THE CHALLENGE:
Site investigations on former farmland that was perceived to be ‘greenfield’ with no contamination or geotechnical constraints identified an expansive infilled pond area on one portion of the site. Soilfix initially proposed a materials management-based solution to mitigate potential ground settlement and gas risks, however the client opted for a more robust option for wholescale removal of all materials underlying proposed house plots, to protect against perceived risks on a high-end development.
Chemical testing suggested all of the fill material would require disposal as Hazardous waste (principally due to >75% of samples showing significant Lead and Zinc), with potentially significant cost implications.
OUR SOLUTION:
Soilfix implemented a commercially robust excavation and disposal solution to mitigate significant cost over-run, including:
- Controlled site strip of 860m3 surface topsoil / vegetation under watching brief, carefully segregating for removal off-site as uncontaminated materials;
- Supplementary trial pitting to further delineate the extent / depths of the infilled pond and increase frequency of contamination data for waste classification purposes;
- Controlled excavation of contaminated soils, segregating into stockpiles for field screening using an X-Ray Fluorescence (XRF) instrument by a qualified Soilfix Engineer to confirm waste classification—based on a methodology pre-agreed in advance with the receiving waste facility to ensure full waste acceptance;
- Riddling / segregation of any bulk deleterious content from excavated fill materials during excavation;
- Periodic tanker visits for recovery of contaminated groundwater from the open excavation – total 40,000 litres removed to a nearby permitted treatment facility;
- Off-site disposal of 1,80 tonnes segregated hazardous and non-hazardous soils & stones to a permitted treatment facility, fully in accordance with Duty of Care; and,
- Importation of a 6F5 aggregate and backfilling of the excavated pond area to required formation levels for construction.
All works were completed on programme to the satisfaction of CALA’s supervising engineer, with no disruption to ongoing construction activities across the remainder of the site. Through controlled excavation and field-screening, the proportion of hazardous waste was reduced by >50%, resulting in an overall saving of £40K on the project cost. This commercial benefit was realised by the client having agreed to re-measure the disposal costs on pre- agreed rates.
