The anhydrite surface - cause of problems in tunnel constructions (new results based on measurements and observations)

Tunnel constructions in the Triassic rocks of north-western Switzerland and southern Germany are often affected by swelling phenomena. The experience shows that these swelling phenomena are technically difficult to manage due to the particular complexity of the initial factors of exceeding strain and deformation in areas of swelling rocks. The following article presents a new approach to the understanding of considerable swelling and deformation in rocks containing sulphate of the "Gipskeuper" formation. In doing so it is focused on the small rock zone of the anhydrite surface with a thickness of only a few meters. The active process of transformation of anhydrite to gypsum and the build-up of swelling pressure mainly occur in this area. The adjustment of stress during tunnel constructions results in a high and far-reaching development ratio of micro cracks in this rock zone and therefore progressive bulking, which produces a distinctive increase in the transformation velocity of anhydrite/gypsum, is promoted.