Permeability of a bentonite–crushed granite rock mixture using different experimental techniques

Abstract

The use of bentonite-based mixtures as a sealing material for deep geologic repositories is common to many different concepts for isolating nuclear fuel waste. In this context, saturated hydraulic conductivity becomes a fundamental parameter when analysing the long-term behaviour of these facilities. Despite that, measurement of hydraulic conductivity is still far from being routine for very low-permeability materials. A mixture of 30% sodium bentonite and 70% crushed granite rock is being considered as a sealing material in a future repository in Sweden. This paper describes the experimental programme carried out to determine the permeability of this mixture. Constant-head tests, pulse tests and oedometer tests have been used for that purpose, but because of the characteristics of the material-large grain size and very low permeability-special test procedures and equipments had to be developed. The paper also presents the analysis of the results, including a numerical simulation of the pulse test by using finite elements. A comparison of all results established an average hydraulic conductivity of 6·5 3 10 À12 m/s for a dry specific weight of the mixture of 15·9 kN/m 3 . Darcy's law was also confirmed for this material over the range of gradients applied (4 to 100). Finally, the paper describes a methodology suitable for the measurement of hydraulic conductivity of lowpermeability laboratory samples based on the pulse test concept.