【作者】 罗勇；

【导师】 龚晓南；

【作者基本信息】 浙江大学 ， 岩土工程， 2007， 博士

【摘要】 土是一种不同于一般材料的极其复杂的颗粒集合体,它既不是理想的弹性材料,也不是理想的塑性材料。岩土工程模拟研究的主要对象是松散的岩土材料,这种材料的典型特点就是其破碎性和流动性。尽管连续介质的假设与实际情况不相符合,目前大多的数值计算方法都是用连续介质力学的思路去分析和研究土的力学性状。人们早就认识到应当从微细观角度研究土这种具有独特的不同于金属等连续介质的应力—应变性状,但是由于这种研究相当复杂,而且依赖比较多的外界因素,比如计算机的处理能力和细观颗粒的本构理论等,故很难建立微观与宏观的定量关系。本文引入颗粒流理论PFC2D和PFC3D并开发颗粒流数值模拟技术,克服传统连续介质力学模型采用宏观连续性假设所导致的不足,建立了细观参数标定宏观参数的近似定量关系,并首次尝试用于模拟基坑开挖的动态过程。并且模拟了颗粒排列成的不同空间结构的力学性状和无粘性土三轴应力-应变关系曲线,将土体微细观结构与宏观力学反应紧密地联系起来,对土体剪切位移场形成和演化等渐进破坏过程有更深入的理解和发现。通过大量的参数试算与分析,建立起细观结构参数与宏观力学参数的函数关系式,近似标定了土体的摩擦角、变形模量和加固土的峰值强度,并在此基础上建立了模型模拟支撑开挖的动态过程,其中灌注桩和钢支撑用颗粒的平行粘结本构模型模拟,土体用线弹性接触本构模型模拟。分析得到了分布开挖的结构水平位移、弯矩、剪力、支撑轴力和作用在结构上的土压力变化和分布规律,并且与现场实测数据进行了比较。土的另一方面的复杂性还在于其空间结构性,研究颗粒的空间结构性问题可以从细观形态入手,研究颗粒排列的几何特征对材料力学特性的影响,有利于探讨颗粒世界的非均质性,各向异性。基于三维离散单元颗粒流理论,引入了颗粒的滑动接触本构模型,建立了金字塔和正交晶系两种典型空间排列结构的颗粒体试样,进行了三轴应力应变数值模拟和平面应力应变数值模拟,与物理实验结果吻合,数值试验基本再现了物理试验的力学宏观行为,峰值摩擦角与理论值之间的相对误差小于2.5%。分析不同典型空间排列结构试验过程中的细观织构(配位数、孔隙率和滑动摩擦接触数)的变化趋势和规律。引入了在极限剪力状态下滑动接触本构模型,建立颗粒体砂土试样,进行三轴应力应变数值模拟。通过大量的颗粒流数值试验,从细观力学角度对砂土的工程力学特性进行了模拟研究和分析;对砂土的室内常规三轴试验及其剪切带的形成和发展进行了数值模拟,分别对比不同围压下颗粒体试样与室内承德中密砂三轴试验的应力—应变关系曲线,基本再现了试样的加载曲线;分析颗粒法向刚度、法向和切向刚度比值、孔隙率,摩擦系数等细观结构参数变化对材料的宏观力学性状的影响;分析了细观织构在加载过程中的变化规律;并研究了围压对剪切位移场性状的影响。更多还原

【Abstract】 Soil is not same general material, which has much complicated properties and highly nonlinear. It is neither perfect elastic material nor perfect plastic material. In geotechnical engineering field the material of modeling is geotechnical material, which has the typical properties of fragmentation, dispersing and flowage. In soil mechanics theory, the soils are regarded as continua as a rule after the substance morphology is fallen into gaseity, liquid and solid. Now the continuum theory is used in most of numerical methods. All soil is discontinuous to some extent. So it has been acquainted early to study the stress and distortion properties of soil form the point of microcosmic view, but such study is too complicated to build quantitative relations between microcosmic and macrocosmic and depend on the power of computer and particle model.Within PFC2D (Particle flow code in two dimensions) and PFC3D (Particle flow code in three dimensions) discrete materials such soils were simulated by disks or by spheres and overcame the defect of the continuum theory to establish the relationships of microcosmic parameters and macroscopical parameters; The excavation process was simulated by using PFC firstly in the paper; PFC simulated a series of triaxial tests on steel spheres with pyramid and rhombic packings and the stress-strain curve of sand. And It can be started with micromechanics to study structured properly of soil, and should be noted to disclose the geometry character of soil particle arranging and the mechanic characters of soil particle to study not only the generation of the shear band but also the stress-stain curve in a quite similar manner to those observed in natural granular soils.From the simulations with different physical tests, establishing the functions between microcosmic parameters and macroscopical parameters including of soil angle of internal friction, modulus of deformation, peak strength of soil reinforced and unconfined compression strength of clay. Based on these functions, the paper simulate the process of excavation, and the sheet pile wall and the strut are simulated by joining together a row of disks of equal radius with parallel bonds, and soil is simulated by particles. The resulting displacements, bending moments and shearing forces within the wall, axial forces of struts and earth pressures acting on the wall are studied as well as the behavior within the soils for subsequent excavation steps. The soil microcosmic spatial structure results in its more complexity. It can be started with micromechanics to study spatial structure properties of soil, and should be noted to disclose the geometry character of particle spatial arranging. At the same time, discuss the heterogeneity and anisotropy of different spatial structures. Numerical simulation of three-dimensional assembly of spheres was carried out based on discrete element method and the theory of particle flow where the slip model was used to constituted PFC mode of packing configurations. This paper simulated a series of triaxial tests on steel spheres with pyramid and rhombic packings corresponding with real laboratory compression test. Numerical simulations captured the observed laboratory response well and the relative difference between numerical and theoretical angle of mobilized does not exceed 2.5%. The microscopic fabric (the coordination number, the porosity and the slip fraction) evolve with the development of axial strain is studiedThe PFC3D model of the sand that is consistent with the lab results of Chengde middle close-grained sand have been formed via vast parameters trial-and-error process. Numerical simulation tests were carried out using the distinct element method (DEM) by paying much attention to the micro-deformation mechanism leading to the development of shear bands and the microscopic fabric (coordination number, porosity and slip fraction) evolved regularly with the development of axial strain. Contrasting the stress-strain curves of the PFC3D sand model with Chengde middle close-grained sand under various confining is done in this paper. Analyzing the influencing rules of the stress-strain curves as the microcosmic parameters, such as particle normal stiffness, ratio of normal stiffness to tangent stiffness, friction coefficient, and initial porosity of the PFC model is done in this paper. And the properties of the shear band are studying.更多还原