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1.
A computational framework is presented for dynamic strain localization and deformation analyses of water‐saturated clay by using a cyclic elasto‐viscoplastic constitutive model. In the model, the nonlinear kinematic hardening rule and softening due to the structural degradation of soil particles are considered. In order to appropriately simulate the large deformation phenomenon in strain localization analysis, the dynamic finite element formulation for a two‐phase mixture is derived in the updated Lagrangian framework. The shear band development is shown through the distributions of viscoplastic shear strain, the axial strain, the mean effective stress, and the pore water pressure in a normally consolidated clay specimen. From the local stress–strain relations, more brittleness is found inside the shear bands than outside of them. The effects of partially drained conditions and mesh‐size dependency on the shear banding are also investigated. The effect of a partially drained boundary is found to be insignificant on the dynamic shear band propagation because of the rapid rate of applied loading and low permeability of the clay. Using the finer mesh results in slightly narrower shear bands; nonetheless, the results manifest convergency through the mesh refinement in terms of the overall shape of shear banding and stress–strain relations. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
2.
简要介绍了颗粒抗转动模型,并将其引入离散元程序中,通过建立挡墙地基模型和合理选取模型参数,分别考虑了地基填土不同密实度和挡墙不同位移模式（被动T模式、RB模式、RT模式）情况下,刚性挡墙被动土压力随挡墙位移增长发展到达临界状态时,土压力系数 随位移发展的变化规律及墙后填土剪切带的形成规律,并与其他学者的研究成果进行对比分析。研究结果表明,土压力系数 随着挡墙位移增长的变化规律与填土的孔隙比（或相对密实度）和挡墙的位移模式紧密相关。随着孔隙比的减小或相对密实度的增大,土压力系数 会逐渐由位移硬化特性过渡为位移软化特性。尽管中密试样在双轴压缩试验中呈现出应变软化特性,而中密样的土压力系数 随着挡墙平动位移的增长可能呈现出位移软化特性,也可能呈现位移硬化特性。随着刚性挡墙向墙后土体推移,试样中的剪应变随之增大,并会在墙后形成应变局部化,即剪切带的出现。与室内试验剪应变云图相似,离散元较好地模拟了土压力临界状态时剪切带分布规律。同时,墙后土体表面不再是光滑的平面,而是逐渐隆起的凹凸面;随着挡墙位移增长,土体表面隆起量越来越大,直至土体破坏。 相似文献
3.
M. A. Hicks 《国际地质力学数值与分析法杂志》2000,24(5):453-476
An adaptive mesh refinement algorithm has been developed for non‐linear computations in geomechanics, based on a smoothed stress–strain finite element formulation. This uses estimates of error in the incremental shear strain invariant to guide the regeneration of unstructured meshes at regular intervals during loading. Following each mesh‐update, no re‐analysis of previous increments with the new mesh is necessary. Algorithm performance has been investigated by analysing a passive earth pressure problem using a linear elastic‐perfectly plastic Mohr–Coulomb soil model. Perfectly drained behaviour has been considered, as have partially drained situations using hydromechanical coupling, while undrained behaviour has been approximated using time steps close to zero. In all cases, mesh adaptivity has been successful in capturing regions of high strain gradient. The results have been compared with analytical solutions. Accurate computations of limit load and shear band orientation have been obtained for a wide range of material dilation angles. Copyright © 2000 John Wiley & Sons, Ltd. 相似文献
4.
Landslides and collapses occurred during the May 12th earthquake in Wenchuan County. Most of these landslides and collapses
were caused by shear bands. Shear band triggers instability on mountain slopes, resulting in debris flow, landslides, or collapse.
According to experimental results, there was only one shear band forming in the soil layer prior to the initiation of debris
flow under shear load, although several fine shear bands appeared. The development of shear bands in saturated soils is numerically
investigated in this paper using the in situ soil from the Weijia Gully, Beichuan County. The evolution of shear banding from
several finite amplitude disturbances (FADs) in pore pressure has been studied. The numerical analysis revealed that the FADs
evolved into a fully developed shear band. It is shown that the shear banding process consists of two stages: inhomogeneous
shearing and true shear banding. 相似文献
5.
