The mechanical behavior of binary granular mixtures strongly depends on their initial packing density, stress level, fine content, particle size distribution, mineralogy, and shape. This research aims to investigate the effect on the mechanical behavior of fine-sand mixtures of fine particle fraction through various features: grain size distribution, fine particle size, and plasticity. The concept of equivalent intergranular void ratio is proposed for this analysis. It is correlated to the micromechanical activation of fines within the sand matrix.Monotonic consolidated undrained triaxial tests are carried out for mixtures of coarse particles (sand) and fine particles (silt or clay), in the sand dominant behavior, having various shapes and grain-size distributions. Loose, medium, and dense mixtures are tested using different fine contents and confining pressures. The undrained response is strongly affected by particle interactions, depending on the packing density, particle size, and plasticity. The active fine fraction captures the active contribution of fine particles in the sand skeleton structure. It influences the equivalent intergranular void ratio estimated in these experiments and associated to the steady state of mixtures. The reliability of equivalent state theory and an original formula proposed to estimate the active fine fraction is demonstrated in the case of fine-sand mixtures having plastic fine particles and confirmed for non-plastic fines.