A coupled hydromechanical system defined through rock mass classification schemes

Abstract

SUMMARYChanges in the hydraulic conductivity "eld, resulting from the redistribution of stresses in fractured rock masses, are di$cult to characterize due to complex nature of the coupled hydromechanical processes. A methodology is developed to predict the distributed hydraulic conductivity "eld based on the original undisturbed parameters of hydraulic conductivity, Rock Mass Rating (RMR), Rock Quality Designation (RQD), and additionally the induced strains. The most obvious advantage of the methodology is that these required parameters are minimal and are readily available in practice. The incorporation of RMR and RQD, both of which have been applied to design in rock engineering for decades, enables the stress-dependent hydraulic conductivity "eld to be represented for a whole spectrum of rock masses. Knowledge of the RQD, together with the original hydraulic conductivity, is applied to determine the e!ective porosity for the fractured media. When RQD approaches zero, the rock mass is highly fractured, and fracture permeability will be relatively high. When RQD approaches 100, the degree of fracturing is minimal, and secondary porosity and secondary permeability will be low. These values bound the possible ranges in hydraulic behaviour of the secondary porosity within the system. RMR may also be applied to determine the scale e!ect of elastic modulus. As RMR approaches 100, the &softening' e!ect of fractures is a minimum and results in the smallest strain-induced change in the hydraulic conductivity because the induced strain is uniformly distributed between fractures and matrix. When RMR approaches zero, the laboratory modulus must be reduced signi"cantly in order to represent the rock mass. This results in the largest possible change in the hydraulic conductivity because the induced strain is applied entirely to the fracture system. These values of RMR bound the possible ranges in mechanical behaviour of the system. The mechanical system is coupled with the hydraulic system by two empirical parameters, RQD and RMR. The methodology has been applied to a circular underground excavation and to qualitatively explain the in situ experimental results of the macropermeability test in the drift at Stripa.