DEC offer Advanced In-situ Immobilisation and Capping Technique for Polluted Sediments

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Courtesy of DEC NV (DEME Environmental Contractors)

In the July 2002 issue dedicated to environmental dredging of World Dredging Mining & Construction, DEC present its newly developed in-situ immobilisation and capping technique for polluted sediments, the 3SI-technique.


DEME Environmental Contractors N.V. (DEC), a Belgium-based company specialising in remediation and recycling or disposal of water bottom sediments, is offering a vast range of new home-grown clean-up technologies to the world's environmental engineering market. The company's core activities include ex-situ and in-situ treatments of sediments and soils, site remediation and recycling, and isolation techniques, using in-house engineered and constructed equipment and plant.

Prompted by the ever more stringent legislative demands of the Flemish Government on standards for land and waterway remediation, DEC has been concentrating its Research and Development effort into the need for more successful and cost-effective site remediation techniques. This effort has resulted in the development of the patent pending system of Surface Sediment Stabilisation by Injection (3SI-System), assisted by a bucket dredger or cutter suction dredger to treat polluted sediments in-situ.

There are other alternatives for the remediation of contaminated sediments like the complete removal and transport of contaminated sediments to a licensed recycling centre or landfill. Licensed landfills for sediments are however rare and it is not always possible to remove all the material without causing stability problems to surrounding structures such as quaywalls and buildings. Furthermore during the removal of contaminated sediment a very large amount of water is normally mixed with the silt, thus increasing the quantity of material requiring treatment or disposal. Though this can be reduced by special dredging techniques, the volumes of materials to be treated remain considerable. Facilities for contaminated sediment treatment are limited in what they can treat and carry an element of environmental risk while treatment techniques such as thermal treatment or incineration are in themselves environmentally unfriendly and expensive.

The other more often used alternative is the in-situ immobilisation and stabilisation technique. The basic principle of this technique is the injection of a cement-based mixture with additives into the subsoil to create columns or blocks, resulting in immobilisation of the contaminants. This batch-wise method utilises a drilling rig to vertically stir the subsoil. At the same time, chemical agents are injected and mixed by means of jets on the rotating drill head, creating a column in the subsoil. This technique is known variously as Deep Soil Mixing (DSM), Very High Pressure Grouting (VHP) or Soft Soil Improvement (SSI).

DEC's 3SI-System makes further progress by utilising the stirring possibilities and higher efficiency of a bucket or cutter-suction dredger for the treatment of the upper layer of sediment. The 3SI-system combines all the parameters of immobilisation with the stirring action of the dredger. It is able to control the quantity of additives added to per m3 of contaminated sediment and to ensure a homogeneous mixing of the additive with the sediment. The consistency of the treatment area is in this way guaranteed. Any slight muddying or disturbance of the water can be further minimised by pre-placing a thin sand layer above the silt.

There is an ever-increasing tendency to remove only a part of the contaminated sediment and to isolate the remaining quantity from the surroundings (should this be a feasible option), so that the contamination cannot spread further into its surroundings. Immobilisation of the contaminated sediment under water and in-situ by injection of additives is thus employed. Immobilisation can sometimes be achieved by treating only the upper layer. If the upper layer is made impermeable to contaminants, there is no longer any danger to the surroundings. This is known as the capping method. With higher capacity and flexibility offered by a dredger, the 3SI-Sytem has made it possible to treat contaminated sediments (and if desired only the upper layer) in-situ in a more efficient and economical manner.

For further information on the 3SI-Sytem, contact DEC's General Manager Export Dirk Poppe or Technical and Commercial Executive Stefaan Vandaele on +32 3 250 54 11 (fax: +32 3 250 52 53, email: export@decnv.com).

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