Strength Characteristics and Stress-Strain Relationship of Bentonite under Different Water Contents

ABSTRACT Bentonite functions as a critical engineered barrier in multibarrier geological repositories, positioned at the interface between radioactive waste canisters and host rock formations. It serves multiple roles: as a geomechanical stabiliser (engineering barrier), a low-permeability sealant (hydraulic barrier), and a geochemical buffer (chemical barrier). However, excavation-induced damage during repository construction can trigger fracture propagation and reactivation within the host rock. Additionally, pre-existing shear-induced microfractures in the rock matrix may form interconnected pathways that facilitate groundwater infiltration. These coupled mechanisms necessitate a comprehensive characterisation of bentonite’s hydro-mechanical behaviour, with particular emphasis on shear strength degradation and the evolution of swelling pressure under varying degrees of saturation. Such investigations are essential for evaluating the long-term geoenvironmental performance and containment reliability of the barrier system. This study investigates the variations in strength and the stress-strain relationship of bentonite samples with varying water contents by conducting direct shear tests. The effects of water content and vertical stress on the stress-strain curve, peak strength, and shear strength indices were analysed and the corresponding models were employed to fit the different behaviours of the stressstrain curves. The results indicated that the strength of bentonite is influenced by both water content and vertical stress. Numerical expressions for the peak and proportional strengths of bentonite were established as functions of water content and vertical stress. As water content increased, the internal friction angle of bentonite decreased linearly, while cohesion showed a quadratic decrease. Additionally, the stress-strain curve transitioned from strain-softening to strain-hardening behaviour with increasing water content. To describe both behaviours effectively, a unified model was proposed, which accurately captured the trends observed in the experimental data. These findings offer valuable insights into the strength characteristics and stress-strain behaviour of bentonite and provide a theoretical basis for developing mathematical models to describe its mechanical response.