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1.
循环荷载下砂土液化特性颗粒流数值模拟 总被引：7，自引：2，他引：5
利用PFC2D常体积循环双轴试验条件,对砂土在不排水循环荷载作用下的液化特性进行了颗粒流数值模拟,数值模拟按等应力幅加荷方式进行。颗粒流数值模拟的优点在于得到试样液化宏观力学表现的同时,通过不同循环加荷时刻试样内细观组构参量（包括配位数、接触法向分布、粒间法向接触力、粒间切向接触力）的演化规律,分析砂土液化过程中细观组构变化与宏观力学响应之间的内在联系,从而可进一步探讨砂土液化的细观力学机制。数值模拟研究结果表明,砂土液化现象在宏观力学表现上反映为超静孔隙水压力的累积上升和平均有效主应力的不断减小,在细观组构上对应于配位数的累积损失和粒间接触力的不断减小。砂土液化细观机制分析表明,试样配位数的减少与循环加荷过程中组构各向异性滞后于应力各向异性有关。 相似文献
2.
This study explores the link between the monotonic and cyclic undrained behaviour of sands using the discrete element method (DEM). It is shown that DEM can effectively capture the flow deformation of sands sheared under both monotonic and cyclic undrained loading conditions. When subjected to cyclic shearing, flow-type failure is observed for a loose sample, while cyclic mobility is observed for a dense sample. A strong correlation between the monotonic and cyclic loading behaviour that has been revealed experimentally is also confirmed in DEM simulations: (a) flow deformation occurs in the compressive loading direction when the cyclic stress path intersects the monotonic compression stress path prior to the monotonic extension stress path, and vice versa; (b) the onset of flow deformation in q–\(p^{\prime }\) space is located in the zone bounded by the critical state line and the instability line determined from monotonic simulations. Hill’s condition of instability is shown to be effective to describe the onset of flow failure. Micro-mechanical analyses reveal that flow deformation is initiated when the index of redundancy excluding floating particles drops to below 1.0 under both monotonic and cyclic loading conditions. Flow deformation induced by either monotonic or cyclic loading is characterized by an abrupt change of structural fabric which is highly anisotropic. The reason why the dense sample dilated during monotonic loading but showed cyclic mobility (temporary liquefaction) during cyclic loading is attributed to the repeating reversal of loading direction, which leads to the periodic change of microstructure. 相似文献
3.
为了真实地描述饱和密砂在循环加载过程中的变形行为,需要引入考虑剪胀阶段组构变化的宏观参量。在已有的基于状态参量的本构模型基础上,引入反映组构变化的剪胀内变量,简称组构-剪胀内变量z。以相变线PTL作为参考线,采用基于相变的状态参量判断砂土在初始时刻和任意时刻体积变形的变化趋势,并通过z对剪胀比d的影响,考虑反向加载过程中塑性变形的累积,建立了一个针对饱和密砂的循环加载的弹塑性本构模型。该模型根据试验现象将已有模型中的塑性剪切模量区分为首次加载模量与再加载模量,能较好地模拟排水情况下砂土循环加载的胀-缩变化过程。最后,针对密砂的三轴排水情况,利用文中模型进行预测,并把预测结果与试验结果进行比较,结果表明该模型能够总体反映砂土循环加载的变形行为。 相似文献
4.
A modification to the nonlinear Pastor–Zienkiewicz–Chan (PZC) constitutive model without any change in the number of model parameters is introduced in order to simulate stiffness degradation of dense sands at dynamic loading. The PZC model is based on generalized plasticity and was verified by good prediction of liquefaction and undrained behavior of saturated sand. The PZC is a robust model that can predict drained dynamic behavior of sands, especially stiffness increase in loose sand at reloading of dynamic loading. Yet, this model does not show stiffness degradation of dense sand at reloading. The modification is made through modifying the stress memory factor, H DM, which is multiplied by the plastic modulus, H L. This modification does not influence reloading behavior of loose sand. The modified PZC model is verified via results of drained cyclic tests. Two cyclic triaxial tests on loose and dense specimens, along with two cyclic plane strain tests on dense sand are utilized for validation. The model simulation shows that the modified PZC model is able to predict the stiffness degradation of dense sand at reloading well. 相似文献
5.
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
6.
