Influence of Fines Plasticity and Water Content on the Mechanical Response of Binary Soil Mixtures: Analysis Using the Equivalent Void Ratio

Abstract The mechanical behavior of sand–fines mixtures, particularly under various fines plasticity and water content, is not yet fully understood. This study experimentally investigated the mechanical behavior of sand–fines mixtures with various fines content (FC), water content, and fines plasticity, reflecting the unsaturated and heterogeneous nature of natural soils. Bender element tests, in conjunction with one-dimensional compression tests, were conducted on mixtures of silica sand with three distinct fines types: nonplastic silica flour, plastic kaolin clay, and plastic Korean red clay. These mixtures were prepared across a range of water contents (0%–10%) and FC of up to 30%. The results reveal that with increasing FC, mixtures containing nonplastic fines consistently exhibited a reduction in stiffness, regardless of the water content. In contrast, those with plastic fines exhibited an increase in the shear wave velocity under low-water-content conditions, a phenomenon attributed to suction-induced interparticle bonding. The contribution of fines to the load-carrying soil skeleton was examined using the equivalent void ratio concept. Through back-analysis, the fitted b parameter, which represents the structural role of fines, demonstrated a strong, nonlinear percolation-type correlation with matric suction. This finding suggests that in partially saturated conditions, the b parameter, when considered in conjunction with matric suction, provides a physically meaningful and practical indicator for assessing the structural contribution of fines in unsaturated granular mixtures. This study advances the understanding of complex soil behavior and has significant implications for the characterization of natural soils.