A micromechanical framework for understanding the role of fines in the monotonic and cyclic response of granular mixtures

The discrete-element method was employed to simulate a series of monotonic and cyclic triaxial tests on host sands (i.e. clean sands) with varying particle gradations, mixed with a range of non-plastic fines. The cyclic liquefaction resistance, critical state, and micromechanical responses of both clean sands and sand-fines mixtures were investigated. The findings reveal that both fines content (FC) and the uniformity coefficient of host sand (C_us) significantly influence liquefaction resistance, the critical state line in e-logp’ space, and force transmission within contact network. However, the critical stress ratio is unaffected by FC and C_us. Microscopic analysis indicates that, under both monotonic and cyclic loadings, sand-sand contacts primarily contribute to the deviatoric stress, while fines-fines contacts, despite their high proportion, contribute negligibly. A new contact state variable, termed the soil skeleton coordination number (MCN_sk), is proposed to capture active contacts within soil skeleton and effectively characterize the critical state behavior and liquefaction resistance of granular mixtures, independent of particle size distribution. Furthermore, liquefaction resistance is well interpreted within both macroscopic and microscopic frameworks of critical state soil mechanics. The integration of macro- and micro-level results enhances understanding of the force transmission network and associated mechanical behavior in sand-fines mixtures with varying particle size distributions.