Influence of specimen size on the small strain dynamic properties and strength of compacted Bentonil-WRK bentonite for nuclear waste repository

This study examines the small-strain dynamic properties and strength of highly compacted Bentonil-WRK bentonite, a candidate buffer material for engineered barrier system (EBS) in South Korea. Free-free resonant column (FFRC) and unconfined compressive tests (UCT) were performed on specimens with varying diameter sizes ( Ø ), dry density ( ρ d ), and degree of saturation ( S r ). FFRC determined shear, unconstrained and constrained compressive wave velocities ( V s , V c , and V p , respectively), small-strain moduli, material damping in shear ( D smin ) and longitudinal ( D ucmin ). The influence of increasing Ø , ρ d , and S r were investigated by utilizing seismic waves to (1) determine the dynamic stiffness, (2) characterize material damping, (3) permit direct calculation of dynamic Poisson's ratio ( v ), and (4) develop moduli and strength correlations. An inverse size effect was observed, wherein increasing Ø reduced velocities, moduli, damping, v , and strength. Conversely, increasing ρ d enhanced mechanical properties, with maximum values at around 70 % S r . Plasticity index and mineral constituents contributed to the observed nonlinearity in D smin and D ucmin . Although Ø and S r partially contributed, mechanical correlations were primarily influenced by ρ d under unconfined conditions. Hence, the proposed wave-based equations enable practical assessment of bentonite buffer blocks after production and prior to installation in nuclear waste repositories.