Distinct Element Method Research Articles

Numerical Analysis on Stability of Caisson-Type Breakwaters under Tsunami-Induced Seepage

In this study, distinct element method (DEM) analysis and slip analysis based on the simplified Bishop method have been carried out in order to investigate stability of a caisson-type breakwater under tsunami-induced seepage and establish the design method of the caisson foundation. The effect of seepage on the bearing capacity of the caisson foundation was introduced as the reduction of the effective weight of the rubble mound in the both analyses. As a result of the distinct element (DE) analysis, it has been revealed that the shear strain of the rubble mound increases due to the tsunami-induced seepage. Moreover, from the slope stability analysis, it has been indicated that the safety factor decreases by 7–10 % due to the seepage force, which is approximately corresponding to the increase rates of shear deformation in DE analysis.

Sampling of Regolith on Asteroids Using Electrostatic Force

AbstractThe authors have developed an electrostatic force–based sampler to reliably and autonomously sample asteroid regolith. After applying a rectangular high voltage between parallel screen electrodes mounted at the lower end of a tube, the resultant electrostatic force acts on nearby regolith particles. Some agitated particles are captured when passing through the screen electrode openings and transported to a collection capsule through the tube. In a microgravity environment, effective particle sampling was expected because particle motions are not affected by the negligible gravitational force. The authors confirmed the sampler’s performance in a microgravity environment through numerical calculations and a model experiment. The calculation using the distinct element method predicted successful regolith capture, including conductive and insulative particles, under air and microgravity. The sampler shows much better performance in vacuum than in air. Lunar regolith simulant was sampled experimentally...

A Basic Study on Protective Steel Structures against Woody Debris Hazards

This paper presents a basic study for protective structures (slit dams) against woody debris hazards. First, a model experiment was carried out to examine the trap efficiency of a slit dam against woody debris with or without roots. Herein, the effects of length of woody debris, flow discharge and opening width of a slit dam on the trap efficiency were investigated. Second, a new three dimensional distinct element method (3D-DEM) was developed to simulate the trap efficiency of woody debris with or without roots by introducing cylindrical elements. Finally, the new 3D-DEM was applied to simulate an actual woody debris disaster site in Hiroshima, Japan.

Kinetics of co-crystal formation with caffeine and citric acid via liquid-assisted grinding analyzed using the distinct element method

The kinetics of co-crystal formation of caffeine (CF) with citric acid (CTA) was evaluated. Ball milling of CF and CTA in molar ratios of 4:1, 3:1, 2:1, 1:1, 1:2, 1:3, and 1:4 was performed by the liquid-assisted grinding (LAG) method. The samples were characterized by powder X-ray diffraction (PXRD), Fourier transform infrared spectroscopy (FT-IR) and differential scanning calorimetry (DSC). Two types of co-crystals (co-crystal-1, a 1:1 CF-CTA co-crystal; and co-crystal-2, a new co-crystal form) were obtained. The kinetic characteristics of this new co-crystal formation were assessed by calculating the ball impact energy and force using the distinct element method (DEM) simulations. The results indicated that co-crystal-2 creation occurred under a condition in which the ball impact force exceeded a certain threshold value. Moreover, the total ball impact energy was positively correlated with co-crystal formation, exhibiting a higher ball impact force than the threshold value. The kinetics of co-crystal-2 formation was almost consistent with the Jander equation. Consequently, co-crystal-2 formation could be explained according to a three-dimensional diffusion mechanism.

Back Analysis of the 2014 San Leo Landslide Using Combined Terrestrial Laser Scanning and 3D Distinct Element Modelling

Landslides of the lateral spreading type, involving brittle geological units overlying ductile terrains, are a common occurrence in the sandstone and limestone plateaux of the northern Apennines of Italy. The edges of these plateaux are often the location of rapid landslide phenomena, such as rock slides, rock falls and topples. In this paper, we present a back analysis of a recent landslide (February 2014), involving the north-eastern sector of the San Leo rock slab (northern Apennines, Emilia-Romagna Region) which is a representative example of this type of phenomena. The aquifer hosted in the fractured slab, due to its relatively higher secondary permeability in comparison to the lower clayey units leads to the development of perennial and ephemeral springs at the contact between the two units. The related piping erosion phenomena, together with slope processes in the clay-shales have led to the progressive undermining of the slab, eventually predisposing large-scale landslides. Stability analyses were conducted coupling terrestrial laser scanning (TLS) and distinct element methods (DEMs). TLS point clouds were analysed to determine the pre- and post-failure geometry, the extension of the detachment area and the joint network characteristics. The block dimensions in the landslide deposit were mapped and used to infer the spacing of the discontinuities for insertion into the numerical model. Three-dimensional distinct element simulations were conducted, with and without undermining of the rock slab. The analyses allowed an assessment of the role of the undermining, together with the presence of an almost vertical joint set, striking sub-parallel to the cliff orientation, on the development of the slope instability processes. Based on the TLS and on the numerical simulation results, an interpretation of the landslide mechanism is proposed.

