2012:51 Technical Note, Rock Mechanics related to long-term repository and site evolution

In general, SKB demonstrates that they have accumulated a significant set of hydraulic, mechanical and thermal data that is essential for constructing a deep geological repository. The method of data acquisition is robust using the internationally- recognized and suggested methods. The methodology of safety assessment is firmly established by applying the state-of-the-art technology and expertise which results in high quality of produced reports.

Conclusions drawn in the reports are closely linked with various individual geological and mechanical processes, which are often independently presented in numerous background reports and published separately. Given the many uncertainties associated with designing and constructing a deep underground repository, the breadth of geological data and safety assessment, the quality of reporting is judged to be high. The content of the reviewed reports clearly demonstrate that SKB is well acquainted with modern rock mechanics and rock engineering. Nonetheless, there are a number of limitations noticed through the review of the four SKB reports.

Results from stress measurements conducted at Forsmark have a large uncertainty. The in situ stress model adopted in the safety assessment was constructed without making full use of the stress measurements with hydraulic methods; therefore a better in situ stress model is required. The effect of shear dilation due to thermal stress was not considered in the modelling work presented by SKB. Further analyses are required to quantitatively estimate the expected shear dilation due to thermal loading and related change of overall permeability in the target area. Importantly, this dilation is not reversible and the increased permeability will not be fully recovered even after cooling of the repository. SKB has not studied seismicity and estimated location and magnitude of events during the thermal phase.

The thermal expansion of the rock mass at repository level is anticipated to generate seismic events, rock spalling, excavation damage, and stress induced permeability change in fractures are all investigated by SKB without considering that thermal stress can initiate new fractures and existing fractures can propagate. Additional investigations are required to consider the alternative stress model, thermally-induced shear dilation and permeability, seismicity due to thermal loading, and fracture mechanics approach.