Influence of Stress-Strain Boundary Conditions on the Swelling Behavior of Bentonite

The swelling characteristics of bentonite play a very important role in ensuring the long-term stability and isolation of high-level radioactive waste (HLW) repositories. A custom-made test apparatus, capable of providing a range of boundary conditions, was designed to investigate the swelling behavior of bentonite. The imposed boundary conditions include constant mean stress (CMS), constant volume (CV) and an intermediate flexible boundary condition called constant stiffness (CS), which applies stress as a specified function of volume increase. The results show that boundary conditions significantly affect the swelling strain and swelling pressure of the bentonite tested. More specifically, the relationship between the range of boundary conditions provided and the corresponding swelling pressures are: CV > CS > CMS, while the relationship between the range of boundary conditions and the corresponding swelling strain is CMS > CS > CV. Based on these results, several swelling equilibrium limit (SEL) curves are developed to index the effect of boundary conditions on soil swelling potential. The methodology can be used to predict the final stress and volume states of bentonite during fluid infiltration under the range of boundary conditions possible in HLW repositories.