The density, porosity and mechanical properties from uniaxial compression and Brazilian tests along with measurements of the P- and S-wave velocities in axial and two lateral directions at unloaded state and at increasing axial load were conducted on cylindrical cores of veined gneiss and granitic pegmatoid. The specimens were sampled close to a tunnel wall (0.1–1.7 m depth) with the aim to find if there is an increasing amount of microcracks due to the excavation in a zone near the surface. Evidence of microcracks was found via the mechanical tests and the wave velocity measurements, due to increased mechanical stiffness and increasing wave velocities at loading. Anisotropy in in the gneiss specimens and also in the granitic pegmatoid was detected and measured via strain and wave velocity measurements. No clear evidence was found of increased amount of microcracks closer to the tunnel wall.
Knowledge of the extent and properties of excavation damaged zone (EDZ) around deposition tunnels in underground repositories for spent nuclear fuel is part of the long-term safety assessment. Several projects for characterizing and studying the properties of EDZ have been carried out by e.g. various nuclear waste management organizations such as Posiva Oy in Finland and SKB in Sweden (Mustonen et al., 2010). The conducted studies indicate that each underground repository site has its own unique characteristics in terms of geological history, in-situ stress state and lithology along with the applied excavation methods, which make the development of EDZ to a large extent site specific.
This study, financed by Posiva Oy, belongs to the on-going investigations into characterization of excavation damage at the ONKALO research facility in Olkiluoto, Finland. The excavation damage was studied below the floor level of a specifically excavated ONK-TKU3620 niche located at the depth of -345 m. The major rock types prevailing in Posiva’s EDZ study volume were foliated veined gneiss (VGN) and granitic pegmatoid (PGR).
