Coupled thermo-hydro-mechanical-chemical (THMC) processes in swelling clay-sulfate rocks

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Clay rocks containing the sulfate mineral anhydrite swell when getting in contact with water. The swelling of clay-sulfate rocks is a threat in tunneling and geothermal drilling. In Staufen, for example, swelling ground with uplift rates exceeding 1 cm/month resulted in more than 250 seriously damaged houses after the installation of geothermal heat pumps. Numerical models that simulate the swelling allow experts to test and optimize countermeasures. However, our knowledge about the processes taking place during swelling is limited. There are contradicting conceptual swelling models, and an accepted mathematical swelling law to quantify the swelling of clay-sulfate rocks is lacking. Although various numerical models for swelling simulations have been proposed so far, all existing approaches have limitations. To overcome them, enhanced numerical models must consider the coupling of thermal, hydraulic, mechanical and chemical (THMC) processes and, at the same time, must be able to include the complexity of the geological setting as well as the impact of engineering activities, such as drilling, tunneling and groundwater pumping. The planned research has therefore two main goals: (1) to complete our present understanding of the swelling, and in particular of the involved coupled THMC processes; and (2) to improve our ability to simulate the swelling processes comprehensively.The project wants to reach these goals by implementing and comparing different contradicting conceptual and mathematical models into coupled numerical codes; and subsequently challenging the models with field observations. For this purpose, the study site Staufen provides very comprehensive thermal, hydraulic, chemical and mechanical data. The different models will be validated or rejected by observation data, and they will also be evaluated by an uncertainty study. The project will finally use the models for the optimization of remediation strategies at the study site to show their applicability.An additional goal of the project is to make the developed modeling tools commonly available and usable for others. Therefore, we will employ an open-source software and publish developed codes; and we will host a symposium and workshop to exchange knowledge and modeling experience in the area of swelling clay-sulfate rocks.The proposed project follows up our previous research, adding and coupling mechanical processes to our already developed hydraulic-chemical approach, while still maintaining the hydrogeological and geochemical complexity of the 3D geological and engineered setting. The project will strongly benefit from our previous project, because site specific processed data (3D geological model, time series of observation data and boundary conditions, hydraulic and geochemical fluid and rock properties) are already at hand.