Compression behaviors of crushed COx argillite used as backfilling material in HLW repository

In high-level radioactive waste(HLW) repository, backfilling material should be pre-compacted. The compression characteristics are very important for the design and security of HLW repository. The Callovo-Oxfodian(COx) argillite(France) is now proposed, after being crushed and compacted, as an alternative backfilling material in the geological HLW repository. In this investigation, the compression behavior of COx argillite with two different grain size distributions(GSDs), which were obtained by two different crush procedures, was studied by running 1D compression tests with several loading-unloading cycles. It is found that the compression curves are significantly influenced by the GSD of the soils. To obtain the same degree of compaction, the axial stress applied to finer soil is much higher than that of coarser soil. In addition, the compression index at initial water content of the finer soil is bigger than that of coarser soil. During the compression, both the compression curves of the finer and coarser soils tend to be close to each other with increasing dry density. It is shown that the effect of GSD on the compression behavior is weakened after the soil is highly compacted. The swelling index at initial water content increases with the dry density and seems to be independent of the GSD. During saturation, the highly compacted coarser soil sample(ρ_d =2.0 g/cm³) shows evident collapse behavior under 7 MPa axial stress. The compression index decreases and the swelling index increases after the saturation.