Undrained deformation of dilative sand generates negative excess pore pressure. It enhances the strength, which is called dilative hardening. This increased suction is not permanent. The heterogeneity at the grain scale triggers localisations causing local volume changes. The negative hydraulic gradient drives fluid into dilating shear zones. It loosens the soil and diminishes the shear strength. It is essential to understand the mechanism behind this internal drainage and to capture it numerically. The purpose of this paper is to develop a macroscopic constitutive relationship for the undrained deformation of saturated dense sand in the presence of a locally fully or partially drained shear band. Separate constitutive relations are generated for the band and intact material. Both time and scale dependence during pore fluid diffusion in saturated sand are captured, eliminating the mesh dependency for finite element implementations. The model is applied to the Gauss points that satisfy the bifurcation criterion. The proposed method is calibrated to recreate the undrained macroscopic response bestowed by an extra-small mesh. The microscopic behaviours inside and outside shear band predicted by this model are qualitatively in good agreement with individual material point behaviours inside and outside the shear band in the extra-small mesh. Depending on the loading rate and the shear band thickness, the response inside the band can be fully or partially drained, which governs the ultimate global strength. The calibrated model is exploited to simulate an upscaled biaxial compression test with semipermeable boundaries. 相似文献
6.
与 Cosserat 理论相比,偶应力理论在一定程度上可以降低数值框架的复杂度,已逐渐应用于岩土体应变局部化分析中。然而,一般的偶应力有限元法需要满足 C1连续性,即单元内部和单元交界面上的应变都需要具有连续性。为了避免开发较为复杂的C1型偶应力单元,在 Cosserat 连续体理论框架下,通过借助罚函数方法对 C1连续性进行松弛来获得偶应力理论的逼近解,建立了基于罚函数的偶应力有限元方法 PCS-FEM。通过平面应变条件下的弹性圆孔应力集中问题对 PCS-FEM 方法的有效性进行了验证,并应用于土体应变局部化分析中。通过对Ottawa砂的平面应变试验进行数值模拟,发现 PCS-FEM 方法获得的应力−应变曲线及剪切带破坏形态与试验结果基本一致,且能够克服经典连续体理论病态的网格敏感性问题,保证应变局部化问题的适定性;通过对承受偏心荷载作用下的土坡应变局部化经典算例进行分析,发现 PCS-FEM 方法同样可以克服土坡应变软化阶段的网格敏感性问题,展现土体的渐进破坏过程。 相似文献
7.
月壤双轴试验的剪切带离散元数值分析 总被引：2，自引：0，他引：2
土体的破坏问题一般都是从其剪切带入手进行研究的。针对真实月壤所处的环境（无水、低重力场、低气压等）和内摩擦角较大的特点,采用最近提出的1种考虑粒间抗转动作用与粒间范德华力2个因素的月壤散粒体力学接触模型,用离散单元法模拟了柔性边界条件下月球环境（含范德华力）与地球环境（不含范德华力）2种试样的双轴压缩试验,通过试样局部变形、速度场、孔隙比及转动场的变化情况研究了2种试样剪切带形成与发展。研究结果表明,剪切带的形成是试样内部应变局部化的结果,同时也伴随着试样内部颗粒相对转动的局部化,月面环境对试样的破坏形式与性状（剪切带的倾角及厚度）有显著的影响,探月工程中必须考虑到月面环境对月壤力学性质的影响。 相似文献
8.
狭窄基坑平动模式刚性挡墙被动土压力分析 总被引：2，自引：0，他引：2
对于地铁车站、地下管道沟槽等狭窄基坑,其被动区土体宽度有限,不满足半无限体的假定,采用经典的库仑、朗肯土压力理论计算挡墙被动土压力是不合适的。首先建立了无黏性土中狭窄基坑刚性挡墙的有限元分析模型,研究了挡墙相对平移时不同宽度土体的被动滑裂面的分布规律;借鉴库仑平面土楔假定,建立了狭窄基坑刚性平动挡墙被动土压力的理论计算模型,推导了被动极限状态下滑裂面倾角及被动土压力系数的解析公式;再采用水平薄层单元法,得到了被动土压力分布、土压力合力作用点高度的理论公式。结合算例,深入研究了这种工程背景下挡墙被动滑裂面倾角的影响因素,以及被动土压力合力、土压力分布及合力作用点位置与经典库仑土压力理论的差别,与数值计算结果的对比验证了该理论方法的合理性。研究发现,当被动区土体宽度小于满足半无限体的临界值、且墙土摩擦角大于0时,被动滑裂面倾角大于传统库仑被动滑裂面倾角,被动土压力大于经典库仑解,合力作用点高度则小于库仑解,且基坑越窄,墙土摩擦角越大,其差别越大。 相似文献
9.
