Fracture characteristics of Hongcheon Gneiss in South Korea assessed from deep drillcore samples

Fractures in a rock mass have an important influence on the mechanical response and hydraulic properties. Investigating and modeling fractures is crucial, especially with regard to the geological disposal of high-level radioactive waste, where long term stability is necessary to avoid a direct threat to society. This study sought to classify joints and faults in the Hongcheon gneiss, South Korea, based on their geometric characteristics. A generic classification model for joints and faults is proposed, dividing them into four types based on propagation pattern and development of fault core and fault effect zone. More than 4,000 joints and 34 faults were analyzed using samples of deep drillcore to investigate the relationships between the properties of these structures and depth or lithology. To validate the model, it was applied to other area where different lithologies are found. A damage index is utillized to visually represent the impact of a fault on the quality of rock mass. The joint patterns can be correlated with lithology, as the mineralogical characteristics and internal structures influence the patterns. The fault zone patterns show a relationship with depth, and the damage index provides a reliable indication of the fault impact. The validation of the fault model will be conducted in subsequent studies. With estimation of the hydromechanical properties of each joint and fault pattern, a practical approach would be provided to characterize fractures from drillcore data and enhance the accuracy of fluid flow and stability models for any given rock mass.