The term "self-sealing" broadly encompasses all hydromechanical, hydrogeochemical and hydro-biochemical processes that lead to a reduction in transmissivity of a fractured rock mass (Tsang et al. 2005). The term "sealing" implies that the reduction in hydraulic conductivity is so significant that transport properties are again comparable to the intact rock mass or simply so low that transport becomes diffusion-dominated (Section 5.8). The prefix "self" implies that the sealing processes occur spontaneously if access to water or brine is granted. Self-sealing can be considered as the key factor for the barrier integrity of the Opalinus Clay and other clay-rich rocks even in tectonically disturbed areas, which is a critical difference to other low-permeability rocks with lower clay-mineral content.
Section 5.7.2 first summarises selected empirical and phenomenological evidence for self-sealing in the Opalinus Clay. The key processes and mechanisms responsible for self-sealing are then explained in Section 5.7.3.
It was highlighted previously that hydraulic conductivity in the Opalinus Clay is low even in fractured and faulted intervals (Section 5.6). The most relevant evidence is results from the hydraulic packer tests in the new deep boreholes, which are consistent with matrix values of unfractured laboratory samples (Fig. 5‑38). Besides the standard packer tests in deep boreholes, relevant new data were also acquired from dedicated special hydraulic tests in the deviated RHE1 borehole, which are discussed in Section 5.7.4.
Self-sealing potential correlates mainly with material properties such as clay-mineral content and bulk density or porosity but depends also on the effective stress field (see Section 5.7.3 for the mechanisms). This stress dependence explains why a landfill in the Opalinus Clay at a few tens of metres depth cannot rely on similar self-sealing potential as can a facility at repository depth. Effective stresses in all the siting regions currently ensure robust self-sealing (Section 5.7.4) for the Opalinus Clay at the level of a future repository. Self-sealing may be limited when the overburden thickness decreases to values < 200 m. This is not expected for the period under consideration (Sections 6.4 and 6.5).
Other factors affecting self-sealing are discussed in Section 5.7.5, and the relevance of mineralised fractures ("veins") for the self-sealing potential in the Opalinus Clay is addressed separately in Section 5.7.6.