Quantitative Measurements of Fracture Aperture and Directional Roughness from Rock Cores

Abstract

The hydro-mechanical behavior of a blocky rock mass near the surface and at shallow depths is more dependent on the characteristics of the system of discontinuities within the rock mass than the characteristics of the intact rock. Discontinuities represent planes of weakness and conduits of enhanced hydraulic conductivity relative to the intact rock. The spatial aperture distribution and roughness of these fractures can have a significant influence on their hydromechanical behavior. In terms of mechanical behavior, the aperture distribution and roughness directly affect the spatial distribution and inclination of contact areas, which in turn influence the stress distribution, deformation, and asperity damage, under normal and shear loading (e.g., Re and Scavia 1999; Gentier et al. 2000; Grasselli and Egger 2003). In terms of hydraulic behavior, the spatial aperture distribution and roughness directly affect the tortuosity and connectivity of flow paths, which in turn influence the hydraulic transmissivity of the fracture (Zimmerman and Bodvarsson 1996; Berkowitz 2002). The evaluation of fracture roughness requires measurements or observations of surface topography coupled with some empirical (e.g., Barton and Choubey 1977), statistical (e.g., Reeves 1985; Maerz et al. 1990) or fractal (e.g., Seidel and Haberfield 1995; Kulatilake et al. 2006) analysis methodologies, which yield one or more roughness parameters. Several contact and non-contact tools and techniques have been used to measure surface topography of discontinuity surfaces in rock. Contact techniques include: