A SURVEY OF THE ENGINEERING PROPERTIES OF SOME ANHYDRITE AND GYPSUM FROM THE NORTH AND MIDLANDS OF ENGLAND

The Permian and Triassic systems in the north and midlands of England contain notable beds of anhydrite and gypsum. These rocks contain small but varying amounts of muddy material which often has some influence on their geomechanical properties. Anhydrite is a strong to very strong rock in terms of both its unconfined compressive strength and point load index, whereas gypsum is of medium strength according to its unconfined compressive strength. Because of the low porosity of these rocks, porosity does not have a significant influence on their strength. However, the strength and hardness of these rocks are strongly correlated. In terms of the slake-durability test, anhydrite has a very high durability and gypsum a high to very high durability, with the stronger rocks yielding higher values of durability. Most anhydrite and gypsum exhibit plastic-elastic-plastic deformation, subsequent plastic deformation occurring at an earlier stage during loading of gypsum than of anhydrite. As far as deformability is concerned, that of anhydrite is very low while that of gypsum varies from low to high. Both rock types experience hysteresis on cyclic loading, that of gypsum being more notable than that of anhydrite and the amount of hysteresis undergone increases with successive cycles of loading and unloading. Anhydrite and gypsum both undergo creep when subjected to constant loading, gypsum being the more prone to creep under constant load.