Summary

The swelling of clay-sulfate rocks often produces large problems in road and tunnel engineering in such rocks. Recently, the swelling problem arose also in a different setting: Geothermal drillings in clay-sulfate rocks triggered ground heave and dramatic damage to more than 250 houses in the town of Staufen. The processes involved in the swelling of clay-sulfate rocks are complex and not yet sufficiently understood. In particular, the hydraulic and geochemical processes in swelling zones, and how these change when affected by engineering activities, are largely unknown.

The main cause of the swelling is the transformation of anhydrite into gypsum in the presence of water, accompanied by a volume increase of up to 60%. This transformation takes place indirectly: first anhydrite dissolves; at the same time, the sulfate concentration of the pore water changes; the dissolved sulfate can be transported with groundwater flow; and finally gypsum precipitates. This progression of processes suggests that groundwater flow and pore water geochemistry play a key role in understanding the swelling processes. We hypothesize that changes in hydraulic conditions by human activities lead to geochemical changes in clay-sulfate rocks that cause the swelling. Therefore, the major aim of the project is the quantitative characterization of groundwater flow and chemical reactions in a swelling zone affected by human activities. In addition, the study aims at determining reaction rates of anhydrite dissolution and gypsum precipitation at the field scale, which may differ from reaction rates known from laboratory experiments.