Regeneration site demonstrates 'suitable for use' approach
Dr Geoffrey B Card, Chief Executive, Card Geotechnics Ltd, describes how the remediation of a former industrial site demonstrates the 'suitable for use' policy, stating that the success of this project was due to collective team co-operation between the developer, regulatory authorities and Card Geotechnics seeking to replace a local eyesore with a sustainable development
The nature of the ground contamination present was consistent with the former uses of the site Elevated concentrations of arsenic, chromium, lead and nickel were identified together with localised "hot spots" of mineral oil. In addition, elevated concentrations of toluene extractable material (TEM), exceeding HSE criteria, were also detected.
Groundwater analyses were undertaken on samples which were visually contaminated with hydrocarbons. In addition, although concentrations of methane and carbon dioxide were found in the ground no detectable gas flows were recorded from the standpipes indicating that the volume of methane and carbon dioxide present is insignificant.
As a result of the ground and groundwater contamination identified on the site, a remediation scheme was designed and a Remediation Method Statement agreed with the Director of Housing and Health, Guildford Borough Council and the Environment Agency, prior to commencing work.
A constraint on remediation was that site levels should remain as existing in order to protect the existing flood plain. Thus it was not possible to provide additional capping layers and raise site levels. The proposed method of site remediation therefore involved part excavation and removal of selective "hot spots" of contamination and encapsulation, using an engineering capping layer, to existing site levels. This was considered to be the most practical and economic solution.
One specific area had widespread contamination of both heavy metals and petroleum hydrocarbons. A general site strip was removed to eliminate the majority of these contaminants which were concentrated at the site surface. The spoil arising was classified as hazardous industrial waste and was removed from site to a licensed disposal facility.
During these excavations the environmental engineer supervising the works observed that localised areas of exposed ground began to generate steam vapour with heating of the soil mass. A probe, used to identify the extent of the heating in the soil, confirmed that an exothermic reaction was taking place in the Made Ground. This was confined to isolated areas comprising black or dark brown fused slag or clinker, which easily crumbles to a fine powder, and the reaction appeared to have been triggered by exposure to moisture and oxygen in the air.
Chemical analysis of the material was carried out and it was judged not to be an environmental risk. Notwithstanding this assessment, on the request of the developer, Richard Cook Ltd, all Made Ground in these areas was removed to expose natural soils. Localised areas were excavated deeper to remove petroleum hydrocarbons, such as mineral oil found in certain trial pits.
It was initially estimated that 3,500m&3; of contaminated material would need to be removed from the site. However, a significant proportion of this material was left on site and placed beneath building units as general fill. As a result, the amount removed from site was reduced to 3,000m3 thereby safeguarding against possible migration and avoiding unnecessary road haulage through the local area.
The purpose of the capping layer is to isolate the contaminated material from future users of the site by covering it with a physical barrier of clean material sufficient to prevent direct contact and upward transfer of the contamination over the long-term use of the site. Where possible, and so long as it was not contaminated, the brickwork and concrete from the demolition of the original buildings was crushed and reused as capping material beneath hardstanding areas.
Based on the measured gas regime, no gas protection measures have been incorporated into the development. Notwithstanding this fact, all ground floors were constructed as suspended slabs with an underfloor ventilated void to allow dispersion of soil gas if any should become present for whatever reason.
The on-site works were undertaken and completed under the inspection of a qualified geo-environmental engineer from Card Geotechnics to ensure that the excavation and encapsulation operations complied with the agreed Remediation Method Statement.
Hence, the site has been remediated and poses no risk of significant harm to
human health or ecological systems. In line with the Government's policy of
"suitable for use approach" the site has been remediated for the planned
end-use for office and commercial development with open landscaping and associated