Understanding the response of sand to complex loading conditions is vital for practical geotechnical engineering. Circular rotational shear is a special stress path where the magnitudes of three principal stresses vary following a circular stress trajectory in the it-plane with their directions fixed. Although experimental studies under such stress paths are limited, the discrete element method appears to be an appealing approach to examine the response of granular materials to varying complex loading paths in numerical virtual tests. This study presents comprehensive numerical simulations of granular samples subjected to a circular stress path under varying conditions, including samples prepared with different bedding-plane angles and densities and subjected to different stress ratios. Both macroscopic and microscopic behaviors are presented and interpreted. A contactnormal-based fabric tensor is adopted in a detailed analysis to measure the internal structure of the granular assembly. The fabric, strain, and strain increment tensors are decomposed with respect to the stress tensor, and the evolutions of these components are presented along with the key influential factors. The results obtained in this study provide useful physical insight for the development of constitutive models for granular soils under general loading conditions.

The constant volume behavior of sands is substantially influenced by the initial stress anisotropy. This research aims to investigate the stress anisotropy effect by conducting a series of bidirectional direct simple shear tests that can apply initial shear stress on a sample in different directions during the consolidation stage. The experimental program provides insights into the impacts of the initial shear stress and the subsequent principal stress rotation (PSR) on some critical aspects of soil behavior, including the onset of instability, brittleness index, phase transformation, and the critical state line. The findings show that the onset of instability and the brittleness indices are significantly dependent on the initial stress anisotropy. In contrast, the critical state and phase transformation lines are not influenced by the initial anisotropy of the stress state, even in the presence of PSR. Furthermore, the study gives particular attention to the non-coaxiality between the major principal stress and strain rates to illustrate how the non-coaxiality decreases with increasing shear strain. The research also explores the non-coaxiality between the resultant shear stress and shear strain rate and suggests a predictive flow rule accordingly. The results reveal that a substantial level of non-coaxiality may exist between the resultant shear stress and the shear strain rate.

This study investigated the impact of major principal stress direction angle (alpha) and intermediate principal stress coefficient (b) on the stress-strain behavior of silt sand soil through directional shear tests under isotropically consolidated drained conditions. Analyzing octahedral stress-strain relationships, shear stress-strain behaviors, radial and circumferential strains, shear stress ratios, and non-coaxial characteristics, findings show that both b and alpha significantly influence strain components with radial strain remaining stable and circumferential strain being dependent on both factors. Anisotropy in circumferential strain is notably affected by alpha and b, while radial strain transitions from tensile to compressive states by increasing b values. Initial loading stages exhibit similar characteristics, but it increased anisotropic with shear stress particularly at b = 0.5 and b = 1. Shear strength is notably influenced by b and alpha, with peak shear stress exhibiting direct proportionality to alpha angles between 0 and 45 degrees, and an inverted relationship beyond 45 degrees. Material strength is significantly impacted by stress orientation with pronounced non-coaxial behavior observed at angles other than 0 degrees, 45 degrees, and 90 degrees. These findings emphasize the intricate relationship between stress coefficients and material behavior providing significant insights into silt sand soil responses under varying stress conditions.