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The concurrent multiscale method, which couples the discrete element method (DEM) for predicting the local micro‐scale evolution of the soil particle skeleton with the finite element method (FEM) for estimating the remaining macro‐scale continuum deformation, is a versatile tool for modeling the failure process of soil masses. This paper presents the separate edge coupling method, which is degenerated from the generalized bridging domain method and is good at eliminating spurious reflections that are induced by coupling models of different scales, to capture the granular behavior in the domain of interest and to coarsen the mesh to save computational cost in the remaining domain. Cundall non‐viscous damping was used as numerical damping to dissipate the kinetic energy for simulating static failure problems. The proposed coupled DEM–FEM scheme was adopted to model the wave propagation in a 1D steel bar, a soil slope because of the effect of a shallow foundation and a plane‐strain cone penetration test (CPT). The numerical results show that the separate edge coupling method is effective when it is adopted for a problem with Cundall non‐viscous damping; it qualitatively reproduces the failure process of the soil masses and is consistent with the full micro‐scale discrete element model. Stress discontinuity is found in the coupling domain. Copyright © 2017 John Wiley & Sons, Ltd. 相似文献
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A coupled 3‐dimensional bonded discrete element and lattice Boltzmann method for fluid‐solid coupling in cohesive geomaterials 
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下载免费PDF全文 This paper presents a 3D bonded discrete element and lattice Boltzmann method for resolving the fluid‐solid interaction involving complicated fluid‐particle coupling in geomaterials. In the coupled technique, the solid material is treated as an assembly of bonded and/or granular particles. A bond model accounting for strain softening in normal contact is incorporated into the discrete element method to simulate the mechanical behaviour of geomaterials, whilst the fluid flow is solved by the lattice Boltzmann method based on kinetic theory and statistical mechanics. To provide a bridge between theory and application, a 3D algorithm of immersed moving boundary scheme was proposed for resolving fluid‐particle interaction. To demonstrate the applicability and accuracy of this coupled method, a benchmark called quicksand, in which particles become fluidised under the driving of upward fluid flow, is first carried out. The critical hydraulic gradient obtained from the numerical results matches the theoretical value. Then, numerical investigation of the performance of granular filters generated according to the well‐acknowledged design criteria is given. It is found that the proposed 3D technique is promising, and the instantaneous migration of the protected soils can be readily observed. Numerical results prove that the filters which comply with the design criteria can effectively alleviate or eliminate the appearance of particle erosion in dams. 相似文献
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Rock failure is observed around boreholes often with certain types of failure zones, which are called breakouts. Laboratory‐scale drilling tests in some high‐porosity quartz‐rich sandstone have shown breakouts in the form of narrow localized compacted zones in the minimum horizontal stress direction. They are called fracture‐like breakouts. Such compaction bands may affect hydrocarbon extraction by forming barriers that inhibit fluid flow and may also be a source of sand production. This paper presents the results of numerical simulations of borehole breakouts using 3D discrete element method to investigate the mechanism of the fracture‐like breakouts and to identify the role of far‐field stresses on the breakout dimensions. The numerical tool was first verified against analytical solutions. It was then utilized to investigate the failure mechanism and breakout geometry for drilled cubic rock samples of Castlegate sandstone subjected to different pre‐existing far‐field stresses. Results show that failure occurs in the zones of the highest concentration of tangential stress around the borehole. It is concluded that fracture‐like breakout develops as a result of a nondilatant failure mechanism consisting of localized grain debonding and repacking and grain crushing that lead to the formation of a compaction band in the minimum horizontal stress direction. In addition, it is found that the length of fracture‐like breakouts depends on both the mean stress and stress anisotropy. However, the width of the breakout is not significantly changed by the far‐field stresses. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
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Erfan G. Nezami Youssef M. A. Hashash Dawei Zhao Jamshid Ghaboussi 《国际地质力学数值与分析法杂志》2006,30(8):783-801
