A unique set of true-triaxial experiments have been used to mimic the loading path that the in situ rock is exposed to during excavation, heating, cooling and unloading through the Posiva's Olkiluoto Spalling Experiment (POSE), at the proposed deep geological repository at Olkiluoto, Finland. Results from testing of rock specimens prepared with their foliation planes oriented with respect to δ1, under true-triaxial conditions, show that 0°, 30° and 60° gneiss samples fail whereas 90° gneiss and isotropic pegmatite specimens do not fail under the same loading path. The pegmatite specimen shows numerous acoustic emission (AE) activities but does not fail during the final stages of the stress path. Temporal and spatial analysis of AE events show that fracture initiation and propagation are sensitive to foliation plane orientation with respect to m and that failure planes coincide with foliation planes in failed specimens. Variation of seismic wave velocities measured along three orthogonal axes as a function of the loading path show that compressional and shear wave velocities measured parallel to foliation planes are higher than perpendicular to such planes characterizing the transversely isotropic nature of Olkiluoto gneiss. The true-triaxial POSE experimental loading and unloading path is complex and results in strength and behavioural characteristics that are not replicated with conventional triaxial testing where δ2 = δ3.


Crystalline rock formations are being considered as a potential host for the geological disposal of radioactive wastes and these formations are characterized with foliation planes resulting in anisotropy of their strength, deformation, permeability and geophysical properties such as in the case of ONKALO, located on Olkiluoto Island, Finland. The ONKALO nuclear waste repository site is characterized mainly by migmatitic foliated gneiss, massive coarse-grained pegmatitic granites together with numerous brittle fracture zones and thrust faulting, which are believed to have affected the state of in situ rock stress indicated by the LVDT -(;ell stress measurements (Johansson et al., 2015). Modelling techniques and theoretical evaluation of 3D in situ stress variation during various stages of excavation at the site show that the three principal stresses change independently in the vicinity of the excavation.

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