Investigating hydromechanical and geochemical processes in swelling clay-sulfate rocks - presentation of a new experimental setup

Due to human interventions such as the construction of tunnels and bridges as well as in geothermal projects, natural groundwater flow paths are disturbed. Within regions of clay-sulfate rock deposits like Southern Germany and Switzerland („Gipskeuper“) as well as Spain („Red Clay“), the mentioned structural interventions may promote the swelling of such rocks. Consequently, damage to buildings and other infrastructure in their vicinity can be triggered.

Until today, the planning of countermeasures that would minimize or prevent the mentioned swelling is difficult, because it is hardly possible to predict the swelling behavior of clay-sulfate rocks connected with geotechnical constructions. One reason is the limited knowledge of geochemical, hydraulic and geomechanical processes taking place during rock swelling.

The swelling process in clay-sulfate rocks is mainly due to the transformation of anhydrite into gypsum, which leads to an increase in rock volume of up to 60 %. The chemical transformation process includes anhydrite dissolution, possibly transport of dissolved sulfate with groundwater flow, and subsequent gypsum precipitation; and has large impact on flow paths within the swelling rock.

To extend the knowledge of hydromechanical and geochemical processes during the swelling of clay-sulfate rocks, a swelling test facility was installed at the chair of Engineering Geology and Environmental Geotechnics (TU Bergakademie Freiberg, Geotechnical Institute). The basis are six independently controllable apparatuses to conduct swelling experiments under oedometric conditions. First experiments with natural rock samples from a tunnel in Southern Germany are currently carried out.

Because groundwater circulation strongly influences swelling processes in clay-sulfate rocks, and vice versa, we developed a modified setup for swelling experiments. To observe and quantify hydraulic, mechanical and chemical processes during rock swelling in the lab, we combine the swelling apparatuses with a flow-through cell (permeameter). The modified setup allows the realization of an active and controlled fluid flow through the rock sample during swelling. As a result, reactive flow processes such as changes in permeability due to swelling can be observed under variable stress and strain conditions. Our contribution presents the experimental setup of the swelling tests with and without fluid flow. Furthermore, first results from the current experimental runs are presented.