1 Introduction

In the geological nuclear waste disposal concept, the knowledge of long-term behavior, especially related to rock strength, is of high importance for long-term safety. An in situ experiment to investigate rock mass strength was conducted in Olkiluoto at Posiva's ONKALO underground characterization facility, located in Western Finland. The Posiva's Olkiluoto Spalling Experiment (POSE) was executed at the depth of -345 m, in a complex rock stress field and lithology. After executing the experiment a back-calculation phase followed to develop the predictive capability (Valli et al. 2015).

2 Methodology

The eight years of experience and research on the rock strength in ONKALO and executing the POSE in situ experiment reveal that the rock mass failure is governed by fracture growth on lithological borders. A two-fold failure criterion was proposed (Siren et al. 2015b) based on the onset of rock mass damage at 40 MPa and a peak rock mass strength of 90 MPa.

The confirming site investigations (Siren et al. 2015b) also indicated that the construction-induced (EDZCI) and stress-induced excavation damage (EDZSI) could be distinguished within the EDZ. An indication of this was the discovery of an EDZSI fracture in the tunnel floor under tensile conditions (Fig. 1) revealed while a section of the floor was wire sawed. The fracture had no characteristics of blasting or natural fracture processes and later Koittola et al. (2016) confirmed with a fracture mechanics simulation that a tensile stress field could be the driving factor of the observed fracture.

In the First Phase of the POSE experiment three new fractures initiated subcritically in the first two experiment holes. The first two fractures were localized, after boring of the first hole (Ø1.5 m), in mica-rich layers and rock type contacts, which were known to be relatively weak. The third, subvertical fracture was observed in the wall of the second hole after boring had been completed. Also in the third Phase, a subcritical failure at a mica-rich layer was observed. (Siren et al. 2015a)

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