Posiva Oy has conducted investigations into excavation damage, including comprehensive laboratory testing of physical properties of rock specimens from excavation damaged rock mass. Laboratory testing was conducted on drill core specimens extracted from the excavated surface of a tunnel located at approximately 345 m depth in Olkiluoto, Finland. A total of 141 drill core specimens of three main rock types, a structurally isotropic coarse-grained pegmatoid (PGR) and structurally anisotropic veined gneiss (VGN) and diatexitic gneiss (DGN), were subjected to petrophysical testing, rock mechanics testing and petrographic analyses. Results from the various tests were subjected to rigorous statistical analysis in order to reveal the effects excavation damage has on the physical properties of the rock mass. Results of the study revealed changes that are credited to excavation damage in resistivity, S-wave velocity and various elastic properties of the rock specimens. Effects of excavation damage and the depth of the excavation damaged zone seem to be different to gneiss compared to pegmatoid. On microscopic level, the extent of excavation damaged zone appears to be 0.2 – 0.4 m depending on the measured property. This means that the deeper excavation damaged layer observed by geophysical surveys may be caused by larger scale fractures.
Extent and properties of excavation damage and its possible effects on hydraulic conductivity are some of the key considerations when evaluating the long-term safety of the deep geological disposal of spent nuclear fuel [1]. Posiva Oy has conducted investigations into excavation damage over the past several decades, including comprehensive laboratory testing of a wide variety of physical properties of rock specimens from excavation damaged rock mass. These results have recently been published in a series of four Posiva Working Reports [2, 3, 4, 5].
Laboratory testing was conducted on drill core specimens extracted from the excavated surface of a tunnel located at approximately 345 m depth from ground surface in Olkiluoto, Finland. A total of 141 drill core specimens from 25 drillholes were subjected to petrophysical testing, rock mechanics testing and petrographic analyses. Specimens had three main rock types: a structurally isotropic coarse-grained pegmatoid (PGR) and structurally anisotropic veined gneiss (VGN) and diatexitic gneiss (DGN). Specimen depths from the excavated surface varied from 0 to 1.8 metres, as this was expected to fully capture the extent of major excavation damage as observed previously with geophysical methods [1].
