In situ strength and failure mechanisms of migmatitic gneiss and pegmatitic granite at the nuclear waste disposal site in Olkiluoto, Western Finland

The primary goals of the experimental work described in this paper were to establish the in situ spalling/damage strength of the intact rock, to establish the state of the in situ stress, and for the work to act as a Prediction-Outcome exercise within the context of confirmatory underground site investigations being conducted at the nuclear waste disposal site at Olkiluoto, in Western Finland. To establish the in situ mechanical properties, Posiva (the Finnish implementer) formulated the in situ Posiva Olkiluoto Spalling Experiment (POSE) in 2009. The outcome of this experiment was that rock failure mainly occurred due to structurally controlled factors, rather than being dictated solely by the expected location of the maximum stress. The POSE experiment also showed that the onset of fracture initiation in the Olkiluoto rock occurs at 40 MPa, and the rock mass strength is ca. 90 MPa-compared to the mean laboratory value of 104 MPa. In view of these observations made at the projected disposal depth, the vertical disposal concept, KSB-3V, will involve initiation of new fractures, although the horizontal disposal concept would be less affected by such fracture initiation. However, neither of the disposal concepts is expected to suffer any major rock mass failure, and the vertical disposal holes are not particularly sensitive to fracture initiation if the disposal tunnels are oriented within 30° trend of the major principal stress direction.