Shear Behavior Of Granular Materials Research Articles

Numerical investigation of the fast shear behaviour of granular materials and its significance for rapid landslides

The shear behaviour of granular materials at high velocities is crucial for understanding the high mobility of rapid landslides and the low-friction mechanisms behind them. Here, a numerical ring shear model driven by granular shear platens was developed, and this model was validated in terms of the kinematics and mechanics of the granular material. The granular material was then accelerated at different accelerations to high shear velocities (ranging from 0.001 m/s to a maximum of 5.0 m/s). The results indicate that with increasing shear velocity, the shear behaviour of granular materials transitions from a single behaviour to a composite behaviour. The composite shear behaviour is important for both the shear flow state and the frictional characteristics. The velocity profile reveals the transition of granular materials from uniform shear flow to composite shear flow consisting of locally high-shear-rate layers and slow creep layers; the volume of granular materials transitions from global expansion at the onset of shearing to local expansion; and particle velocity fluctuations and contact force fluctuations change from being uniformly distributed and relatively small at the beginning of shearing to rapid growth in local areas. Furthermore, with increasing shear velocity, the frictional characteristics become nonuniform. Local areas exhibit positive velocity-related friction effects, whereas the friction of the particles in other regions slightly decreases. Particle fluctuations represent an important factor that leads to the composite shear behaviour of granular materials. This study provides valuable insights into the shear behaviour of particles in rapid landslides.

Mechanical effects of nanomaterials on the residual shear strength of dry sandy mixtures: Implication for the high mobility of rock avalanches

Nanoparticles found in rock avalanches and natural faults were inferred to be the reason for the ultra-low shear resistance of the materials along the shear surface. Nevertheless, this inference has not been physically verified yet. To investigate the role of nanoparticles in the shear behavior of granular materials, we conducted a series of ring shear tests on silicon dioxide nanoparticles (SN), silica sand (S8), and mixtures of the S8 with different contents of SN, under varying normal stresses (σ) and shear velocities (V). Our results showed that the shear strength (μ) of SN is high when σ is small (≤ 215 kPa at our test range), even though the maximum shear velocity is about 210 cm/s, but shows a great decrease under higher σ (≥ 619 kPa). The shear strength of mixtures varied with SN content. For S8 (M0) and the mixture of 10% SN (M10), when sheared under small σ, μ first decreased slightly with increasing V when V is smaller than 1 cm/s, after that it increased slightly with further increase of V. Nevertheless, this kind of shear-rate dependent behavior was not observed in the tests under higher σ. When the content of SN in the mixture was <20%, μ increased with the increase of SN content. However, with further increase of SN content, μ decreased. Under high normal stress conditions, distinctive powder rolls were observed on the shear surface formed on the tests. When the mixtures with lower SN content (≤50%), the powder rolls were not observed. We inferred that powder rolls can be produced under high SN content and high normal stress conditions, and this structure altered the particle movement mode from sliding friction to rolling resistance, resulting in a significant decrease in the shear strength. These findings provide valuable insights into the shear behavior of granular materials with nanoparticles and then provide evidence for a better understanding of the high mobility of rock avalanches.

Systematic investigation into the role of particle multi-level morphology in determining the shear behavior of granular materials via DEM simulation

This study aims to numerically investigate the dependence of shear behaviors of granular materials on particle multi-level morphology, encompassing form (mean of elongation and flatness), angularity, and roughness. To this end, four series of particles—ellipsoidal particles with varying elongation indices (EI) and flatness indices (FI), and equiaxed concave particles with varying angularity indices (AI) and roughness (RG)—are adopted to isolate the effects of particle morphology at each level. Through discrete element method (DEM) simulations, granular assemblies with varying values of EI, FI, AI, and RG are consolidated to achieve the maximum packing density under the same confining pressure. Subsequently, they are subjected to triaxial drained compression tests until reaching a critical state. Based on numerical results, the independent effects of EI, FI, AI, and RG on the macro- and micro-mechanical response of granular materials are examined and compared in detail. Furthermore, using the ‘Stress-force-fabric’ (SFF) relationship, the anisotropic origins of the peak- and critical-state shear strengths induced by EI, FI, AI, and RG are also analyzed.

