In Situ Rock Stress-Strength Comparison: Posiva’s Olkiluoto Spalling Experiment (POSE)

As part of the preparations for the design of an underground repository for radioactive waste at Posiva's site on the west coast of Finland, there has been considerable site investigation work. One of the key elements of this work has been the estimation of the in situ stress field and determination of the mechanical properties of the crystalline rocks present. The stress field is required as the boundary condition for numerical modelling and to determine a suitable orientation for the repository tunnels. The mechanical rock properties are required, inter alia, to establish whether there is any potential for rock spalling at the ca. 400–450 m anticipated depth of the repository. A key aspect of the analysis is the understanding of the in-situ spalling strength in order to be able to predict the spalling potential, not only during deposition tunnel and deposition hole excavation but also in the longer term when the rock mass is subjected to canister heating up to temperatures close to 60 °C. The in situ spalling strength is of the order of 60% of the uniaxial compressive strength. Accordingly, an in situ rock mechanics experiment has been designed and conducted in a niche tunnel off the main ONKALO ramp at the Olkiluoto site in Finland. The experiment has the acronym POSE: Posiva's Olkiluoto Spalling Experiment. The experiment has involved the drilling of two closely located full-scale simulated deposition holes, 1.524m in diameter, which concentrate the in situ stress and enable rock spalling to be observed at close hand. In addition, heaters have been used to simulate the rock temperature increase due to radioactive canister heat generation, further altering the in situ stress field. The preliminary POSE experimental results and the thermo-mechanical simulations are illustrated and presented in this paper.