A continuum approach for estimating permeability in naturally fractured rocks

Many models for predicting permeability in rock masses have been proposed. However, most of these models do not relate the conductivity-closure behavior to fracture geometry parameters and geostati stresses. In this study an attempt is made to estimate the permeability tensor as well as the relation between permeability and depth on naturally jointed rocks. The new model includes effects of hydromechanical coupling, normal closure and fracture geometry. The fracture tensor related only to the fracture geometry (aperture, size and orientation) is introduced to formulate a permeability tensor. Fracture aperture, estimated by a simple field investigations, is defined as a function of joint roughness coefficient (JRC), joint wall compression strength (JCS) and geostatic stress. An actual rock mass of jointed andesite rock on the Lan-Yu site, Taiwan was studied to predict the permeability and conducting aperture with depth. Through a comparison of the presented model with three existing models [(1) the simple one parallel fracture set model, (2) the three orthogonal fracture set model, and (3) the random disc fracture model], it was shown that permeability-depth relations for the three orthogonal fracture set model and the new model are almost the same.