The remediation of the former gas works at Castlegreen Street in Dumbarton took place in two phases and included a significant change in the proposed end use.
The site was typical of its kind, with contamination predominantly comprising coal tars and heavy hydrocarbons. The first stage of the works comprised the investigation and exposure of below-ground tar storage tanks, extending to over 5.5m deep. Once safe access had been secured, all residual tar-impacted materials within the structures were excavated, stabilised and removed off-site. The concrete structures were subsequently broken out and recovered or made safe. All structures and surface concrete were broken out and crushed; gaining in excess of 500m3 of material suitable for re-use on site.
The works subsequently progressed to the treatment of contaminated ground. In total, over 6,500m3 of contaminated material was excavated. The initial strategy, applied during the first period on site, was to treat the material by ex-situ bioremediation, carried out directly by our own resources. We aimed to achieve either re-classification of contaminated soils to allow local disposal, or an end product suitable for re-use on site. To suit space and time constraints, approximately half of the ground was treated during the first visit.
During our time off site, the Client’s requirements were modified to reflect a change in the proposed end use of the site. Working closely with the Client and the Designer to address the revised SSTLs, we altered the strategy for the second portion of the works, to one of selective excavation, physical stabilisation, minimisation and disposal to landfill.
To mitigate against shallow groundwater at the site and large volumes of perched water, excavation phases were divided into a number of individual cells to eliminate the potential recontamination of clean backfill. Cells were formed by the installation of an impermeable clay cut-off wall around the perimeter, tied into underlying clay strata. Any water encountered during excavation of individual cells was pumped to a site treatment system. Once treated water achieved pre-determined targets, it was released for discharge to public sewer.
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