: The PFC Model for Rock is employed to predict damage formation adjacent to a circular test hole in gneissic tonalite subjected to compressive loading. The simulation results demonstrate that the PFC2D model can produce plausible predictions of excavation-induced damage. The predictions provide information about the detailed distribution of microcracks, including microcrack intensity, location and orientation, as well as the progressive evolution of such damage. The failure mechanisms exhibited by the PFC2D model include the formation of breakout notches (in compressive regions) and tensile fractures (in tensile regions) adjacent to the excavation. The microproperties of the PFC2D material were chosen to match the elastic modulus, crack-initiation stress and unconfined compressive strength, as well as the anisotropy in modulus and strength of gneissic tonalite. No attempt was made to match the strength envelope or the fracture toughness of the material; however, it is not clear to what extent these latter properties will influence the damage that forms adjacent to an excavation.
As part of the development of disposal technology for spent nuclear fuel, an in-situ failure test is planned for execution in the gneissic tonalite of the Research Tunnel at Olkiluoto, Finland (Autio et al. 1999a). One objective of this test is to assess if numerical modeling techniques can predict rock failure and associated cracking (Potyondy & Cundall 2000). In the proposed test, two horizontal slots, located approximately 350 mm above and below a 100-mm diameter test hole, are pressurized via expansive grout. The combination of in-situ stresses and the stresses induced by slot pressurization is expected to produce damage around the inner surface of the test hole. The expected damage consists of (1) sidewall breakout on the opposing sides in the compressive region and (2) radial tensile fractures on the opposing sides in the tensile region. Continuum-based numerical modeling performed with the elastic-plastic code FLAC3D (Autio et al. 1999b) generally supports these damage predictions, but the detailed distribution of microcracks and macroscopic fractures that comprise the damage cannot be predicted from such an analysis. To make such predictions, a discontinuum model, in which cracks are represented explicitly, is employed. The discontinuum model used here is referred to as the PFC Model for Rock.