共查询到19条相似文献，搜索用时 234 毫秒
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单调荷载下砂土变形过程数值模拟及细观机制研究 总被引：6，自引：0，他引：6
采用离散单元法的颗粒流理论,模拟了松砂和密砂在单调荷载作用下的变形过程,研究了砂土渐进破坏过程中的宏观力学行为和细观组构参量的演化规律。采用PFC的FISH语言开发了细观组构统计程序,通过记录加载不同时刻试样的细观参量,如配位数、接触法向分布、粒间法向接触力、切向接触力等的演化,分析了砂土变形过程中细观组构变化与宏观力学响应之间的内在联系。应用表征上述量的组构参数研究了砂土的诱发各向异性,探讨了松砂剪缩、密砂剪胀的细观机制。研究成果对于揭示砂土变形的细观机制以及建立砂土的细观力学模型具有重要意义。 相似文献
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基于数字图像相关法的砂土细观直剪试验及其颗粒流数值模拟 总被引：1，自引：0，他引：1
砂土等天然颗粒材料具有的剪胀性、压硬性、各向异性等特殊力学性质受控于其内部的微细结构及其演化,如何利用试验与数值模拟等手段对砂土细观组构进行量化,对深入研究砂土变形机制尤为重要。基于数字图像相关方法的光学测量技术与土工试验相结合的新手段,通过对直剪仪的可视化改造,开发了能初步实现土体宏细观力学性状联合测量的“砂土变形细观瞬时光学测量系统”,并利用该系统对福建标准砂进行了室内细观直剪试验。随后进行了基于颗粒流软件PFC2D直剪数值试验,提取室内试验还难以获得的细观信息,作为对室内试验的补充。通过对宏观力学性质和位移场、应变场、颗粒定向、速度场、颗粒配位数和接触力链等细观的室内与数值试验结果进行剖析,探讨了砂土力学性质的细观机制。 相似文献
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《岩土力学》2021,(4)
采用颗粒流软件模拟了饱和砂土在不排水条件下的循环剪切试验,研究了不同因素对液化的影响,并进一步分析了饱和砂土液化后宏观变形的基本规律。在此基础上,从孔隙分布角度解释了砂土液化后的大变形的细观物理机制。通过自编程序对颗粒排列和孔隙分布的演化过程进行定量描述,给出孔隙率标准差作为液化后体积收缩势的度量,并研究了孔隙率标准差与液化后大变形的关系。结果表明,试样初始状态并不会影响液化后的极限状态,仅对初始液化所需循环次数有影响。数值模拟可再现室内试验中的宏观现象,证实了室内试验中饱和砂土液化后的有限剪切大变形是客观真实的材料响应。土体体积收缩势的累积所导致的孔隙均匀化以及土颗粒间自由空隙增大正是饱和砂土液化后循环剪应变逐渐增大的细观机理。孔隙率标准差作为孔隙均匀化的量化指标,与循环剪应变各周次幅值有良好相关性。 相似文献
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《岩土力学》2020,(4)
采用颗粒流软件模拟了饱和砂土在不排水条件下的循环剪切试验,研究了不同因素对液化的影响,并进一步分析了饱和砂土液化后宏观变形的基本规律。在此基础上,从孔隙分布角度解释了砂土液化后的大变形的细观物理机制。通过自编程序对颗粒排列和孔隙分布的演化过程进行定量描述,给出孔隙率标准差作为液化后体积收缩势的度量,并研究了孔隙率标准差与液化后大变形的关系。结果表明,试样初始状态并不会影响液化后的极限状态,仅对初始液化所需循环次数有影响。数值模拟可再现室内试验中的宏观现象,证实了室内试验中饱和砂土液化后的有限剪切大变形是客观真实的材料响应。土体体积收缩势的累积所导致的孔隙均匀化以及土颗粒间自由空隙增大正是饱和砂土液化后循环剪应变逐渐增大的细观机理。孔隙率标准差作为孔隙均匀化的量化指标,与循环剪应变各周次幅值有良好相关性。 相似文献
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砂土力学性质的细观模拟 总被引：23，自引：9，他引：14
基于颗粒流理论,对砂土的室内双轴试验和砂土剪切带形成与发展进行了数值模拟,基本再现了砂土试样应力-应变关系。主要研究了颗粒粒径、颗粒摩擦系数等细观参数变化时试样宏观性质的变化情况。对比分析了室内试验和颗粒流数值模拟试验的砂土剪切带特征。同时对比研究了模型试样颗粒粒径、颗粒刚度和摩擦系数等细观参数的变化,对剪切带的形成与发展过程的影响。结果表明：通过颗粒流数值模型试验可以有效模拟砂土剪切带的形成与发展机理。 相似文献
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采用颗粒流软件模拟了饱和砂土在不排水条件下的循环剪切试验,研究了不同因素对液化的影响,并进一步分析了饱和砂土液化后宏观变形的基本规律。在此基础上,从孔隙分布角度解释了砂土液化后的大变形的细观物理机制。通过自编程序对颗粒排列和孔隙分布的演化过程进行定量描述,给出孔隙率标准差作为液化后体积收缩势的度量,并研究了孔隙率标准差与液化后大变形的关系。离散元细观数值模拟再现了室内试验中的宏观现象,证实了室内试验中饱和砂土液化后的有限剪切大变形是客观真实的材料响应。土体体积收缩势的累积所导致的孔隙均匀化以及土颗粒间自由空隙增大正是饱和砂土液化后循环剪应变逐渐增大的细观机制。孔隙率标准差作为孔隙均匀化的量化指标,与循环剪应变各周次幅值有良好的相关性。 相似文献
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基于泥质粉砂岩室内双轴压缩试验，建立PFC2D颗粒流数值模型，以此来探究泥质粉砂岩破坏的细观机理。考虑组成泥质粉砂岩试样的颗粒形状，根据电镜扫描图勾选出5种典型颗粒形状，与圆形颗粒一起生成给定孔隙率的稳定数值试样。颗粒间选用平行粘结接触模型，选取弹性模量、泊松比、峰值应力分别对饱和、天然状态的泥质粉砂岩试样进行不同围压下的细观参数标定，然后进行双轴压缩试验模拟，分析试样在双轴压缩试验过程中颗粒法向接触力、切向接触力、配位数、孔隙率等细观组构参数的分布特征和演化规律。试验结果表明:双轴压缩试验的细观参数标定可不考虑抗剪强度指标黏聚力c和内摩擦角ϕ值的影响。破坏前后，试样各方向统计范围内都存在法向接触力和切向接触力。围压的存在影响试样的起始配位数、孔隙率和试样破坏后的稳定配位数和孔隙率，对配位数和孔隙率的改变速率影响很小。试样空间孔隙率的演化在一定程度上反映了试样破坏时内部结构的变迁，能更加直观地反映试样的破坏模式。 相似文献
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颗粒流理论模拟砂土的力学性质 总被引：1，自引：0，他引：1
文中采用颗粒流理论模拟了砂土双轴试验和砂土剪切带的形成与发展机理。通过颗粒流数值模拟试验再现了砂土应力－应变关系曲线，同时研究了砂土细观参数变化时其宏观力学性质的反应。对比研究了室内试验与颗粒流试验砂土剪切带的形成与发展机理，同时研究了当细观参数如颗粒大小、刚度、摩擦系数变化时，砂土剪切带的变化规律。 相似文献
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随着地下能源工程、热法地基处理和核废料处理等技术越来越多的开展,热力耦合对土体的影响已经成为当今岩土工程领域的热门研究课题之一。基于颗粒体积膨胀法,通过PFC2D模拟不同变温幅值下（20、40、60℃）松散颗粒（相对密实度Dr=0.2）的循环热固结试验。在宏观角度上发现颗粒在温度循环过程中产生热固结现象,即试样随温度循环圈数的增加,其孔隙比不断累积降低。研究表明：变温幅值越大将导致长期温度循环过程中的颗粒材料越密实,且孔隙比、侧向应力和竖向应力的分布出现一定的不均匀性。与此同时,通过微观角度的分析,总结了温度循环和变温幅值对颗粒间接触数量、法向接触力和切向接触力所存在的影响,得出热固结试验中温度循环的幅值对试样的各向异性有较大的影响。 相似文献
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Thallak. G. Sitharam 《Geotechnical and Geological Engineering》2003,21(4):297-329
