The in situ stress state is a key rock mechanics parameter and it must be well understood for the design of an underground repository of spent nuclear fuel. The Swedish Nuclear Fuel and Waste Management Company (SKB) has selected the Forsmark site for the construction of a repository at a depth of -470 m. So far approximately 130 overcoring and 240 hydraulic stress measurements have been completed and interpreted while also making use of borehole breakout, core disking and non-linear strain rock sample data. This interpretation indicated notable variation in both horizontal stress components but did not address the variability.

The primary goal of this study was to understand the observed and expected variation in stress magnitudes and orientations at repository depth, but also to verify rock parameters. The volume to be studied and the brittle fault geometry was defined by the local geological model, resulting in a total of 110 faults. Work was performed in two phases: fault geometry and shear strength were studied in Phase 1 and the in situ stress state was applied according to the current interpretation. Phase 2 simulations involved only undulating fault geometries while the interpreted stress state was induced using boundary conditions, resulting in a better overall response. Glaciation was also simulated. Results indicated that to be able to simulate the observed variation in the in situ stress state, the stress state has to be established with thrust boundary conditions and undulating fault geometry yields a more realistic response.

For a more detailed description of the study please see the published report.