Hydro-mechanical and gas transport behaviour of crushed Opalinus Clay

Deep geological repositories generate large volumes of excavated host rock that can be reused as backfill, yet knowledge of the hydro-mechanical and gas transport behaviour of crushed shales remains limited. Opalinus Clay (OPA), the selected host rock in Switzerland, was tested in crushed form (OPA c ) through an integrated experimental campaign combining swelling and one-dimensional compression tests, water retention measurements, gas injection experiments, and microstructural analyses along controlled stress paths. As-compacted OPA c exhibits a hierarchical pore system formed by assemblages of shale fragments and clay aggregates. Upon hydration, swelling was strongly boundary-dependent, suggesting that in repository conditions OPA c could fill technological gaps while developing only limited swelling pressure. Mechanical loading progressively closed macropores, while intra-fragment micropores remained largely unaffected. This transition towards a single pore mode enhanced water retention and reduced intrinsic permeability. Gas injection tests showed breakthrough pressures comparable to intact OPA, confirming the governing role of micropores. However, the much higher gas permeability of OPA c was attributed to its larger overall porosity, particularly the persistence of macropores that facilitated gas flow after breakthrough. Together, these results demonstrate the link between microstructural evolution and macroscopic flow in OPA c and indicate that remaining macropores provide efficient pathways for gas dissipation post-breakthrough. These findings support the evaluation of OPA c as a locally available and sustainable material for backfills in deep geological repositories for radioactive waste. • Crushed Opalinus Clay's hydro-mechanical and gas transport behaviour is assessed. • Fabric exhibits assemblages of shale fragments and clay aggregates. • Microstructural evolution affects hydro-mechanical and gas transport behaviour. • Macropores provide efficient pathways for gas dissipation.