With the increasing demand for discrete element simulations with larger number of particles and more realistic particle geometries, the need for efficient contact detection algorithms is more evident. To date, the class of common plane (CP) methods is among the most effective and widely used contact detection algorithms in discrete element simulations of polygonal and polyhedral particles. This paper introduces a new approach to obtain the CP by employing a newly introduced concept of ‘shortest link’. Among all the possible line segments that connect any point on the surface of particle A to any point on the surface of particle B, the one with the shortest length defines the shortest link between the two particles. The perpendicular bisector plane of the shortest link fulfils all the conditions of a CP, suggesting that CP can be obtained by seeking the shortest link. A new algorithm, called shortest link method (SLM), is proposed to obtain the shortest link and subsequently the CP between any two polyhedral particles. Comparison of the analysis time between SLM and previously introduced algorithms demonstrate that SLM results in a substantial speed up for polyhedral particles contact detection. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
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A succinct 3D discrete element model, with clumps to resemble the real shapes of granular materials, is developed. The quaternion method is introduced to transform the motion and force of a clump between local and global coordinates. The Hertz–Mindlin elastic contact force model, incorporated with the nonlinear normal viscous force and the Mohr–Coulomb friction law, is used to describe the interactions between particles. The proposed discrete element model is used to simulate direct shear tests of the irregular limestone rubbles. The simulation results of vertical displacements and shear stresses with a mixture of clumps are compared well with that of laboratory tests. The bulk friction coefficients are calculated and discussed under different contact friction coefficients and normal stresses. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
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The ability of discrete element models to describe quantitatively (and not only qualitatively) the constitutive behaviour of a dense sand is assessed in this paper. Two kinds of 3D discrete models are considered. Both consider spheres as elementary particles. Nevertheless, the first model implements a contact law with rolling resistance whereas the second takes into account clumps made of two spheres. The discrete models are calibrated and validated from mechanical tests performed on a dense Hostun sand with a true triaxial apparatus. The calibration is carried out from axisymmetric drained compression tests, while the validation is discussed from monotonic and cyclic stress proportional loading paths and from a circular stress path in the deviatoric stress plane. The quality of the predictions of the discrete models are evaluated by comparison with the predictions given with advanced phenomenological constitutive relations, mainly an incrementally non-linear relation. Predictions given by the discrete models are remarkable, particularly when it is put in perspective with respect to the very few number of mechanical tests required for their calibration. However, these results and conclusions were reached in enabling conditions, and some limitations of such discrete models should be kept in mind. 相似文献
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Numerical simulation of desiccation cracking in a thin clay layer using 3D discrete element modeling
Desiccation cracking of clay soil is of critical importance in many applications, such as industrial waste containment, hydraulic barriers, road embankments, and agricultural operations. The factors that influence cracking are known qualitatively, but it is not clear how to predict the initiation and propagation of cracks. This study presents a discrete element approach to modeling desiccation cracking in thin clay layers, considering material property changes. First, an aggregate shrinkage model based on the aggregate structure of clay was proposed, and the drying shrinkage of clay soil was modeled by imposing drying shrinkage kinetics for each aggregate at the micro-scale. Second, the clay soil was represented by an assembly of aggregates linked by bonds, and desiccation cracking of the clay layer was modeled using a three-dimensional discrete element code (PFC3D), with the aid of the embedded programming language FISH. When the clay layer is sufficiently thin, the water content gradient along the section can be neglected; thus, the shrinkage kinetics are the same for all of the grains of clay. In the model based on the discrete element method (DEM), the bond strength and contact stiffness changed during drying. Their changes were determined by matching the simulation results with the experimental data. Third, the DEM approach was validated by reproducing experimental desiccation tests performed on a thin clay layer in a disk shape. The geometric parameters of surface cracks were quantified using image analysis techniques and were compared with experimental observations. Fourth, some factors of influence, such as the sample thickness, the properties of the soil–base interface, micro-mechanical parameters, and shrinkage parameters, were investigated using the DEM model. The results obtained from the DEM analyses were compared with the results of prior research in this field of study. The approach used in this study is very promising for simulating desiccation cracking in thin clay soil because the model captures the initiation and propagation mechanism of desiccation cracks. Although this study was carried out on surface cracking in a thin clay layer, the extension of this methodology is of potential benefit not only for predicting three-dimensional desiccation cracking in real clay liners but also for modeling cracking in other materials with properties that vary with water content or temperature, such as concrete and rock. 相似文献
