Degradation of a Granular Base under a Flexible Pavement: DEM Simulation

Flexible pavements are composed by an asphalt concrete layer, granular base and subbase layers, and a natural subgrade. The granular materials forming part of the granular layers are subjected to static and dynamic loads during their engineering life. As a result of these loads particle crushing may occur depending on the strength of the particles forming the granular layers. Particle crushing is important since it is associated with several detrimental effects such as settlements and a reduction in hydraulic conductivity. A computer simulation using the discrete element method (DEM) is presented in order to understand and visualize how crushing initiates and develops inside a simulated pavement structure.     

Experimental and Numerical Study of Railway Ballast Behavior under Cyclic Loading

This paper presents the results of the influence of frequency on the permanent deformation and degradation behavior of ballast during cyclic loading. The behavior of ballast under numerous cycles was investigated through a series of large-scale cyclic triaxial tests. The tests were conducted at frequencies ranging from 10–40 Hz, which is equivalent to a train traveling from 73 km/h to 291 km/h over standard gauge tracks in Australia. The results showed that permanent deformation and degradation of ballast increased with the frequency of loading and number of cycles. Much of breakage occurs during the initial cycle; however, there exists a frequency zone of 20?Hz ? f ? 30?Hz where cyclic densification takes place without much additional breakage. An empirical relationship among axial strain, frequency and number of cycles has been proposed based on the experimental data. In addition, discrete-element method (DEM) simulations were carried out using PFC2D on an assembly of irregular shaped particles. A novel approach was used to model a two-dimensional (2D) projection of real ballast particles. Clusters of bonded circular particles were used to model a 2D projection of angular ballast particles. Degradation of the bonds within a cluster was considered to represent particle breakage. The results of DEM simulations captured the ballast behavior under cyclic loading in accordance with the experimental observations. Moreover, the evolution of micromechanical parameters such as a distribution of the contact force and bond force developed during cyclic loading was presented to explain the mechanism of particle breakage. It has been revealed that particle breakage is mainly due to the tensile stress developed during cyclic loading and is located mainly in the direction of the movement of ballast particles.     

Time Effects Relate to Crushing in Sand

Based on previously obtained experimental results, a mechanistic picture of time effects in granular materials is presented. Accordingly, time effects are caused by grain crushing, which in turn is time dependent, as indicated by static fatigue of brittle materials. Triaxial compression tests have been performed on Virginia Beach sand at high pressures, where grain crushing is prevalent, to study effects of initial loading strain rates on subsequent amounts of creep and stress relaxation. Grain size distribution curves were determined after each test and the amount of crushing, as characterized by Hardin’s breakage factor, is related to the energy input to the triaxial specimens. A pattern emerges that indicates the importance of crushing for the axial and volumetric strains, while rearrangement and frictional sliding between intact grains play much smaller roles in the stress-strain and volume change behaviors of granular materials at high stresses and shear strains. Because particle crushing is a time-dependent phenomenon described as static fatigue or delayed fracture, the close relation between time effects and crushing in granular materials is established.     

DEM Simulation of Particle Damage in Granular Media — Structure Interfaces

Using the discrete element method (DEM) with clustering, a novel means of numerically modeling damage of particles is presented. Damage, such as grain crushing, is treated by allowing clusters to break apart according to a failure criterion based upon sliding work. If the accumulated work done on an individual DEM particle of a cluster exceeds a threshold, that particle is allowed to break from the cluster. A value for the critical energy density is determined by comparing the degree of particle breakage from numerical simulations to data from laboratory tests. Numerical simulations were also conducted to determine the impact of particle damage on interface behavior. It was found that a very distinct shear zone was evident when particle damage was considered and that this occurred without significant reduction of the maximum shear strength of the medium. Also, the degree of damage was shown to be related to the angularity of the clusters.     

3D Shape Characterization and Image-Based DEM Simulation of the Lunar Soil Simulant FJS-1

This paper describes a procedure used to characterize the three-dimensional (3D) grain shape of lunar soil and undertake simulations of lunar soil by image-based discrete element method (DEM). Given that detailed 3D grain-shape information is unavailable for real lunar soil, a simulant material, FJS-1, is used in this study. We use the high-resolution micro X-ray CT system at SPring-8, a synchrotron radiation facility in Japan, to visualize precise 3D images of the granular assembly of FJS-1. A newly developed image-analysis procedure is then applied to identify individual grains. Using the obtained grain-shape data, a sufficient number of FJS-1 grains are directly modeled for DEM simulation using an efficient modeling scheme. A series of particle flow simulations are then performed with the modeled grains. The resulting slope angles are in good agreement with experimental results. We discuss the effect on the slope angle of grain parameters such as contact stiffness, restitution coefficient, and interparticle friction.     

