Abstract. The long-term safe containment of high-level radioactive waste in a repository
in rock salt is ensured if the geological barriers in conjunction with the
geotechnical barriers are permanently impermeable to fluids. As such, an
essential factor in underground disposal is to confirm that the interfaces
between the biosphere and the lithosphere, i.e., shafts, boreholes, and
galleries, created during the excavation of underground cavities can be
sufficiently tightly sealed. An essential element of the sealing system
required to this end is shaft closure. All shaft closure concepts developed thus far include sealing and supporting
elements in repository shafts, but differ in the arrangement of these
structural elements and the materials used. The materials currently proposed
and 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 the
German Federal Ministry for Economic Affairs and Energy
(Bundesministerium für Wirtschaft und Energie, BMWi) is
investigating the possibility of integrating a layer of salt cut bricks
several decameters to 100 m thick into the shaft closure system as a
sealing element that provides the option of allowing the geological barrier to
heal in the long term. Like the surrounding rock mass, the salt cut bricks are made of
natural rock salt. According to this plan, the profile of the bricks is
designed to minimize joint volume as far as possible by cutting them to match
the geometry of the shaft. The joints between the salt cut bricks can either
be 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 of
the salt cut bricks. Once the salt solution has hardened (cooling of the
solution, evaporation of the mixing water), only the pore space in the
crushed-salt joint sealant filled with salt solution or air, or the joint
volume resulting from the mismatch between individual rock salt bricks are
susceptible to a reaction. This means that the sealing element consisting of
salt cut bricks develops early supporting pressure against the creeping rock
salt of the rock mass compared to crushed salt, has a low initial porosity and
already shows a strong sealing effect in the short term (regression of the
loosened zone). One can also assume that cohesive bonding between the
surrounding rock and the sealing element can already be achieved by
introducing the joint filling or by wetting the contact surfaces of the salt
cut bricks (no or reduced separation planes in the contact zone). Essential prerequisites for the investigation of the
geomechanical-geohydraulic effectiveness of a sealing element made of salt cut
bricks included the development and construction of a pilot plant to analyze
the mechanical and hydraulic material properties of the bonding system
comprising salt cut bricks and joint sealing (FKZ 02E11223, FKZ 02E11425), as
well as preliminary investigations on the production of salt cut bricks and
joint sealant (→ cutting technique/processing of salt cut
bricks; maufacture/workability of jointing material) and on the spatial
arrangement of the salt cut bricks (→ avoidance of continuous
axial 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 first
successful test results demonstrating the functionality of sealing elements
made of salt cut bricks.
The Application of a Sonic Probe Extensometer for the Detection of Rock Salt Flow Field in Underground Convergence Monitoring