Discrete Element Modeling (DEM) of cyclic behavior of granular material has been attempted to understand liquefaction behavior of sands. A series of cyclic biaxial tests in both undrained and drained conditions with constant stress and strain amplitudes were performed on assemblage of loose and dense systems. Tests are conducted on monodisperse (uniform) and polydisperse (well graded) samples. From this study, it has been shown that DEM can simulate the cyclic behavior of sands very satisfactorily. Characteristic features, i.e., occurrence of large plastic strains and changing over from contractile to dilative behavior beyond the phase transformation angle, anisotropy of reduced strength on the extension side etc are very well reflected in numerical simulations. Liquefaction of loose assemblage seems to be mainly due to continued and cumulative loss of co-ordination number under each cycle as there is a reversal of loading direction and hence a continuous reorientation of fabric. There is no cumulative loss of co-ordination number in dense states because the stress ratios are mostly higher than the phase transformation level where the fabric has reached a limiting orientation. Micro mechanical explanations to the macroscopic behavior of the disc assemblage under cyclic loading are presented in terms of the force and fabric anisotropy coefficients. 相似文献
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DEM study of fabric features governing undrained post-liquefaction shear deformation of sand 总被引：3，自引：3，他引：0
In an effort to study undrained post-liquefaction shear deformation of sand, the discrete element method (DEM) is adopted to conduct undrained cyclic biaxial compression simulations on granular assemblies consisting of 2D circular particles. The simulations are able to successfully reproduce the generation and eventual saturation of shear strain through the series of liquefaction states that the material experiences during cyclic loading after the initial liquefaction. DEM simulations with different deviatoric stress amplitudes and initial mean effective stresses on samples with different void ratios and loading histories are carried out to investigate the relationship between various mechanics- or fabric-related variables and post-liquefaction shear strain development. It is found that well-known metrics such as deviatoric stress amplitude, initial mean effective stress, void ratio, contact normal fabric anisotropy intensity, and coordination number, are not adequately correlated to the observed shear strain development and, therefore, could not possibly be used for its prediction. A new fabric entity, namely the Mean Neighboring Particle Distance (MNPD), is introduced to reflect the space arrangement of particles. It is found that the MNPD has an extremely strong and definitive relationship with the post-liquefaction shear strain development, showing MNPD’s potential role as a parameter governing post-liquefaction behavior of sand. 相似文献
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Jianxiu Wang Yansheng Deng Yule Shao Xiaotian Liu Bo Feng Linbo Wu Jie Zhou Yao Yin Na Xu Haihua Peng 《Environmental Earth Sciences》2018,77(12):471
Medium-coarse sands (CS) were dredged and exhausted in land reclamation. However, the remaining silty-fine sands (FS) were wasted. The liquefaction behavior of dredged silty-FS and the possibility of utilizing the remaining silty-FS as dredger fill source for land reclamation should be investigated. Cyclic consolidation-undrained triaxial tests were performed to investigate the liquefaction resistance of dredged silty-FS under different influencing factors. The cyclic stress ratio (CSR) of dredged silty-FS increased with the increase in initial relative