The stress directional dependence of soil, including the material direction (material anisotropy) and the stress direction (rotation of principal stress axis), is a hot and difficult issue of soil mechanics. Both the experiments and theoretical studies show that the anisotropy is the inner reason of the effect of principal stress axes rotation, and the complex stress state is the external reason of the anisotropy of soil. Therefore, these two factors should be considered in constitutive simulations. Related constitutive models can be roughly divided into three categories: (1) Macro constitutive models based on the theory of continuum mechanics and macro experimental phenomena; (2) Micro constitutive models based on micro-mechanism analysis; (3) Constitutive models based on the reasonable correlation between micro-mechanism and macro-phenomenon. By sorting out the modeling ideas of representative constitutive models, the treatment methods of the stress directional dependence of soil are analyzed, and the advantages and disadvantages of different modeling methods are summarized. It is expected to deepen the understanding of the stress directional dependence of soil and provide reference for geotechnical workers.

Given the problem that most of the existing shear tests with fixed principal stress direction ignore the change of intermediate principal stress coefficient b. The loading parameters of the four external loads of the hollow cylindrical apparatus were strictly deduced based on the mapping relationship between different force systems. The shear tests with fixed principal stress direction were carried out under two groups of confining pressures using remodeled Fujian standard sand specimens. The differences in mechanical properties such as deformation, strength, and non-coaxiality of the sand were studied when the principal stress direction α was different. Test results show that the stress path, strength, and other mechanical properties of sand have a significant stress direction dependence. With the principal stress direction changes from 0° to 90°, the stress path shows two variable phases, the peak strength decreases and then increases, and the minimum value of strength near α of 67.5°. The specimen has an obvious non-coaxiality when the deviatoric stress ratio is small, according to a comparison of the major principal strain increment direction and the major principal stress direction. However, as the deviatoric stress ratio increases, the direction of the principal strain increment gradually approaches the principal stress direction.

Red clay is widely distributed in Southwest China. Restricted by the preparation technology of remolded clay hollow cylindrical sample, the phenomenon and mechanism of the influence of the change of principal stress direction on the mechanical properties of red clay are still not clear. In this paper, saturated remolded red clay hollow cylindrical samples were prepared by the self-developed sample preparation device using the layered under compaction method. Pure principal stress axis rotation tests and shear tests with fixed principal stress axis were carried out. The results show that macro-mechanical behaviors of the specimen, such as deformation, strength, and morphology, are closely related to the rotation angle around the second principal stress, and these macro-mechanical behaviors show obvious segmentation characteristics. The calculation of stress paths shows that the loading direction of the extra load plays a decisive role in the tension and compression state of the sample. It results in the phenomena of "compression and swelling", "pure torsional failure" and "tensile and necking" and determines the macroscopic deformation. The reason for the above phenomenon may be that the compaction of the remolded specimen along the axial direction is stronger than that in the radial and circumferential directions. Therefore, the modulus, strength and other parameters of the specimen along the axial direction are different from those in the horizontal direction. It means that specimens have obvious cross-isotropy. The stress applied in different directions "activates" such difference, which in turn makes the specimen exhibit different mechanical properties in macro. Therefore, the research on the influence of stress direction on soil deformation and failure should comprehensively consider the stress direction and specimen anisotropy.