REC Ltd was commissioned by Silver DCC on behalf of LHA-ASRA and Durkan Ltd, main contractor to conduct Phase I and Phase II site investigations at a former Thames Water site in Plumstead, London.
The aim of the investigations were to determine any environmental risk presented by the historical use of the site and to establish geotechnical information on ground conditions across the site to assist with foundation design and assess the suitability of site won materials for re-use.
The proposed development was for 44 residential units comprising two terraced blocks of houses with private gardens along with two adjoined blocks of low rise flats, associated car parking and communal open space.
The site had previously been owned and operated by Thames Water, with historical maps indicating the presence of three reservoirs (subsequently modified into underground water storage tanks), a single well and an air raid shelter.
REC carried out a review of BGS well records which indicated the presence of 3 wells on-site. Records indicated the wells were brick lined and extended to a depth of 130m bgl at a diameter of approximately 2.50m. Records also indicated a sizeable gallery and a number of horizontal tunnels at 130m bgl. Liaison with Thames Water, the Environment Agency and the British Geological Survey revealed that the wells were believed to have been backfilled, but that no decommissioning report existed detailing the methods employed.
REC alerted the client to the presence, approximate location and construction of the wells, and the implications these structures presented.
REC recommended suitable investigation methods to determine the exact location and geotechnical properties of the backfill material and potential remediation measures that could be employed.
The Phase I investigation identified the presence of elevated concentrations of PAHs in areas of soft landscaping and the Phase II investigation determined that these elevated concentrations were present in near surface soils across the site. REC suggested potential remediation measures that could be implemented during the development phase to sever the pollution linkage and remove the risk posed to human health.
REC were able to demonstrate that the natural sands underlying the site were a suitable founding material for traditional strip foundations and were also suitable for re-use as both an engineered fill and a general fill across the site, thus delivering potential cost savings to the client.
The Phase I investigation identified the presence of a significant thickness of Made Ground confined to one area of the site. The Phase II investigation proved the thickness of the Made Ground to be in excess of 3.0m by the Phase II investigation with a 1.20m thick brick floor slab being encountered at approximately 2.20m bgl. REC recommended piled foundations and supplied preliminary pile loadings for this area of the site. REC were also able to provide the client with founding depths for underground tanks and air raid shelter.