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Geologic repositories for radioactive waste are designed as multi-barrier disposal systems that perform a number of functions including the long-term isolation and containment of waste from the human environment, and the attenuation of radionuclides released to the subsurface. The rock laboratory at Mont Terri (canton Jura, Switzerland) in the Opalinus Clay plays an important role in the development of such repositories. The experimental results gained in the last 20 years are used to study the possible evolution of a repository and investigate processes closely related to the safety functions of a repository hosted in a clay rock. At the same time, these experiments have increased our general knowledge of the complex behaviour of argillaceous formations in response to coupled hydrological, mechanical, thermal, chemical, and biological processes. After presenting the geological setting in and around the Mont Terri rock laboratory and an overview of the mineralogy and key properties of the Opalinus Clay, we give a brief overview of the key experiments that are described in more detail in the following research papers to this Special Issue of the Swiss Journal of Geosciences. These experiments aim to characterise the Opalinus Clay and estimate safetyrelevant parameters, test procedures, and technologies for repository construction and waste emplacement. Other aspects covered are: bentonite buffer emplacement, highpH concrete-clay interaction experiments, anaerobic steel corrosion with hydrogen formation, depletion of hydrogen by microbial activity, and finally, release of radionuclides Geosci (2017) 110:3-22 DOI 10.1007 into the bentonite buffer and the Opalinus Clay barrier. In the case of a spent fuel/high-level waste repository, the time considered in performance assessment for repository evolution is generally 1 million years, starting with a transient phase over the first 10,000 years and followed by an equilibrium phase. Experiments dealing with initial conditions, construction, and waste emplacement do not require the extrapolation of their results over such long timescales. However, experiments like radionuclide transport in the clay barrier have to rely on understanding longterm mechanistic processes together with estimating safety-relevant parameters. The research at Mont Terri carried out in the last 20 years provides valuable information on repository evolution and strong arguments for a sound safety case for a repository in argillaceous formations.

The research and development programme on geological disposal for high-level and long-lived waste (HLW) in Belgium was initiated in 1974. A deep tertiary clay formation, the Boom Clay, present under the Mol-Dessel nuclear site, was selected as a reference host formation for experimental purposes. The construction of the underground laboratory HADES (at a depth of 223 m, initiated in 1980 and extended in 2002) allowed the building of a valuable geotechnical database, and led to the development of improved excavation techniques that significantly reduce the excavation-damaged zone (EDZ). Since the operational start of HADES about 25 years ago, many geotechnical measurements have been performed around excavations. Comparison between in situ measurements and modelling results allowed a continuous improvement of our knowledge on the Boom Clay behaviour. Important issues for interpreting the measurements correctly are good control of the excavation parameters and the boundary conditions. An extensive characterisation of the hydromechanical response of Boom Clay around an excavation for short- and long-term conditions has been performed. One important finding was the occurrence of measurable hydraulic effects at a distance of about 60 m (12·5 tunnel diameters) ahead of the tunnel excavation.

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