Evolution of small-strain stiffness and nonlinear behavior in compacted bentonite during dehydration

Compacted bentonite undergoes mechanical degradation during thermally induced dehydration, which can affect its performance in engineered barrier systems. This study investigates the evolution of mechanical behavior in compacted bentonite during drying at 110 °C using ASTM C215-based resonant frequency (RF) testing under free-free boundary conditions. Two complementary resonance-based parameters were monitored: the small-strain Young's modulus ( E ), representing the specimen-scale stiffness response, and a nonlinear acoustic parameter ( α ), representing the sensitivity of resonant frequency to excitation amplitude. The mechanical response of pure compacted bentonite was first characterized, followed by an assessment of basalt fiber–reinforced bentonite with varying fiber contents. X-ray micro-computed tomography (CT) was employed after drying to visualize internal crack patterns and provide microstructural context for the resonance-based measurements. Results show that E decreased gradually during early drying and then underwent a pronounced reduction as the water content approached approximately 0.09, near the estimated shrinkage-limit range of the bentonite. This transition is interpreted as a possible change in moisture-dependent interparticle interactions and contact-network behavior during dehydration. In contrast, α increased progressively during active drying and reached a plateau once volumetric contraction became limited, suggesting stabilization of the mechanically active contribution of microcracks, weak contacts, and crack-like interfaces. Basalt fiber reinforcement delayed stiffness degradation before the shrinkage limit range, but the nonlinear response varied nonmonotonically with fiber content. These results demonstrate that global small-strain stiffness E , nonlinear acoustic response α , and post-test CT imaging provide complementary descriptors of dehydration-induced mechanical changes in compacted bentonite.