The anisotropy effect is exhibited more prominently in sedimentary depositions, and it relates the soil’s mechanical specifications to the directions of imposed loads. Even though this phenomenon has been comprehensively explored in silica sands, few research has been conducted for studying the anisotropic behavior of marine carbonate sands. To bridge this gap, the present study investigates the anisotropy effect on the mechanical behavior of Bushehr carbonate sand acquired from the north shelf of the Persian Gulf in Iran. Toward this end, some undrained principal stress rotation tests are conducted using a hollow cylinder shear torsional apparatus in such a manner that the direction of the applied principal stresses are fixed along a desired orientation and the total mean stress and intermediate principal stress ratio are kept constant. Furthermore, prior to shearing, the samples are consolidated under three confining pressures and two isotropic and anisotropic states. The results show that dilative behavior is observed in all loading directions after initial contraction; this contradicts the response observed in silica sands. The anisotropy response of soil follows two different trends in the contractive and dilative phases. The relation of soil’s mechanical properties shows a descending trend with the angle of maximum principal stress in the contractive phase; on the other hand, the anisotropy behavior shows a dominant parabola trend in the dilative phase, where the maximum ultimate pore pressure and minimum soil strength occur in the stress direction with an angle of α?=?30°. By increasing the confining pressure in the soil element, the intensity of the anisotropy in some mechanical properties except the soil deformation is reduced. Furthermore, the deviatoric-to-effective mean stress ratio in the phase transformation state from contraction to dilation is independent of the loading direction and consolidation stress state, and it is considered one of the intrinsic properties of sand. 相似文献
7.
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
8.
An advanced understanding of the strain accumulation phenomenon in granular soils subjected to low amplitude cyclic loading with relatively high frequency is needed to enhance the ability to predict the settlement of granular soils induced by vibrations. In the current study, the discrete element method is used to study this phenomenon. A loose and a medium dense sample composed of a relatively large number of spheres are considered. A series of stress controlled cyclic triaxial tests with different excitation amplitudes and frequencies is performed on these samples at different static stress states. The response of these samples at the macroscopic and microscopic scales is analyzed. The sample density, the cyclic stress amplitude and the static stress state importantly affect strain accumulation. However, the cyclic excitation frequency has a small effect on strain accumulation. At the microscopic scale, frictional sliding occurring at a few contacts continuously dissipates energy and the fraction of these contacts varies periodically during cyclic loading. The coordination number of these samples increases slightly as strain accumulates. However, the anisotropy remains almost constant during low amplitude cyclic excitation. A qualitatively good agreement between numerical and experimental results is found. 相似文献
9.
Simple Shear Behavior of Calcareous and Quartz Sands 总被引：2，自引：1，他引：1
H. G. Brandes 《Geotechnical and Geological Engineering》2011,29(1):113-126
The monotonic and cyclic simple shear behavior of several loose skeletal calcareous sands is investigated and compared to
that of common Nevada and Ottawa quartz sands in order to draw contrasts between these types of materials and to develop stiffness
and modulus degradation curves for quasi-nonlinear ground response analysis. Mobilized frictional resistance and cyclic strength
are generally higher for the calcareous sands, whereas shear modulus and damping are larger for quartz sand at strain levels
between 0.05 and 1%. Degradation properties do not appear to be very sensitive to the initial cyclic stress ratio. Differences
in behavior between the calcareous and quartz sands are presumably due to contrasts in grain geometry, hardness, gradation
and the amount of intraparticle voids. 相似文献
10.
以福建标准砂为材料,采用宏细观模型试验方法,分析了转速比对松砂和密砂地基中螺旋挤扩钻具成孔特性的影响。宏观上,研究了竖向阻力、扭矩和土体应力随贯入深度的变化规律;细观上,结合环氧树脂砂土固化方法与数字图像分析技术,探讨了孔周土体相对密实度变化及其组构演化规律。试验结果表明,无论松砂还是密砂,钻具贯入时的竖向阻力、扭矩及孔周土体最大径向应力均随着转速比的增加而减小;钻具贯入导致松砂地基中钻头深度和孔周土体均发生挤密,而对于密砂则均发生不同程度的体胀;无论松砂还是密砂,孔周砂土切块环向面上的接触法向各向异性程度均随着转速比的增加而减小,而转速比对其径向面上的接触法向各向异性分布影响不大。该试验结果有助于提升对螺旋挤土桩成孔过程宏细观机制的认识。 相似文献
11.
12.
Hypoplastic model for sands with loading surface 总被引：3，自引：3，他引：0
Although the hypoplastic models for sands have exhibited good predictive capability in monotonic loading, they are not able to reproduce memory effects and predict excessive plastic accumulation under cyclic loading. To overcome these issues, a loading surface has been incorporated into a hypoplastic model. This surface is capped and has two hardening variables. Notions from the bounding surface plasticity were borrowed in order to formulate the hardening functions. With this novel model, some salient features can be described: the model can account for the accumulation of plastic deformation, a memory effect is provided by the new surface, and stress-induced anisotropy effects observed in sands are successfully simulated. A short calibration guide of the parameters is given, and some simulations for Hostun RF loose sand and Toyoura sand are presented. 相似文献
13.
