双应力主轴旋转条件下砂土扭剪试验与本构模拟研究

The principal stress axis rotation refers to the phenomenon that the stress rotates around its principal axes, which is widely exists in various geotechnical engineering, and is a hot and difficult problem in soil mechanics. Recently, the problem of uniaxial rotation of principal stress has been well studied. However, restricted by experimental techniques and modeling methods, the problem of the biaxial rotation of principal stress is still rarely studied. It believes that the principal stress axis rotation is rooted in the directionality of stress and the directionality of the material (or anisotropy). Therefore, both experimental design and theoretical research need to be considered comprehensively. In this project, the preparation method of hollow cylindrical sand sample with inclined sedimentary surface will be developed fristly. Then, the stress paths, rotating around the second and the third principal stress axes, can be realized by controlling the rotation angle of the sedimentary surface around the circumferential direction and the stress around the radial direction respectively. Therefore, the influence of the biaxial rotation of principal stress on the mechanical properties of sand can be revealed. Based on the experiments and the previous research, the equivalent stress tensor can be constructed to reflect the stress magnitude, fabric magnitude and the relationship between stress and fabric in three-dimension. Finally, a general method for establishing the constitutive model of sand under the biaxial rotation of principal stress will be proposed. The development of this project has important academic significance for further revealing the influence of stress tensor on soil mechanics characteristics, which might bring new development opportunities for the theory of soil mechanics. It can also further enrich the theory of soil mechanics and provide theoretical support for reasonable design of geotechnical engineering.

应力主轴旋转是指应力绕其主轴发生旋转的现象，广泛存在于各类岩土工程中，是土力学的热点与难点问题。目前，针对单应力主轴旋转的问题已开展了较为深入的研究，然而受试验技术和建模方法的制约，对双应力主轴旋转的研究仍鲜有报道。应力主轴旋转根源在于应力的方向性与材料的方向性（即各向异性），试验设计与理论研究均需加以综合考虑。本项目通过研发沉积面倾斜的砂土空心圆柱试样制备方法，分别控制试样沉积面绕环向旋转的角度和应力绕径向旋转的角度，实现绕第二、三应力主轴旋转的应力路径，进而揭示双应力主轴旋转对砂土力学特性的影响。在试验和前期研究的基础上，构建能综合反映三维空间中组构大小、应力大小以及组构应力关系的等效应力张量，提出采用等效应力张量建立双应力主轴旋转条件下砂土本构模型的一般性方法。项目的开展对进一步揭示应力张量对土力学特性的影响有重要的学术意义，可进一步丰富土力学理论，并为岩土工程合理设计提供理论支撑。

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；二是应力的方向性，也即通常所说的“应力主轴旋转效应”。以材料各向异性主轴和应力主轴互为参考，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。相关本构模型可大致分为宏观本构模型、细观本构模型和宏细观结合的本构模型。通过控制砂土颗粒下落时的重力和离心力，设计研发了一种沿径向倾斜沉积的砂土空心圆柱制样装置，图像分析和统计学分析表明，所制备试样的沉积面偏差分布较符合正态分布，试验所制试样的界面倾角与理论计算值较为接近，倾斜沉积砂土空心圆柱试样制样装置的制样效果较好。系统开展了试样沉积面倾角固定、绕第二应力主轴旋转角度固定的双应力主轴定轴剪切试验。结果表明，有效应力路径和孔压的发展具有明显的应力方向依赖性，即有效应力路径和孔压受到试样倾角和应力旋转的耦合作用影响，试样倾角和应力旋转角度的增大均会使孔压差值增大，导致有效应力路径的差距也越大，其中，应力旋转为主要因素，试样倾角则为次要因素。在连续介质力学基础上，紧扣土体有效应力的基本内涵，从土骨架应力角度，建立了宏微观应力的定量关联，提出了各向异性土体的有效应力张量，称为等效应力张量。采用等效应力张量对既有模型中的有效应力张量进行替换，建立了等效Lade破坏准则、等效UH模型，新模型只增加了一个组构参数。与已有试验结果对比表明，新准则/模型可以较好地反映应力大小和应力方向对强度、变形、孔压等各向异性的耦合影响。