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A novel three‐dimensional particle‐based technique utilizing the discrete element method is proposed to analyze the seismic response of soil‐foundation‐structure systems. The proposed approach is employed to investigate the response of a single‐degree‐of‐freedom structure on a square spread footing founded on a dry granular deposit. The soil is idealized as a collection of spherical particles using discrete element method. The spread footing is modeled as a rigid block composed of clumped particles, and its motion is described by the resultant forces and moments acting upon it. The structure is modeled as a column made of particles that are either clumped to idealize a rigid structure or bonded to simulate a flexible structure of prescribed stiffness. Analysis is done in a fully coupled scheme in time domain while taking into account the effects of soil nonlinear behavior, the possible separation between foundation base and soil caused by rocking, the possible sliding of the footing, and the dynamic soil‐foundation interaction as well as the dynamic characteristics of the superstructure. High fidelity computational simulations comprising about half a million particles were conducted to examine the ability of the proposed technique to model the response of soil‐foundation‐structure systems. The computational approach is able to capture essential dynamic response patterns. The cyclic moment–rotation relationships at the base center point of the footing showed degradation of rotational stiffness by increasing the level of strain. Permanent deformations under the foundation continued to accumulate with the increase in number of loading cycles. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
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应用离散元强度折减对复杂边坡进行稳定性分析 总被引：1，自引：0，他引：1
介绍了离散元（DEM）及强度折减法基本原理,利用3DEC软件结合强度折减法对某水电站高边坡进行稳定性分析。对岩体强度参数进行折减,以关键点位移与时间关系曲线发散时的折减系数作为边坡安全系数,根据位移矢量图确定滑面和破坏形态。通过与Dijkstra极限平衡有限元法及Sarma法的计算结果对比,验证该方法的可靠性。在二维计算基础上建立并简化三维模型,模拟边坡三维应力场。结合工程关注区域的应力及位移趋势,采用3DEC中的辅助节理截取典型坡段单宽模型,在已有的三维应力场的基础上对其进行强度折减,弥补了二维强度折减未考虑三维应力场对边坡稳定性影响的不足,把握了工程重点及力学特性,为类似工程提供了一种合理高效的稳定性分析手段。 相似文献
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A three-dimensional Voronoi tessellation model based on the distinct element method (DEM) is proposed to model the representative part of the microstructures of granular brittle rocks. Regularization is employed to decrease the frequency of polyhedrons with large edge ratio and contributes to a higher efficiency for element meshing. Sensitivity analyses are performed for a series of micro contact parameters in accordance with the macro responses observed in laboratory experiments (e.g. the uniaxial compression test, the Brazilian disc test and the triaxial compression test). Verifications by simulating the spalling test and plate impact test indicate that the 3D polycrystalline discrete element method (3PDEM) can be employed for efficiently simulating nonlinear mechanical behaviors, large deformation, strain softening and rock dynamics. 相似文献
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Coupling of solid deformation and pore pressure for undrained deformation—a discrete element method approach 下载免费PDF全文
下载免费PDF全文 This paper presents a numerical scheme for fluid‐particle coupling that uses the discrete element method by taking into consideration solid deformation and pore pressure generation. A new water particle element is introduced to calculate pore water pressure due to porosity changes. The water particle element has the same size and shape as the solid element and experiences the same amount of deformation. On the basis of the effective stress principle at the element contact, the total force is equal to the sum of the force transmitted through the solid element contact and the water particle force due to pore water pressure. Analytical solutions of traditional soil mechanics problems, such as isotropic compression and consolidated triaxial undrained test, are used to quantitatively validate the proposed model. The numerical results show good agreement between the model and the analytical solutions. The model therefore provides an effective method to calculate pore pressure in a porous medium in discrete modeling. 相似文献
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We investigate the quasi‐static simple shear flow of a two‐dimensional assembly of cohesionless particles using discrete element method (DEM) simulations. We focus on the unsteady flow regime where the solid would experience significant evolution of stresses, mobilised shear strength and dilation. We construct the DEM model using a discretised‐wall confined granular cell where the apparent boundary is allowed to dilate or contract synchronously with the confined solid. A rather uniform simple shear field is achieved across the whole assembly, which benefits rheological studies in generalising constitutive laws for continuum methods. We examine two aspects of the simple shear behaviour: macroscopic stress and strain rate evolution, particularly the non‐coaxiality between the principal directions of the two; and micromechanics such as evolution of fabric. For an initially anisotropic specimen sheared under constant normal pressure, the direction of principal stress rotates towards that of the principal strain rate, gradually reducing the degree of non‐coaxiality from about 45° to fluctuating around 0°. The rate in approaching coaxiality is slower in samples with larger initial porosity, stress ratio and mean stress. Generally, a faster rate in approaching coaxiality in simple shear is observed in a more dilatant sample, which often shows a larger degree of mobilised fabric anisotropy, suggesting the possible important role of instantaneous internal friction angle. The evolution of principal fabric direction resembles that of the principal stress direction. © 2013 The Authors. International Journal for Numerical and Analytical Methods in Geomechanics published by John Wiley & Sons, Ltd. 相似文献
