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

Abstract

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 (<i/>Ø</i>), dry density (<i>ρ_d</i>), and degree of saturation (<i>S_r</i>). FFRC determined shear, unconstrained and constrained compressive wave velocities (<i>V_s</i>, <i>V_c</i>, and <i>V_p</i>, respectively), small-strain moduli, material damping in shear (<i>D_smin</i>) and longitudinal (<i>D_ucmin</i>). The influence of increasing <i/>Ø</i>, <i>ρ_d</i>, and <i>S_r</i> 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 (<i>v</i>), and (4) develop moduli and strength correlations. An inverse size effect was observed, wherein increasing <i/>Ø</i> reduced velocities, moduli, damping, <i>v</i>, and strength. Conversely, increasing <i>ρ_d</i> enhanced mechanical properties, with maximum values at around 70 % <i>S_r</i>. Plasticity index and mineral constituents contributed to the observed nonlinearity in Dsmin and Ducmin. Although <i/>Ø</i> and Sr partially contributed, mechanical correlations were primarily influenced by <i>ρ_d</i> 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.