Contamination and subsidence mar 150-year-old redevelopment

Established during the 1850s, production continued at Gas Hill until its closure in the 1970s. Past investigations have revealed a history of sand, lime and chalk workings in the surrounding area. These, together with the presence of tunnels and solution features have, not surprisingly, led to various incidents of subsidence being reported.

May Gurney initially undertook extensive investigations on the site – which is being redeveloped for housing. These included the excavation of a series of trial pits and the installation of boreholes to a maximum depth of 25m below existing ground level, enabling long-term monitoring of groundwater levels and landfill-type gases. The strata present was found to consists of made ground to a maximum depth of 5m, overlying mainly fine to course sand drift deposits, in turn overlying chalk. Groundwater was encountered at a depth of between 12 and 15m – believed to be in hydraulic continuity with the nearby River Wensum. Analysis of the soils and groundwater has shown them to be heavily contaminated with a cocktail of cyanides, lead, mercury and hydrocarbons, consistent with the known land-use history.

Risk assessment

A programme of qualitative and quantitative risk assessment was carried out in order to develop the remediation strategy for the site, in line with current guidance and legislation. All constituent materials on site, from re-deposited fill material to natural strata, were classified according to material type and the concentration of contaminants each contained. The associated risk each would pose to people, water and new development was a key element of the assessment. May Gurney worked closely with the Environment Agency to establish acceptable proposals, which resulted in a pragmatic and cost-effective approach to reclamation for the client.

Therefore, the remediation strategy included the removal of shallow contamination across the entire site together with isolated hotspots of lead, mercury and cyanide present within made ground. Areas of hard standing were crushed and screened to be re-used as fill material. In addition, continual testing of the soils throughout the earthworks operation has enabled recycling of other uncontaminated material.

A chemical precipitation process has been incorporated as part of a pump and treat system on site to remove cyanide from groundwater. When concentrations are below the derived remediation target, the treated water is returned to the ground via an infiltration trench. Migration of polluted leachates is being controlled with a 150m low permeability in-ground containment barrier, installed along the northern perimeter of the site, to an average depth of 7m. The remediation phase of the project is due to be completed in 6 weeks, allowing for the follow-on works programme. This will include the installation of a series of continuous flight auger piles into the chalk stratum as part of the foundation package.