An elastoplastic constitutive model is proposed for saturated sands in general stress space using the middle surface concept (MSC). In MSC, different features of stress–strain response of a material are divided into different pseudo‐yield surfaces. The true‐yield surface representing the true response is established by using various links between the yield surfaces. In this MSC sand model, several well‐known features of sand response are represented by three different pseudo‐yield surfaces, which are developed in a simple and straightforward way. These features include the critical state behaviour, the effects of state parameter, unloading and reloading plastic deformation, the influence of fabric anisotropy, and phase transformation line related behaviour. Finally, the model predictions and test results are compared for two different types of sands under a variety of loading conditions and good comparisons are obtained. The application of MSC to saturated sand modelling shows the versatility of MSC as a general concept for modelling stress–strain response of materials. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
14.
Anisotropy in the onset of liquefaction and post liquefaction under cyclic loading condition causes the change in response behavior of the soil. The proposed model operates within the integration of sliding/opening/closing framework of 17 predefined planes as local deformation. This leads to the use of better stress/strain multilaminate histories with many directional effects on soil behavior specifically internal mechanism during pre and post-liquefaction. The ability of multilaminate model for fabric anisotropy has been proven by comparison with the experimental results under drain and undrained conditions and monotonic and cyclic loading. The effects of induced anisotropy was also investigated. 相似文献
15.
Ciantia Matteo O. Arroyo Marcos O’Sullivan Catherine Gens Antonio 《Acta Geotechnica》2019,14(5):1337-1356
In granular soils grain crushing reduces dilatancy and stress obliquity enhances crushability. These are well-supported specimen-scale experimental observations. In principle, those observations should reflect some peculiar micromechanism associated with crushing, but which is it? To answer that question the nature of crushing-induced particle-scale interactions is here investigated using an efficient DEM model of crushable soil. Microstructural measures such as the mechanical coordination number and fabric are examined while performing systematic stress probing on the triaxial plane. Numerical techniques such as parallel and the newly introduced sequential probing enable clear separation of the micromechanical mechanisms associated with crushing. Particle crushing is shown to reduce fabric anisotropy during incremental loading and to slow fabric change during continuous shearing. On the other hand, increased fabric anisotropy does take more particles closer to breakage. Shear-enhanced breakage appears then to be a natural consequence of shear-enhanced fabric anisotropy. The particle crushing model employed here makes crushing dependent only on particle and contact properties, without any pre-established influence of particle connectivity. That influence does not emerge, and it is shown how particle connectivity, per se, is not a good indicator of crushing likelihood.
相似文献16.
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
17.
SHARAFUTDINOV Rafael 《岩土力学》2022,43(10):2873-2886
砂土的密度和应力状态对其刚度有很大的影响。计算岩土工程中许多硬化土体模型都是基于邓肯−张模型得出的,没有考虑到密度对砂土刚度的影响。而在极致密或松散的砂土的三轴压缩过程中,剪切应变的上升会引起密度的显著变化。为了评估粒径分布、密度及应力状态对砂土刚度的影响,使用统计和回归方法对来自莫斯科和明斯克的15个建筑工地的962个土壤样本的各向同性三轴试验数据进行分析。基于密度和应力状态参数的影响,提出了评估不同粒径砂土刚度的经验方程。对来自欧洲、印度和美国的冲积土和陆地土试验的比较分析表明,其砂土的刚度与莫斯科和明斯克的砂土在同一范围。所提出的方程可用于初步估计有限元法计算里的刚度参数,也可应用于岩土工程模型（允许考虑刚度的变化、水平和垂直分布）。此外,还提出了基于邓肯−张模型的半经验关系。当密度的变化影响土的刚度时,该半经验关系可为受到大变形和（或）复杂加载路径影响的松散和非常致密的砂土提供更为真实的结果。一般来说,岩土工程师可将获得的结果应用于更为复杂的土体模型设计中。 相似文献
19.
An anisotropic hardening model for sand or other granular materials is presented. The state of initial densification is represented by a configuration surface and the state of loading by a loading surface in the stress space. The domain of elastic response may not exist and the irreversible strain is assumed to occur for both active and reverse loading. The derived constitutive equations are applicable both for dense and loose sands. Some model predictions are compared with available experimental data for triaxial compression and extension under undrained condition. 相似文献
20.