Although stress measurements have been conducted in numerous locations in the ONKALO facility located in Olkiluoto, the prevalent trend of the maximum horizontal stress has yet to be defined to an extent where it is acceptable. As Posiva is responsible for the safe deposition of spent nuclear fuel in Finland, the definition of the in situ stress state, particularly at deposition depth, is a vital element to achieving this. Stress-geology interaction modelling work reported in Valli et al. (2011) was updated to support on-going Posiva's Olkiluoto Spalling Experiment (POSE) back-analyses.
The model was updated to and oriented according to the interpreted orientation of δH detailed in the 2011 Site Report (112°). In addition to changes in the simulation approach the effect of pore water was also included for the first time, according to pore pressure values detailed in the upcoming report Suikkanen et al. 2015. Likewise in situ stress state changes resulting from glaciation were simulated based on glacial stress additions detailed in Fäith & Hökmark (2014). Finally, BFZ parameters were changed to be based on all available brittle fault zone intersection (BFI) data, with RQD 100 values filtered from the data.
Results indicated that the in situ stress field is interacting with brittle deformation zones unless an unrealistically high fault plane cohesion of 6 MPa is considered. With more realistic parameters interaction is apparent up to a depth of ca. -350 m. The maximum variation of stress magnitude and orientation is ca. 11 MPa and 70°. Brittle deformation zones were, however, modelled as best-fit planes and the effect of true discontinuous, curved I bending /undulating faults remains unclear until the next planned model revision. Additionally, local parameter variation along a BFZ should possibly not be averaged across the zone and may need to be coupled with the geometry of the zone. Further, it is possible that LVDT-stress measurements conducted close to the excavation surface have suffered from excavation damage and/or time dependent stress damage observed in ONKALO.
