Abstract: The bentonite/sand mixture is an optional buffer/backfill material in the geological disposal of high level nuclear waste. Understanding the volume change characteristics of this mixture under coupled hydro-mechanical condition is highly significant for evaluating the long-term safety of the repository. In this investigation, six groups of bentonite/sand mixtures were prepared with different contents of quartz sand, i.e. 0%, 10%, 20%, 30%, 40% and 50%. Then a series of experiments including uniaxial confined compression (to a final dry density of 1.7 Mg/m3), saturation (under 0.2 MPa vertical pressure) and reloading tests was carried out in sequence. The volume change behavior of bentonite/sand mixtures was understood, in addition, the effect of sand content was also analyzed emphatically. It is found that: (1) The compression behaviour of mixture significantly depends on sand content, saturation state and density. As the mixture is unsaturated and at relative low dry density (?d <1.7 g/cm3), the compression index decreases linearly with increasing sand content. However, as the mixture is saturated and at relative high dry density (?d <1.7 g/cm3), the compression index of mixture is independent of sand content, and the corresponding value is much less than that at unsaturated and loose state. (2) The final swelling strain of the mixture decreases exponentially with increasing sand content, while increases exponentially with effective bentonite dry density. As the sand content in the mixture is higher than 40%, volume collapse can occur during saturation, and collapse extent is enhanced by increasing sand content. (3) The observed volume change characteristics of the mixture under different hydro-mechanical conditions or at different sand contents are mainly related to the distribution of bentonite and sand in the mixture, and their dominant effect on soil skeleton. Generally, the higher sand content and higher density, the volume change behavior the mixture is more conditioned by sand. (4) The swelling pressure of mixture decreases with increasing sand content. A parameter terms as “effective bentonite dry density” is introduced to quantitatively describe the swelling pressure of the mixture, and an exponential relationship between these two parameters is developed which can be used to predict mixture swelling pressure. Moreover, the swelling mechanism of bentonite/sand mixture is further analyzed based on volume change ratio and mass distribution ratio of montmorillonite in the mixture. In conclusion, this work is a reference for the selection and performance optimization of buffer/backfill materials.

Key words: nuclear waste disposal, bentonite-sand mixture, buffer/backfill material, compression behaviour, swelling behaviour, swelling pressure