Testing and simulation of the Sandwich sealing system at the Mont Terri rock laboratory

Shaft seals constitute an integral part of the multi-barrier system within deep geological repositories for radioactive waste, serving to restrict fluid migration between the repository, the overlying rock mass, and the biosphere. A distinctive feature of German shaft seal concepts (Kudla & Herold 2021) is the Sandwich sealing system. Unlike monolithic bentonite seals, this approach utilizes sealing segments (DS) composed of bentonite interlayered with equipotential segments (ES) made from mineral materials exhibiting (at least one order of magnitude) higher hydraulic conductivity (Nüesch et al. 2002). When formation water infiltrates the hydraulic seal (particularly via preferential flow paths), it is captured by the equipotential segments and redistributed uniformly across the seal. This mechanism ensures homogeneous hydration and swelling of the bentonite. The Sandwich sealing system is investigated in an ongoing international project implementing experimental testing on multiple scales and hydro-mechanical simulation (Emmerich et al. 2019; Wieczorek et al. 2024). The laboratory experiments range from swelling pressure tests for bentonite and MiniSandwich experiments to semi-technical scale tests. The Sandwich (or SW-A) experiment at the Mont Terri rock laboratory brings the Sandwich sealing system to in-situ scale. It consists of two experimental shafts with a diameter of 1.18 m and depths of 12.6 m (Shaft 1) and 10 m (Shaft 2), respectively, constructed in the sandy facies of the Opalinus Clay. Vertical hydraulic Sandwich sealing systems have been installed in both shafts which can be hydrated by pressure chambers located at the shaft bottoms via inclined boreholes connecting the shafts to the niche. The sealing systems and the surrounding rock are intensely monitored. The simulation work in the project comprises the calibration of swelling models describing the complex hydro-mechanical behavior of bentonite and interpretative modelling of the Sandwich sealing system taken as a whole, including the interaction with the surrounding host rock. The experimental upscaling approach is transferred to the simulation schedule, i.e., models of laboratory and semi-technical scale experiments, as well as different two- and three-dimensional models of the in-situ test with increasing complexity are studied. Particularly, these modeling perspectives were combined to design a simulation task for the international model development and validation project DECOVALEX-2027, in which several modeling teams use different simulation codes and model approaches with the final goal to predict the hydro-mechanically coupled processes in the Sandwich in-situ test. This contribution provides an overview of the SW-A experiment including the interpretation of recent measurement data. Moreover, simulation results related to the modeling of the in-situ test are presented and connected to the measured data.