【作者】 张勋；

【导师】 温晓贵； 周建；

【作者基本信息】 浙江大学 ， 岩土工程， 2010， 硕士

【摘要】 通过对国内外学者关于基坑开挖卸荷过程中及交通荷载、地震荷载、波浪荷载作用下地基土单元应力状态变化研究成果的总结,发现主应力轴旋转是土单元应力状态变化的显著特征。显然,主应力轴旋转路径下土体的剪切破坏特性及破坏标准是实际工程设计关注的重要问题之一。基于此,根据我国东南沿海广泛分布软粘土地区工程建设的需要,采用空心圆柱扭剪仪、常规试验三轴仪、SEM电镜扫描仪,以典型的杭州原状软粘土为研究对象,进行了考虑初始应力状态变化的主应力轴旋转系列宏微观试验。主要工作及成果如下:1.开展了主应力轴单向、正逆向、连续循环旋转及剪应力幅值与主应力轴方向同时变化路径下的固结不排水试验。试验结果表明:(1)主应力轴旋转阶段的应力应变开展水平很小,主应力轴旋转阶段赋予后期定向剪切阶段的大主应力方向角对整个剪切阶段应力应变开展特征起控制作用。(2)主应力轴单向旋转对整个剪切阶段孔压开展特征影响很小,但是,主应力轴正逆向、连续旋转及剪应力幅值与主应力轴方向同时变化对整个剪切阶段孔压开展特征影响较大。(3)主应力轴旋转路径对整个剪切阶段原状软粘土的强度变化特征影响较小,其强度的各向异性受后期定向剪切阶段的大主应力方向角控制。2.开展了考虑初始主应力方向角、中主应力系数、初始剪应力、超固结比变化的主应力轴旋转系列宏微观试验。试验结果表明:(1)初始主应力方向角决定了试样剪切变形状态,以初始主应力方向角等于45°为界。(2)随着中主应力系数的增大,产生的孔压增量先减小后增大,但粘土的归一化强度没有呈规律性的变化,这是因为中主应力系数不同时影响原状软粘土归一化强度各向异性的微观变化因素不同。(3)初始剪应力与超固结比变化对剪切过程中孔压的开展特征影响较大,从孔压角度建立破坏标准时应当考虑初始剪应力与超固结比变化的影响。3.在分析主应力轴旋转与初始应力状态变化对原状软粘土剪切破坏特性影响的基础上,对复杂应力路径下原状软粘土的剪切破坏标准进行了研究,得到以下结论:(1)对Lade-Duncan破坏准则做了进一步修改,使其适用本文试验控制体系下原状软粘土剪切破坏特征的分析。(2)采用修正Lade-Duncan破坏准则计算不同剪切方向的粘土参数,并对极限孔隙水压力、有效主应力比峰值、给定广义剪应变、主应力差峰值剪切破坏标准进行了对比分析,发现复杂应力路径下原状软粘土剪切试验应采用主应力差峰值作为剪切破坏标准。更多还原

【Abstract】 Through a summary of research results belonging to domestic and foreign scholars on stress state change for foundation soil unit in the process of excavation unloading of pit and under traffic loads, seismic loads and wave loads, I found that rotation of principal stress axis was salient features in stress state change for foundation soil unit. Clearly, characteristics of shear failure and failure criterion in stress path for rotation of principal stress axis were one of the important issues related to practical engineering design. Based on this, according to the needs of construction projects in southeastern coastal areas of widely distributed soft clay, Hollow cylinder apparatus, Instrument for triaxial test and Scanning electron microscopy were used to carry out series of macro-micro tests for Hangzhou intact soft clay including the rotation of principal stress axis and the initial stress state changes. The main work and achievements were as follows:1. Undrained tests including one-way. forward& reverse, continuous cycle for rotation of principal stress axis and meanwhile changes for amplitude of shear stress and direction of principal stress axes had been done, and it was found that(1) the level of stress and strain was very small during rotation of principal stress axis. and major principal stress direction angle of late shear stage with fixed principal stress direction that was given by early rotation of principal stress axis controlled stress-strain characteristics for the complete shear stage.(2)one-way rotation of principal stress axes carried out little impact for pore pressure characteristics of the complete shear stage, but forward& reverse, continuous cycle for rotation of principal stress axis and meanwhile changes for amplitude of shear stress and direction of principal stress axes carried out greater impact for that.(3)rotation of principal stress axis carried out little impact for shear strength characteristics of intact soft clay in the complete shear stage, and the anisotropy of shear strength was controlled by major principal stress direction angle in late shear stage with fixed principal stress direction.2. Series of macro-micro tests for rotation of principal stress axis considering changes that included the initial principal stress direction angle, coefficient of intermediate principal stress, initial shear stress and over-consolidation ratio had been done, and it was found that(1)the initial principal stress direction angle determined the characteristics of shear deformation, and the initial principal stress direction angle equal to 45 degrees was the cut-off point.(2)when coefficient of intermediate principal stress was increasing, the increment of pore pressure generated was increased after the first decrease, but normalized shear strength of clay had not reduce or increase regularly, because micro-change factors which influenced anisotropy of normalized shear strength of intact soft clay were different in different coefficients of intermediate principal stress.(3)the changes of initial shear stress and over-consolidation ratio had a greater impact to pore pressure characteristics, so the establishment of failure criterion from the perspective of pore water pressure should consider the effects of changes of initial shear stress and over-consolidation ratio.3. Based on the analysis of impacts of rotation of principal stress axis and the initial stress state changes to shear characteristics of intact soft clay, failure criterion for intact soft clay under complex stress path was researched, the conclusions was that(1)Lade-Duncan failure guidelines after the authors had done a further revision might analysis characteristics of shear failure in stress control system of this test.(2)revised Lade-Duncan failure guidelines was used to calculate clay parameters in different shear directions, and the limit pore water pressure, the peak of effective principal stress ratio, the generalized shear strain and the peak of principal stress difference as failure criterion was carried out a comparative analysis, then the shear test for intact soft clay under complex stress path should use the peak of principal stress difference as failure criterion.更多还原