Future uplift and subsequent erosion will reduce the overburden thickness of the Opalinus Clay (Section 6.4.4). This may influence the geomechanical and hydrogeological properties of the host rock and confining units, and the self-sealing of fractures.
As long as the overburden thickness above the Opalinus Clay remains greater than 200 m, erosional unloading has only a very minor impact on the hydraulic conductivity (Section 5.6). This is because unloading of the Opalinus Clay is not associated with brittle deformation (Nagra 2014h), because existing fractures are held sufficiently tightly closed (Section 5.7.4). Furthermore, changes in porosity when unloading from repository depth to 200 m would be negligible (of the order of 0.5 to 1 vol.-%, Sections 5.3.2 and 5.5.2) and would not result in a relevant change in matrix hydraulic conductivity. However, with progressive erosional unloading to shallower depth, fractures may be induced and effective normal stress on the fractures may be insufficient for robust self-sealing.
For the most likely erosion scenario, reliable self-sealing conditions are predicted for all sites, as the host rock overburden thickness remains > 200 m during the time under consideration (Section 6.4.4). The full range of erosion scenarios shows that NL is the most robust in this regard. In JO, the remaining overburden thickness depends more substantially on the evolution of the drainage system and associated topography (Section 6.4.4, Fig. 6‑44 and Fig. 6‑47).
A study evaluating numerous hydraulic tests in various shallow boreholes in the Opalinus Clay demonstrates a marked increase in hydraulic conductivity of the Opalinus Clay when the overburden thickness is less than approximately 30 – 50 m, with the exact depth of this transition depending on multiple factors, in particular the topographic setting (Hekel 1994). Detailed investigations in the Lausen borehole revealed that a significant increase in hydraulic conductivity (the uppermost 30 m) broadly coincides with the depth at which significant decompaction-induced fracturing starts to occur (Fig. 6‑46; Vogt et al. 2017, Crisci 2019, Mazurek et al. 2023b). The investigations also show that a significant increase in matrix porosity occurs only at very low effective stress (< 1 MPa), equivalent to an overburden thickness of just a few tens of metres (Fig. 6‑46). The mineralogy and rock geochemistry were only altered in the uppermost few metres, and substantial geochemical evidence was gathered indicating that transmissive fractures self-sealed even in the lower weathered zone (approximately 15 m depth; Mazurek et al. 2023b). In summary, a substantial increase in hydraulic conductivity in the Opalinus Clay only occurs for overburden thicknesses < 30 – 50 m.
The clay-mineral-rich confining units are predicted to show a similarly low sensitivity of hydraulic conductivities to unloading as the Opalinus Clay. For carbonate-rich parts of the upper confining units, the depth-dependent hydraulic conductivity is less constrained. In the Effingen Member (Wildegg Formation), for example, the increased hydraulic conductivities observed down to a depth of around 300 m can possibly be explained by decompaction effects (Nagra 2014f).
Fig. 6‑46: Rock properties of the Opalinus Clay in the shallow decompaction zone: Synopsis of the observations in the Lausen case study
Figure based on Mazurek et al. (2023b) and Vogt et al. (2017). Dark colours indicate the depth range of alteration. TDS: Total dissolved solids.