Analysis of the dynamics of the FT4 powder rheometer

Traditional powder flow measurement devices, such as shear cells, operate in the quasi-static regime of shear strain rate. The FT4 powder rheometer of Freeman Technology, developed over the last two decades, has provided a clearer differentiation of powder flowability in some instances. This has been attributed to the instrument operating in the dynamic regime of shear strain rates, a feature that has yet to be established. We report an analysis of the dynamic behaviour of a bed of glass beads made cohesive by silanisation and subjected to standard FT4 testing procedure, where a rotating blade is driven into a cylindrical bed, using a combination of experimental measurements and numerical simulations by the Distinct Element Method (DEM). The DEM analysis underestimates the flow energy measured experimentally, although the agreement is improved when sliding friction is increased. The shear stress of the powder in front of the blade is shown to be roughly constant along the radial direction and increasing as the impeller penetrates the bed, suggesting that a characteristic shear stress can be determined for a powder under a given test condition in the FT4. For ease of simulations large beads were used (1.7–2.1mm). Future work will investigate the influence of particle properties and operational conditions on the prevailing stresses and strain rates.

Coupled LBM–DEM Micro-scale Simulations of Cohesive Particle Erosion Due to Shear Flows

In this paper, the erodibility of cohesive micro-scale particles is investigated for a range of surface shear stresses. Compacted layers of particles, formed by compressing 100 cohesive spheres together, are subjected to shear flow conditions. Fluid–particle interactions are solved using the lattice Boltzmann method (LBM), while the particle–particle interactions are treated by coupling with the distinct element method (DEM). Results reveal a clearly defined critical surface shear stress, beyond which erosion occurs, and a linear relation between the rate of erosion and excess surface shear stress. Further to this, an interesting mechanism by which detached particles gain upward movement is observed. This work also serves to highlight the potential of the coupled LBM–DEM approach for modelling dynamic erosion processes in three dimensions.

Stress evolution during caldera collapse

The mechanics of caldera collapse are subject of long-running debate. Particular uncertainties concern how stresses around a magma reservoir relate to fracturing as the reservoir roof collapses, and how roof collapse in turn impacts upon the reservoir. We used two-dimensional Distinct Element Method models to characterise the evolution of stress around a depleting sub-surface magma body during gravity-driven collapse of its roof. These models illustrate how principal stress orientations rotate during progressive deformation so that roof fracturing transitions from initial reverse faulting to later normal faulting. They also reveal four end-member stress paths to fracture, each corresponding to a particular location within the roof. Analysis of these paths indicates that fractures associated with ultimate roof failure initiate in compression (i.e. as shear fractures). We also report on how mechanical and geometric conditions in the roof affect pre-failure unloading and post-failure reloading of the reservoir. In particular, the models show how residual friction within a failed roof could, without friction reduction mechanisms or fluid-derived counter-effects, inhibit a return to a lithostatically equilibrated pressure in the magma reservoir. Many of these findings should be transferable to other gravity-driven collapse processes, such as sinkhole formation, mine collapse and subsidence above hydrocarbon reservoirs.

Modeling failure of jointed rock slope with two main joint sets using a novel DEM bond contact model

In order to investigate the failure processes and mechanisms of jointed rock slopes, Distinct Element Method (DEM) was applied with a novel bond contact model. The bond contact model is validated by a series of numerical direct shear tests on jointed models and the comparisons to laboratory test results. DEM jointed rock slope models were then generated and two arrangements of joint sets and two types of slope surface were chosen to test the capabilities of the bond contact model. The slope models were destabilized progressively by the critical gravity approach, and both the failure onset and the slope collapse process were analyzed. Microscopic information reveals that the initiation of slope failure can be divided into three phases and is induced from stress concentration within the rock bridges, occurrence of tension cracks, and their propagation and coalescence. The slope surface properties reflected by different damping affect both post-failure configuration and propagation distance of the rock slope. The study demonstrates that the novel bond contact model implemented in the DEM code is able to tackle the fundamental problems of jointed rock slope failure and helps to better understand the slope failure mechanisms from a macroscopic and microscopic level.

