土的应力方向依赖性包含材料方向性（各向异性）与应力方向性（应力主轴偏转）两层含义，是已得到国内外广泛重视但仍没解决好的热点和难点课题。申请者认为土的方向依赖性是材料方向性（内因）和应力方向性（外因）共同作用的结果，本构模拟须加以综合考虑并走宏细观结合的道路。本项目拟摒弃传统的修正屈服面、流动法则与硬化定律的本构建模方法，首先通过室内土工试验、细观成像分析与离散元数值模拟试验，揭示土的材料方向性与应力方向性协同作用的细观机理；然后建立“骨架-胞体”物理模型，提取包含组构张量与应力张量的等效应力张量，以综合反映材料方向性和应力大小与方向，实现土的细观结构与宏观力学特性的合理关联；最后以等效应力张量作为宏细观关联的桥梁，从应力新角度提出对现有的各向同性破坏准则与本构模型进行“应力方向依赖性”改造的一般性方法——等效应力法。项目的开展既可进一步丰富现代土力学，又可使现有的土的本构模型焕发生机。

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 which has been widely regarded at home and abroad but has not been solved yet. Applicants believe that the stress directional dependence of soil is the result of the interaction of the material direction (internal) and the stress direction (external). Therefore, the above two reasons should be taken into consideration and the method of micro-macro combination should be adopted in the constitutive modeling. The traditional constitutive modeling method, which is to modify the yield surface, flow rule and hardening law, is abandoned in this study. Instead, the micro-mechanism of the interaction of the material direction and the stress direction will be revealed through the indoor geotechnical test, microscopic image analysis and discrete element numerical simulation test. Then, the “Skeleton-Cell” physical model is constructed, and the equivalent stress tensor including the fabric tensor and the stress tensor could be extracted in order to reflect the material direction and the stress magnitude and direction synthetically and associate the meso-structure and the macro-mechanics reasonably. Finally, taking the equivalent stress tenor as a bridge, a general method of extending the existing isotropic failure criterion and constitutive model to stress directional dependence, named “Equivalent Stress Method”, can be put forward from the new perspective. To carry out the project can not only further enrich the modern soil mechanics, but also brings vitality to existing constitutive models of soil.