Percolation and permeability of fracture networks in excavated damaged zones

Generally, the excavation process of a gallery generates fractures in its immediate vicinity. The corresponding zone which is called the excavated damaged zone (EDZ), has a larger permeability than the intact surrounding medium. Therefore, some of its properties are of crucial importance for applications such as the storage of nuclear wastes. Field observations suggest that the fracture density is an exponentially decreasing function of the distance to the wall and that the fracture orientation is anisotropic and well approximated by a Fisher law whose pole is orthogonal to the wall. Numerical samples are generated according to these prescriptions and their percolation status and hydraulic transmissivity are systematically determined for a wide range of decay lengths and anisotropy parameters. All the numerical data are presented and discussed. A heuristic analytical expression for the percolation threshold is proposed which unifies and accurately represents all the numerical data. A simple parallel flow model yields an explicit analytical expression for the transmissivity as a function of the density, heterogeneity, and anisotropy parameters; the model also successfully accounts for all the numerical data.