考虑主应力轴循环旋转效应的冻结粘土破坏准则研究

Most sedimentary soils are inherently anisotropic. Their response to loading will therefore depend on the directions of principal stresses in relation to the deposition direction. In nearly all geotechnical problems, principal stress directions gradually rotate during loading. In some cases, such as traffic and earthquake loading, these rotations are continuous and cyclic in nature. As a consequence, deformations occur not only because of the change in the magnitudes of the principal stresses but also because of the change in their directions. Ignoring principal stress changes in directions might lead to underestimated settlements. Numerous studies have already been carried out to analyze the influence of principal stress axis rotation on unfrozen soil mechanical behavior. However, limited by the laboratory measurement technology, the existing knowledge about frozen soil mechanical behavior was obtained from the triaxial compression tests, generally employing axial compression loading without principal stress axis rotation. Consequently, the effect of principal stress axis rotation on the frozen soils mechanical behavior has not been fully understood until now. This project intends to carry out a series of experiments which include the directional shear test, the principal stress axis rotation tests and the principal stress axis cyclic rotation tests, to investigate the stress-strain and strength characteristics of frozen clay under different temperature. All the tests will be performed by a means of frozen soils hollow cylinder apparatus, which allows independent control of the magnitudes of the principle stress and rotation of the major-minor principal stress axes. By analyzing the frozen clay stress-strain behavior and its strength properties under different frozen soils temperatures and stress paths, to explore the development law of frozen soils cumulative deformation and the stiffness attenuation on the premise that the effect of major principal stress direction, intermediate principal stress coefficient, shear stress level, initial stress state, the principal stress rotation direction are considered. It is hoped that the study results of this project will provide insight to the cold region engineering design from which a predictive strength criterion model may evolve.

针对寒区大部分土木工程在其建造和运行过程中普遍会发生应力主轴旋转的事实，在建立冻土主应力主轴旋转试验研究体系的基础上，利用新研制的冻土空心圆柱仪，在不同的负温度条件下开展饱和冻结粘土定向剪切试验、主应力主轴纯旋转以及主应力主轴循环旋转试验，确定不同应力路径下冻结粘土弹性模量、剪切模量等力学参数，分析冻土温度、大主应力方向、中主应力系数、剪应力水平、初始应力状态、主应力旋转方向以及循环次数对冻结粘土应力-应变-强度特性的影响，探索主应力方向循环旋转时冻结粘土累积变形发展规律、刚度衰减规律。建立计算主应力轴循环旋转引起冻结粘土累积变形的方法，并提出考虑主应力轴旋转效应的冻结粘土破坏强度预测方法，进而为冻土地基灾变技术和高速交通重大工程长期服役性能的提高提供重要参考。

以主应力方向连续循环旋转为核心的土的力学参数、破坏特征、本构模型研究是目前土力学研究的焦点，考虑主应力方向变化的冻结土的应力-应变-强度特性，不仅成为解决寒区公路、铁路等路基工程建设中地基土体各向异性对地基变形、承载力影响的关键所在，而且为准确阐明复杂应力路径条件下冻土单元体的力学特征、变形规律，完善和发展冻土力学理论奠定基础。本项目首先通过对黏土饱和含水率与干密度、土体冻结过程中水分迁移与环境温度关系的研究，研发了冻土空心圆柱试样制样装置，确定冻土空心圆柱试样的制样模式并建立复杂应力路径条件下冻土力学试验的研究平台；在此基础上，通过对空心圆柱仪试验能力的分析，确定轴力、扭矩、内外围压的加载方式，使空心圆柱仪能实现定向剪切试验、主应力旋转试验和主应力循环旋转试验等多种加载应力路径。然后以此为平台，在不同的负温度条件下开展饱和冻结粘土定向剪切试验、主应力主轴纯旋转以及主应力主轴循环旋转试验，研究结果表明：虽然定向剪切应力路径下冻结粘土应力-应变曲线均呈现出应变硬化特征，但大主应力方向角、中主应力系数以及平均主应力对冻结粘土的应力-应变关系及强度特征都有一定的影响；在轴向荷载和扭剪荷载的作用下，试样的破坏以扭剪破坏为主，且主要取决于土颗粒和冰的胶结力；主应力轴方向单向旋转对轴向应变影响较小，而对剪应变影响较大；对于主应力轴方向往复旋转过程中剪应力-剪应变曲线的研究表明，第一个循环旋转产生的剪应变增量最大，约为第二个循环的2~3倍，然后逐渐减小；累积剪应变会随旋转周数的增加近似线性增加。最后，在分析子午面和偏平面上的破坏函数后，把温度效应和时间效应引入建立的强度模型中，获得了全应力空间下随温度和时间变化的冻结粘土破坏曲面，构建了全应力空间下耦合时温效应的冻结粘土强度准则。