Experimental simulation of boundary condition effects on bentonite swelling in HLW repositories

On the basis of textural and hydro-mechanical characteristics, bentonite has been proven to be an effective buffer/backfill material for long-term containment of high-level radioactive waste (HLW) in deep geological repositories. Herein, the results of experiments performed to investigate the swelling equilibrium limit (SEL) of bentonite under various boundary conditions are presented. A special apparatus was employed to simulate various stress–strain boundary conditions, including constant volume (CV), constant vertical stress (CVS), and constant stiffness (CS). Bentonite samples were prepared with various initial dry densities ranging from 1.5 to 1.7 g/cm³ and vertically stressed to different levels. During wetting, they were subjected to different boundary conditions before the swelling strain or swelling pressure reached equilibrium. Test results indicate that stress–strain boundary conditions have significant effects on the measured swelling strain and swelling pressure of the tested bentonite. More specifically, the relationship between the sequence of swelling pressure under different boundary conditions is CV > CS > CVS, while the relationship between the sequence of swelling strain is CVS > CS > CV. In addition, the characteristics of SEL curves are governed by the initial dry density and vertical stress with the effect of dry density being more significant. Based on these results, several SEL curves were developed to index the effects of boundary conditions on swelling potential of bentonite. They can be used to evaluate the final stress and volume states of bentonite during fluid infiltration under the range of boundary conditions possible in HLW repositories.