Strain Accumulation in Sand under Long-Term Cyclic Loading: Effects of Fines Content and Drainage Conditions

Abstract This study investigates the long-term cyclic response of sand containing controlled amounts of nonplastic silt, with the objective of quantifying how fines content influences strain accumulation, stiffness degradation, and pore-pressure generation. The loading conditions considered represent repeated actions in granular foundations, such as traffic, wind, wave, and machine-induced vibrations, under both drained and undrained conditions. While most previous studies have focused on clean sands or plastic fines under low-cycle or liquefaction conditions, the influence of nonplastic fines in the long term, low-amplitude cyclic loading remains insufficiently understood. To address this gap, a comprehensive experimental program was conducted, including monotonic and high-cycle triaxial tests (3,000 cycles) on mixtures with 0%, 15%, and 30% fines. Both sand and silt were derived from the same parent rock to ensure consistent mineralogy. Tests were performed under varying mean effective stresses (126.6–253.3 kPa), cyclic stress amplitudes (30–100 kPa), loading frequencies (0.2–0.6 Hz), and consolidation anisotropy values of η ave = 0 – 0.63 . The results demonstrate that fines content fundamentally governs cyclic response. Below a threshold fines content, the soil maintains a sand-dominated structure with limited strain accumulation, whereas beyond this threshold, the response transitions to a fines-influenced behavior with significantly increased deformation and stiffness degradation. For example, doubling mean effective stress increased accumulated strain by ∼ 23 % in clean sand but up to ∼90% at 30% fines, while increasing cyclic stress amplitude from 40 to 60 kPa produced progressively larger strain increases (14.5%, 35%, and 175% for 0%, 15%, and 30% fines, respectively). The cyclic response is strongly controlled by loading amplitude and stress-path proximity to the critical state line (CSL), while frequency effects remain negligible within the quasi-static range investigated. Under undrained conditions, fine-rich mixtures exhibited rapid pore-pressure buildup and reached ∼5% axial strain within approximately 800 cycles, in contrast to the stable response of clean sand. A comparative discussion situates these findings within prior work and highlights their relevance for long-term performance assessments of granular foundations subjected to repeated loading.