Evaluating cyclic liquefaction of soil from the perspective of energy dissipation provides a more comprehensive insight into its liquefaction mechanism. This study conducted a series of undrained cyclic triaxial tests using discrete element method to investigate the influence of plastic fines content (FC) on the dynamic characteristics of sand-clay mixtures. A new evaluation index, the Viscous Energy Dissipation Ratio (VEDR), is introduced to assess the energy dissipation performance of sand-clay mixtures. Macroscopically, it is shown that when FC 30 %, the trend reverses. In terms of energy dissipation, as the fines content increases, VEDR gradually transitions from the sand-like to the clay-like mode, exhibiting a unique transitional mode when FC = 50 %. Microscopically, the development of bond breakage is highly similar to that of VEDR. The bond breakage facilitates particle sliding and rolling, which is the fundamental factor causing the differences of energy dissipation between pure sand and sand-clay mixtures. This paper contributes to the mechanistic study of liquefaction criteria based on energy theory by establishing the connection between microscopic particle behavior and macroscopic energy dissipation during the cyclic liquefaction process.

Evaluating the cyclic strength development using energy-based methods is a novel concept in studying the dynamic properties of sand-clay mixtures under cyclic loading. In this study, a series of undrained cyclic triaxial tests were conducted on sand-clay mixtures, and the performance of different fine-grain contents on the dynamic properties and the energy dissipation of sand-clay mixtures was investigated based on the energy-based methods. The results demonstrated a gradual increase in the cyclic strain amplitude and the residual axial strain with increasing fines content (FC) under cyclic loading with a controlled cyclic stress ratio; in contrast, the accumulation of pore-water pressure slowed down. An initial decrease in the cyclic strength of the mixtures was observed with an increase in their fines contents; however, further increasing the FC enlarged the cyclic strength of the sand-clay mixtures. This transition was observed when the threshold fines content reached about 30%. The viscous energy dissipation ratio (VEDR), which is a nondimensional energy ratio based on the relationship between cyclic stress and strain and reflects the characteristics of dynamic properties, was utilized to compare three critical phase transition points, namely, VEDRvalley, VEDRpeak, and VEDR5%strain, in the energy dissipation of the sand-clay mixtures. Based on the VEDR results, the cyclic strength development indexes were established. Furthermore, low-vacuum environmental scanning electron microscopy revealed that as the FC increased, the particle composition of the sand-clay mixtures transitioned from predominantly coarse-grained to fine-grained, resulting in a change in the cyclic behavior of the mixtures from sandlike to claylike. The cyclic strength development indices provided further insights into and quantified the effect of fines contents of the sand-clay mixtures on their cyclic strength development process.

The dynamic stability of offshore foundations in the marine environment is one of the major technical challenges. Millions of loading cycles from waves and winds act on the foundations and seabed in their entire service life. The mixing of adjacent soil layers due to naturally occurring sediment or the construction process raises more difficulties in the design. Previous studies have focused on the dynamic constitutive relations of idealized clean sand or clay under cyclic loading, while the cyclic characteristics of sand-clay mixtures are still unclear. In this study, the dynamic responses of sand-clay mixtures under a large number of shear cycles were investigated through 25 constant-volume cyclic direct simple shear tests on sand-clay mixtures with sand contents of 0 %, 25 %, 50 %, and 75 %. The results indicate that high sand contents and high cyclic stress ratios accelerate the reduction of effective stress in sand-clay mixtures, resulting in a more rapid increase in shear strain. As the sand content increases from 0 % to 75 %, the normalized shear modulus decreases by 35.9 % at a shear strain of 1 %. In contrast, the normalized dynamic shear strength is reduced from between 0.490 and 0.525 to between 0.181 and 0.325 after over 5000 loading cycles.