Hydraulic conductivity test system for compacted, 2-mm-thick bentonite specimens

Both the design and safety assessment of radioactive waste disposal facilities demand an accurate evaluation of the hydraulic conductivity of the bentonite materials, especially compacted bentonite. For permeability tests of bentonite materials, the lengthy time necessary for specimen saturation and measurement may present a bottleneck. The permeability behavior of bentonite, such as the effects of the water quality and the exchangeable cations, has not been systematized sufficiently. For the present study, a hydraulic conductivity test system with 2-mm-thick specimens was developed. Its applicability was evaluated in terms of test accuracy. Six specimens of compacted Japanese sodium bentonite, with dry densities of 1.34–1.79 Mg/m³, were subjected to falling head hydraulic conductivity tests. The results showed that the hydraulic gradient set for this study did not affect the hydraulic conductivity, indicating that the macroscopic hydraulic behavior was consistent with Darcy's law. Furthermore, it was possible to reduce the test period considerably, by about one-tenth, compared to that using 10-mm-thick specimens. The obtained hydraulic conductivity was found to be similar to that in earlier studies. Furthermore, the values showed less variation particularly in terms of the consolidation test results. The results demonstrated that 2-mm-thick specimens are useful for hydraulic conductivity measurements of compacted bentonite.