Dry granular masses impacting on rigid obstacles: numerical analysis and theoretical modelling

Acta Geotechnica - The assessment of the time evolution of the impact force exerted by dry flowing masses on rigid obstacles is mandatory for the dynamic design of sheltering structures and the...     

DEM assessment of impact forces of dry granular masses on rigid barriers

In the design of sheltering structures/embankments for the mitigation of the risk due to rapid and long spreading landslides, a crucial role is generally played by the assessment of the impact force exerted by the flowing mass on the artificial obstacle. This paper is focused on this issue and in particular on the evaluation of the maximum impact force on the basis of the results obtained by performing an extensive numerical campaign by means of a 3D discrete element code, in which a dry granular mass is schematised as a random distribution of rigid spherical particles. The granular mass is generated just in front of the obstacle: its initial volume, velocity distribution, height, length and porosity are arbitrarily assigned, and the impact process is exclusively analysed. The initial conditions are varied to take a large variety of geometrical/mechanical factors, such as the initial front inclination, its height, the initial void ratio, the length of the impacting mass and the inter-particle friction angle, into consideration. A design formula is also proposed on the base of the obtained results and critically compared with the literature data.     

Effects of particle size of mono-disperse granular flows impacting a rigid barrier

Understanding the interaction between complex geophysical flows and barriers remains a critical challenge for protecting infrastructure in mountainous regions. The scientific challenge lies in understanding how grain stresses in complex geophysical flows become manifested in the dynamic response of a rigid barrier. A series of physical flume tests were conducted to investigate the influence of varying the particle diameter of mono-dispersed flows on the impact kinematics of a model rigid barrier. Particle sizes of 3, 10, 23 and 38 mm were investigated. Physical tests results were then used to calibrate a discrete element model for carrying out numerical back-analyses. Results reveal that aside from considering bulk characteristics of the flow, such as the average velocity and bulk density, the impact load strongly depends on the particle size. The particle size influences the degree of grain inertial stresses which become manifested as sharp impulses in the dynamic response of a rigid barrier. Impact models that only consider a single impulse using the equation of elastic collision warrant caution as a cluster of coarse grains induce numerous impulses that can exceed current design recommendations by several orders of magnitude. Although these impulses are transient, they may induce local strucutral damage. Furthermore, the equation of elastic collision should be adopted when the normalized particle size with the flow depth, δ/h, is larger than 0.9 for Froude numbers less than 3.5.     

坡度对碎屑流冲击立式拦挡墙力学特征的影响

由坡度和挡墙倾角的改变造成碎屑流冲击力学模型的改变是目前被忽略的问题。在碎屑流冲击倾式拦挡墙物理试验的基础上,利用离散元数值计算方法研究了坡度对碎屑流冲击立式拦挡墙（墙面与地面的夹角为90°）力学特征的影响,依据死区颗粒堆积特征,流动层颗粒冲击特征以及二者的相互作用特征提出了两种新力学模型：由倾斜冲击挡墙向坡面堆积转变的力学模型和考虑流动层对死区冲切摩擦作用的水平直接冲击力学模型。对不同冲击力学模型进行了验证分析,结果表明：坡度和挡墙倾角改变了死区的堆积特征从而改变了流动层的冲击方向和冲击力大小。当坡度小于40°时,碎屑流流动层首先沿死区上覆面倾斜冲击挡墙,在最大冲击力作用时刻,流动在坡面层状堆积,最大法向冲击合力可按静土压力公式估算。随着坡度的增大,在最大冲击力时刻,流动层颗粒直接冲击挡墙,但由于死区颗粒对流动层颗粒具有摩擦缓冲减速作用,大幅降低了流动层对挡墙的直接冲击力。此时死区对挡墙的作用力主要包括3个部分：流动层沿坡面冲击死区,由死区传递至挡墙的冲击力、流动层对死区的冲切摩擦力以及死区自重的静土压力。死区对挡墙作用力占最大法向冲击合力的比例增大至90%左右。当坡度由40°增大到50°时,在最大法向冲击合力作用时刻,流动层对死区的冲切摩擦力占最大冲击力的比例由15%增大到49%,流动层与死区之间的摩擦系数由滚动摩擦系数转变为静摩擦系数。提出的流动层对死区的冲切摩擦力为碎屑流冲击刚性挡墙力学计算模型提供了新的研究思路。     

