Testing and simulation of a large-scale Sandwich shaft sealing system

Shaft seals are part of the multi-barrier system of a deep geological repository for high-level radioactive waste. They are designed to limit the fluid exchange between the repository and the overlying rock mass and biosphere. A particular component of German shaft seal designs is the Sandwich sealing system which can improve the sealing performance of bentonite seals by combining them with intermediate equipotential segments of mineral materials with a higher hydraulic conductivity. This leads to an even distribution of intruding fluids via the equipotential segments across the seal and a homogeneous hydration and swelling of the bentonite (Nüesch et al. 2002, Schuhmann et al. 2009).

The international Sandwich project investigates the behavior of the Sandwich sealing system by experimental testing on multiple scales and hydro-mechanical simulation, see (Emmerich et al. 2019, Wieczorek et al. 2024) and the synthesis (Hinze et al. 2025). The laboratory experiments range from swelling pressure measurements and MiniSandwich experiments to semi-technical scale tests. Moreover, a large-scale experiment has been conducted at the Mont Terri rock laboratory since July 2019. The in-situ test consists of two experimental shafts with Sandwich sealing systems installed in each. Both are hydrated from the bottom with artificial pore water and closely monitored, alongside the surrounding rock. The simulation work in the project comprises the calibration of swelling models describing the complex hydro-mechanical behavior of bentonite and interpretative modeling of the Sandwich sealing system itself including the interaction with the surrounding host rock.

This contribution is particularly related to the in-situ test and its numerical simulation. It provides and interprets recent measurement data (comprising water content (acquired by electrical resistivity tomography and time-domain reflectometry), relative humidity and temperature, pore pressure, axial and radial stress, and displacements) and relates them to simulation results for models of the experimental shafts and the surrounding rock mass.

 

References

Emmerich, K. et al.: Joint project: Vertical hydraulic sealing system based on the sandwich principle – preproject (Sandwich-VP). Final report, Karlsruhe Institute of Technology (KIT) & Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) gGmbH, https://doi.org/10.2314/KXP:1692488228, 2019.

Hinze, M. et al.: Multi-scale testing and simulation of a vertical hydraulic Sandwich sealing system, Waste Management 2025 Conference, Phoenix, Arizona, USA, March 9 - 13, 2025. 

Nüesch, R., Brandelik, A., Hübner, C. & Schuhmann, R.: Verschlussstopfen und Verfahren zum Verschließen von untertätigen Hohlräumen, German Patent DE 10149972 C1, 2002.

Schuhmann, R., Emmerich, K., Kemper, G. & Königer, F.: Verschlusssystem mit Äquipotenzialsegmenten für die untertägige Entsorgung (UTD und ELA) gefährlicher Abfälle zur Sicherherstellung der homogenen Befeuchtung der Dichtelemente und zur Verbesserung der Langzeitstabilität: Schlussbericht, 117 S., https://doi.org/10.2314/GBV:637752392, 2009.

Wieczorek, K. et al.: Sandwich-HP – Vertical hydraulic Sandwich sealing system, Final Report, Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) gGmbH & Karlsruhe Institute of Technology (KIT) & TU Bergakademie Freiberg (TUBAF), Technical Report GRS – 745, https://www.grs.de/de/aktuelles/publikationen/grs-745, 2024.