Compaction of crushed salt for safe containment – overview of the KOMPASS project
Authors: Friedenberg Larissa , Bean James , Czaikowski Oliver , Düsterloh Uwe , Müller-Hoeppe Nina , Laurich Ben , Lerch Christian , Lerche Svetlana , Lüdeling Christoph , Mills Melissa , Popp Till , Reedlunn Benjamin , Stührenberg Dieter , Svensson Kristoff , Zemke Kornelia , Zhao Juan 
Published: November 2021

Abstract. In Germany, rock salt formations are possible host rock candidates for a
repository for heat-emitting radioactive waste. The safety concept of a
repository in salt bases on a multibarrier system consisting mainly of the
geological barrier salt and geotechnical seals ensuring safe containment. Crushed salt will be used for backfilling of cavities and sealing measures in
drifts and shafts due to its favourable properties and its easy availability
(mined-off material). The creep of the rock salt leads to crushed salt
compaction with time. Thereby, the crushed salts' porosity is reduced from the
initial porosity of 30 %–40 % to a value comparable to the porosity
of undisturbed rock salt (≤1 %). In such low porosity ranges,
technical impermeability is assumed. The compaction behaviour of crushed salt is rather complex and involves
several coupled THM processes (Kröhn et al., 2017; Hansen et al.,
2014). It is influenced by internal properties like humidity and grain size
distribution, as well as boundary conditions such as temperature, compaction
rate or stress state. However, the current process understanding has some important gaps referring
to the material behaviour, experimental database and numerical modelling. It
needs to be extended and validated, especially in the low porosity range. The objective of the KOMPASS project was development of methods and strategies
for the reduction of deficits in the prediction of crushed salt compaction
leading to an improvement of the prognosis quality. Key results are as follows (KOMPASS Phase 1, 2020):
selection of an easily available and permanently producible synthetic crushed salt mixture, acting as a reference material for generic investigations; development and proof of different techniques for producing pre-compacted samples for further investigations; establishment of a tool of microstructure investigation methods to demonstrate the comparability of grain structures of pre-compacted samples with in-situ compacted material for future investigations; execution of various laboratory experiments using pre-compacted samples, e.g. long-term creep tests which deliver reliable information about time- and stress-dependent compaction behaviour; development of a complex experimental investigation strategy to derive necessary model parameters considering individual functional dependencies. Its technical feasibility was successfully verified; benchmarking with various existing numerical models using datasets from three different triaxial long-term tests. The result was not entirely satisfactory; however, the number of influencing factors is small and further validation work has to be done.
Overall, the KOMPASS project has made significant progress in the approaches to solving the outstanding question, building the basis for further investigations.