Based on the discrete element particle flow program PFC3D, an undrained cyclic triaxial numerical model is established to investigate the large strain dynamic characteristics and liquefaction behaviors of the loose sand under stress amplitude-controlled and strain amplitude-controlled tests. The results demonstrate that the value of micro parameters at the initial liquefaction moment are the same under the two control modes. The whole cyclic loading process of both loading control methods can be divided into different zones based on the evolution of the micro parameters. In studying the movement state of soil particles after initial liquefaction, the strain amplitude-controlled test is more comprehensive to observe the development process of microstructure. The peak value of the damping ratio calculated by the typical symmetrical hysteresis loop method is around 0.5% of the deviatoric strain, while is around 1.0% of the deviatoric strain when considering the asymmetry of the stress-strain hysteresis loop. In stress amplitude-controlled tests, the phase transition and large flow-slip behavior of the loose sand will result in an unclear peak of the damping ratio. In this context, strain amplitude-controlled tests can be advantageous for the study of loose sand.

Seismic events and wave action can induce volumetric strain (ev) accumulation in saturated sandy soils, leading to damage to the ground surface and structures. A quantifiable relationship exists between the generation of ev in sandy soils under drained conditions and the development of pore water pressures under undrained conditions. In this study, the impact of relative density (Dr), cyclic stress path, and stress level on the characteristics of volumetric strain (ev) generation in saturated coral sands (SCS) was evaluated through drained tests employing various cyclic stress paths. The test findings demonstrate that the rate of ev accumulation in SCS is notably affected by the cyclic stress path. The rise in peak volumetric strain (evp) in SCS, as a function of the number of cycles, conforms to the arctangent function model. The unit cyclic stress ratio (USR) was employed as an indicator of complex cyclic loading levels. It was determined that coefficient (evp)u is positively correlated with USR at a specific Dr. At the same Dr, coefficient CN1 exhibits a positive correlation with USR, while coefficient CN2 displays a negative correlation with USR, following a power-law relationship. Irrespective of cyclic loading conditions, evp rises with an increase in generalized shear strain amplitude (yga). A power function model was established to represent the relationship between evp and yga. The coefficient 41 decreases as Dr increases. Comparisons were drawn between evp and yga for Ottawa sand and SCS. The results indicate that, as Dr of Ottawa sand increases from 30 % to 70 %, the coefficient 41 decreases from 1.54 to 0.73, representing a reduction of approximately 53 %. In contrast, under identical conditions, the coefficient 41 of SCS exhibits a less pronounced decrease, from 1.16 to 0.79, corresponding to a reduction of roughly 32 %. These observations suggest that variations in Dr have a more substantial impact on generating evp in Ottawa sand compared to SCS.