Comparative constitutive modelling of anisotropy, pore-pressure response and strain localization in Opalinus clay

Several countries are considering argillaceous rocks as host formations for the geological disposal of radioactive waste. These rock types possess favourable properties for the retardation of radionuclide migration from the deep geological repository (DGR), including very low permeability, low molecular diffusivity, high sorption capacity and self-healing characteristics. However, several internal and external events and processes, such as excavation of the DGR, thermal loading from the waste, and gas generation, can mechanically disturb the host rock and might adversely affect its favourable properties. The assessment of the host rock performance as a barrier to radionuclide migration necessitates the development of robust constitutive models. In the present paper, constitutive models developed by six research teams of the international DECOVALEX project are presented and compared. The models were developed using a comprehensive experimental database from laboratory tests performed on Opalinus Clay, an argillaceous host rock formation in the Swiss DGR concept. The laboratory tests consist of uniaxial compression tests, Brazilian tensile tests, and consolidated-undrained (CU) triaxial tests with pore pressure measurements, followed by microscopic observations of the deformation and failure modes. The models reproduced the main features of the experimental observations, including anisotropy due to bedding, deviation from linearity well before the peak strength, and localization of deformation and damage. These constitutive relations are being implemented by the teams to simulate gas migration tests in the same rock type in the dilatancy flow regime, where localized damage occurs and provides a preferential pathway for gas migration.