考虑主应力轴旋转的粉细砂变形特性试验与本构理论研究

The non-coaxial behavior of sands caused by rotation of principal stress axes would make the macro-scopic mechanical properties of soils “soften”, which has important significance for safety of engineerings. But at present，the researches about the non-coaxial behavior and constitutive models for sands are not sufficient, which impact on the reliability of the design theory. To make up for the deficiency, a series of tests on saturated fine sands would be carried out with HCA. The stress paths under fixed,continuous rotation, cyclic rotation of principal stress and increasing principal stress direction angle and deviatoric stress would be designed to study the non-coaxiality behaviors and influence factors.Then the three-dimensional non-coaxial plasticity theory would be introduced based on the traditional constitutive theories for sands and a three-dimensional non-coaxial constitutive model would be built to describe the deformation behavior of fine sands under rotation of principal stress axes. Finally, the implicit integration of the non-coaxial constitutive model and the consistent elastoplastic matrix would be proposed, and a reliable method for finite element analysis based on the non-coaxial constitutive model would be formed. The research results have important theoretical significance and practical value for the deformation and stability analysis on fine sand foundations under wave, earthquake and traffic loads.

主应力轴旋转引起的砂土的非共轴现象能够使得土体宏观力学特性 “变软”，对工程安全意义重大。但目前对于砂土的非共轴变形特性及本构模型的研究还很欠缺,使得现有设计理论存在安全隐患。为弥补理论不足,本课题以饱和粉细砂为研究对象,设计包括主应力轴固定单调剪切、主应力轴连续旋转、往复循环旋转以及主应力方向角和剪应力同时增大等不同应力路径下的排水试验，分析和探讨饱和粉细砂的非共轴变形特性及其影响因素。进而在传统弹塑性本构理论框架的基础上引入三维非共轴塑性流动理论,建立能够描述主应力轴旋转条件下粉细砂非共轴特性的弹塑性本构模型,并进一步提出非共轴本构模型的隐式积分方法和一致弹塑性矩阵，形成一种可靠的基于非共轴本构模型的有限元分析方法。研究成果对于波浪、交通、地震等荷载作用下粉细砂地基的变形稳定分析具有重要的理论意义和工程应用价值。

地震、交通以及波浪荷载等的作用能够使得土体的主应力轴发生持续不断的旋转，使得土体产生明显的非共轴现象，从而造成土体强度显著降低,宏观力学响应整体“变软”，对工程安全意义重大。但目前对于砂土的非共轴变形特性及本构模型的研究还很欠缺,使得现有设计理论存在安全隐患。为弥补理论不足,本课题以饱和粉细砂为研究对象,采用空心圆柱扭剪仪，开展了包括主应力轴固定单调剪切、主应力轴连续旋转、往复循环旋转以及主应力方向角和剪应力同时增大等不同应力路径下的主应力轴旋转排水剪切实验，分析和探讨了饱和粉细砂的非共轴变形特性及其影响因素。实验结果表明，主应力轴旋转过程中粉细砂将产生一定程度的塑性变形，初始剪应力水平、中主应力系数、循环起始角度、偏应力增长幅度等因素对粉细砂的变形特性均有一定的影响，在主应力轴固定单调剪切应力路径条件下砂土的非共轴现象不可忽略。在本构理论方面，课题将三维应力状态下的非共轴应变率引入传统的砂土弹塑性本构理论框架，并对Mohr-Coulomb强度准则的形状函数进行了改进，建立了三维应力空间的破坏准则。在此基础上建立了适用于粉细砂的三维非共轴弹塑性本构模型，使之更加适合砂土三维应力空间的变形特性。通过对粉细砂空心圆柱扭剪试验结果的数值模拟，表明模型能够较好地反映主应力轴旋转过程粉细砂的变形规律，验证了模型的合理性和有效性。