Isle of Sheppey, Kent

National Housebuilder

Geo Environmental Group were commissioned by a National Housebuilder to undertake a Phase I Desk Study and Phase II Intrusive Investigation of a 2.3 hectare largely 'greenfield' site on the Isle of Sheppey in Kent. The Phase I assessment confirmed the likely 'greenfield' history of the site, although a small former structure was identified that could potentially have been associated with a local historical RAF airfield. The site was shown to be underlain directly by London Clay, with no superficial deposits indicated. The Phase II intrusive investigation incorporated 5 No. window sample boreholes to maximum depths of 5.0m with in-situ geotechnical testing and 3 No. standpipe installations for subsequent gas/water monitoring. In addition, 20 trial pits were undertaken to determine consistency of the strata, for both geotechnical and environmental parameters. 

As with all 'greenfield' sites, there is always the potential for localised filling to raise levels of depressions (for flood avoidance for example) with imported contaminated material. On this site, although localised made ground was found to depths of almost a metre, no contamination was identified. Underlying the topsoil/made ground London Clay was proven to maximum depth of the investigation across the site. Limited gas monitoring of the site indicated a 'green' condition in accordance with CIRIA C665, and although perched groundwater showed a presence of localised hydrocarbons, the concentrations were not considered significant and are likely to have derived from farm machinery. No residual pesticides were encountered in either the soils or the groundwater. 

GEG concluded that generally traditional shallow strip foundations would be suitable founding on the London Clay for the proposed 90 plots. However, the London Clay was locally softened near the surface and therefore a few plots required the depth of the strip footings extending to more competent strata. Due to the absence of ground gases and contamination, no gas protective dpc membranes or remedial works were recommended.