Evaluation of the swelling characteristics of bentonite-sand mixtures

© 2015 Elsevier B.V. The swelling characteristic of bentonite-sand mixtures is an important index in evaluating the long term performance of the deep geological repository of nuclear waste. In this study the swelling characteristics of different types of bentonite-sand mixtures with various sand contents are compared. It is concluded that for pure bentonite and bentonite-sand mixtures with less sand than the critical sand content, the relationship between montmorillonite void ratio and vertical stress can be expressed as a unique line in a double logarithmic plot, and volumetric strain under a given vertical stress and swelling pressure at a constant volume after full saturation can be predicted based on this line; however, for mixtures with more sand than the critical sand content, the above relationship deviates from the line when the stress is larger than the starting deviation stress, which can be determined using the sand skeleton void ratio. Before the sand skeleton forms, the vertical stress is borne mainly by montmorillonite particles and the amount of swelling at full saturation is determined by the content of montmorillonite per unit volume, however, after the sand skeleton forms the vertical stress is borne by both the montmorillonite and the sand skeleton. The stress distribution coefficient is proposed to present a proportion of the vertical stress that is borne by both parts. A predictive method, verified by the swelling tests on bentonite-sand mixtures with various sand contents, is suggested to predict the amount of swelling due to saturation in the full range of sand content.