Rock mass failure of foliated rock at the nuclear waste disposal site in Olkiluoto, Western Finland

In Finland and Sweden, the geological nuclear waste disposal is heading for the implementation phase in the next ten years or so. In the disposal concept, the knowledge of the rock mass strength and in situ rock stresses are important for long-term safety analyses. The rock mass strength of the nuclear waste disposal site in Olkiluoto, Western Finland, was researched as a part of Posiva’s Spalling Experiment (POSE). The POSE experiment was conducted as an in situ rock mass strength test that revealed the rock mass failure in Olkiluoto being governed by fracture growth at lithological borders instead of spalling type of failure commonly encountered in sparely fractured crystalline rock conditions. Based on the POSE experiment results, a structurally controlled failure is distinguished as a new rock mass failure mechanism. A back-calculation campaign was established to study the new rock failure mechanism with multiple modeling techniques, of which the results of fracture mechanics code FRACOD simulations are presented in this paper. Fracture Mechanics is able to mimic failure conditions, however after excessive calibration.