饱和砂土局部变形带模拟的有限元数值实现 总被引：1，自引：0，他引：1
基于有限变形理论,推导了Newton-Raphson 迭代算法在k+1步增量表达的矩阵形式,实现了饱和砂土变形局部化的有限元数值计算,得到了饱和砂土发生局部化变形的准则。基于Galerkin 方法,得到了位移场和应力场的空间离散化矩阵方程;由土体局部变形带的连续性条件,引入第1切线算子,推导出了砂土等颗粒状媒介发生局部化变形的必要条件。基于此核心算法,编制了有限元计算程序,模拟了饱和砂土在不排水条件下平面压缩过程中剪切带的形成与发展;通过比较分析,研究了有限元网格粗细对于土体局部变形带的影响,结果表明,网格粗细的病态依赖只是微小的,它只与变形条带的宽度有关,对于土体所表现出来的其他力学特性没有影响。 相似文献
10.
不同应力路径下超固结黏土试样变形局部化分析 总被引：2，自引：1，他引：1
基于改进伏斯列夫面超固结黏土三维本构模型,利用有限元软件ABAQUS材料子程序接口,采用回映应力更新算法,实现了该模型在有限元分析中的应用。通过该模型与比奥固结理论的耦合,对超固结比为8的超固结黏土在三轴压缩、三轴伸长及平面应变应力条件下的变形局部化问题,进行了水-土耦合弹塑性有限元分析。分析结果表明：剪切带带内、带外点经历不同应力路径;剪切带带外单元经历了体缩、剪胀及被吸水体缩过程,而剪切带带内单元一直保持剪胀趋势;剪切带的形成伴随着剪胀,剪切带内、外出现了负的孔压,且孔压的分布也具有局部化特性。关于剪切带带内、外的孔隙水压及体变变化趋势与剪切速率有关,而平面应变介于三轴压缩与三轴伸长之间,但平面应变较早出现剪切带。孔隙水的迁移速度影响剪切带带内单元的剪胀,进而影响剪切带的形成及发展;而围压和弱单元位置也对剪切带的形成也有影响。 相似文献
11.
为探究挡墙前存在斜坡临空面条件下土体破坏与侧土压力特征,采用强度折减法研究了不同临坡距与嵌入深度下的挡墙前侧有限斜坡土体的破坏特征,并用水平层分析法与静力平衡法,推导了一种考虑斜坡坡度、临坡距及临空斜坡内土体层间剪切力的被动土压力理论计算公式。通过与室内试验、数值模型及其他计算理论对比,建议方法同模型试验、数值解及其他理论计算结果基本吻合,证明了建议方法对计算有限斜坡条件下被动土压力的有效性,最后分析了斜坡坡度、临坡距对被动土压力与临空斜坡土层中层间剪力的影响。研究表明:平动模式下的有限斜坡土体破坏面主要沿墙底与斜坡坡脚附近破坏,这与半无限空间条件的破坏特征明显不同;斜坡条件下的被动土压力随深度呈指数增加规律,且随临坡距减小与坡度增大,被动土压力均出现了一定程度的减小,其中临坡距为0时,被动土压力相比半无限空间条件时降低幅度达到30% ~50%;平动模式下的临空斜坡土体中的层间剪切系数为0.07~0.1;当墙背光滑且临坡距足够大时,建议方法可简化为理想条件的朗肯被动土压力公式。 相似文献
12.
An efficient finite–discrete element method applicable for the analysis of quasi‐static nonlinear soil–structure interaction problems involving large deformations in three‐dimensional space was presented in this paper. The present method differs from previous approaches in that the use of very fine mesh and small time steps was not needed to stabilize the calculation. The domain involving the large displacement was modeled using discrete elements, whereas the rest of the domain was modeled using finite elements. Forces acting on the discrete and finite elements were related by introducing interface elements at the boundary of the two domains. To improve the stability of the developed method, we used explicit time integration with different damping schemes applied to each domain to relax the system and to reach stability condition. With appropriate damping schemes, a relatively coarse finite element mesh can be used, resulting in significant savings in the computation time. The proposed algorithm was validated using three different benchmark problems, and the numerical results were compared with existing analytical and numerical solutions. The algorithm performance in solving practical soil–structure interaction problems was also investigated by simulating a large‐scale soft ground tunneling problem involving soil loss near an existing lining. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
13.
