Royal Arsenal, Soil Washing

The reclamation of the Royal Arsenal, a site situated on the banks of the Thames, was carried out as a joint venture between VHE and Mowlem Civil Engineering. VHE's role in the project was to carry out the site remediation whilst Mowlem constructed the roads and installed services and drainage.

Phase One of the project involved the washing of 130,000 cubic metres of material, Phase Two extended the washing to a further 45,000 cubic metres (amounting to an additional 75,000 tonnes) and added £1.6 million to the contract value.

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. This 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.

During Phase One, daily outputs of 1,000 tonnes were not uncommon and between 250 and 500 tonnes were normal. Between 80 and 85 per-cent of the input material was re-used, of which 70 per-cent was gravel and 30 per-cent 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 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 was predominantly coarse grained, which results in a lower surface area to volume ratio and therefore increases the effectiveness of soil washing.

Following the completion of Phase Two of the project, London Development Agency, together with Berkeley Homes, plans 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 used for the development of new homes, offices, bars, restaurants and leisure facilities, together with businesses and light industrial accommodation.