The position of larger seismically mappable faults will be taken into account when planning the location of the disposal areas. However, subseismic faults (small-scale and/or micro faults, Tab. 4‑1) may also occur in areas free of seismically mappable faults. The effects of tectonic overprinting on the Opalinus Clay and the confining units are less pronounced than in the more competent over- and underlying formations (Section 4.3), which can be related to the mechanical layering (Section 4.6) at larger scale.

Here the focus is on the deformation behaviour and typical fault geometries of the Opalinus Clay and clay-mineral-rich confining units. It is emphasised that these are favourable to maintaining barrier integrity over longer timescales, consistent with hydraulic packer test results (Section 4.5). Compaction and moderate diagenesis of the Opalinus Clay in the siting regions has led to overall higher strength than at the Mont Terri rock laboratory (Section 5.5.3), whilst still maintaining good swelling capacity (Section 5.5.2).

The understanding of the detailed constitutive behaviour of the Opalinus Clay (e.g. Nagra 2014h, Marschall & Giger 2016) has not changed significantly with the recent exploration campaign. However, the mechanical database in the siting regions is now substantially improved, much more robust and demonstrates a relatively small variation in mechanical properties for the Opalinus Clay. Numerical modelling of the constitutive behaviour was further advanced to integrate both aniso­tropic stiffness and strength (Section 5.5.3).

In Section 5.5.4, the low tectonic overprint in the Opalinus Clay is discussed. Only small-scale or micro-scale faults were encountered in the deep boreholes. Analyses of outcrops and 3D seismic reflection data and comparison with similar rocks shows that deformation in clay-rich rocks is generally soft-linked, and fractures poorly interconnected. The expansive volumetric behaviour of the Opalinus Clay and clay-mineral-rich confining units as elaborated in Section 5.5.2 further counteracts fracture-dominated flow, as will be emphasised in Section 5.7 (self-sealing).