土的应力方向依赖性(I):概念与现象

土的应力方向依赖性是指土的力学特性随应力方向改变而改变的性质,包含两层含义:一是材料的方向性,即土的各向异性,是应力方向依赖性产生的内在原因;二是应力的方向性,即通常所说的“应力主轴旋转效应”,是应力方向依赖性的外部客观条件。以材料各向异性主轴和应力主轴的位置关系分类,相关试验研究可划分成“应力不转、试样旋转”和“旋转应力、试样不转”两类。诸多证据表明,材料的方向性与应力的方向性之间存在某种密切的关联。随着各向异性主轴和应力主轴发生相对旋转,试样所表现出的力学性质存在显著的差异。旋转角度相同时的剪切试验研究表明(包括直剪试验、三轴试验、定轴剪切试验等),旋转角度、潜在滑移面位置和试样的强度与变形之间存在某种特定的关系。通过对比梳理两类试验的试验原理与重要结论,明晰了土应力方向依赖性的试验研究框架,为相关研究提供一些新的思路,以期抛砖引玉,引起广大岩土工作者对该问题的思考。

The Stress Direction Dependence of Soil (I): Concepts and Phenomena

The stress direction dependence of soil refers to the property that the mechanical properties of soil change with the change of stress direction. It has two aspects: one is the directionality of material, also called the material anisotropy, which is the inner reason for the dependence of stress direction; the other is the directionality of stress, commonly known as the principal stress rotation, which is the external objective condition of stress direction dependence. Taking the principal axis of anisotropy and the principal axis of stress as reference to each other, relevant experimental researches can be divided into two categories: "stress unrotated, specimen rotates" and "stress rotates, specimen unrotated". Lots of evidence show that there is a close relationship between the directionality of materials and the directionality of stress. The mechanical properties exhibited by the specimens are significantly different with the relative rotation of the anisotropic principal axis and the stress principal axis. There is a specific relationship between the rotation angle, the position of the potential slip surface, and the strength and deformation of the sample in shear tests with the similar rotational angle, such as the directional shear test, triaxial shear test, shear test with fixed principal stress axes, etc. By comparing and combing the experimental principles and important conclusions of the two types of tests, the experimental research framework of the direction dependence of soil stress is clarified, and some new ideas are provided for related research. It is hoped that geotechnical workers will pay more attention to this issue.