To characterise the Opalinus Clay, we investigate its hydrogeological, geochemical, and rock-mechanical properties. In particular, we are interested in fluid and material transfer processes and deformation mechanisms. Through experiments, we acquire relevant characteristic values, such as hydraulic permeability, diffusion coefficients and thermal properties. The heterogeneity of the characteristic values and the influence of layering (anisotropy) play key roles in the characterisation. Moreover, investigation of the excavation-damaged zone around a tunnel structure in Opalinus Clay is a main target of on-going research. Here, we study and clarify the question of the self-sealing of natural and excavation-produced cracks in the tunnel walls.