Characterization of Fines Produced by Sand Crushing

Sand particle crushing generates coarse fragments with size d ≥ 75?μm and fine fragments, i.e., “fines,” with size d<75?μm. Yet, postcrushing fines are seldom characterized due to testing constraints. An experimental study was conducted to examine the size distribution evolution of fine fragments generated by crushing two uniform sands with contrasting degrees of mineral composition heterogeneity, in one-dimensional compression. The determination of fine fragment sizes was made possible by using a particle size analyzer that employs a small sample. The results indicate that the degree of mineral composition heterogeneity affects the load–deformation behavior of crushing sands and the resulting amounts and size distribution evolutions of the produced coarse and fine fragments. In particular, the trends gathered suggest that fines generation occurs by abrasion of parent particles, coarse fragment breakage, and subsequent breakage of fine fragments with sizes larger than the comminution limit.     

Micromechanical Analysis of the Shear Behavior of a Granular Material

A distinct element analysis of the behavior of a granular material was performed by simulating direct shear tests of a dense and a loose 2D sample of 1,050 cylinders. Macroscopic results exhibit typical features of the shear response of granular materials: a perfect plasticity state that does not depend on the initial density, a peak stress and a dilatant behavior in the case of the dense sample, and a contractant behavior of the loose sample. A micromechanical analysis of the shear behavior was carried out based on the simulation results. Using the particle displacements and rotations, a shear band is located within the sample. Special attention is focused on the evolution of particle∕particle contact orientation as well as on the direction of particle∕particle contact forces. The shear process induces a clear change of contact and contact force orientations. A strong correlation between the induced anisotropy of the microstructure and the macroscopic loading is evident in the simulation results.     

Lateral Resistance of Short Single Piles and Pile Groups Located Near Slopes

Many transmission towers, high-rise buildings, and bridges are constructed near steep slopes and are supported by large-diameter piles. These structures may be subjected to large lateral loads, such as violent winds and earthquakes. Widely used types of foundations for these structures are pier foundations, which have large diameter with high stiffness. The behavior of a pier foundation subjected to lateral loads is similar to that of a short rigid pile, because both elements seem to fail by rotation developing passive resistance on opposite faces above and below the rotation point, unlike the behavior of a long flexible pile. This paper describes the results of several numerical studies performed with a three-dimensional finite-element method (FEM) of model tests and a prototype test of a laterally loaded short pile and pier foundation located near slopes, respectively. Initially, in this paper, the results of model tests of single piles and pile groups subjected to lateral loading, in homogeneous sand with 30° slopes and horizontal ground were analyzed by the three- dimensional (3D) finite-element (FE) analyses. Furthermore, field tests of a prototype pier foundation subjected to lateral loading on a 30° slope was reported. The FE analyses were conducted to simulate these results. The main purpose of this paper is the validation of the 3D elasto–plastic FEM by comparisons with the experimental data.     

离散元法在磨矿设备及参数优化研究中的应用现状

介绍了离散元法（DEM）在磨矿领域的应用背景；解释了DEM中Hertz-Mindlin 接触模型和颗粒黏结模型的基本原理；分类综述了DEM在球磨机、搅拌磨机和自磨机/半自磨机等三类磨矿设备及参数优化研究中的应用现状；指出了DEM在磨矿领域研究中的独特优势及其发展方向。      

Full-Scale Field Load Testing of Storm-Water Storage Chamber Structures

One of the latest solutions available for storm-water storage purposes is the underground chamber structure. Recently, a research team at Ohio University conducted a series of field load tests on the underground chambers. Four chamber structures were placed side by side in an excavated area, backfilled with coarse granular soil, buried under a soil cover of 0.46?m (18?in.), instrumented with sensors, and subjected to a series of controlled live load tests at a field project site. The sensor readings and visual inspection results indicate that the chamber with the specified minimum soil cover had no problem in supporting a wheel load of 52.3?kN (11.8?kips) and an axle load of 109.0?kN (24.5?kips) in both the transverse and longitudinal directions. Maximum reduction in the rise dimension was only approximately 2.3% when the chambers were subjected to the live loads. The vertical soil pressure readings measured at the chamber crowns were within 15% of the values given by a live load spreading formula included in the AASHTO LRFD specifications.     