DEM study on the effect of particle shape on the shear behaviour of granular materials

This study investigates the effects of particle convexity, sphericity and aspect ratio (AR) on the behaviour of sheared granular materials using two-dimensional discrete element method simulations. Isotropic, dense and loose assemblies with different particle shapes were prepared and subjected to drained shearing via biaxial compression until the critical state was reached. Macroscopic characteristics such as strength and dilatancy are presented. The factors underlying the macroscopic behaviour are then investigated by considering the coordination number, fabric anisotropy, particle moment, friction mobilisation at contacts and particle rotation. For the range of shapes considered here, the data indicate that the shear strength decreases as particle convexity and sphericity increases while the shear strength increases with increasing AR. The shear strength and convexity are weakly correlated, however a stronger correlation is observed between AR and strength. The volumetric strain at large strains tends to increase with increasing AR. There is a stronger correlation between the critical state strength and both the critical state coordination number and the critical state mechanical void ratio than there is between the critical state void ratio and the critical state strength. The contact fabric anisotropy, the magnitude of the moment transmitted by particles and the friction mobilised at the contacts are important factors underlying strength. The critical state strength increases as both the mean particle moment and the mean mobilised friction increased. Analysis of particle rotation provides insights into the response of the granular materials to shearing.

Combined Influence of Particle Shape and Fabric on the Shear Behaviour of Granular Materials

The anisotropy feature is an important characteristic of granular materials in natural life and is caused by two facts: the anisotropy feature of particle shape, such as an elongated or flattened shape, and the anisotropy feature of the packing fabric, such as the preferable orientation of particle alignment. The discrete element method has been commonly used in the study of meso-mechanics of granular materials and is in our study to simulate the direct shear test with particles of various aspect ratios under different initial orientation alignment conditions for assessing the coupled influence of anisotropy from particle shape and fabric of particle packings on the shear behaviour of granular materials. Analysis results show that anisotropy from the particle shape has the most significant influence on the shear behaviour of a granular packing when the packing has the initial anisotropy fabric of an orientational alignment perpendicular to the shear direction. Moreover, the initial fabric anisotropy of a granular packing has an increasing influence with the rise in anisotropy of particle shape. A combined anisotropic factor is finally introduced to reflect the coupled influence of the shape anisotropy of particles and the fabric anisotropy of particle packings.

Uticaj trostruke veličine čestica na mehaničko ponašanje zrnastih materijala primeno metode diskretnih elemenata (MDE)

Granules are used in various industries such as medicine and agriculture, and their behavior is influenced by the characteristics of the constituent particles. The Discrete Element Method (DEM) is a technique for characterizing the mechanical behavior of granular materials by building a mechanical model that describes the impacted parameters, including particle shape, which is being one of these parameters. As a result, the discrete element method is applied to investigate the macro-and micro-mechanical shear behavior of granular materials. For this study, a gravitational disposition for geometrical arrangement model has been used to model various triple particle sizes for a direct shear test using (EDEM®), which is a three-dimensional (3D) program based on (DEM). Different triple particle sizes were used to create an assembly. The results revealed that the size index affected the relationship between shear strength, angular velocity, dilation, coordination number (CN), and volumetric strain.

Study of shear behavior of granular materials by 3D DEM simulation of the triaxial test in the membrane boundary condition

This paper studies the shear behavior of granular materials by using a three-dimensional (3D) discrete element method (DEM) simulation of the triaxial test. The experimental triaxial tests were conducted on glass beads samples for verification. DEM simulations of the triaxial test were carried out in the membrane boundary condition consisting of 37,989 membrane particles. A new method that divides the irregular sample shape into two parts of cones and parts of three-dimensional simplexes is used to follow the volume change of irregular deformation of samples. The free rotatable upper platen is considered during the shearing process, which influences the shear behavior of samples especially in the residual stage and formations of a single shear band or X-shape shear band. The confining pressures have been demonstrated to influence the rotation angle and angular velocity of the upper platen. Moreover, the timing of replacing a rigid wall boundary condition with the membrane boundary condition is investigated, which affects the porosity of samples before shearing and the mechanical strength. The DEM model in the membrane boundary condition reflects well the evolution of irregular sample deformation and shear band in the shearing process. From the perspective of micro structures, the normal force decreases and the tangential stress increases during the shearing stage. This study greatly improves the accuracy of DEM simulations of the triaxial test in the membrane boundary condition.

