Uneven rockhead caused by Chalk dissolution. Dissolution pipes within the Chalk are infilled with soft clay and sand from the overlying strata, which creates potential problems for foundation design.

Chalk is a very distinctive, pure form of limestone, and is the most widespread carbonate rock in the country. It occurs across much of southern and eastern England, and is of immense importance for water supply as it forms Britain's most important aquifer. In places, the development of solutionally enlarged fissures and conduits can potentially cause problems for groundwater supply by creating rapid contaminant pathways though the aquifer.

This is particularly important as the Chalk underlies many major transport corridors and urban areas. Chalk dissolution also generates subsidence hazards and difficult engineering conditions with the development of clay-filled pipes and fissures. These problems include irregular rockhead, localised subsidence, increased mass compressibility and diminished rock mass quality.