Abstract: To investigate the characteristic differences between soft clay cyclic softening and silt liquefaction, as well as the effects of plasticity on dynamic strength and pore water pressure, and the influence trend of pore water pressure on the dynamic modulus and damping ratio of fine-grained soils, a typical silt and two types of silty clay from Handan, along with silts mixed with varying montmorillonite clay content, were tested using a cyclic triaxial apparatus. The test results indicate that the silt sample (FT) exhibits typical liquefaction behavior, with a pore pressure ratio reaching 1.0. Once the ratio exceeds 0.6, dynamic stress attenuates significantly, and the stress-strain hysteresis loop becomes progressively flatter. For silts with 5%, 10%, 15%, and 20% clay content, the pore pressure ratio exceeds 0.8, and the dynamic stress and hysteresis loop characteristics are similar to those of the silt sample (FT). The peak pore pressure of silty clays does not reach the confining pressure, with pore pressure ratios below 0.7. These samples exhibit significant pore pressure fluctuations, slow attenuation of dynamic stress, and broader hysteresis loops. The cyclic strength of silts with varying clay content and constant dry density first decreases and then increases. The strength of silty clays is significantly higher than that of silts. The pore pressure ratio for cycles producing 5% double amplitude axial strain decreases gradually with increasing plasticity. The modulus ratio of the five silts decreases rapidly as pore pressure increases. Once the pore pressure ratio reaches 0.8, the modulus approaches 0 and tends to stabilize. The modulus ratio of the two silty clays is higher than that of the five silts at similar strain levels. The damping ratio of the most cohesive silty clay (FN1) increases with rising pore pressure. Other samples initially increase and then decrease in damping ratio, while the damping ratio of the silt sample (FT) approaches 0 upon liquefaction. In this study, the damping ratio of soils during cycles producing 5% double amplitude axial strain increases gradually with increasing plasticity.

Key words: saturated fine-grained soils, plasticity, cyclic triaxial test, pore water pressure, dynamic modulus, damping ratio