Analytical evaluation of post-excavation hydraulic conductivity field around a tunnel

Changes in hydraulic conductivity field around a tunnel that accompany the underground excavation are difficult to characterize due to the nature of hydromechanical coupling processes in fractured rock masses. A methodology is developed to represent changes in the post-excavation hydraulic conductivity as a result of excavation-induced strains. Relationships linking changes in hydraulic conductivity and excavation-induced strain are defined through the assumption of an equivalent porous medium. These relationships may be used to represent a whole spectra of fractured rock masses by assigning different R_m (modulus reduction ratio) values. When the modulus reduction ratio, R_m = 1, the rock mass and intact rock material moduli are identical, and the strain is uniformly distributed between fractures and matrix. This results in the smallest possible change in conductivity. When R_m = 0, the strain is applied entirely to the fracture system and precipitates the largest possible change in conductivity. These values bound the possible ranges in behavior of the system in a natural and mechanistically defensible manner. Analytical solutions of post-excavation hydraulic conductivity field are obtained by applying these relations to fractured rock masses around a circular tunnel. The analytical solutions are then used to develop a dimensionless chart which defines the relationship between normalized ratios of post-excavation to pre-excavation hydraulic conductivities and the dimensionless radium. This chart is verified against field results from Buffer Mass Test at the Stripa.