Although the known evaporite minerals amount to more than 25, anhydrite and gypsum are the most conspicuous from the engineering geological point of view. They present considerable difficulties in the construction of tunnels, dams and other structures because of their solubility and swelling when treated with water. Swelling phenomena observed in anhydrites have been exaggerated before the 1960's. During those years anhydrite has been thought to be solely responsible for all kinds of harmful swelling effects. The results of observations and investigations made after 1960, showed that besides anhydrite, clay minerals such as corrensite and montmorillonite have the property of swelling and that of stress releasing owing to gravitational and tectonic stresses and also preconsolidation properties which are responsible for deformation.