A simple three‐dimensional distinct element modeling of the mechanical behavior of bonded sands

SummaryThis paper presents a simple three‐dimensional (3D) Distinct Element Method (DEM) for numerical simulation of the mechanical behavior of bonded sands. First, a series of micro‐mechanical tests on a pair of aluminum rods glued together by cement with different bond sizes were performed to obtain the contact mechanical responses of ideally bonded granular material. Second, a 3D bond contact model, which takes into account the influences of bond sizes, was established by extending the obtained 2D experimental results to 3D case. Then, a DEM incorporating the new contact model was employed to perform a set of drained triaxial compression tests on the DEM bonded specimens with different cement contents under different confining pressures. Finally, the mechanical behavior of the bonded specimens was compared with the available experimental results. The results show that the DEM incorporating the simple 3D bond contact model is able to capture the main mechanical behavior of bonded sands. The bonded specimen with higher cement content under lower confining pressure exhibits more pronounced strain softening and shear dilatancy. The peak and residual strengths, the apparent cohesion and peak/residual friction angles, and the position and slope of the critical state line increase with increase in cement content. Microscopically, bond breakage starts when the system starts to dilate and the maximum rate of bond breakage coincides with the maximum rate of dilation. Bond breakage is primarily due to tension‐shear failure and the percentage of such failures is independent of both confining pressure and cement content. Copyright © 2015 John Wiley & Sons, Ltd.

Mechanics of Nanocrystalline Particles With the Distinct Element Method

In geomechanics and civil engineering, the distinct element method (DEM) is employed in a top-down manner to simulate problems involving mechanics of granular media. Because this particle-based method is well adapted to discontinuities, we propose here to adapt DEM at the mesoscale in order to simulate the mechanics of nanocrystalline structures. The modeling concept is based on the representation of crystalline nanograins as mesoscopic distinct elements. The elasticity, plasticity, and fracture processes occurring at the interfaces are captured with contact models of interaction between elements. Simulations that rely on the fitting of the peak stress, strain, and failure mode on the experimental testing of Au and CdS hollow nanocrystalline particles illustrate the promising potential of mesoscopic DEM for bridging the atomistic-scale simulations with experimental testing data.

DEM investigation of weathered rocks using a novel bond contact model

The distinct element method (DEM) incorporated with a novel bond contact model was applied in this paper to shed light on the microscopic physical origin of macroscopic behaviors of weathered rock, and to achieve the changing laws of microscopic parameters from observed decaying properties of rocks during weathering. The changing laws of macroscopic mechanical properties of typical rocks were summarized based on the existing research achievements. Parametric simulations were then conducted to analyze the relationships between macroscopic and microscopic parameters, and to derive the changing laws of microscopic parameters for the DEM model. Equipped with the microscopic weathering laws, a series of DEM simulations of basic laboratory tests on weathered rock samples was performed in comparison with analytical solutions. The results reveal that the relationships between macroscopic and microscopic parameters of rocks against the weathering period can be successfully attained by parametric simulations. In addition, weathering has a significant impact on both stress–strain relationship and failure pattern of rocks.

Overcoming the limitations of distinct element method for multiscale modeling of materials with multimodal internal structure

• We propose general expressions describing interaction between simply deformable elements. • Realization of various rheological and fracture models is discussed. • Models accounting for grain and phase boundaries within the DEM framework are proposed. • Examples of DEM application to materials with multiscale internal structure are shown. This paper develops an approach to model the deformation and fracture of heterogeneous materials at different scales (including multiscale modeling) within a discrete representation of the medium. Within this approach, molecular dynamics is used for the atomic-scale simulation. The simply deformable distinct element method is applied for simulating at higher length scales. This approach is proposed to be implemented using a general way to derive relations for interaction forces between distinct elements in a many-body approximation similar to that of the embedded atom method. This makes it possible to overcome limitations of the distinct element method which are related to difficulties in implementing complex rheological and fracture models of solids at different length scales. For an adequate description of the mechanical behavior features of materials at the micro- and mesoscales, two kinds of models that consider grain and phase boundaries within the discrete element framework are proposed. Examples are given to illustrate the application of the developed formalism to the study of the mechanical response (including fracture) of materials with multiscale internal structure. The examples show that the simply deformable distinct element method is a correct and efficient tool for analyzing complex problems in solid mechanics (including mechanics of discontinua) at different scales.

