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Solid argument

CONCRETE ENGINEERING - With 'dig and dump' on its last legs as a way of dealing with contaminated earth, a group of key industry bodies is sounding its support for soil stabilisation and solidification techniques. Alan Bromage reports.

Dumping contaminated soil into landfill sites is becoming an increasingly unattractive option for contractors as costs rise and EU legislation tightens.

The Concrete Centre, working closely with the British Cement Association and the British Lime Association, believes that soil stabilisation and solidification (S/S) offers a fast and cost effective remediation solution.

S/S is a civil engineering technique used to improve the physical properties of soil. Cement or lime-based binders are mixed on site with contaminated soils to form a new solid material. The result is that contaminants are rendered practically immobile - and therefore non-leachable.

The most commonly used principal binders in the UK are cement, lime, ground-granulated blast furnace slag and fly ash.

S/S involves the chemical modification of contaminants by absorption, preprecipitation of more soluble forms and the formation of low-solubility salts.

S/S also physically alters the soil to make it more stable and less prone to being affected by external agents such as passing groundwater which could mobilise the contaminants.

Additives are used to modify and fine-tune the effectiveness of the process. These include pH/ Redox modifiers such as wetting agents, flocculants and sorbents.

The binders and additives are mixed into the soil using rotovator-type plant; modified excavation plant; augurs and sub-surface injection; single pugmill; or a 'treatment train' that involves a range of processes.

The method of mixing depends on the soil and contamination type.

Alan Bromage is head of civil engineering at The Concrete Centre.

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