Effect of particle type on the shear behaviour of granular materials

In discrete element method (DEM) simulations, multi-sphere (MS) clumped and convex particles are two main particle models that are used to study the mechanical behaviours of granular materials. Of interest is the evaluation of the effect of multiple contacts between clumped particles or single contacts between convex particles on the mechanical behaviours of granular materials. In this context, a series of drained triaxial compression tests were conducted on convex true (CT) ellipsoids and MS ellipsoids with aspect ratios (ARs) ranging from 1.0–2.0. The microscale results indicate that at a given AR, the critical friction angle φc changes with the particle type, whereas the peak friction angle φp is nearly independent of the particle type. The anisotropic analysis provides underlying mechanisms of the shear strength evolution from two perspectives. First, the anisotropies of granular materials are essential to shear strength as the deviatoric (q)-to-effective mean (p′) stress ratio can be expressed as the sum of the anisotropies, i.e., q/p′ ≈ 0.4ac+ 0.4an + 0.6at, where ac, an and at are the normal contact anisotropy, normal contact force anisotropy and tangential contact force anisotropy, respectively. For all samples, ac and an underpin the shear strength and are influenced by the particle type. The similar φp displayed by the CT and MS ellipsoids does not translate to similar an and ac but similar ac+an for the two particle types. In addition, owing to their larger ac+an, the CT ellipsoids have a higher φc than the MS ellipsoids. Second, there is a satisfactory linear relationship between q/p′ and ac within strong and non-sliding (sn) contacts acsn (i.e., q/p′ = kacsn), where k is the fitting parameter. Accordingly, in the peak state, the subtle difference in shear strength is attributed to the greater acsn in the CT ellipsoids than in the MS ellipsoids that is counteracted by the smaller k. However, in the critical state, the greater difference in acsn between the CT and MS ellipsoids is partially offset by the smaller difference in k, causing a higher φc in the CT ellipsoids than in the MS ellipsoids.

Exploring the effect of particle shape caused by erosion on the shear behaviour of granular materials via the DEM

The fragments from crushed rock are initially very angular. The erosion process causes the fragment shape to evolve from angular to well rounded and affects the mechanical behaviour of granular materials. This report explores the effect of particle shape caused by erosion (Er) on macroscopic and microscopic behaviour under triaxial compression tests of granular materials via the discrete element method. A convex rigid block was employed to model particles with different erosion degrees. After being prepared in the densest condition, all particle assemblies were subjected to axial compression until a critical state was reached. The macroscopic characteristics, including the shear strength and dilatancy response, were investigated. As Er increased, the shear strength generally decreased, and the volumetric strain monotonically decreased. Then, the microscopic characteristics, including the coordination number, particle rotation, percentage of sliding contacts and contact force, were examined. Finally, analysis-related anisotropy coefficients of the entire contact network were evaluated to probe the microscopic origins of the peak and residual shear strength related to Er.

DEM study on the effect of roundness on the shear behaviour of granular materials

Abstract This report investigated the effects of irregular particles on macroscopic and microscopic behaviour under triaxial compression tests for a range of various roundnesses. A multi-sphere method was employed to model the three-dimensional irregular particles generated by rotation-invariant spherical harmonics. After being prepared in the densest conditions, all assemblies were subjected to axial compression until a critical state was reached. The macroscopic characteristics, including the shear strength and dilatancy response, were investigated. We found that the shear strength generally decreased and the volumetric strain linearly and monotonically decreased with increasing roundness. Then, the microscopic characteristics, including the coordination number, particle rotation and percentage of sliding contacts, were examined. An increase in the coordination number obstructed the rotation of particles and led to an increase in the percentage of sliding contacts. Finally, analysis of the related anisotropy coefficients of the entire contact network was carried out to probe the microscopic origins of macroscopic behaviour. A detailed analysis revealed the microscopic mechanism of the dependence of the peak and residual shear strengths on roundness.

The influence of particle elongations on direct shear behaviour of granular materials using DEM

This study examines the influence of particle elongation on the direct shear behaviour of granular materials using the discrete element method. A series of numerical direct shear test simulations were performed, and both the macroscopic and microscopic behaviour of elongated assemblies at the critical state were examined. The macroscopic response of elongated particles exhibits an initial hardening followed by post-peak strain softening, prior to reaching the critical state. The peak state friction angles initially increase and stay stable as the dimensionless elongation parameter ($$\eta$$) increases, whereas the critical state friction angles increase with the increase of $$\eta$$. Independent of the applied normal stresses, all samples reach a critical state at a unique normalized stress ratio (i.e., $$\tau /\sigma = 0.51$$) after ~ 25% shear strain. The stress-fabric relationship is mainly governed by the strong force subnetwork which is more affected by the change of η than the weak force subnetwork. Particle elongation generates a downward shifting of critical state lines (CSLs) in $$e - p^{{\prime }}$$ space. Furthermore, the correlations between CSL parameters and $$\eta$$ are well-fitted by a second-order polynomial function. These findings highlight the significance of particle elongation on direct shear behaviour of granular materials.

