Abstract Crushed claystone produced by excavation of repository openings has been investigated as backfill/seal material. The raw coarse-grained claystone can be used for backfilling repository openings and, in mixture with bentonite, for sealing boreholes, drifts and shafts. The investigation programme focused on characterizing the thermo-hydro-mechanical properties of the excavated Callovo-Oxfordian claystone and the compacted claystone–bentonite mixtures, including (1) mechanical compaction, (2) gas and water permeability as a function of porosity, (3) water retention and saturation, (4) swelling capacity and (5) thermal properties of the materials. The major results are presented in this paper.