Under the influence of water anhydrite will transform into gypsum by dissolution. During this hydration the volume may theoretically expand up to 60.8% of its original size. If the expansion of this process is obstructed, considerable pressure will develop. Especially in unleached Gypsum Keuper containing anhydrite the interaction of the hydration of anhydrite and the swelling of argillaceous rock can cause enormous damages to subsurface excavations, mainly at their floors. In this diploma thesis the expansion mechanisms of the hydration of anhydrite are exemplified, in addition laboratory tests and field tests of the determination of hydration behaviour on clay sulphate rock are described. The difficulties in using the results of laboratory tests for the judgement and description of in situ swellings are pointed out. The various possibilities of different origins of the water causing this swelling phenomena are elaborated on. Floor constructions according to the resistance and give-way principle, which prevent damage to subsurface excavations are presented on the basis of examples and compared one to another.