Investigation of relations between the geometrical properties and hydraulic properties of jointed rock through numerical simulation.

Abstract

Effect of joint geometry parameters and joint transmissivity on the hydraulic properties of jointed rock including the equivalent permeability tensor is investigated through a series of numerical experiments. The equivalent continuum hydraulic behavior of jointed rock was found to be sensitive to the orientation of the joint sets in a joint network system. The chances for the equivalent continuum behaviour were found to increase with the increase of joint density and joint size. It was found that for some joint systems, the equivalent continuum behavior does not exist irrespective of the rock block size; joint systems with low orientation angles, low joint densities and small joint sizes seem to have chance belonging to this category. 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). If the F0 value for a given joint network is more than the cut-off value, then the joint system can be approximated by an equivalent continuum. For the joint systems investigated in this study, values between 10 and 30 were found for cut-off F0. A threshold value of F0 between 3 and 6 was found to achieve a non-zero average block permeability (K0). Both the threshold value and the cut-off value of F0 were found to depend on the relative orientations of joint sets. Chances to reach equivalent continuum behavior for a rock mass having a certain joint configuration increases with the increase of block size. REV size for a joint system was found to depend on the orientation of the joint sets and on the distribution of joint size and density. REV size seems to decrease with increasing joint density. The block size corresponding to the aforementioned cut-off F0 can be considered to provide the REV size for a given joint configuration. For a considered joint network, K0 increases with the increase of block size until the block size is equal to REV size. The K0 value at the REV size for a joint system increases with increase in joint size and joint density. The relation between K0 and F0 seems to depend on the relative orientation of the joint sets in the joint network. Joint hydraulic conductivity plays a major role on the equivalent continuum behaviour and permeability anisotropy. A non linear relation seems to exist between the directional permeability and the fracture tensor component for the connected joint configuration when rock blocks contain minor discontinuities. For rock blocks containing major discontinuities this relation becomes linear.