2013:37 Technical Note, Rock Mechanics – Evolution of fracture transmissivity within different scenarios in SR-Site – Main Review Phase

The current review assignment consists of a literature review and independent modeling for assessing the importance of fracture transmissivity change especially driven by shear loading due to SKB’s thermal, glacial and earthquake loading scenarios for the KBS-3 repository at Forsmark.

In the first part of the report, SKB’s laboratory experiments on shear fracture dilation and extensive literature review on shear fracture dilation in-situ and in laboratory under moderate normal stress show that dilation can become important even under moderate normal stress of about 20 MPa. Importantly, the increase of transmissivity induced by shear dilation would not recover to its initial state after cooling of the repository because the process occurs at shear failure, as demonstrated by the experiments in the literature. This permanent change of transmissivity can impact on the safety assessment of the repository for issues related to buffer resaturation time and radionuclide transport.

In the second part of the report, a modeling study focuses on the transmissivity change from thermally-induced shearing of fracture around repository on three types of geometries with a single fracture, realistic Discrete Fracture Network (DFN) models for Forsmark, and a far-field model with large scale deformation zones at the Forsmark site. The thermal shearing analysis was conducted by means of a discrete element method code, UDEC. DFNs were independently generated based on the fracture data provided by SKB. It was shown that transmissivity increase can be up to 2 orders of magnitude for an initial fracture aperture of 30 μm. These large transmissivity changes can occur around 10 m from thedeposition tunnel. This indicates that significant changes are possible anywhere between adjacent deposition tunnels because dilation occurs and shear stresses can exceed the frictional strength of the fractures and deformation zones. Furthermore, because the frictional strength of some fractures was exceeded, irreversible shear dilation occurs, which does not recover to its initial state in the models after cooling of the repository. The magnitude and spatial extent of the transmissivity change is greater than the investigations reported by SKB. The significance and limitation of the modeling was discussed in view of parameters from fracture characterization, two-dimensionality of the used DFNs and numerical code. In the far-field model, the gently dipping deformation zones were most prone to shear dilation under thermal loading.

The third and fourth part of the report concern the effect of glacial and earthquake loading on transmissivity change. Investigations were made on the same DFN models used for the thermal study. The effect of glaciation on fracture transmissivity change was negligible due to the fact that glacial loading were increased in an almost isotropic manner, which does not promote shear slip. Furthermore, induced shear displacements in the models recovered after the retreat of the ice, which indicates that shear behavior of the fractures during a glacial cycle is mainly elastic. Earthquake modeling was conducted by applying synthetically generated ground motion to the models surrounded by viscous boundaries. Earthquake loading on a conceptual model containing a single fracture with size of 5, 10 and 15 m shows that there can be up to 20 μm of permanent aperture increase when the stress condition is close to shear failure. This result shows that the effect of an earthquake needs to be investigated, not only in terms of canister integrity, but also in terms of fracture trans- missivity change. Earthquake modeling on DFN models, however, showed that actual shear dilation is negligible due to the fact that fracture orientation is not favorable to shearing. Further systematic investigation may be required on the effect of various DFN statistics and earthquake conditions.