The Palaeocene Basal Sands of the London Basin are usually in hydraulic continuity with the underlying Cretaceous Chalk aquifer. They are a minor aquifer rarely utilized for water supply. Instead, production wells are completed in the Chalk aquifer, which provides the bulk of the water. The conven tional well design is for cemented casing to extend into the top few metres of the Chalk, with the rest of the Chalk section below left unlined (open hole). Although this has generally proved to be successful, there have been occasions where failures have occurred owing to sand entering the wells from the Basal Sands. The pathways are not always evident, but can include fractures in the Chalk, damaged casings or poor grout seals. Setting casings deep into the Chalk does reduce the risk of sand ingress, but yields may be diminished. In some cases, screening of the Basal Sands and the very top part of the Chalk has been used as an alternative. Detailed data from geological and geophysical logs can greatly assist in the design process, and the risk of sand ingress can be reduced by following good drilling and construction practice and ensuring that casings have been properly grouted. In this paper the terms wells, production wells, boreholes and water supply boreholes are used interchangeably.
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