DEM modelling analysis of tree root growth in street pavements

The coexistence of urban green spaces and infrastructure is often difficult. Trees find inhospitable environments in urban areas, whereas tree root systems can damage sidewalk, street and parking lot. The main form of failure concerns the growing of tree roots beneath the pavements, which produce upheavals and displacements on wearing course. The main objective of this study was a distinct element method modelling analysis of the potential interactions between tree root systems and street pavements. This calculation code allowed us to simulate the dynamic behaviour of the ground-pavement system as result of the roots growth. The performed modelling, representative of various planting strategies commonly used in urban contexts, have highlighted the progressive reduction of deformations with the increase of the roots penetration depth. Such displacements were strongly influenced by other factors such as the porosity and the grain-size distribution of the soil and the type of pavement considered.

Numerical Modelling of Thermal Cracking of Pavements

Thermal cracking of pavements may occur due to the top of the pavement being exposed to cold atmospheric conditions. The formation of cracks may cause a deterioration of ride quality and ingress of water to the base. Developing of capabilities to model thermal cracking of pavements is beneficial because it will allow the assessment of susceptibility of pavement materials and geometries to thermal cracking. Fracture resistant asphalt mixtures can then be identified. A methodology is proposed by which numerical modelling of thermal cracking of pavements is carried out using the distinct element method and cohesive cracks. The distinct element method efficiently handles the subdivision of the originally intact material. The cohesive crack method allows the inclusion of experimentally determinable fracture properties into the formulation. Results obtained were consistent with an analytical model available in literature. Thermal cracking observed in a field experiment could then be numerically replicated.

A study of submarine steep slope failures triggered by thermal dissociation of methane hydrates using a coupled CFD-DEM approach

This paper presents an extended numerical study on submarine landsliding induced by instantaneous thermal dissociation of a methane hydrate layer, by considering different dissociation scenarios in terms of position and volume of dissociated gas hydrates. A novel scheme of coupling computational fluid dynamics and distinct element method (CFD–DEM) is implemented. The behavior of fluid is simulated by CFD incorporating an empirical equation of state for slightly compressible liquid, whereas that of sediment skeleton with methane hydrates by DEM through a two-dimensional thermo-hydro-mechanical bond contact model. The scenario of whole hydrate dissociation identifies four typical stages of submarine landsliding: the initiation of landslide after hydrate dissociation, the onset of the submarine landslide, the sliding process, and the final deposition and stabilization, which is characterized by some microscopic variables (e.g. particle displacement, fluid velocity, excess hydrostatic pressure, particle-scale energy input and dissipation) so to attain a complete analysis of the landslide process. Reasonable representation of the slope instability phenomenon as triggered by thermal dissociation of gas hydrates is obtained. Besides, slope failure scenarios with different positions and volumes of the occurring MH dissociation are presented to better understand their influence on the evolution of the instability. Four landslide types are generated by the different dissociation scenarios: fall, slump, flow and a combination of slump and flow, which are further analyzed by outputting some microscopic information (e.g. force-chain, averaged pure rotation rate and bond distributions).

Contrasting fracture patterns induced by volume-increasing and -decreasing reactions: Implications for the progress of metamorphic reactions

