The Sandwich seal systems: A large-scale shaft sealing experiment at the Mont Terri rock laboratory – Installation and materials

Shaft seals are part of the engineered barrier system (EBS) that belongs to the multi-barrier system for deep underground repositories of high-level radioactive waste. The German regulator demands that with respect to the reliability of containment, the interplay of barriers has to be optimized in diverse redundancy and shaft seals should also be constructed of diverse redundant components. The design of shaft seals for generic site models of Germany contains hydraulic sealing elements that should be realized as Sandwich sealing systems. In contrast to conventional hydraulic seals of monolithic bentonite the Sandwich sealing system consists of sealing segments (DS) of bentonite and hydraulically conductive equipotential segments (ES) (Nüesch et al., 2002). Formation water that is penetrating the hydraulic seal via preferential flow paths is contained in the ES and evenly distributed over the cross section of the seal. Thus, a more homogeneous hydration and swelling of the DS is obtained.

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 and to assess the sealing effectiveness. The in-situ experiment consists of two experimental shafts of 1.18 m diameter and 10 ‑ 12.6 m depth in the sandy facies of the Opalinus Clay. The DS in both shafts are constructed of German Ca-bentonites from Bavaria (Calcigel) and Westerwald region (Secursol), respectively. Both Sandwich sealing systems are hydrated with Pearson water type A3 and are intensely monitored, together with the surrounding rock. The in-situ experiment is accompanied by extensive laboratory work and numerical modelling. The laboratory work comprises mineralogical analyses, Oedometer tests, MiniSandwich experiments and semi-technical scale experiments. The variety of experiments on different scales and with different model geometries allow us to recognize and understand different scale-dependent and nonlinear effects on the system behaviour.

The presentation will focus on the differences of the two German bentonites with respect to HMC properties and the installation of the Sandwich sealing systems in both shafts at MTRL. The bentonite from Westerwald is characterized by a high smectite content and had to be blended with non-swellable material to adapt expected maximum swelling pressure to the specific site conditions of the in-situ experiment. Both bentonites were then installed as binary mixtures of bentonite pillows and bentonite granular material with similar EMDD, water content and dry density.

Acknowledgment

The Sandwich pre-project and the Sandwich in-situ experiment were/are funded by the German Federal Ministry for Economic Affairs and Energy under contracts 02E11587 and 02E11799.