在土石坝工程中越来越重视岸坡与坝料之间的接触特性,将土体本构模型中使用较多的双曲线型硬化规律运用至接触面模型,提出了一个简洁的接触面本构模型,推导了接触面模型刚度矩阵表达式。通过对4组试验的预测结果与试验结果的对比,表明该模型可以较好地预测粗粒料的界面剪切试验。将提出的接触面模型嵌入有限元程序,应用于如美心墙坝河谷与坝料的接触分析。三维有限元计算结果表明：坝体两侧岸坡的剪切位移变化规律都是中间部分剪切滑移量较大,岸坡边缘部分相对较小,陡坡一侧最大滑移量大于较缓一侧。这些均符合粗粒料滑移的基本规律,可为岸坡?坝料接触特性研究以及土石坝工程计算提供参考。 相似文献
14.
Large deformation soil behavior underpins the operation and performance for a wide range of key geotechnical structures and needs to be properly considered in their modeling, analysis, and design. The material point method (MPM) has gained increasing popularity recently over conventional numerical methods such as finite element method (FEM) in tackling large deformation problems. In this study, we present a novel hierarchical coupling scheme to integrate MPM with discrete element method (DEM) for multiscale modeling of large deformation in geomechanics. The MPM is employed to treat a typical boundary value problem that may experience large deformation, and the DEM is used to derive the nonlinear material response from small strain to finite strain required by MPM for each of its material points. The proposed coupling framework not only inherits the advantages of MPM in tackling large deformation engineering problems over the use of FEM (eg, no need for remeshing to avoid mesh distortion in FEM), but also helps avoid the need for complicated, phenomenological assumptions on constitutive material models for soil exhibiting high nonlinearity at finite strain. The proposed framework lends great convenience for us to relate rich grain-scale information and key micromechanical mechanisms to macroscopic observations of granular soils over all deformation levels, from initial small-strain stage en route to large deformation regime before failure. Several classic geomechanics examples are used to demonstrate the key features the new MPM/DEM framework can offer on large deformation simulations, including biaxial compression test, rigid footing, soil-pipe interaction, and soil column collapse. 相似文献
15.
The driving response of thin‐walled open‐ended piles is studied using numerical simulation of the wave propagation inside the soil plug and the pile. An elastic finite element analysis is carried out to identify the stress wave propagation in the vicinity of the pile toe. It is found that the shear stress wave has the highest magnitude above the bottom of the soil plug. Below the bottom of the soil plug, the vertical stress wave has the highest magnitude. Although the shear stress wave propagating in the radial direction is similar in magnitude to the vertical stress wave at the bottom of the soil plug, it decays rapidly while travelling downwards. The highest vertical stress at the bottom of the soil plug appears after the vertical stress wave interacts with the shear stress wave travelling in the radial direction. Initially, the vertical stress wave propagates with the dilation wave velocity in both the radial and vertical directions. After it interacts with the shear stress wave, the vertical stress wave starts to propagate with the shear wave velocity in the radial direction and with the axial wave velocity downwards. It is concluded that at the bottom of the soil plug, the interaction between the waves travelling in radial and vertical directions is important. The capabilities of several one‐dimensional pile‐in‐pile models to reproduce the driving response given by a two‐dimensional axisymmetric finite element model is studied. It is seen that when the base of the soil plug fails, a one‐dimensional pile‐in‐pile model can be used to achieve results in agreement with the finite element model. However, when the pile is unplugged, where the base of the soil plug does not fail, a reduced finite element mesh that permits the radial wave propagation inside the soil plug must be used. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
16.
Vegetation contributes to weak soil stabilisation through reinforcement of the soil, dissipation of excess pore pressure and increasing the shear strength by induced matric suction. This paper describes the way vegetation influences soil matric suction, shrinkage and ground settlement in the vadose zone through transpiration. A mathematical model for the rate of root water uptake, including the root growth rate considering ground conditions, type of vegetation and climatic parameters, has been developed. A finite element approach is employed to solve the transient coupled flow-deformation equations. The finite element mesh is built using partially saturated soil elements capable of representing the salient aspects of unsaturated permeability and the soil water characteristic curve. The model formulation is based on the effective stress theory of unsaturated soils. Based on this proposed model, the distribution of the ground matric suction profile adjacent to the tree is numerically analysed. Current field measurements of soil matric suction and moisture content collected from Miram site located in Victoria State, Australia by the authors are compared with the numerical predictions. The results indicate that the proposed root water uptake model incorporated in the numerical analysis can be used for prediction of ground properties influenced by tree roots. 相似文献
17.
