During recent seismic events, such as 2010 Darfield and 2016 Ecuador earthquakes, widespread liquefaction has been observed in sand deposits with silt content. Nevertheless, the presence of non-plastic fines implies variable liquefaction resistance of sands. The goal of this research is the assessment of the influence of non-plastic fines and mixture-packing conditions on liquefaction triggering. A series of monotonic and cyclic consolidated undrained triaxial as well as resonant column tests is carried out on reconstituted soil specimens. The behavior of loose, medium and dense silty sands is analyzed, using different fine contents and confining pressures. The results show that the behavior of mixtures strongly depends on the packing configuration of coarse and fine particles. The experimental results are analyzed in terms of equivalent intergranular void ratio, which is identified in the literature as an adequate state parameter to characterize the global effect of fine particles. The estimation of the equivalent intergranular void ratio requires the determination of the active fine fraction b participating in the force transfer. An original formula is proposed for the parameter b based on packing configuration. The validation of the proposed formula is undertaken through comparisons with the present experimental results but also with results reported in the literature. The proposed expression to estimate the active fine fraction b allows a satisfactory prediction of liquefaction triggering in sand-fines mixtures independently from the fine content.