Study of the interaction between dry granular flows and rigid barriers with an SPH model

This study uses an incompressible smoothed‐particle hydrodynamics model to investigate the interaction between dry granular material flows and rigid barriers. The primary aim is to summarise some practical guidelines for the design of debris‐resisting barriers. The granular materials are modelled as a rigid‐perfectly plastic material where the plastic flow corresponds to the critical state. The coupled continuity equation and momentum equation are solved by a semi‐implicit algorithm. Compared with flows in controlled flume experiments, the model adequately reproduces both the kinetic of the flows and the impact force under various conditions. Then the numerical simulations are used to study the detailed interaction process. It is illustrated quantitatively that the interaction force consists of two parts, ie, the earth pressure force caused by the weight of the soil and a dynamic force caused by the internal deformation (flowing mass on top of a dead zone). For the estimation of impact load, this study suggests that an increased earth pressure coefficient depending on the Froude number should be incorporated into the hydrostatic model.     

On the impact of dry granular flow against a rigid barrier with basal clearance via discrete element method

Landslides - Rigid barriers are prevalent structure countermeasures for mitigating granular flow hazards in mountainous areas. To reduce maintenance for barriers, a basal clearance is commonly set...     

Analysis of rigid rafts supported by granular piles

Solutions for the magnitude and rate of settlement of rigid foundations supported by soil reinforced with granular piles are presented. An analytic solution using the theory of elasticity is developed for the settlement of the foundation and expressions for evaluating the moment and shear distributions across the foundation are given. A solution for the increase of the rate of settlement due to the presence of the granular piles has been found by a numerical solution of Biot's equations of consolidation.     

Microscopic interpretation of one-dimensional compressibility of granular materials

The one-dimensional compressibility of granular materials is investigated analytically in the framework of micromechanics and simulated using the discrete element method. The results show that the oedometric modulus can be expressed in terms of the material properties of particles and fabric-related variables. By relating the coordination number to different quantities, two forms of the compression curve equations are discussed.     

Estimating the impact force generated by granular flow on a rigid obstruction

Flowing sediments such as debris and liquefied soils could exert a tremendous amount of force as they impact objects along their paths. The total impact force generally varies with slope angle, velocity at impact, and thickness of the flowing sediment. Estimation of the impact force of flowing sediments against protective measures such as earth retaining structures is an important factor for risk assessment. In this paper, we conduct small-scale laboratory physical modeling of sand flow at different slopes and measure the impact force exerted by this material on a fixed rigid wall. We also conduct numerical simulations in the Eulerian framework using computational fluid dynamics algorithms to analyze and reproduce the laboratory test results. The numerical simulations take into consideration the overtopping of the wall with sand, which influenced the measured impact force–time history responses. In addition, the numerical simulations are shown to capture accurately the change of the impact force with slope angle. Finally, the modeling approach conducted in this study is used to estimate the quasi-static force generated by the sediment as it comes to rest on the wall following impact.     

