Salinity-dependent ultimate shear strength of compacted Wyoming-type bentonite investigated by triaxial tests

Abstract

Ion composition in groundwater at the spent nuclear fuel repositories is a relevant uncertainty for predicting the shear behavior of the bentonite buffer. However, only a few studies investigated the range of shear strength variation with in-situ groundwater conditions. This paper examines the ultimate shear strength of compacted Wyoming-type bentonite, focusing on the effect of pore water salinity. Triaxial tests were performed under effective confining pressures up to 10 MPa with different types of pore water representing: 1) extremely low, 2) current, and 3) maximal salinity conditions at the repositories, while tracking the evolution of exchangeable cation composition. The results revealed that pore waters with typical Ca²⁺ concentrations at the repositories strengthen the sodium bentonite through Na⁺-Ca²⁺ cation exchange. The observed salinity-dependent strength of the bentonite was described by the ultimate shear strength line and its apparent mobilization in the deviatoric stress-mean effective stress plane. The upper limit of the increase in strength was found to be 0.74 MPa. Previous triaxial test results with deionized water were compared to refine the shape of the ultimate shear strength line. Lessons learned from the triaxial testing of saturated bentonite were elucidated to offer recommendations to address the challenges associated with high swelling potential and low permeability. The findings in this paper have implications for accurate assessment of shear behavior in the deep geological disposal system as well as effective laboratory measurements of the shear strength parameters of compacted bentonite.