Abstract. Repositories for high-level radioactive waste in geological formations require
knowledge on thermal, mechanical and fluid transport properties of the whole
repository system, including the engineered barriers and backfill
materials. For about 30 years, crushed salt has been considered the most
suitable geotechnical barrier material to backfill cavities and encapsulate
radioactive waste in rock salt repository sites (e.g., Czaikowski et al.,
2020). Over time, when the surrounding cavity walls converge by the creep of
salt, it can become strongly compacted and safely encapsulates radioactive
waste from any fluid flow. Hence, crushed salt has been characterized in
detail for its physical material properties and its response to environmental
controls (stress, temperature and moisture). This characterisation provides a
basis for long-term numerical simulations (e.g., Liu et al., 2018), which
verify so-called safety cases in radioactive waste disposal. Displacement-controlled oedometric compaction tests mimic the long-term in
situ behaviour of crushed salt. The tests show that it can be compacted to a
state comprising physical rock properties similar to natural rock salt. In
general, compaction is easier with an increase in humidity and temperature
(e.g., Stührenberg, 2007; Kröhn, et al., 2017). Triaxial test series
address the compactions' response to differing confining pressures and help to
identify generalized constitutive equations for crushed salt. Both BGR
procedures, the oedometric and the triaxial compaction, are verified by the
German accreditation body (DAkkS). Figure 1 illustrates the history of oedometric tests at the BGR laboratory
since 1993, which examined crushed salt from various origins and differing
temperature conditions. Most tests focused on material from the Asse mine,
revealing the compactions' response to the materials' humidity and to brine
flow. Moreover, systematic test series with synthetic grain size distributions
and bentonite additives provided a basis for barrier material design. More
recent tests on bedded salt formations (e.g., Teutschenthal and Sondershausen
mines) allow the differentiation from characteristics from domal salt deposits
(e.g. Gorleben). The current research continues the history of oedometric and triaxial tests,
but has a new focus on late compaction stages with marginal remaining
porosities (<5 %). The approach of systematic material characterization
under best-controlled conditions essentially benefits from the international
research collaboration in the KOMPASS project (Czaikowski et al., 2020). The
aim of its current phase two is to synthetically generate, identify and
quantify dominant grain-scale deformation processes in response to changes in
environmental controls. Subsequently, these laboratory results will be
embedded in numerical models on the long-term in situ rheology of crushed
salt.
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