Mechanical and seismic anisotropy of rocks from the ONKALO underground rock characterization facility

Abstract

Olkiluoto is an island that was selected as the site for a final nuclear waste disposal facility in Finland. ONKALO underground research facility was constructed for the purpose of rock characterization. A hybrid stress path, incorporating the uncertainties of the near-field stress state estimated for the different Posiva Olkiluoto Spalling Experiment (POSE) phases, was developed as the basis for boundary conditions for a series of true-triaxial tests. Evolution of each principal stress component as a result of excavations and heating of the experimental holes to understand the phenomenon of excavation damage zone (EDZ) development was evaluated. The analysis of the temporal and spatial acoustic emission (AE) activity, construction of 3D damage fracture network images from X-ray CT, and thin section studies performed on tested specimens are presented here. AE events are used to evaluate the chronological occurrence of the damage fracture networks within the induced fractures preferably initiating and propagating within the foliation zones characterized by mica grains, and predominantly avoiding quartz and feldspar grains. Veined gneiss (VGN) samples with a 30° foliation angle relative to the maximum loading direction, and the minimum loading direction perpendicular to the strike of foliation, exhibited the lowest strength, showing shear type fracture initiating parallel to the foliation planes at 55 MPa of axial stress. This value corresponds to the stress state at the POSE experiment hole wall when the first shear type damage was observed to be parallel to the foliation planes. Comparison of the evolution of AE events and seismic velocity measured at the field site for granitic pegmatite (PGR) with its counterpart laboratory specimen was found to be in good agreement. PGR is the strongest member of the rock mass and shows significant AE activity for specimens subjected to the simulated in situ loading without development of macroscale fractures.