distribution centre.

But hauling large quantities of contaminated soil off site was considered too costly. So the challenge facing design and build contractor McNicholas was to find a more economic alternative.

'The cost of this element alone would have exceeded £1M and was the deciding factor in whether the client would proceed with this particular development or not, ' says McNicholas engineering design manager Adam Tyrie.

Groundwork contractor, Keller Ground Engineering, solved the problem by opting for dynamic deep compaction (DDC) - a system of high energy tamping which involves dropping heavy weights from a controlled height. 'With the costs involved, every extra 100mm of level reduction was critical to the project's overall viability, ' says Tyrie.

To support the structural frame and ground slab of the development, Keller proposed 2,300, 380mm diameter driven cast insitu displacement piles, with a 960m long, 600mm wide and 7m deep slurry cut-off wall to control leachates. A range of measures, including reduced energies and exclusion zones around existing properties and the perimeter slurry wall, were proposed by Keller to keep any impact on surrounding properties to a minimum.

Using two dynamic compaction rigs, Keller completed compaction of the first 20,000m 2in five weeks, producing on average 750mm of settlement. Following residents' concerns, Keller used a high energy impact compaction (HEIC) technique for the area closest to neighbouring cottages.

New to the UK, the system is offered by Landpac in collaboration with Keller, and uses a three or five-sided dual roller module pulled by a tractor unit.

Slocombe explains: 'As the drums roll, so the sides rise and drop, providing a rapid impact system that can induce compaction up to depths of 3m.'

Once the ground was prepared, the 17m long piles were driven into the 8m of refuse, a thin layer of gravel and the underlying stiff London Clay.

The solution satisfied the Environment Agency's concerns about potential vertical migration of leachates.

Reduced displacement from this option kept down the amount of spoil generated. The system's heavy driving capability was able to combat any impediments encountered within the refuse.

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