Developing ground models for chalk engineering: links across the disciplines

Changes in our understanding of chalk geology since the Brighton 1989 International Chalk Symposium (ICS 89) in terms of stratigraphy, sedimentology, structure, micro-fabric, descriptive method and engineering classification are reviewed. The first major development has been the application of a ‘Formation’ stratigraphy to mapping chalk at outcrop and subcrop in England. At subcrop under the London Basin, and offshore, marker beds provide the keys for borehole correlation and linking the geology to geophysics and geotechnical properties. Conceptual ground models are illustrated for complex geology including dry valleys, potential karst areas and the offshore environment for wind turbine foundations. Such models require detailed multidisciplinary knowledge of chalk and Quaternary processes including where chalk has been subjected to glacial and periglacial weathering. Field experiments using geophysical and hydrogeological techniques developed for European research projects including PROTECT and FLOOD1 are used to quantify the physical properties of the engineering ground profile. Laboratory tests linked to the field experiments illustrate lithological controls on behaviour, especially dual porosity and permeability and potential mechanical properties. These results emphasise the need for accurate stratigraphical, lithological and chalk-mass descriptions applying the CIRIA C574 methods.

Recent research has also re-looked at the micro-fabric of chalk and has identified the role of water at a molecular level controlling the bonding and disaggregation of chalk granular structure. It is the start of a new era of chalk research within a well-defined geological framework linked closely to properties and geophysics. The ultimate objective is increasingly consistent descriptions, classifications, test results and accurate predictions of mechanical and groundwater behaviour of the chalk material and rock mass.