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

Safety of deep underground repositories for high-level radioactive waste is ensured by a multi-barrier system consisting of the geological barrier (host rock) and the engineered barrier system (EBS). Shaft seals are a major part of the EBS. The German regulator demands that the interplay of barriers has to be optimized in diverse redundancy to increase reliability and robustness of the barrier system. Shaft seals should also be constructed of diverse redundant components. The design of shaft seals for generic site models of Germany includes hydraulic sealing elements that are to 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 bentonites 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, after a two-years planning phase (Emmerich et al., 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 monitoring techniques, and to assess the sealing effectiveness. The in-situ experiment is supported by laboratory experiments at different scales and by numerical simulation.

The in-situ experiment consists of two experimental shafts of 1.18 m diameter and 10 ‑ 12.6 m depth located in the sandy facies of the Opalinus Clay. The DS in both shafts are constructed of German Ca-bentonites, while the ES consist of fine-grained sand. Both Sandwich sealing systems are hydrated with Pearson water type A3. Both shafts were drilled in 2020 and Shaft 1 was installed subsequently. Hydration of Shaft 1 started in May 2021. Shaft2 was installed about two years later so that experience from Shaft 1 operation could be included. Additionally, an excavation damaged zone had the chance to develop close to the shaft wall.

The presentation will focus on the operation phase of the two shafts. Measurement systems and monitoring results, in terms of the evolution of water content, stress, pore pressure, and relative humidity in the sealing systems will be presented.

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.