Continuum hydrodynamics of dry granular flows employing multiplicative elastoplasticity

We present a Lagrangian formulation for simulating the continuum hydrodynamics of dry granular flows based on multiplicative elastoplasticity theory for finite deformation calculations. The formulation is implemented within the smoothed particle hydrodynamics (SPH) method along with a variant of the usual dynamic boundary condition. Three benchmark simulations on dry sands are presented to validate the model: (a) a set of plane strain collapse tests, (b) a set of 3D collapse tests, and (c) a plane strain simulation of the impact force generated by granular flow on a rigid wall. Comparison with experimental results suggests that the formulation is sufficiently robust and accurate to model the continuum hydrodynamics of dry granular flows in a laboratory setting. Results of the simulations suggest the potential of the formulation for modeling more complex, field-scale scenarios characterized by more elaborate geometry and multi-physical processes. To the authors’ knowledge, this is the first time the multiplicative plasticity approach has been applied to granular flows in the context of the SPH method.     

Stress-induced evolution of anisotropic thermal conductivity of dry granular materials

Various factors, such as the volumetric fraction of constituents, mineralogy, and pore fluids, affect heat flow in granular materials. Although the stress applied on granular materials controls the formation of major pathways for heat flow, few studies have focused on a detailed investigation of its significance with regard to the thermal conductivity and anisotropy of the materials. This paper presents a numerical investigation of the stress-induced evolution of anisotropic thermal conductivity of dry granular materials with supplementary experimental results. Granular materials under a variety of stress conditions in element testing are analyzed by the three-dimensional discrete element method, and quantitative variations in their anisotropic effective thermal conductivity are calculated via the network model and conductivity tensor measurements. Results show that the directional development of contact area and fabric under anisotropic stress conditions leads to the evolution of anisotropy in thermal conductivity. The anisotropy induced in thermal conductivity by shear stress is higher than that induced by compressive stress because shear stress causes more significant changes in microstructural configurations and boundary conditions. The shear-stress-induced evolution of anisotropy between principal thermal conductivities depends on dilatancy as well as shearing mode, and the shear-driven discontinuity localizes the conductivity. Factors involved in the stress-induced evolution and their implications on the thermal conductivity characterization are discussed.     

Stress level effect on mobility of dry granular flows of angular rock fragments

Landslides - Granular flows of angular rock fragments such as rock avalanches and dense pyroclastic flows are simulated numerically by means of the discrete element method. Since large-scale flows...     

Mechanical behaviour and failure of cohesive granular materials

We investigate the stress–strain behaviour and failure of a cohesive granular material both by experiments and numerical simulations. The material is an assembly of aluminium rods glued together by means of an epoxy resin. The behaviour of cohesive bonds (force–displacement relationship, failure conditions) is characterized by performing simple loading tests (tension/compression, shear…) on a couple of rods. Then, this local behaviour is introduced in a numerical code based on a discrete element method in order to perform numerical compression tests on large samples. The validation of this approach was the main goal of the present investigation that is essentially achieved by a direct comparison between the numerical results and similar experimental tests. As a basic application, we derive the macroscopic cohesion and friction characteristics of random cohesive materials by systematic numerical simulations in a biaxial geometry. Copyright © 2004 John Wiley & Sons, Ltd.     

Research on acceleration signal of granular column collapse under dry and wet conditions

Landslides - The blockage of rivers and reservoirs caused by disasters such as landslides and collapses has always been a troublesome problem. How to quickly capture this disaster information and...     

Analysis of wave propagation in dry granular soils using DEM simulations

In this paper, a three-dimensional particle-based technique utilizing the discrete element method (DEM) is proposed to study wave propagation in a dry granular soil column. Computational simulations were conducted to investigate the soil response to sinusoidal motions with different amplitudes and frequencies. Three types of soil deposits with different void ratios were employed in these simulations. Different boundary conditions at the base such as rigid bedrock, elastic bedrock, and infinite medium were also considered. Analysis is done in time domain while taking into account the effects of soil nonlinear behavior. The computational approach is able to capture a number of essential characteristics of wave propagation including motion amplification and resonance. Dynamic soil properties were then extracted from conducted simulations and used to predict the response of the soil using the widely used equivalent linear method program SHAKE and compare its predictions to DEM results. Generally, there was a good agreement between SHAKE and DEM results except when the exciting frequency was close to the resonance frequency of the deposit where significant discrepancy in computed shear strains between SHAKE predictions and DEM results was observed.     

