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.

1.1 The problem of swelling

It is generally known that the transformation of anhydrite/ gypsum due to the absorption of H2O causes an increase of volume of approximately 60% and that its restraint results in the build-up of swelling pressure up to 6–7MPa (confirmed by laboratory experiments), see Fig. 1. During tunnelling in anhydrite containing rocks it is therefore essential to ensure that no water encounters the anhydrite. However, it is technically extremely difficult to realise a dry construction procedure. On the one hand in tunnel construction water is always to be expected in the encountered rock layers and their structures. On the other hand any excavation induces deformation due to load relieving whereby the water conductivity around the opening increases in general. Depending on the availability ofwater swelling phenomena are expected in middle- or long-term due to the transformation of anhydrite/gypsum.

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