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.

1. Introduction

In Finland, Posiva Oy is currently in process of building a high active spent nuclear fuel repository in Olkiluoto Island, located at Western Finland. At the moment, detailed site characterization studies of the Olkiluoto Island are being conducted through ONKALO underground research facility at the corresponding site. Also several rock mechanics in situ experiments have been executed lately.

The third Phase of Posiva's Olkiluoto Spalling experiment (POSE) was executed in ONKALO in 2013, where an experiment hole (Ø1.524 m) located at depth of −345 m was heated from inside while rock failure and deformation was observed (Valli et al., 2014). Predecessors of the test were the spalling experiments conducted both in the URL at Canada and ÄSPÖ in Sweden. In previous tests, it was observed that the in situ strength of sparsely fractured crystalline rocks is in the order of 40% – 60% lower than the measured uniaxial compressive strength in laboratory testing conditions (Martin et al., 1997; Andersson 2007). The mechanism leading to the macro scale rock mass failure in the in situ tests reported both in Martin et al. (1997) and Andersson (2007) was observed to be of spalling type.

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