density and consolidation stress ratio and decreased with the increase in silt content and consolidation stress. The CSR first decreased with the increase in clay content up to a threshold value and increased with the increase in clay content. A regression model was created to estimate the relationship between CSR and silt content, clay content, initial relative density, consolidation stress, consolidation stress ratio, and cyclic resistance ratio. Response surface methodology (RSM) was employed to investigate the mutual influence among the five independent variables. On the basis of cyclic triaxial tests, particle flow code models were introduced to investigate the microscopic internal fabric changes of dredged silty-FS and the influence of extended factors on liquefaction. The average microscopic contact force and coordination number between particles controlled the macroscopic mechanical behavior of sands. Sand liquefaction was due to the cumulative loss of coordination number under cyclic loading. The average contact force between particles was linearly decreased to 0 and the coordination number sharply decreased when the sample reached initial liquefaction. On the basis of numerical tests, CSR increased with the increase in D50 and vibration frequency. The influence of vibration frequency was relatively small. In addition, the CS–FS and CS–FS–CS combination layers showed greater liquefaction resistance than the FS layer. In the filling process, the interbed of FS and CS improved the liquefaction resistance of dredged silty-FS to a certain extent. 相似文献
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The strain space multiple mechanism model idealizes the behavior of granular materials on the basis of a multitude of virtual simple shear mechanisms oriented in arbitrary directions. Within this modeling framework, the virtual simple shear stress is defined as a quantity dependent on the contact distribution function as well as the normal and tangential components of interparticle contact forces, which evolve independently during the loading process. In other terms, the virtual simple shear stress is an intermediate quantity in the upscaling process from the microscopic level (characterized by contact distribution and interparticle contact forces) to the macroscopic stress. The stress space fabric produces macroscopic stress through the tensorial average. Thus, the stress space fabric characterizes the fundamental and higher modes of anisotropy induced in granular materials. Herein, the induced fabric is associated with monotonic and cyclic loadings, loading with the rotation of the principal stress, and general loading. Upon loading with the rotation of the principal stress axis, some of the virtual simple shear mechanisms undergo loading whereas others undergo unloading. This process of fabric evolution is the primary cause of noncoaxiality between the axes of principal stresses and strains. Although cyclic behavior and behavior under the rotation of the principal stress axis seem to originate from two distinct mechanisms, the strain space multiple mechanism model demonstrates that these behaviors are closely related through the hysteretic damping factor. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
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低填方桩承式路堤在交通荷载下的动力响应特性与路堤的承载能力及其稳定性密切相关。基于前人的室内模型试验,采用PFC2D软件建立了桩承式路堤离散元数值分析模型;采用数值双轴试验、数值单向压缩试验分别对路堤填料和桩间土的细观参数进行了标定;同时,根据路堤荷载传递效率对模拟结果与试验数据进行了对比分析,验证了离散元数值模型的正确性及适用性。在此基础上,在低填方路堤表面施加正弦波循环荷载,并对路堤荷载传递效率、接触力分布及路堤沉降等的变化规律进行了分析。模拟结果表明：在循环荷载作用下,低填方路堤中土拱结构的承载力先逐渐弱化并最终趋于稳定;该弱化作用在宏观上表现为路堤荷载传递效率的逐渐降低,而在细观上则表现为桩顶及桩间土上方接触力差异程度的逐渐减小;同时,土拱结构的弱化将不断加剧路堤表面的不均匀沉降。 相似文献