Capturing strain localization in reinforced soils 总被引：2，自引：1，他引：1
Lade’s single hardening soil model with Cosserat rotation embodied in the finite element method is employed to investigate
the behavior of geosynthetic reinforced soils with special attention to the development of shear banding. The ability of the
finite element model to detect shear banding in a reinforced soil is examined against three high quality small-scale laboratory
plane strain tests on Toyoura sand with and without reinforcement. These three tests were chosen because of the clear failure
surfaces that developed in the soil during loading. The FEM analyses were able to reasonably simulate the plane strain laboratory
tests including both unreinforced and reinforced cases. The FEM analyses gave reasonably good agreement with the experimental
results in terms of global stress–strain relationships and shear band occurrences. Furthermore, and based on FE analyses of
a hypothetical geosynthetic reinforced soil (GRS) retaining wall, it is shown that the geosynthetic reinforcements are very
effective in hindering the formation of shear bands in GRS retaining walls when small spacing between the reinforcement layers
was used. When used properly, the geosynthetic reinforcements made the soil behave as a truly reinforced mass of considerable
stiffness and strength. 相似文献
18.
In this work, the interface behavior between an infinite extended narrow granular layer and a rough surface of rigid body is investigated numerically, using finite element method in the updated Lagrangian (UL) frame. In this regard, the elasto‐plastic micro‐polar (Cosserat) continuum approach is employed to remove the limitations caused by strain‐softening of materials in the classical continuum. The mechanical properties of cohesionless granular soil are described with Lade's model enhanced by polar terms, including Cosserat rotations, curvatures, and couple stresses. Furthermore, the mean grain diameter as the internal length is incorporated into the constitutive relations accordingly. Here, the evolution and location of shear band, within the granular layer in contact with the rigid body, are mainly focused. In this regard, particular attention is paid to the effects of homogeneous distribution and periodic fluctuation of micro‐polar boundary conditions, prescribed along the interface. Correspondingly, the effects of pressure level, mean grain diameter, and stratified soil are also considered. The finite element results demonstrate that the location and evolution of shear band in the granular soil layer are strongly affected by the non‐uniform micro‐polar boundary conditions, prescribed along the interface. It is found that the shear band is located closer to the boundary with less restriction of grain rotations. Furthermore, the predicted thickness of shear band is larger for higher rotation resistance of soil grains along the interface, larger mean grain diameter, and higher vertical pressure. Regarding the stratified soil, comprising a thin layer with slightly different initial void ratio, the shear band moves towards the layer with initially higher void ratio. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
19.
The flow stress in the yield surface of plastic constitutive equation is modified with a higher order gradient term of the effective plastic strain to model the effect of inhomogeneous deformation in granular materials. The gradient constitutive model has been incorporated into the finite element code ABAQUS and used to simulate biaxial shear tests on dry sand. It is shown that the shape of the post-peak segment of the load displacement curve predicted by the numerical analysis is dependent on the mesh size when gradient term is not used. Use of an appropriate gradient coefficient is shown to correct this and predict a unique shape of the load displacement curve regardless of the mesh size. The gradient coefficient required turns out to be approximately inversely proportional to the mesh elemental area. Use of the strain gradient term is found to diffuse the concentration of plastic strains within shear band resulting in its consistent width. The coefficient of the higher gradient term appears as a function of the grain size, the mean confining stress, and the plastic softening modulus. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
20.
Passive earth pressure calculations in geotechnical analysis are usually performed with the aid of the Rankine or Coulomb theories of earth pressure based on uniform soil properties. These traditional earth pressure theories assume that the soil is uniform. The fact that soils are spatially variable leads to two potential problems in design: do sampled soil properties adequately reflect the effective properties of the entire soil mass and does spatial variability in soil properties lead to passive earth pressures that are significantly different from those predicted using traditional theories? This paper combines non-linear finite element analysis with random field simulation to investigate these two questions. The specific case investigated is a two-dimensional frictionless passive wall with a cohesionless drained soil mass. The wall is designed against sliding using Rankine's earth pressure theory. The unit weight is assumed to be constant throughout the soil mass and the design friction angle is obtained by sampling the simulated random soil field. For a single sample, the friction angle is used as an effective soil property in the Rankine model. For two samples, an average of the sampled friction angles is used. Failure is defined as occurring when the Rankine predicted passive resistance acting on the wall, modified by a factor of safety, is greater than that computed by the random finite element method. Using Monte Carlo simulation, the probability of failure of the traditional design approach is assessed as a function of the factor of safety using and the spatial variability of the soil. 相似文献