Regeneration site demonstrates ‘suitable for use’ approach

Set on a two-acre island bounded by the River Wey, the site of the Abbey Mill

Business Park has a long history of manufacturing and industrial use. From the

late 19th Century to the 1930s the site was a woollen mill. Subsequently, the

site and former mill were used as an engineering works.

Investigation

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.

Remediation

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.

Excavation

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.

Encapsulation

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.

Validation

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

hardstanding.