Effect of Block Size and Joint Geometry on Jointed Rock Hydraulics and REV

Abstract

The effect of joint geometry parameters and the size of jointed rock block on the hydraulic properties of jointed rock, including the equivalent continuum behavior, was investigated through numerical experimentation. The chance to reach equivalent continuum behavior for a rock mass having a certain joint configuration increases with the increase of block size. The REV (representative elementary volume) size to show hydraulic equivalent continuum behavior for a jointed rock system was found to depend on the orientation of the joint sets; the REV size seems to decrease with increasing joint density and joint size. The REV does not seem to exist for some rock masses having joint systems with low relative orientation angles (systems with two joint sets) and low densities. The average block permeability (K0) value at the REV size for a joint system increases with increase in joint size and joint density. The equivalent continuum behavior of a joint system can be expressed with respect to a cut-off value for the first invariant of fracture tensor (F0). The block size corresponding to the aforementioned cut-off F0 (between 10 and 30 for the joint systems investigated in this study) can be considered to provide the REV size for a given joint configuration. A threshold value for F0 (a joint network having two perpendicular joint sets produced 2.75) can be used to find the chance for a given joint network to have nonzero block permeability. A strong power functional relation seems to exist between the directional permeability and the fracture tensor component for the connected joint configuration when rock blocks contain minor discontinuities.