The influence of particle geometry and the intermediate stress ratio on the shear behavior of granular materials

The behavior of granular materials is very complex in nature and depends on particle shape, stress path, fabric, density, particle size distribution, amongst others. This paper presents a study of the effect of particle geometry (aspect ratio) on the mechanical behaviour of granular materials using the discrete element method (DEM). This study discusses 3D DEM simulations of conventional triaxial and true triaxial tests. The numerical experiments employ samples with different particle aspect ratios and a unique particle size distribution (PSD). Test results show that both particle aspect ratio (AR) and intermediate stress ratio $$(b=({\upsigma }_{2}'-{\upsigma }_{3}')/({\upsigma }_{1}'-{\upsigma }_{3}'))$$ affect the macro- and micro-scale responses. At the macro-scale, the shear strength decreases with an increase in both aspect ratio and intermediate stress ratio b values. At the micro-scale level, the fabric evolution is also affected by both AR and b. The results from DEM analyses qualitatively agree with available experimental data. The critical state behaviour and failure states are also discussed. It is observed that the position of the critical state loci in the compression $$(e-p')$$ space is only slightly affected by aspect ratio (AR) while the critical stress ratio is dependent on both AR and b. It is also demonstrated that the influence of the aspect ratio and the intermediate stress can be captured by micro-scale fabric evolutions that can be well understood within the framework of existing critical state theories. It is also found that for a given stress path, a unique critical state fabric norm is dependent on the particle shape but is independent of critical state void ratio.

Scaling behavior of immersed granular flows

The shear behavior of granular materials immersed in a viscous fluid depends on fluid properties (viscosity, density), particle properties (size, density) and boundary conditions (shear rate, confining pressure). Using computational fluid dynamics simulations coupled with molecular dynamics for granular flow, and exploring a broad range of the values of parameters, we show that the parameter space can be reduced to a single parameter that controls the packing fraction and effective friction coefficient. This control parameter is a modified inertial number that incorporates viscous effects.

Fast shear behavior of granular materials in ring-shear tests and implications for rapid landslides

The high mobility of rapid landslides is one of the most important subjects of both theoretical and practical interest to engineers and scientists. The idea that ultralow resistance could explain the high mobility inspires researchers to examine the shear behavior of granular materials under a wide range of conditions, but the response of granular materials to fast loading rates is largely unknown. The motivation for this study was to examine several fundamental issues of particle properties and mechanical conditions on the fast shear behavior of granular materials. Two granular materials were studied in the oven-dried state and were sheared by employing a ring-shear apparatus. Results indicated that angular particles (silica sand) had higher shear strength parameters than spherical particles (glass beads). In addition, the dilative process was observed during shearing, which depended on normal stress and particle shape. A slightly negative shear-rate effect on shear strength was observed for both granular materials under a certain range of shear rates. Furthermore, cumulative shear displacement had a significant effect on the degree of particle crushing. Fast ring-shear tests also revealed that shear rate had a slightly negative effect on particle crushing. Based on these experimental results, the possible applications of dynamic grain fragmentation theory to assess the high mobility of rapid landsliding phenomena were discussed. It was indicated that the magnitude and release rate of elastic strain energy generated by grain fragmentation played important roles on the dynamic process of landslide mobility.

Experimental Study of Shear Zones Formed at Sand/Steel Interfaces in Axial and Torsional Axisymmetric Tests

The interface shear behavior of granular materials is central to many engineering applications, including the performance of structures like deep foundations, landfills, and retaining walls. Consequently, it is paramount to understand the behavior of construction material-soil interfaces involved in these applications. Furthermore, it has been shown that the study of interface behavior, in the laboratory and in-situ, can provide robust information about the soil's properties and engineering performance. This paper presented laboratory evaluations of micro and meso-scale shear deformation of medium-sized sands aimed at developing an improved fundamental understanding of granular-continuum stress-strain behavior. A comparison of interface testing results from two different shear directions—axial and torsional—demonstrated that the evolution and progression of shear zone formation was affected differently by changes in the interface surface roughness and particle angularity. In particular, it was observed that torsional shear is a more dilative process that induces a larger degree of soil shearing and is greatly affected by particle angularity. Studies of shear-induced volume changes also revealed that the influence zone for torsional shearing is larger than that for axial shearing, with soil dilation occurring inside the shear zone in contact with the material counterface and soil contraction in a surrounding outer zone. Fundamental micromechanical processes that aim to explain the differences between the behavior of axial and torsional tests are proposed.

