Excavation damage zones, fracture mechanics simulation and in situ strength of migmatitic gneiss and pegmatitic granite at the nuclear waste disposal site in Olkiluoto, Western Finland

In Finland and Sweden, the geological nuclear waste disposal is heading for the implementation phase in the next ten years or so. In the disposal concept, the knowledge of the in situ stress state, the excavation damage zone (EDZ), rock mass strength, hydraulic conductivity and other rock mass properties are important for long-term safety. In this dissertation, the EDZ, rock strength and rock failure mechanisms have been researched in Posiva's ONKALO underground characterization facility, located in Olkiluoto, Western Finland. The experiments included a prediction–outcome (P–O) component in order to test the current predictive capability. Fracture mechanics modelling in anisotropic rock has been developed and tested, in relation to the in situ experiments. The ground penetrating radar measurements and observations both in ONKALO and the Äspö hard rock laboratory (HRL) indicate that excavation induced damage can be distinguished into a construction-induced excavation damage zone (EDZCI) caused by blasting or by mechanical excavation and into a stress-induced excavation damage zone (EDZSI) as a result of the evolution of the secondary stress state. The EDZCI is evident around the full tunnel perimeter, whereas the effects of the EDZSI are typically noticeable in areas where stress peaks or tensile conditions exist. Previous experience and research of rock strength in ONKALO and executing the POSE in situ experiment revealed that rock mass failure in Olkiluoto is governed by fracture growth at lithological borders. A two-fold failure criterion is proposed in this dissertation based on the onset of rock mass damage strength at 40 MPa and the rock mass strength 90 MPa. By comparing the experiment prediction and outcomes, the fracture mechanics prediction can capture the fracture growth realistically in the anisotropic rock mass, although the in situ experiments revealed a behaviour that was unpredictable. Based on the POSE experiment the structurally controlled failure is distinguished as a new rock mass failure mechanism. In this dissertation the applicability of the horizontal and vertical disposal concepts were evaluated for nuclear waste disposal. Based on the in situ experiments and modelling, the vertical disposal concept is not particularly sensitive if the trend of the tunnel is within 30° of the direction of the major principal stress. In the thermal period, the vertical disposal concept may suffer from the initiation of new fractures sometime after excavation, but neither of the disposal concepts is expected to be affected by any significant rock mass failure. Based on the dissertation, rock damage in Olkiluoto is modest and the site is well-suited for nuclear waste disposal from a rock mechanics perspective.