Exact Buckling Solutions For Rectangular Plates Under Intermediate and End Uniaxial Loads

This paper is concerned with the elastic buckling of rectangular plates subjected to both intermediate and end uniaxial loads. The rectangular plates have two simply supported opposite edges that are perpendicular to the in-plane load direction, while the other two plate edges can have free, simply supported, or clamped edges. The solution procedure involves the use of the Levy approach, the domain decomposition technique, and the state-space concept. The method furnishes exact stability criteria; samples of which are presented in a graphical form for plates with various boundary conditions. These results will be useful to engineers who design plates (or walls) that support intermediate floors.     

Triaxial Load Testing of Metal and FRP Roof-to-Wall Connectors

The allowable capacity of conventional roof-to-wall metal connectors is based on results of unidirectional component tests that do not simulate triaxial aerodynamic loading effects induced by high-wind events. The results of wind and wind-driven rain tests conducted at a full-scale facility were used to create a database on aerodynamic and aerohydrodynamic load effects on roof-to-wall connectors. Based on these results, three axial mean force components (triaxial mean loads) were combined into a series of resultant mean force vectors. A new test protocol was then developed for roof-to-wall connectors under simulated triaxial loading as opposed to simple uniaxial loading. The findings confirm that current testing methods tend to overestimate the actual load capacities of metal connectors. The performance of a nonintrusive roof-to-wall connector system using fiber-reinforced polymer (FRP) ties was also tested and compared with that of a traditional metal connector under simulated aerodynamic loads. The test results demonstrated the validity of FRP ties as an alternative to hurricane clips for use in timber roof-to-wall connection systems.     

Quantification and Simulation of Particle Kinematics and Local Strains in Granular Materials Using X-Ray Tomography Imaging and Discrete-Element Method

Microfeatures of granular materials have significant effects on their macrobehaviors. Unfortunately, three-dimensional (3D) quantitative measurements of microfeatures are rare in literature because of the limitations of conventional techniques in obtaining microquantities such as microdisplacements and local strains. This paper presents a new method for quantifying the particle kinematics and local strains for a soft confined compression test using X-ray computed tomography and compares the experimental measurements with the simulated results using the discrete-element method (DEM). The experimental method can identify and recognize 3D individual particles automatically, which is essential for quantifying particle kinematics and local strains. 3D DEM simulations of the soft confined compression test were performed by using spherical particles and irregular particles. The simulated global deformations and particle translations that were based on irregular particles showed better agreement with the experimental measurements than those that were based on spherical particles. The simulated movements of spherical particles were more erratic, and the material composed of spherical particles showed larger vertical contraction and radial dilation.     

Wedge Failure Analysis of Soil Resistance on Laterally Loaded Piles in Clay

A fundamental study of pile-soil systems subjected to lateral loads in clay soil was conducted by using experimental tests and a lateral load-transfer approach. The emphasis was on an improved wedge failure model developed by considering three-dimensional combination forces and a new hyperbolic p-y criterion. A framework for determining the p-y curve on the basis of both theoretical analysis and experimental load test results is proposed. The proposed p-y method is shown to be capable of predicting the behavior of a large-diameter pile under lateral loading. The proposed p-y curves with an improved wedge model are more appropriate and realistic for representing a pile-soil interaction for laterally loaded piles in clay than the existing p-y method.     

Deflection of Partial Ring Segments Subjected to Concentrated Transverse or Radial Inward Forces

Exact analytical solutions, based on the Hamilton principle, are obtained for the response of supported partial ring segments subjected to concentrated loads. Loading may be applied in the lateral direction, perpendicular to the plane of the ring, or radially, in the plane of the ring. Results of a number of experimental tests are included. Very good agreement between theory and experiment was obtained. It is shown that the theoretical model is easy to apply. There are a number of industrial applications for this solution technique, some of which are discussed.     

