【作者】 史旦达；

【导师】 周健；

【作者基本信息】 同济大学 ， 岩土工程， 2007， 博士

【摘要】 土是由多相介质组成的离散颗粒集合体，尽管研究者早已在土的散粒状属性上达成共识，但关于土体力学性质的研究还经常沿用连续介质力学方法，这种基于连续体小变形假设的方法在求解实际土体非线性、非连续、大变形、应变局部化问题时往往无法得到精确解答。离散单元法以非连续介质力学理论为基础，以单个块体或颗粒为基本单元，单元之间无需满足位移连续和变形协调条件，适用于求解岩土体的非连续变形问题。对土力学试验的模拟是颗粒离散元法的一项重要应用。一方面，通过与试验结果的对比分析，可以验证数值模型的可靠性；另一方面，作为对试验研究的细观补充，数值分析法可以较容易地获得试样受荷过程中内部细观组构参量的空间分布和统计平均，从而为研究土体力学性质的细观机理提供了条件。本文利用颗粒离散元法中的二维颗粒流方法，以试验研究结果为基础，从宏细观结合的角度对单调与循环加荷条件下砂土的力学性质进行了细观模拟。首先，利用常规三轴试验结果对接触本构模型进行标定，得到了二维条件下可用于福建标准砂力学性质模拟的细观参数组合；其次，利用Hertz-Mindlin非线性接触模型模拟了砂土的振动液化特性，并与标准砂循环三轴试验结果进行了对比；接着，针对纯圆颗粒模拟的缺陷，利用“团颗粒”方法开发了形状近似椭圆的“椭圆团颗粒”和形状接近实际角粒砂的“角粒团颗粒”，研究了颗粒形状对砂土力学性质的影响及其细观机理；最后，利用接触粘结模型生成“簇颗粒”单元，分析了高应力一维压缩条件下砂土的颗粒破碎特性。研究结果表明：Hertz-Mindlin非线性接触模型适用于变围压条件下砂土力学性质模拟；纯圆颗粒模拟能够定性地反映饱和砂土振动液化的一般规律；颗粒长短比变化对砂土力学性质的影响具有区间效应，其原因与粒间初始平均接触数有关；不同形状随机混合试样能够从定性规律上反映振动液化过程中砂土的细观组构演化规律；数值模拟不仅可得到颗粒破碎宏观响应，而且可以分析内部接触力变化和实现对粘结破裂位置的追踪，从而可进一步研究颗粒破碎细观力学机制。更多还原

【Abstract】 Soil is a multiphase particulate medium. In spite of the granular nature, soil has traditionally been modeled as a continuum. This continuum mechanics cannot get the precise solution while soil’s nonlinear, discontinuous, large deformation, and strain localization behavior are considered. The discrete element method（DEM） is based on discontinuum theory, and is pioneered by Cundall. DEM treats each block or particle as an element, and the motion of element need not conform to displacement continuity and deformation compatibility conditions, so it is a perfect method to capture the discontinuous behavior of soil.It is an important application of particle discrete element method to simulate the laboratory tests in soil mechanics. On the one hand, through comparing the macromechanical results of lab tests and numerical tests, the validity of numerical model can be proved. On the other hand, numerical tests are the effective supplement to lab tests in microscope, because the wealth of micromechanical and statistical information can be generated by the simulations together with the macroscopic response.In this paper, one of the particle discrete element method named particle flow code in 2-dimension(PFC^2D) is used. Based on the results of lab tests, sand behavior under monotonic and cyclic loading is simulated from the aspect of association of macro and micro scope. Firstly, the calibration of numerical model is carried out by the results of conventional triaxial tests, and the micromechanical parameters for the simulation of Fujian standard sand are achieved. Secondly, the liquefaction behavior of sand under cycling loading is simulated by using Hertz-Mindlin contact model. Thirdly, to cover the shortage of circular element simulations, elliptical element and angular element are developed by using CLUMP logic in PFC^2D, and the effect of particle shape on sand properties is numerically analyzed. Finally, cluster element is developed by using contact-bond model in PFC^2D, and the particle breakage property of sand under high pressure 1D compression condition is investigated. It is found that compared with liner contact model, Hertz-Mindlin contact model is more suitable to use in the simulations of sand properties under the variable stress level. Numerical simulations by assemblies of circular panicles can qualitatively reflect the general characteristics of liquefaction behavior of sand. The interval effect of aspect ratio of elliptical panicle on the mechanical behavior of numerical sample is obtained, and the reason is related to the initial average co-ordination number. The regularity of evolution of fabric in the process of sand liquefaction under cyclic loading can be qualitatively achieved by using the numerical sample randomly mixed of panicles with different shapes. Numerical simulations can not only demonstrate the macro phenomenon of particle breakage, mostly importantly, it can also expose the evolution of particle breakage microscopically, and would contribute to further research of the micromechanics of particle breakage.更多还原