Engineering geological classification of flints

The petrographic and mechanical properties of flints from the Burnham (North Landing, Yorkshire, UK), Seaford (East Sussex, UK, and Dieppe, France), and Lewes Nodular (Mesnil-Val, France) Chalk formations have been investigated. Microtexture and mineral composition of flints are studied to understand how the geological and petrophysical properties of the flint affect drilling responses to the rock and investigate any spatial variation. The flints are categorized based on physical observation into white crust and light brownish grey, dark brownish grey and grey flints. Scanning electron microscopy shows textural variation in the classes. The white crust, light brownish grey, brownish grey and grey flints from the Burnham Chalk Formation from North Landing contain more calcite and have coarser, more poorly cemented silica spherules in comparison with similar classes of flint from the Seaford and Lewes Chalk formations from the Anglo-Paris Basin. In these latter flints, the structure is dominated by massive quartz cement with trace calcite independent of location. Strength tests show that the grey flints from North Landing are weaker than equivalents from the Anglo-Paris Basin. It is suggested that variation in engineering properties between grey and the dark brownish grey flints is caused by mineral composition, microtexture, structure and the local or site geology of flint materials.