Micromechanical Modeling of the Viscoelastic Behavior of Asphalt Mixtures Using the Discrete-Element Method

This paper presents a methodology for analyzing the viscoelastic response of asphalt mixtures using the discrete-element method (DEM). Two unmodified (neat) and seven modified binders were mixed with the same aggregate blend in order to prepare the nine hot mix asphalt (HMA) mixtures used in this study. The HMA microstructure was captured using images of vertically cut sections of specimens. The captured grayscale images were processed into black and white images representing the mastic and the aggregate phases, respectively. These microstructure images were used to represent the DEM model geometry. Rheological data for the nine binders were obtained using the dynamic shear rheometer. These data were used to estimate the parameters of the viscoelastic contact models that define the interaction among the mix constituents. The DEM models were subjected to sinusoidal loads similar to those applied in the simple performance test (SPT). The DEM model predictions compared favorably with the SPT measurements. However, the simulation results tended to overpredict the dynamic modulus, E*, for mixtures made with neat binders and underpredict E* for those that consisted of modified binders. The DEM models gave mix phase angles, ?mix, higher than the experimental measurements.     

Identification and Validation of a Discrete Element Model for Concrete

The use of a three-dimensional discrete element method (DEM) is proposed to study concrete structures submitted to dynamic loading. The aim of this paper is to validate the model first in the quasistatic domain, and second in dynamic compression, at the sample scale. A particular growing technique is used to set a densely packed assembly of arbitrarily sized spherical particles interacting together, representing concrete. An important difference from classical DEMs where only contact interactions are considered, is the use of an interaction range. First, the correct identification of parameters of the DEM model to simulate elastic and nonlinear deformation including damage and rupture is made through quasistatic uniaxial compression and tension tests. The influence of the packing is shown. The model produces a quantitative match of strength and deformation characteristics of concrete in terms of Young’s modulus, Poisson’s coefficient, and compressive and tensile strengths. Then, its validity is extended through dynamic tests. The simulations exhibit complex macroscopic behaviors of concrete, such as strain softening, fractures that arise from extensive microcracking throughout the assembly, and strain rate dependency.     

初始温度条件下全尾胶结膏体损伤本构模型

膏体充填料到达采场初始温度不同是矿山存在的普遍现象，不同初始温度条件下膏体力学特性及应力-应变关系直接影响到矿山采充周期及相邻采场开采时贫化指标.通过对初始温度为2、20、35和50℃的硬化膏体进行单轴抗压强度试验，获得不同初始温度下充填体应力-应变演化曲线.根据理论推导和试验结果，建立了不同初始温度下膏体损伤本构模型，通过本构模型参数回归，提出膏体温度-时间耦合损伤本构模型.最后，采用Comsol数值模拟软件，将温度-时间耦合损伤本构模型嵌入solid mechanics模块，对单轴抗压试验进行数值模拟，模拟应力-应变曲线与试验结果较为吻合，验证了所提出本构模型的可靠性.      

烧结矿颗粒DEM参数的测定与标定

摘要：为了获取烧结矿颗粒在离散元法中所涉及的关键运动参数,通过实验确定了不同粒径范围的颗粒密度、弹性模量、泊松比、弹性恢复系数以及静摩擦因数。基于Hertz Mindlin接触模型,通过实验和离散单元法仿真对比堆积的静止角,得到了适用于球形颗粒模拟状态下,烧结矿的滚动摩擦因数。结果表明：烧结矿密度、弹性模量及颗粒间动摩擦因数与粒径大小近似正线性相关;颗粒间弹性恢复系数与静摩擦因数随粒径增大逐渐减小。通过转鼓实验,对比了混合粒径状态下模拟和实验形成的自由表面,表明实验结果与计算结果吻合较好,误差在5%以内,进而表明所测离散单元参数准确程度较高,可用于后续烧结矿布料及余热竖罐内下移的模拟研究。     

Breakage Prediction of Laminated Glass Using the “Sacrificial Ply” Design Concept

Laminated architectural glass has proven to be well suited for use in glazing systems that must resist wind-borne debris impacts. When the inner glass ply in a laminated window unit remains unbroken after wind-borne debris impacts on the outer glass ply, the integrity of the building envelope is preserved. A mechanics-based analytical model is developed to predict the cumulative probability of inner glass ply breakage in laminated architectural glass subjected to simulated wind-borne debris impacts on the outer glass ply. A nonlinear dynamic finite-element analysis is employed to compute stresses in each layer of the laminate due to impact. Based on the cumulative damage theory, the two-parameter Weibull distribution is used to characterize the cumulative probability of inner glass ply breakage. The analytical predictive model is calibrated using available experimental data on material parameters. Cumulative probabilities of inner glass ply breakage predicted by the analytical model are in agreement with the corresponding experimental data.