In fractured rock masses, apertures of discontinuities play a key role in controlling flow through the rack mass and hence the permeability of fractured rock mass can change due to change in apertures of discontinuities which in turn change due to applied stress. In this study, stress-dependent permeability of fractured rock mass in foundation of Roudbar dam is investigated using 3D distinct element method program, 3DEC. A large three dimensional model of rock mass in dam foundation was established and aperture distribution due to insitu stress was obtained. Using aperture distribution in various parts of rock mass, permeability distribution and its change with depth were investigated. Results show that there is an asymptotic value (dasymptotic) for depth after which aperture don't change with depth and in depth less than this value variations of aperture are higher than other regions. Apertures of discontinuities in depth less than dasymptotic decrease as the depth increase which can be represented with a logarithmic curve. Results also show that aperture distribution in rock mass is strongly controlled by discontinuities stiffness so that an increase in discontinuities stiffness leads to an increase in apertures of discontinuities. The calculated permeability change with vertical stress change shows the decrease of permeability corresponding to the increase of insitu stress in depth less than almost ½ dasymptotic. In order to validate Obtained results, statistical distribution of permeability in rock mass obtained from numerical modeling was compared with that from experiments in the field and good agreement between two results has been observed.
Permeability in rock masses is an important issue in many engineering applications such as mining, civil and oil projects. Fractured rock masses are composed of intact rock materials and discontinuities with the later acting as the main pathways of fluid flow, especially in hard rocks. Apertures of discontinuities can change due to normal stress-induced closures or openings and due to shear stress- induced dilations. Hence, the permeability of fractured rock masses is stressdependent. Stress induced changes in permeability can be large and irreversible under perturbations resulting from various natural and human activities. Human activities include underground constructions causing stress redistributions close to the openings, geothermal energy and oil/gas reservoir productions where injections and extractions of fluids entail significant changes in effective stress underground. However the current insitu stress and permeability distributions are results of Natural activities including past and ongoing geological processes. In recent years, stress-dependent permeability of rock masses has been studied through analytical and numerical methods. Analytical models of stress-dependent permeability of fractured rock masses based on orthogonal or persistent discontinuities sets are available that consider the normal closures of discontinuities and constant shear dilations in both fractured and fractured-porous media. Oda in 1985 developed a permeability tensor considering stressdependency in complex fracture networks . However, when the shear failures and dilations of discontinuities are to be considered, analytical solutions do not exist.