As a result of the high self-sealing potential of the Opalinus Clay, and of clay-rich rocks in general, fractures have a much lower significance as a threat to barrier integrity than fractures in other low-permeability rocks also considered as host rocks for a deep geological repository (e.g. granitic rocks).

Clay-mineral content is a very good indicator for the self-sealing potential of the Opalinus Clay and the confining units for the disposal depths considered in all the siting regions. Rocks with a clay-mineral content greater than about 40 wt.-% have a high self-sealing potential, and rocks with clay-mineral contents less than about 20 wt.-% have low or negligible self-sealing potential.

At the tunnel scale (several metres), self-sealing potential in the Opalinus Clay is high throughout the formation at all the sites. In contrast, some facies in the confining units have low or even negligible self-sealing potential over distances much larger than the tunnel scale.

The other key variable controlling the self-sealing potential is the stress field. The stress field can be considered favourable for robust self-sealing of the Opalinus Clay in all the siting regions at current depths.

The timescale over which self-sealing mechanisms operate depends on the ability of the rock to equilibrate water pressure in the disturbed area. Fracture closure can occur immediately upon a reduction in fluid pressure (or effective stress) and is followed by slower time-dependent mechanisms (especially swelling) over days to years.

For the long-term evolution of the repository, a residual overburden thickness of 200 m can be considered reliable for self-sealing in the Opalinus Clay. If the overburden thickness is reduced to 50 – 100 m, self-sealing may be limited, and if the overburden thickness is only 20 – 30 m, it is hardly or no longer effective.