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循环加载方式与应力路径对砂土的抗液化强度有很大的影响。利用GDS空心圆柱扭剪仪对南海珊瑚砂进行了一系列复杂加载条件下均等固结不排水循环试验，探讨了90°突变应力路径下主应力方向角对珊瑚砂抗液化强度的影响。试验结果发现：以循环应力比（CSR）作为应力水平指标，当不控制中主应力系数b的变化时，主应力方向角 对珊瑚砂的抗液化强度并无显著影响；当控制b始终保持0.5时，珊瑚砂的抗液化强度随着 的增加呈现出先减小后增大的趋势，且在 45°时的抗液化强度最低。基于分析循环荷载引起的土单元大、小循环主应力 、 变化，定义了单元体循环应力比（USR）作为一个新的物理指标，发现不同循环加载方式与应力路径条件下施加于珊瑚砂试样的USR与引起液化所需的循环次数NL存在事实上的唯一性关系。通过引自文献的4种无黏性土原始试验数据的再处理，独立地验证了以USR表征砂类土液化强度的适用性。 相似文献
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The paper examines the mechanics and physics of granular material responses at the macroscopic and microscopic levels during both monotonic and cyclic loadings. A numerical analysis referring to a long retaining wall is conducted using a two-dimensional discrete element model representing a granular system with a free top surface. On one of the lateral boundaries referring to the retaining wall, both active and passive loadings were applied monotonically as well as cyclically. First, the development of sheared zones and classic failure wedges resulting from active and passive monotonic displacements are discussed with respect to Rankine’s and Roscoe’s solution angles. Then, a series of loading cycles were performed using slow small-amplitude displacements at different stress states chosen before the occurrence of failure along the passive monotonic stress response curve. Particular interest is focused on the ultimate asymptotic cyclic response of the granular system, the occurrence of a high-mobility (convective) zone and a detailed macroscopic and microscopic analysis. Finally, major kinematical features that are displayed during cyclic loading from different starting stresses to eventually reach the same asymptotic state were elucidated through particle vortex-like flux formations, including contact rotations. The change in material stiffness was also investigated based on the evolution of strong and weak contact networks, together with the analysis of fabric anisotropy within the entire domain, including the high-mobility zone considered separately.
相似文献19.
Liquefaction is associated with the loss of mean effective stress and increase of the pore water pressure in saturated granular materials due to their contractive tendency under cyclic shear loading. The loss of mean effective stress is linked to loss of grain contacts, bringing the granular material to a “semifluidized state” and leading to development and accumulation of large cyclic shear strains. Constitutive modeling of the cyclic stress-strain response in earthquake-induced liquefaction and post-liquefaction is complex and yet very important for stress-deformation and performance-based analysis of sand deposits. A new state internal variable named strain liquefaction factor is introduced that evolves at low mean effective stresses, and its constitutive role is to reduce the plastic shear stiffness and dilatancy while maintaining the same plastic volumetric strain rate in the semifluidized state. This new constitutive ingredient is added to an existing critical state compatible, bounding surface plasticity reference model, that is well established for constitutive modeling of cyclic response of sands in the pre-liquefaction state. The roles of the key components of the proposed formulation are examined in a series of sensitivity analyses. Their combined effects in improving the performance of the reference model are examined by simulating undrained cyclic simple shear tests on Ottawa sand, with focus on reproducing the increasing shear strain amplitude as well as its saturation in the post-liquefaction response. 相似文献