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

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 × 10^–12 m/s for a dry specific weight of the mixture of 15·9 kN/m³. 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 low-permeability laboratory samples based on the pulse test concept.