Karst and mining geohazards with particular reference to the Chalk outcrop, England

Naturally formed karst landforms and historical man-made underground cavities occur widely across the Chalk outcrop. Spatial analysis reveals the broad-scale pattern of occurrence and the influence of causal factors. The Chalk underlies many large towns and cities of southern and eastern England, and each year additional subsidence sinkholes and crown holes appear without warning, causing damage to the built environment. The situation puts residents at risk, raises concerns for local authorities and has financial implications for insurers. These subsidence problems are triggered mostly by the actions of man involving an escape of water or excessive loading at the surface. Similar subsidence behaviour is exhibited by other rock types in the UK and internationally, although detailed characteristics can vary. Field observations and intrusive investigations of chalk karst and man-made subsidence features show a variety of distinctive characteristics. Ground subsidence dimensions and geological setting often provide clues about the cause and nature of movement. Back-analysis of remedial ground stabilization provides data on the typical subsurface void space available within natural cavities. Using these results an interpretive ground model is proposed to explain how sinkholes of varying size occur upon the Chalk. This forms a basis for understanding the differences between the sizes of recently formed sinkholes and geologically old sinkholes. Assessment of recorded subsidence event dimensions and causes provides guidelines on the scale of karst and mining geohazards posed to sites underlain by chalk. These have implications for foundation construction advice and disposal of surface water drainage.