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.The disposal concepts developed by many countries for repositories in clay formations are based on the principle of a multi-barrier system, which comprises the natural geological barriers provided by the host rock and its surroundings, and an engineered barrier system (EBS). After emplacement of radioactive waste containers, the disposal boreholes, drifts and shafts must be backfilled and sealed with suitable materials to prevent a release of radionuclides into the biosphere. The geotechnical properties of each EBS component must be specified with regard to their functional requirements. In most concepts, bentonite-based materials are considered for use in filling and sealing disposal boreholes. For instance, the Swiss concept (Nagra 2002) provides for waste containers being emplaced on compacted bentonite blocks positioned on the drift floor and the remaining space being backfilled with granular bentonite. Meanwhile, according to the French concept (Andra 2005), the drifts are backfilled with excavated claystone and sealed with compacted bentonite-based seal core confined between concrete plugs.Crushed claystone produced by the excavation of repository openings may be a favourable alternative as backfill/seal material; its use has many advantages, including chemical-mineralogical compatibility with the host rock, availability in sufficient amounts, low cost of material preparation and transport, and there is no need for it to occupy surface space if it is used as backfill for the excavated claystone. Indeed, crushed claystone with coarse grains can be considered for use for backfilling repository openings and, in mixture with bentonite, for sealing boreholes, drifts and shafts. Recently, a research programme has been conducted at the GRS geotechnical laboratory to characterize the geotechnical properties of the excavated Callovo-Oxfordian claystone and the compacted claystone -bentonite mixtures under relevant repository conditions. The following properties of the backfill/seal materials are most important in terms of their barrier functions and have thus been investigated:(1) The compaction behaviour of the backfill material in underground openings. This controls mechanical interactions with support linings (if existing) and with the surrounding rock and, in turn, determines the evolution of backfill porosity and permeability....