非饱和土强度的三维颗粒流模拟

发布时间：2018-03-08 10:00

本文选题：非饱和土　切入点：三维颗粒流　出处：《北京交通大学》2017年硕士论文　论文类型：学位论文

【摘要】：土是一种颗粒集合体,自然界中的土绝大多数是非饱和土,由固、液、气三相组成,这种多相颗粒体系决定了其在受力特性和破坏特性上的复杂性。室内试验和数值模拟是研究土体力学特性的两种重要手段,与试验方法相比,数值模拟方法不仅能够保持初始试样的同一性,而且可以任意改变边界,实现不同条件下的加载数值试验。本文基于已有的室内试验研究成果,引入三维颗粒流理论并开发颗粒流数值模拟技术,对非饱和土的三轴剪切试验和单轴拉伸试验进行数值模拟研究。数值试验过程分三个步骤开展:建立高效且能够反映真实试样颗粒级配的数值模型;确定合理的细观参数使模型与土体实际性状相一致;从细观角度出发揭示土体宏观力学性质的内在机理。本文主要研究工作及成果如下:(1)基于颗粒流理论和现有的试验研究成果,提出了一种建立土体宏细观参数之间关系的方法,并以屈服应力为桥梁建立了颗粒粘结强度随含水率的变化关系;通过编制离散元程序,对现有的PFC3D离散元程序进行了改进,包括在颗粒生成时采用多次动态膨胀以减小初始接触力、采用应力边界来模拟橡皮膜的柔性边界、剪切过程中实时监测颗粒位置以防止其逃逸、采用所建立的宏细观参数之间的关系来反映含水率对土体性质的影响等,从而实现了现有的PFC3D离散元程序在模拟非饱和土力学行为上的功能;基于所提出的方法,可建立不同孔隙比和含水率等初始条件下的非饱和土在不同应力路径下的离散元计算模型。(2)基于所提出的非饱和土试样生成方法和现有的试验研究成果,通过生成不同孔隙比和含水量条件下的非饱和土数值试样,建立了非饱和土三轴剪切试验的离散元计算模型;通过开展固结排水剪切试验的离散元数值模拟研究,揭示了非饱和土试样的三轴剪切应力-应变关系、强度特性等宏观力学性质的演化规律,同时获取了试样的速度场、力链分布、粘结破坏数量和能量变化等细观信息;通过与现有试验结果的对比,验证了所建立模型和计算程序在模拟非饱和土三轴剪切特性方面的适用性。(3)基于所提出的非饱和土试样生成方法和现有的试验研究成果,通过生成不同孔隙比和含水量条件下的非饱和土数值试样,建立了非饱和土单轴拉伸试验的离散元计算模型;分别针对黏土和砂土,通过开展单轴拉伸的离散元数值模拟研究,再现了不同初始孔隙比和含水率的黏土和砂土试样的单轴拉伸应力-应变关系,揭示了黏土和砂土的单轴拉伸强度特性随孔隙比和含水率的变化规律,并对细观参数敏感性进行了分析;通过与现有试验结果的对比,验证了所建立模型和计算程序在模拟非饱和土拉伸行为上的适用性。
[Abstract]:Soil is a kind of granular aggregate, and most of the soil in nature is unsaturated soil, which is composed of solid, liquid and gas three phases. This multiphase particle system determines the complexity of its mechanical and failure characteristics. Laboratory tests and numerical simulation are two important means to study the mechanical properties of soil, compared with the test methods. The numerical simulation method can not only keep the identity of the initial specimen, but also change the boundary arbitrarily to realize the loading numerical test under different conditions. The three-dimensional particle flow theory is introduced and the numerical simulation technology of particle flow is developed. The numerical simulation of triaxial shear test and uniaxial tensile test of unsaturated soil is carried out. The process of numerical test is divided into three steps: establishing a highly efficient numerical model which can reflect the particle gradation of real sample; The reasonable mesoscopic parameters are determined to make the model consistent with the actual behavior of soil. The main work and results of this paper are as follows: (1) based on the theory of particle flow and the existing experimental results, a method to establish the relationship between macro and mesoscopic parameters of soil is proposed. The relationship between grain bond strength and moisture content is established with yield stress as bridge, and the existing PFC3D discrete element program is improved by compiling discrete element program. The dynamic expansion is used to reduce the initial contact force, the stress boundary is used to simulate the flexible boundary of rubber film, and the position of particles is monitored in real time during shearing to prevent the particles from escaping. The relationship between macro and mesoscopic parameters is used to reflect the influence of moisture content on soil properties, and the function of the existing PFC3D discrete element program in simulating the mechanical behavior of unsaturated soil is realized. The discrete element calculation model of unsaturated soil under different stress paths under different initial conditions, such as void ratio and moisture content, can be established. The discrete element calculation model of unsaturated soil triaxial shear test is established by generating unsaturated soil numerical samples with different void ratio and water content, and the discrete element numerical simulation of consolidation drainage shear test is carried out. The evolution laws of macroscopic mechanical properties such as triaxial shear stress-strain relationship and strength characteristics of unsaturated soil samples are revealed. The meso-information such as velocity field, force chain distribution, bond failure quantity and energy change are obtained. The applicability of the established model and calculation program in simulating the triaxial shear characteristics of unsaturated soil is verified by comparing with the existing experimental results. A discrete element model for uniaxial tensile test of unsaturated soil is established by generating numerical samples of unsaturated soil under different void ratios and water contents, and the numerical simulation of uniaxial tensile test is carried out for clay and sand, respectively. The uniaxial stress-strain relationship of clay and sand samples with different initial void ratio and moisture content is reappeared, and the variation of uniaxial tensile strength of clay and sand with void ratio and moisture content is revealed. The sensitivity of mesoscale parameters is analyzed, and the applicability of the established model and calculation program in simulating the tensile behavior of unsaturated soils is verified by comparing with the existing experimental results.
【学位授予单位】：北京交通大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TU43

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