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当前盐岩的宏观力学模型通常是唯象模型,不能很好地解释盐岩受力变形破坏的真正物理基础。盐岩是由于化学沉积而形成的矿物集合体,是一种主要由NaCl和少量杂质组成的多晶体,其变形机制主要由晶粒与晶界的力学特性控制。通过扫描电镜（SEM）,获得盐岩晶粒的微细观结构特征,采用分子动力学方法和纳米压痕技术,确定盐岩晶粒和晶界的微细观力学参数;将盐岩晶粒作为块体,基于Voronoi多边形技术,建立盐岩的微细观数值模型;利用离散元方法,对盐岩试件在单轴压缩和直剪条件下的宏观力学行为进行了数值模拟。数值模拟结果与宏观力学试验结果吻合度高,表明基于盐岩微细观晶粒结构特征并结合离散元数值模拟的方法能够较好地研究盐岩的宏观力学性能及其材料物理基础。 相似文献
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Antoinette Tordesillas Sebastian Pucilowski David M. Walker John F. Peters Laura E. Walizer 《国际地质力学数值与分析法杂志》2014,38(12):1247-1275
Recent analysis of data from triaxial tests on sand and discrete element simulations indicate the final pattern of failure is encoded in grain motions during the nascent stages of loading. We study vortices that are evident from grain displacements at the start of loading and bear a direct mathematical connection to boundary conditions, uniform continuum strain and shear bands. Motions of three grains in mutual contact, that is, 3‐cycles, manifest vortices. In the initial stages of loading, 3‐cycles initiate a rotation around a region Ω* where the shear band ultimately develops. This bias sets a course in 3‐cycle evolution, determining where they will more likely collapse. A multiscale spatial analysis of 3‐cycle temporal evolution provides quantitative evidence that the most stable, persistent 3‐cycles degrade preferentially in Ω*, until essentially depleted when the shear band is fully formed. The transition towards a clustered distribution of persistent 3‐cycles occurs early in the loading history—and coincides with the persistent localisation of vortices in Ω*. In 3D samples, no evidence of spatial clustering in persistent 3‐cycle deaths is found in samples undergoing diffuse failure, while early clustering manifests in a sample that ultimately failed by strain localisation. This study not only delivered insights into the possible structural origins of vortices in dense granular systems but also a tool for the early detection of the mode of failure—localised versus diffuse—a sample will ultimately undergo. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
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The discrete element method (DEM) is crucial in investigating and modeling the elementary behavior of granular materials under varying loading conditions, especially those that cannot be adequately investigated via conventional laboratory testing. However, the application of the DEM in simulations that involve complex loading paths under undrained conditions is scarce, primarily owing to the inability to maintain a constant-volume condition. This paper presents a unified discrete element approach that can apply arbitrary loading paths while satisfying the equivalent undrained condition. The proposed method comprises two parts: (1) a novel strategy that determines the virtual pore pressure under complex undrained loading conditions, and (2) an advanced undrained servomechanism that can simultaneously control each stress component independently. Numerical algorithms corresponding to three new undrained loading paths, that is, true triaxial test, rotational shear, and traffic loading path that have never been simulated using DEM are successfully implemented in a unified manner. Macroscale simulation results under these loading paths are qualitatively in good agreement with their experimental counterparts, thereby confirming the practicality and robustness of the proposed approach. Furthermore, in-depth discussions on the DEM results from these three new loading paths are presented from microscopic perspective. 相似文献
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Earthquakes are recognized as resulting from a stick–slip frictional instability along faults. Based on the node‐to‐point contact element strategy (an arbitrarily shaped contact element strategy applied with the static‐explicit algorithm for modelling non‐linear frictional contact problems proposed by authors), a finite element code for modelling the 3‐D non‐linear friction contact between deformable bodies has been developed and extended here to analyse the non‐linear stick–slip frictional instability between deformable rocks with a rate‐ and state‐dependent friction law. A typical fault bend model is taken as an application example to be analysed here. The variations of the normal contact force, the frictional force, the transition of stick–slip instable state and the related relative slip velocity along the fault between the deformable rocks and the stress evolution in the total bodies during the different stages are investigated, respectively. The calculated results demonstrate the usefulness of this code for simulating the non‐linear frictional instability between deformable rocks. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献