The Dual Role of Low-Plasticity Fines in the Cyclic Behaviour of Sand–Clay Mixtures

Understanding how sand–fines mixtures respond cyclically is crucial for assessing liquefaction risks and how soil stiffness decreases under seismic forces. Fines, especially low-plasticity clays, greatly influence the buildup of excess pore pressure and strain during cyclic loading. However, their mechanical role at moderate fines levels is not yet fully understood.

This study presents findings from a series of stress-controlled undrained cyclic triaxial tests performed on clean sand and sand–clay mixtures. The base material consisted of a uniformly graded sand combined with low-plasticity kaolinite clay, with fines content of 10% and 15% by dry weight. In order to accurately determine the full role of fines content on the mechanical response, grading entropy coordinates where calculated for each mixture.

Cyclic loading involved applying a sinusoidal deviator stress of constant amplitude under undrained conditions. Throughout the tests, axial strain development and excess pore pressure were continuously monitored. Liquefaction was identified using two complementary criteria: (i) initial liquefaction, indicated by the complete loss of effective stress caused by excess pore pressure, and (ii) strain-based criteria, which relied on different double-amplitude axial strain thresholds.

The results demonstrate that higher fines content slows the development of excess pore pressure and delays the onset of liquefaction compared to clean sand. Both sand–clay mixtures showed less strain accumulation during initial cyclic loading, due to changes in pore space compressibility and drainage caused by low-plasticity clay. Nevertheless, at higher strain levels, significant cyclic softening, notable stiffness loss, and increased residual pore pressures were observed.

The findings emphasise the dual role of low-plasticity fines: moderate fines levels can improve cyclic resistance, while higher fines contents may weaken the granular framework and hinder effective stress transfer. The study underscores the importance of detailed analysis of void ratio and soil structure for accurately assessing the cyclic behaviour and liquefaction potential of sand–fines mixtures.