Linking stress-dependent effective porosity and hydraulic conductivity fields to RMR

Abstract Relations are developed to define changes in effective porosity and hydraulic conductivity that result from the redistribution of stresses and strains in disturbed rock masses. In each instance, changes in porosity and directional conductivities are determined from pre-disturbance porosities and conductivities, knowledge of the number of joint sets, and the indices of Rock Quality Designation (RQD) and Rock Mass Rating (RMR), defining the rock mass structure. Measured magnitudes, or estimates, of the applied strain distribution are the final required parameter. These parameters allow porosity and conductivity changes to be straightforwardly evaluated for a broad spectrum of rock mass qualities, including the representation of granular media. The model is applied to an effective stress analysis of conductivity changes that develop around a unlined circular drift in a biaxial stress field. Large increases in tangential conductivity, and reductions in radial conductivity are shown to result. These results are corroborated against the drift macropermeability test at Stripa where increases in hydraulic conductivity of the order of 1000–10000 times were measured in a 0.5–1.0 m wide zone adjacent to the excavation.