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. The long-term safe containment of high-level radioactive waste in a repositoryin rock salt is ensured if the geological barriers in conjunction with thegeotechnical barriers are permanently impermeable to fluids. As such, anessential factor in underground disposal is to confirm that the interfacesbetween the biosphere and the lithosphere, i.e., shafts, boreholes, andgalleries, created during the excavation of underground cavities can besufficiently tightly sealed. An essential element of the sealing systemrequired to this end is shaft closure. All shaft closure concepts developed thus far include sealing and supportingelements in repository shafts, but differ in the arrangement of thesestructural elements and the materials used. The materials currently proposedand planned for the construction of the sealing elements include:clay/bentonite, asphalt/bitumen, crushed salt, and salt/sorel cement. In addition to the materials mentioned above, a research project funded by theGerman Federal Ministry for Economic Affairs and Energy(Bundesministerium für Wirtschaft und Energie, BMWi) isinvestigating the possibility of integrating a layer of salt cut bricksseveral decameters to 100 m thick into the shaft closure system as asealing element that provides the option of allowing the geological barrier toheal in the long term. Like the surrounding rock mass, the salt cut bricks are made ofnatural rock salt. According to this plan, the profile of the bricks isdesigned to minimize joint volume as far as possible by cutting them to matchthe geometry of the shaft. The joints between the salt cut bricks can eitherbe filled with, for example, a supersaturated salt solution, Magnesium building materials, molten salt,crushed salt, etc., or directly brought into contact by wetting the surface ofthe salt cut bricks. Once the salt solution has hardened (cooling of thesolution, evaporation of the mixing water), only the pore space in thecrushed-salt joint sealant filled with salt solution or air, or the jointvolume resulting from the mismatch between individual rock salt bricks aresusceptible to a reaction. This means that the sealing element consisting ofsalt cut bricks develops early supporting pressure against the creeping rocksalt of the rock mass compared to crushed salt, has a low initial porosity andalready shows a strong sealing effect in the short term (regression of theloosened zone). One can also assume that cohesive bonding between thesurrounding rock and the sealing element can already be achieved byintroducing the joint filling or by wetting the contact surfaces of the saltcut bricks (no or reduced separation planes in the contact zone). Essential prerequisites for the investigation of thegeomechanical-geohydraulic effectiveness of a sealing element made of salt cutbricks included the development and construction of a pilot plant to analyzethe mechanical and hydraulic material properties of the bonding systemcomprising salt cut bricks and joint sealing (FKZ 02E11223, FKZ 02E11425), aswell as preliminary investigations on the production of salt cut bricks andjoint sealant (→ cutting technique/processing of salt cutbricks; maufacture/workability of jointing material) and on the spatialarrangement of the salt cut bricks (→ avoidance of continuousaxial joints in the bonding system, Fig. 1). The presentation includes the results of the research work on the development,construction and commissioning of the pilot plant, as well as the firstsuccessful test results demonstrating the functionality of sealing elementsmade of salt cut bricks.

Pilot plant tests to demonstrate the functionality of sealing elements made of salt cut bricks