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 smal l but varying amounts of muddy material which often has some influence on their geomechanical properties. Anhydrite is a strong to very stro ng rock in terms of both its unconfined compressive strength and point load index, whereas gypsum is of medium strength according to its unc onfined compressive strength. Because of the low porosity of these roc ks, porosity does not have a significant influence on their strength. However, the strength and hardness of these rocks are strongly correla ted. In terms of the slake-durability test, anhydrite has a very high durability and gypsum a high to very high durability, with the stronge r rocks yielding higher values of durability. Most anhydrite and gypsu m exhibit plastic-elastic-plastic deformation, subsequent plastic defo rmation 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 type s experience hysteresis on cyclic loading, that of gypsum being more n otable than that of anhydrite and the amount of hysteresis undergone i ncreases with successive cycles of loading and unloading. Anhydrite an d gypsum both undergo creep when subjected to constant loading, gypsum being the more prone to creep under constant load.