DEM simulations of polydisperse media: efficient contact detection applied to investigate the quasi-static limit

Discrete element modeling (DEM) of polydisperse granular materials is significantly more computationally expensive than modeling of monodisperse materials as a larger number of particles are required to obtain a representative elementary volume, and standard contact detection algorithms become progressively less efficient with polydispersity. This paper presents modified contact detection and inter-processor communication schemes implemented in LAMMPS which account for particles of different sizes separately, greatly improving efficiency. This new scheme is applied to the inertial number (I), which quantifies the ratio of inertial to confining forces. This has been used to identify the quasi-static limit for shearing of granular materials, which is often taken to be I=10−3. However, the expression for the inertial number contains a particle diameter term and therefore it is unclear how to apply this for polydisperse media. Results of DEM shearing tests on polydisperse granular media are presented in order to determine whether I provides a unique quasi-static limit regardless of polydispersity and which particle diameter term should be used to calculate I. The results show that the commonly used value of I=10−3 can successfully locate the quasi-static limit for monodisperse media but not for polydisperse media, for which significant variations of macroscopic stress ratio and microscopic force and contact networks are apparent down to at least I=10−6. The quasi-static limit could not be conclusively determined for the polydisperse samples. Based on these results, the quasi-staticity of polydisperse samples should not be inferred from a low inertial number as currently formulated, irrespective of the particle diameter used in its calculation

Strength and stiffness properties of an unsaturated clayey silt: experimental study at high degrees of saturation

Unsaturated constant water content triaxial compression tests with suction measurement, using an Imperial College tensiometer, and saturated consolidated undrained tests were conducted on reconstituted brickearth, a naturally unsaturated clayey silt from London. The results show that the saturated effective stress can be applied to the critical state line (CSL) and normalized stiffness for unsaturated brickearth but that Bishop's effective stress variable gives a slightly improved CSL. The stiffness derived from local instrumentation demonstrates that Bishop's effective stress is also beneficial for normalizing the stiffness modulus over the small-strain range (up to axial strains of about 3%)

A novel transversely isotropic strength criterion for soils based on a mobilized plane approach

The peak shear strength rules of transversely isotropic soils are stress state dependent and dependent on relative orientation between bedding plane and principal stress. Accordingly, the shear strength of transversely isotropic soils exhibits two primary characteristics: (i) the strength curve on the deviatoric plane is asymmetrical with respect to three principal stress axes; (ii) the shear strength changes with the direction angle of the bedding plane when the intermediate principal stress coefficient is a constant. In this paper, the mobilized plane is introduced and used to reveal the failure mechanism of soils. By projecting the microstructure tensor of transversely isotropic soils onto the normal of the mobilized plane, the directionality of the transversely isotropic soils is introduced into the friction rules on the mobilized plane, and a transversely isotropic strength parameter is proposed. The proposed strength parameter can extend isotropic strength criteria into transversely isotropic strength criteria. This mobilized plane approach is used to establish a novel transversely isotropic nonlinear unified strength criterion (TI-NUSC). The difficulty to establish a unified description of the asymmetrical strength curve and its evolution with direction angle is overcome by the established criterion. Comparisons between available test results and the TI-NUSC shows that the TI-NUSC can successfully describe these two primary peak strength characteristics

Effect of depositional water content on the collapsibility of a reconstituted loess

Loess, a wind-blown silty soil, can be deposited under a variety of moisture conditions, including dry deposition, wet deposition and gravitational settling of aggregations formed in moist air by capillary forces at grain contacts. This experimental study uses single and double oedometer tests to assess the effect of depositional water content on the collapse potential of reconstituted samples of the Langley Silt Member, known as Brickearth, a natural loessic soil. A freefall sample preparation technique was used to mimic loess formation and environmental scanning electron microscopy was used to relate the observed behaviour to sample fabric. The results show that loess deposited at higher water contents has a greater collapse potential, which is shown to be related to its looser, more granular fabric

