Soil washing cleans up at Royal Arsenal site

The process of washing contaminated earth on-site is a relatively new development in the land reclamation industry. Typically, projects such as the Royal Arsenal clean-up would have been handled by excavating contaminated earth and removing it to an off-site licensed containment cell. However, because of green issues and the rising cost of landfill, the traditional dig-and-dump method is neither as economical nor as environmentally viable an option as it once was. The reclamation of the Royal Arsenal, a site situated on the banks of the Thames, demonstrated how soil washing techniques can offer significant advantages, including, in this particular case, the reduction of over 100 daily import and export lorry movements in a busy corner of south-east London.

Phase 1 of the £15 million project commenced in February 1999 and was completed in June this year. With 130,000m³ of material washed, phase 2 extended the washing to a further 45,000m³ (amounting to an additional 75,000 tonnes) and added £1.6 million to the contract value. The continuation of the soil washing process reflects the economic benefits of keeping plant on the site, offering significant cost savings when applied to large-scale projects.

Soil washing experience

South Yorkshire based VHE Construction’s role in the project was to carry out the site remediation whilst joint venture partners Mowlem Civil Engineering constructed the roads, and installed services and drainage. VHE drew its expertise in the area of soil washing from a contract it carried out at the former Basford Gas Works in Nottingham in 1998. At Basford, outputs of 2,500 tonnes per week were achieved whereas during phase 1 at Woolwich, daily outputs of 1,000 tonnes were not uncommon and between 250 and 500 tonnes were normal.

The Basford project, which used European specialist plant, was the largest operation of its kind at the time and gave VHE the skills and capability to handle the more challenging Royal Arsenal clean up.

The washing plant at Woolwich was designed and built entirely in-house by VHE, with guidance from subsidiary company, VHE Technology. During the scheme, the Environment Agency granted VHE a Mobile Plant Licence. This meant that the plant can now be used on other sites, subject to the Environment Agency’s local site specific licensing requirements, and without the delay normally associated with waste management licence applications.

Complex operation

The strategy employed at the Royal Arsenal, which was formulated by consulting engineer Campbell Reith Hill, was a relatively complex and technologically driven process. Following screen testing for explosive residues and the separation of materials of different sizes, soil was washed, crushed and screened to allow as much as possible to be re-used in the subsequent re-engineered ground works operation. The washing involved attrition, scrubbing, de-sanding, de-silting, fines flocculation and sludge de-watering by centrifuge.

A significant advantage of this technique is that it left only a small amount of residual sludge which, along with material from other existing on-site hot-spots, had to be removed and safely disposed of in licensed landfill sites. Because less material was removed to landfill sites, the need for back-fill was vastly reduced, offering cost benefits by lowering disposal charges and reducing the cost of importing material. Between 80 and 85% of the input material was re-used, of which 70% was gravel and 30% was sand. Washed soil was routinely monitored by Campbell Reith Hill for compliance with the chemical and geotechnical acceptance criteria required in order to allow re-use, ensuring that the site was safely and thoroughly remediated and ready for redevelopment.

The client, English Partnerships, opted for soil washing due to the physical nature of the affected material and the planning requirement to keep lorry movements to a minimum. The soil at the Royal Arsenal is predominantly coarse grained, which results in a lower surface area to volume ratio and therefore increases the effectiveness of soil washing. Consequently, only a small amount of very fine-grained material, which is much harder to wash, had to be removed off-site. Soil washing is a cost-effective technique where there is a range of substances to be treated on generally granular material.

Following the completion of stage two of the project, English Partnerships, together with Berkeley Homes, plan to redevelop the fully reclaimed site for mixed leisure, residential and business usage. The former munitions production complex, which has been closed to the public for over 300 years, will eventually be put to beneficial use by new homes, offices, bars, restaurants and leisure facilities, together with businesses and light industrial accommodation.