Observed Effects of Interparticle Friction and Particle Size on Shear Behavior of Granular Materials

AbstractThis paper presents an experimental study on the shear behavior of granular materials, focusing on the effects of interparticle friction and particle size, which are of fundamental importance but are not yet well understood. The experimental program consisted of a large number of direct shear tests on glass beads of varying sizes and interparticle friction conditions, performed under a range of packing densities and normal stress levels. Test data were interpreted in terms of the stress–dilatancy relationship and shear strength parameters. The study indicates that under otherwise similar testing conditions, oil-lubricated glass beads tend to have substantially lower shear strength as compared with water-lubricated, water-flooded, and dry glass beads. It has also been found that at similar particle size uniformity, increasing mean particle size (d50) leads to more dilatant shear response and higher shear strength. A generalized stress–dilatancy relation is proposed, which introduces a variable dila...

Localized deformation in sands and glass beads subjected to plane strain compressions

In order to investigate shear behavior of granular materials due to excavation and associated unloading actions, load-controlled plane strain compression tests under decreasing confining pressure were performed under drained conditions and the results were compared with the conventional plane strain compression tests. Four types of granular material consisting of two quartz sands and two glass beads were used to investigate particle shape effects. It is clarified that macro stress-strain behavior is more easily influenced by stress level and stress path in sands than in glass beads. Development of localized deformation was analyzed using photogrammetry method. It was found that shear bands are generated before peak strength and shear band patterns vary during the whole shearing process. Under the same test condition, shear band thickness in the two sands was smaller than that in one type of glass beads even if the materials have almost the same mean particle size. Shear band thickness also decreased with increase of confining pressure regardless of particle shape or size. Local maximum shear strain inside shear band grew approximately linearly with global axial strain from onset of shear band to the end of softening. The growth rate is found related to shear band thickness. The wider shear band, the relatively lower the growth rate. Finally, observed shear band inclination angles were compared with classical Coulomb and Roscoe solutions and different results were found for sands and glass beads.

Effect of opening on the shear behavior of granular materials in direct shear test

This paper examines the effect of the opening between the shear boxes on the shear behavior in the direct shear box test (H=20 mm×D=60 mm) under a constant vertical stress using seven types of granular material. It is observed that the magnitude of shear strength and dilatancy decreases due to the outflow of sample with an increase in the opening. The two notions of the Threshold Point (TP) as the limit opening size and the Threshold Line (TL) offering the guideline of opening size is defined according to the mean particle size of soil samples. However, similar shear behaviors were observed regardless of the opening size because there was no outflow of sample through applying the Teflon sheet in the direct shear box tests for Toyoura sand. Finally, the shear mode on the circular specimen applying the Teflon sheet is also examined in comparison with the results from other tests.

Particle Shape and Crushing Effects on Direct Shear Behavior Using DEM

Numerical analyses using the PFC2D are conducted to study the relative changes of particle crushing and the shear behavior of granular materials according to the quantified particle shapes. A total of seven particle shapes are standardized and quantified. Three different particle models, including a circular particle model, a non-crushing particle model, and a crushing particle model, are developed and analyzed. The results show that shear strength is mobilized in size order: 3 ball>6 ball_T>2 ball>6 ball_R>4 ball>9 ball>1 ball model, corresponding to triangle>rectangle>square>circle shape in both the non-crushing particle model and the crushing particle model. Within the same shape but with a different number of sub-particles, it is found that an increase in the number of sub-particles within a particle coincides with smaller shear strength per model. The non-crushing particle model shows the increase of porosity not only in the shear band, but also in other layers. However, in the case of the crushing particle model, the increase of porosity is mainly focused within the shear band. It is found that with a larger circularity and convexity, that is, as a particle becomes more circular in shape, the shear strength decreases, regardless of particle crushing. It can be concluded that the standardized particle shape model suggested in this research has broader applications for future studies.

DEM analysis of soil fabric effects on behaviour of sand

The effects of initial soil fabric on the shear behaviour of granular materials are investigated by employing distinct element method (DEM) numerical analysis. Soil specimens are represented by an assembly of non-uniform sized spherical particles. DEM specimens that have different initial contact normal distributions were prepared at two different densities (loose and dense) and then isotropically consolidated triaxial tests were simulated with the following conditions: (a) different drainage conditions (undrained/drained), (b) different modes of shearing (compression/extension), and (c) different directions of shearing (vertical/horizontal). The numerical analysis results are compared qualitatively with the observed experimental data and the effects of initial soil fabric on resulting soil behaviours are discussed. The discussions include the effects of specimen reconstitution methods, effects of large preshearing, and anisotropic characteristics in drained and undrained conditions. The effects of initial soil fabric on the quasi-steady state are also investigated. The numerical analysis results can systematically explain that the observed experimental behaviours of sand are due principally to the effects of initial soil fabric. The outcome provides insights into the observed phenomena in microscopic view.