Hydration and dehydration reactions during metamorphism cause drastic changes in porosity and fluid pressure in a rock, and some of these reactions proceed with fracturing. For this paper we have developed a model for coupled hydraulic–chemical–mechanical processes, using the distinct element method, in order to understand the relationships among chemical reactions, fluid flow, and fracturing during metamorphism. The model considers fluid advection along fractures and the dependence of reaction rates on fluid pressure. With the help of the model, volume-decreasing dehydration reactions and volume-increasing hydration reactions are investigated in detail as analogs of prograde and retrograde reactions, respectively. Both types of reaction proceed inwards from drained boundaries or fractures, but they show contrasting fracture patterns. A volume-decreasing dehydration reaction produces tree-type fractures, with the new fractures generated as branches of a pre-existing fracture. A volume-increasing hydration reaction produces a polygonal network of fractures, where new fractures nucleate at sites far from the pre-existing fracture, and extend to form T-junctions. These contrasting fracture patterns are essentially controlled by solid volume changes during reaction rather than by the fluid pressure gradient. In the case of the tree-type fractures, the fractures are continuously generated at the reaction front, and the reaction proceeds smoothly by positive feedbacks between reaction, fracturing, and fluid flow. In contrast, in a volume-increasing hydration reaction, fracturing initially occurs in response to the irregular boundary shape, but as the reaction progresses, a compressive stress field is generated, which inhibits further fracture generation. The compressive stress field also prevents fluid flow by closing the pre-existing fractures, which slows down the reaction. These contrasting feedback systems between volume-decreasing dehydration and volume-increasing hydration reactions help to explain why prograde metamorphic reactions proceed pervasively, and why the progress of a retrograde hydration reaction tends to be localized along fractures so that relics of peak metamorphism are commonly preserved.

Numerical simulation of gas–liquid–solid three-phase flow using particle methods

We want to simulate, based on particle methods, the dynamic behavior of multi-phase flows in a gas–solid–liquid mixture system. With the governing equations discretized within the finite volume particle method, the effects of contact and collision between solid particles were modeled by the distinct element method. Applicability of the viscosity model and an empirical drag force model were confirmed for the hydrodynamic interactions between solid particles and fluid. Simulations were performed of a single bubble rising in a tank of stagnant solid particle–liquid. The results for the dynamic behavior indicate that the present computational framework of particle-based simulation method may be useful for numerical simulations of multi-phase flow behavior in a solid particle–fluid mixture system.

Influence of brick–mortar interface on the mechanical behaviour of low bond strength masonry brickwork lintels

A study of the influence of the brick–mortar interface on the pre- and post-cracking behaviour of low bond strength masonry wall panels subjected to vertical in plane load is presented. Using software based on the Distinct Element Method (DEM), a series of computational models have been developed to represent low bond strength masonry wall panels containing an opening. Bricks were represented as an assemblage of distinct blocks separated by zero thickness interfaces at each mortar joint. A series of sensitivity studies were performed supported with regression analysis to investigate the significance of the brick–mortar interface properties (normal and shear stiffnesses, tensile strength, cohesive strength and frictional resistance) on the load at first cracking and ultimate load that the panel can carry. Computational results were also compared against full scale experimental tests carried out in the laboratory. From the sensitivity analyses it was found that the joint tensile strength is the predominant factor that influences the occurrence of first cracking in the panel, while the cohesive strength and friction angle of the interface influence the behaviour of the panel from the onset of cracking up to collapse.

A conceptual model of pore-space blockage in mixed sediments using a new numerical approach, with implications for sediment bed stabilization

In mixed sediment beds, erosion resistance can change relative to that of beds composed of a uniform sediment because of varying textural and/or other grain-size parameters, with effects on pore water flow that are difficult to quantify by means of analogue techniques. To overcome this difficulty, a three-dimensional numerical model was developed using a finite difference method (FDM) flow model coupled with a distinct element method (DEM) particle model. The main aim was to investigate, at a high spatial resolution, the physical processes occurring during the initiation of motion of single grains at the sediment–water interface and in the shallow subsurface of simplified sediment beds under different flow velocities. Increasing proportions of very fine sand (D50=0.08 mm) were mixed into a coarse sand matrix (D50=0.6 mm) to simulate mixed sediment beds, starting with a pure coarse sand bed in experiment 1 (0 wt% fines), and proceeding through experiment 2 (6.5 wt% fines), experiment 3 (10.5 wt% fines), and experiment 4 (28.7 wt% fines). All mixed beds were tested for their erosion behavior at predefined flow velocities varying in the range of U 1-5=10–30 cm/s. The experiments show that, with increasing fine content, the smaller particles increasingly fill the spaces between the larger particles. As a consequence, pore water inflow into the sediment is increasingly blocked, i.e., there is a decrease in pore water flow velocity and, hence, in the flow momentum available to entrain particles. These findings are portrayed in a new conceptual model of enhanced sediment bed stabilization.