Laboratory studies supporting the Sandwich in-situ experiment and insights from the semi-technical scale tests HTV-7 and HTV-9

Shaft and drift seals close exploited underground mines and are part of the engineered barrier system that is an integral part of the multi-barrier system for deep geological repositories for hazardous toxic wastes like high level radioactive wastes (HLW). A distinctive feature of German shaft seal concepts for HLW repositories (Kudla & Herold 2021) is the Sandwich sealing system developed at the Karlsruhe Institute of Technology by the Competence Center for Material Moisture (CMM) (Nüesch et al. 2002). In contrast to monolithic bentonite seals the Sandwich sealing system contains equipotential segments (ES) that are sandwiched between sealing segments (DS) of bentonite. The ES consist of silty to sandy mineral materials with a slightly higher hydraulic conductivity than the bentonite in DS that is installed as a binary mixture of compacted bentonite pillows and bentonite granular material (BGM). During hydration any formation water that infiltrates the hydraulic seal via preferential flow paths is captured by the ES, redistributed uniformly across the seal cross section and available for hydration of adjacent DS. This mechanism ensures homogeneous hydration and swelling of the bentonite resulting in a tight sealing and enables monitoring with lower risk of breakthroughs along sensor cables. The functionality of the Sandwich sealing system was initially proofed at laboratory scale (Schuhmann et al. 2009). After a joint international planning pre-project (Emmerich et al. 2019) in July 2019 a large-scale experiment was launched at the Mont Terri rock laboratory (MTRL) to demonstrate the feasibility of installation, to investigate the saturation process, to qualify instrumentation and to assess the sealing effectiveness in interaction with a potential clay host rock via monitoring (Wieczorek et al. 2024) and possible sampling after termination. The in-situ experiment is accompanied by extensive laboratory work and computational chemistry studies as well as interpretive and predictive numerical modelling at different scales. The variety of experiments on different scales and with different model geometries together with theoretical calculations will enable an in-depth understanding of hydro-mechanical-chemical (HMC) coupled processes in the Sandwich sealing system. Together with interpretive numerical modelling, scale-dependent and nonlinear effects on the system behavior can be recognized to improve the predictive modelling. This contribution focuses on laboratory work, summarizing material characterization and providing an overview of the experiments. The paper is complementary to Hinze et al. (2026). There an overview of the in-situ experiment at MTRL and related numerical simulations is provided.