Key points:

• Geomechanical properties of the Opalinus Clay are comparable in all three siting regions. The high clay-mineral content in combination with deep maximum burial and significant overburden today (> 400 m) contribute to the low variability in mechanical properties (e.g. compared to Mont Terri).
• The relatively low stiffness and low «brittleness» of the Opalinus Clay and clay-mineral-rich confining units is consistent with more distributed deformation compared to more competent Mesozoic units.
• Localised fault-related flow is generally impeded by this type of deformation and is further counteracted by the expansive volumetric behaviour of clay-rich rocks when unloaded.