This study examines the behavior of anisotropically consolidated granular assemblies under undrained cyclic true triaxial loading paths. To achieve this, the Discrete Element Method (DEM) is conjugated with the Coupled Fluid Method (CFM) to account for fluid-solid interaction in undrained conditions. The examined loading paths include two phases: anisotropic consolidation and undrained cyclic true triaxial loading. During consolidation, samples are sheared at various Lode angles to reach a spectrum of initial static shear stress levels. In the second stage, undrained cyclic loading is applied with constant shear stress amplitudes at various Lode angle values. The results indicated that the monotonic and cyclic Lode angle, initial static shear stress, and amplitude of deviatoric stress have pronounced effects on the secant shear modulus degradation and the rate of excess pore water pressure generation of granular assemblies. In tandem with macro-scale observations, the evolution of the microstructure within assemblies is analyzed using the coordination number, redundancy index, inter-particle contact fabric tensor, and particle orientation fabric tensor. The micro-scale findings confirm that the anisotropy induced by changes in the loading direction significantly impacts the shear strength of the assemblies. Additionally, the fabric of assemblies aligns along the preferential direction corresponding to the major principal stress, influencing the dilative response.

Stratified soil is a type of widely distributed special soil, consisting of alternating interlayered soils with distinct properties in both terrestrial and marine sedimentation conditions. It is endowed with anisotropic physical properties and mechanical behavior by its unique laminar structure features. So far, its mechanical behavior has not been fully understood. To systematically investigate the laminar structure effects of stratified soil, artificially prepared stratified soil samples of silty clay interlayered by silty sand were studied. First, the laminar structure features of stratified soil in Yangtze River floodplain deposits at Nanjing, China, were summarized. Then, based on the laminar structure features, preparation method for stratified soil samples was proposed by stacking soil layers one by one, which was basically an integration of soil paste plus consolidation method for silty clay layer preparation and water pluviation plus freezing method for silty sand layer preparation. After verification of the sample preparation method, a series of consolidated-undrained triaxial compression tests were carried out to study the mechanical behavior of stratified soil. The effects of thickness of constituent layers, consolidation conditions (isotropic or anisotropic consolidation), and loading paths (conventional triaxial compression, constant-p compression, and lateral extension) were investigated. The results show that the mechanical behavior of stratified soil (including stress-strain curves, excessive pore pressure accumulation, sample failure modes, and strength index) generally falls in between the behavior of the two constituent layers of soil, i.e., a normally consolidated silty clay and a medium-dense silty sand. The silty clay layer thickness (with fixed silty sand layer thickness), consolidation conditions, and loading paths together determine the stratified soil behavior, either silty sand dominant or silty clay dominant. Laminar structure can improve volumetric dilation trend and thus increase undrained shear strength of stratified soil. The presence of silty clay layer would suppress shear banding development in stratified soil. The strength of stratified soil can be underestimated by experiments using disturbed or remolded samples where the laminar structure is partially or completely lost.

Previous studies, mostly experimental, have reported conflicting observations on the effect of the initial anisotropic consolidation stress ratio (eta 0) on the stress ratio at the onset of instability (eta f) for drained and undrained loadings that involve static liquefaction. They indicate that as eta 0 increases, eta f could decrease, increase, or remain invariant, albeit without providing potential mechanistic factors. In this study, the anisotropic critical state theory (ACST) is used to investigate potential factors, including compressibility, state, and fabric anisotropy, that can explain the influence of eta 0 on eta f under undrained and drained loading. The assessments consider numerical simulations with the ACST-based SANISAND-F model, insights from SANISANDF-based instability criteria, the instability surface concept, and available experimental observations. Our findings show that compressibility and fabric anisotropy (and its evolution) are key factors in explaining the influence of eta 0 on eta f. In particular, the results show that if fabric evolution is not substantial, an increase in eta 0 decreases eta f for materials with significant compressibility, whereas the effect of eta 0 in low compressibility materials is minimal. On the other hand, for materials that promote fabric evolution, the results suggest that the effect of fabric changes counteract compressibility effects, potentially increasing eta f. Finally, the study also highlights the need for further research to better understand potential coupled effects of compressibility and fabric evolution on materials prone to significant loading-induced fabric changes.

The stress-induced anisotropy of sand significantly affects the liquefaction susceptibility. This study systematically investigates the undrained behavior of saturated sand under both monotonic and cyclic loading through a series of torsional shear tests, considering the effects of initial anisotropic consolidation states, including extensional and compressional consolidation states. Special attention is paid to the evolution of effective stress paths, deformation pattern, pore water pressure generation, and the rotation of the principal stress direction during the torsional shear process. The experimental results show that specimens under different initial stress states and loading conditions exhibit two typical failure modes, i.e., residual strain accumulation and cyclic mobility. The evolutions of principal stress direction under both monotonic and cyclic loading are presented, which can provide useful insights into the underlying mechanism of the occurrence of different failure modes. Based on the experimental results, a new index, unified cyclic stress ratio (USR), is proposed by correlating the number of cycles required for failure (Nf) with the initial anisotropic stress state and the shear strength at the phase transformation point. The proposed index USR can serve as a unified criterion for the evaluation of liquefaction resistance of sand considering anisotropic consolidation conditions.