DEM filtration modelling for granular materials: Comparative analysis of dry and wet approaches

A detailed study comparing two – dry and wet – numerical approaches to model filtration processes at stake in actual granular filters is presented using the discrete element method (DEM). In the first approach, the migration of fines is provided by gravitational forces, while in the second, hydrodynamic forces induce their movement. Numerical filtration tests were performed on granular filters involving materials with different gradings and porosities. The study demonstrated that the wet filtration approach generates higher tortuosity due to the possibility for fines to deviate from direct paths towards more open sideways. It leads to a lower coefficient of retention for the filter than if it were characterised using a dry filtration approach. However, the intensity of this feature greatly depends on the grading and the porosity of the granular filter. Finally, an enhanced dry filtration model designated as the “equivalent cyclic wet filtration model” is presented, which better mimics the results obtained through the preferable wet filtration model compared to the original dry filtration model. This new model constitutes a valuable alternative tool for studies of filtration properties in granular materials.     

Experimental study of the dynamic behavior and segregation of density-bidisperse granular sliding masses at the laboratory scale

From the understanding of dynamics and processes of rapid granular flows and the granular-segregation mechanism in gravity-driven flow, we can clarify the particle-composition structure in the downstream areas of avalanches in geophysical contexts, such as landslides, rock falls, and snow-slab avalanches. Such dynamics also provide a basis for geophysical studies. This study experimentally investigates the dynamic behavior and segregation phenomena of a density-bidisperse, rapid, granular flow down a quasi-2D, rough, inclined rectangular chute. Particles with two density ratios are used to investigate the mechanism of density-induced segregation, and four chute-inclination angles are tested to examine the influence of driving forces. The dynamics of the mixture flow—which includes the flow-depth evolution, stream-wise and depth-wise velocity profiles, shear rate, and granular temperature in the upper high-shear band of the flow—are obtained from particle image velocimetry (PIV) measurements. The two-dimensional concentration distributions of the particles in the stream-wise direction are also obtained using 2D image processing to determine the segregation state. In the upstream region, the variation in the concentration of heavier particles is defined as the strength of the density-induced segregation state, S_d. Our results indicate that the mixture-flow parameter—particularly the shear rate and the granular temperature in the upper high-shear band—crucially influence the strength of particle segregation in granular avalanches. In the upstream region, a higher shear rate and a higher granular temperature in the upper high-velocity band result in a smaller drag force in the mixture flow, causing stronger density-induced particle segregation. These results well describe the entire processes of dense granular flows, from upstream initiation to the downstream steady state. Therefore, they reveal the structure of the mixed flow in the depth direction and are expected to explain various gravity-driven mixture granular flows.

     

循环荷载下干燥与饱和砂岩力学特性及能量演化

对取自陕北榆横矿区小纪汗煤矿顶板砂岩的干燥及饱和试件进行单轴循环加、卸载试验,分析了干燥与饱和状态下岩石的强度和变形特征以及岩石破坏过程中的能量演化与能量分配情况。结果表明:(1)循环加、卸载导致干燥试件的峰值强度较单轴压缩时下降19.47%;(2)饱水对砂岩的强度和变形均有影响,饱和试件的平均峰值强度比干燥试件低39.55%,同时饱和试件具有更加明显的压密阶段;(3)循环加、卸载下砂岩的加载和卸载弹性模量均呈现增大趋势,但饱和试件的弹性模量低于干燥试件;(4)通过峰值强度归一化处理后发现,在应力加载的不同阶段,饱和试件的弹性应变能Ue和耗散能Ud均低于干燥试件;(5)加载过程中,饱和试件弹性应变能Ue的占比高于干燥试件,该比例随着加载过程逐渐下降并在破坏前与干燥试件的Ue的占比达到近似水平。