Fracture Initiation and Propagation in Low Porosity Crystalline Rocks: Implications for Excavation Damage Zone (EDZ) Mechanics

Abstract

The Deep Geological Repository (DGR) is the globally preferred concept for long-term storage of nuclear waste, in which the combination of engineered barriers and the surrounding geology are designed to safely and securely contain the radioactive waste. The construction processes of underground openings in hard rocks at depth are commonly associated with introducing some damage to the surrounding rock mass through the initiation and propagation of fractures parallel to the excavation boundary due to disturbance to the in situ stress condition, as well as the excavation process quality. The induced fractures in depth away from the excavation surface can provide a migration path for the radionuclides in case of damage to the engineered barriers. Therefore, different damage zones are defined according to the crack density and change in hydraulic permeability of the rock mass surrounding the waste storage tunnel and rooms, these are commonly referred to as the Excavation Damage Zones (EDZs). Crack initiation (CI) and propagation (CD) characteristics of massive rock masses have a significant influence on the mechanics and geometry of EDZs, and this research aims to provide a better understanding of these parameters for brittle rocks. This research addresses the short- and long-term in situ behaviour of massive brittle rock masses through characterization of the micro-mechanics of crack initiation and crack damage progress for rock specimens in the laboratory. Particular attention is paid to formulize the laboratory estimation of the crack damage thresholds. The effect of inherent fabric in intact rocks on the fabric-guided micro-fracturing is investigated through laboratory tests on different rock types. Similarly, the influence of the excavation process (i.e. confinement) and geological processes such as the glacial cycle (i.e. stress fatigue) are investigated for different brittle rocks in the laboratory. A developed approach for a three-dimensional discontinuum Grain-Based Model (GBM) complemented the laboratory investigation of crack damage in brittle rocks. The developed approach provides an in-depth understanding of the micro-mechanics of crack damage and demonstrates great potential as a predictive design tool. Finally, the laboratory and numerical investigations are discussed within the context of the short- and long-term behaviour of excavation damage zones (EDZs).