In general, the reviewers find that the level of geomechanics data collected and synthesized to date has been impressive in scope and high in quality. The data collected has been incorporated into interlinked analyses concerning construction and operational performance of the repository. There are a number of uncertainties in the data and subsequent analyses that are unavoidable when dealing with earth materials. An effort has been made to understand the impact of propagation of these uncertainties through the analysis and design process. The reviewers find, however, that the main shortcoming of the reporting is a lack of coherent structure required to understand the relative reliability of the conclusions central to the design premises. Major concerns beyond data and interpretive clarity include:
- Challenges posed by conflicts between very tight construction and operational tolerances and the geological uncertainty that is likely to create geometric variability at different scales.
- While significant detail and confidence exists for the main rock types expected, there is room for additional data or interpretation concerning less common but likely rock types within the repository footprint.
- There is a concern that the rejection criterion for the Deposition Holes (EFPC) may not adequately incorporate the interaction of spall damage and existing fractures and that the rejection rate may be higher in practice than proposed.
- Significant uncertainty exists with respect to the in-situ stresses. While this is to be expected for deep projects such as this, conservatism is required at this stage with respect to adopted ranges for stress within the context of EDZ (Excavation Damage Zone) generation around excavations and deposition holes.
- The Observational Method is proposed to deal with uncertainties. There is a need to be clear about how deviations will be dealt with and whether this approach will facilitate adequate and timely adjustments to the design as deviations from expected conditions are encountered