On the progressive nature of grain crushing

In this work acoustic emission (AE) is used as experimental evidence of the progressive nature of grain crushing. Stress controlled high pressure oedometric compression test are carried out on 1.2 mm monodisperse samples of glass beads. It was observed that the granular assembly starts to experience particle breakage at a vertical stress of about 25MPa. When this yield pressure is exceeded the glass beads start to break emitting loud impulsive sound and the vertical displacement increases rapidly. The load was increased beyond the yield stress and at each increment while the vertical stress remained constant the sample continued to emit sound. The emission of sound at a constant vertical stress indicates that crushing is a progressive failure mechanism; once the first crushing event occurs, the structure starts to rearrange causing other crushing events to occur and additional settlement. In particular, two signal processing algorithms are used on the samples of the acoustic signal to obtain two additional metrics of the crushing evolution. The first is the cumulative energy versus time. The second is the number of crushing events versus time, which is based on the automatic detection of the peaks of the sound signal envelope. There is a clear correlation between the cumulative acoustic energy emitted and the observed sample displacement. Using laser scanning, the evolution of the particle size distribution and particle shape are measured in detail so that a link between the acoustic data and the crushing intensity is established. The crushing intensity was controlled using materials with different strengths

Comparison between a μFE model and DEM for an assembly of spheres under triaxial compression

This paper presents a simple case of a Face Centred Cubic (FCC) array of 2,000 spheres under triaxial compression to compare the results obtained using the Discrete Element Method (DEM) and a micro finite element model (μFE). This μFE approach was developed so that the internal structure of the soil can be obtained using x-ray computed tomography and converted into a numerical fabric. The individual grains are represented as continuum deformable bodies and the inter-granular interaction based on the defined contact laws. In order to demonstrate the simple contact constitutive behaviour used in this μFE model, the response for two contacting elastic spheres is compared with theoretical equations. The strength at failure of the packing of 2,000 spheres is seen to yield similar values for DEM, μFE and the analytical solution. When comparing the evolving void ratio, a good agreement between the two numerical models was observed for very small strains but as the strain increases, the values start to diverge, which is believed to be related with the rigidity of the grains used in DEM

Coupled DEM-CFD Analysis of the Initiation of Internal Instability in a Gap-Graded Granular Embankment Filter

Internal instability is a form of internal erosion that can occur in embankment dams or flood embankments where the finer fraction of the material is washed out under the action of seepage flow; if undetected this process can progress to cause embankment collapse. Gap-graded materials are particularly susceptible. Skempton and Brogan [1] proposed that a key contributor to instability is the reduced stress transmitted by the finer fraction and that the magnitude of this reduced stress could be inferred from the hydraulic gradients observed at the initiation of particle migration in experiments. Here Skempton and Brogan’s hypothesis is assessed at the particle scale using a discrete element method (DEM) model coupled with computational fluid dynamics (CFD). This contribution discusses validation of the coupled DEM-CFD software prior to describing the simulation of a permeameter experiment. The simulation generated particlescale data at the initiation of instability by considering a gap-graded sample subject to at a hydraulic gradient of 1.0 (upward flow). The results provide insight into the instability mechanism, most notably showing that while the particles that move under seepage flow do indeed transmit relatively small effective stress, a finite proportion of the particles that move transfer relatively large stresses

Coupled DEM-CFD Analysis of the Initiation of Internal Instability in a Gap-Graded Granular Embankment Filter

Internal instability is a form of internal erosion that can occur in embankment dams or flood embankments where the finer fraction of the material is washed out under the action of seepage flow; if undetected this process can progress to cause embankment collapse. Gap-graded materials are particularly susceptible. Skempton and Brogan [1] proposed that a key contributor to instability is the reduced stress transmitted by the finer fraction and that the magnitude of this reduced stress could be inferred from the hydraulic gradients observed at the initiation of particle migration in experiments. Here Skempton and Brogan’s hypothesis is assessed at the particle scale using a discrete element method (DEM) model coupled with computational fluid dynamics (CFD). This contribution discusses validation of the coupled DEM-CFD software prior to describing the simulation of a permeameter experiment. The simulation generated particlescale data at the initiation of instability by considering a gap-graded sample subject to at a hydraulic gradient of 1.0 (upward flow). The results provide insight into the instability mechanism, most notably showing that while the particles that move under seepage flow do indeed transmit relatively small effective stress, a finite proportion of the particles that move transfer relatively large stresses