Loose Granular Materials Research Articles

Propagation velocity of landslide-induced liquefaction and entrainment of overridden loose, saturated sediments

Loose saturated granular materials are particularly susceptible to instability, resulting in deviatoric strain softening, and static liquefaction. When instability occurs in the context of a landslide, the consequences in terms of the mobility of the debris and risk to life and property can be catastrophic. Physical model landslides initiated in a geotechnical centrifuge under rising groundwater conditions were used to trigger instability and static liquefaction. Four experiments with a loose contractile soil and two experiments with a dense dilative soil were performed. The velocity of propagation of the liquefaction front within the loose granular soils at the base of a landslide was quantified using a dense sensor network of pore water pressure sensors and high-speed imaging. On triggering of a landslide, a localized toe failure was observed to shear and liquefy the soil at the base of the landslide. However, the velocity of this initial failure (0.5 m/s) was an order of magnitude slower than the subsequent 4.2 m/s propagation velocity of the liquefaction front. These experiments demonstrated and quantified how a localized failure onto a liquefiable deposit may propagate liquefaction much farther than simply the runout of the localized failure, and highlight the potential implications and consequences of such an occurrence.

Microscopic analysis of the nonlinear stiffness of granular materials at small-to-medium strain

The nonlinear stiffness of dense and loose granular materials at small-to-medium strain is investigated using the three-dimensional discrete element method (DEM). The threshold strain of the nonlinear elasticity of granular materials, i.e., the Y2 surface in the kinematic yielding surface framework, is obtained from the simulated drained triaxial tests. The analytical stress-force-fabric relationship is used to quantitatively examine the evolution of fabric anisotropy. Results show that the after reaching Y2 surface, the contact normal anisotropy begins to increase from a steady low level, and evident slippage begins to occur at strong contacts. The threshold strains of the contact normal anisotropy and strong contact slippage increase linearly with the confining pressure, which is consistent with the threshold strain of the nonlinear elasticity of granular materials. The creation of new contacts and disruption of existing contacts during loading are investigated. A three-stage micromechanism of the observed nonlinear stiffness behavior is proposed.

Packing properties and steady strength of cemented loose granular materials

We study samples composed of loose cemented assemblies of particles under isotropic compression and biaxial shearing by means of a discrete-element approach. Compression tests are undertaken by consolidation of grains initially not presenting contacts under varying level of cementation and increasing confining pressure. We find a nonlinear evolution of the solid fraction with pressure that is described using the evolution of granular connectivity and the collapse of pores under homogeneous load. The poral space is characterized in terms of probability of void size number and volume distribution which, surprisingly, can contain voids as 30 times the size of an average particle. Under steady flow, the shear strength turned out to evolve non linearly with the cementation level. For cementation strengths below the confining pressure, the cementation between particles has little effect upon macroscopic friction angle. For greater values of cementation, a rapid increase of macroscopic friction occurs despite a drop in grain connectivity. Macroscopic cohesion is, in turn, small when compared with the interparticle bonding strength for highly cemented samples. The increment of macroscopic strength is found to deeply depend on the anisotropy of contact forces despite a homogeneous distribution of contact orientations and lower connectivity for highly cemented samples.

Quasistatic response of loose cohesive granular materials

DEM-simulated model cohesive assemblies of spherical grains of diameterd, with contact tensile strengthF0, once prepared in loose states, are quasistatically subjected to growing isotropic pressureP, and then to triaxial compression, maintaining lateral stresses σ2= σ3=Pwhile increasing axial stress σ1=P+qand strain є1. Reduced pressureP*=d2P/F0varies from 0.1 (cohesion dominated case, for which systems typically equilibrate with solid fraction Ф ≃ 0.35), to large values for which the cohesionless behavior is retrieved. In triaxial compression, while the moderate strain response (є1~ 0.1) is influenced by initial coordination numbers and mesoscale heterogeneities, the approach to the critical state, as bothq(deviator) and Ф steadily increase, gets slower for smallerP*. Critical ratioq/P strongly increases for decreasingP*, as roughly predicted in an “effective stress” scheme. Anomalously small elastic moduli are observed in the gel-like structures. While extensive geometric rearrangements take place, no shear banding is observed. Loose cohesive granular assemblies are thus capable of large quasistatic stable plastic strains and ductile rupture.

Numerical Investigation of Mineral Grain Shape Effects on Strength and Fracture Behaviors of Rock Material

Rock is an aggregate of mineral grains, and the grain shape has an obvious influence on rock mechanical behaviors. Current research on grain shape mostly focuses on loose granular materials and lacks standardized quantitative methods. Based on the CLUMP method in the two-dimensional particle flow code (PFC2D), three different grain groups were generated: strip, triangle, and square. Flatness and roughness were adopted to describe the overall contour and the surface morphology of the mineral grains, respectively. Simulated results showed that the grain shape significantly affected rock porosity and further influenced the peak strength and elastic modulus. The peak strength and elastic modulus of the model with strip-shaped grains were the highest, followed by the models with triangular and square grains. The effects of flatness and roughness on rock peak strength were the opposite, and the peak strength had a significant, positive correlation with cohesion. Tensile cracking was dominant among the generated microcracks, and the percentage of tensile cracking was maximal in the model with square grains. At the postpeak stage, the interlocking between grains was enhanced along with the increased surface roughness, which led to a slower stress drop.

Instability of loose dry granular slopes observed in centrifuge tilting table tests

Landslides in loose saturated granular materials can experience significant deviatoric strain softening leading to static liquefaction and flow failure. This research study aimed at understanding the instability conditions leading to softening has been conducted under triaxial or plane strain conditions on loose granular soils brought to failure under undrained or drained stress paths. In this paper, it is hypothesised that the onset of granular instability should also be reproducible in tilt-table tests of loose dry granular soils. A centrifuge tilt-table apparatus was developed to test this hypothesis in which the stress path imposed on the soil layer could be inferred from an infinite slope analysis and the shear and volumetric strain be measured through the transparent side walls. Experiments performed at 1g, 20g, 40g and 60g illustrate that this approach can capture the constitutive behaviour of the soil layer. Critically, as the tilt-table test is a load-control test, instability was observed as a rapid acceleration in shear strain rate accompanied by a rapid volumetric collapse of the pore space. The tilt-table test therefore not only enables a measurement of the angle of repose, but provides a method to characterise the instability behaviour of granular materials at low stress levels found in shallow landslides.

Improving slip prediction on Mars using thermal inertia measurements

Rovers operating on Mars have been delayed, diverted, and trapped by loose granular materials. Vision-based mobility prediction cannot reliably distinguish hazardous sand from safe sand based on appearance alone. Unlike surface appearance, the thermal inertia of terrain is directly correlated to the same geophysical properties that control slip. This paper presents a quantitative analysis that shows improvement in rover slip prediction when considering thermal inertia based on data from the Curiosity rover. Thermal inertia is estimated for each slip measurement in sand using both on-board and orbital instruments. Slip models are learned using a mixture of experts approach where the experts are identified using thermal inertia. Two-expert models are compared to a single-expert, vision-only model to show that slip predictions are improved by separating high-slip, low thermal inertia sand from low-slip, high thermal inertia sand. Simulated experiments are also presented to show that thermal inertia has the potential to identify sand even when it is beneath a thin layer of surface duricrust. These results support the hypothesis that the consideration of thermal inertia improves mobility estimates for rovers on Mars.

Micro‐inertia origin of instabilities in granular materials

SummaryWithin the framework of the second‐order work theory, the onset of instabilities is explored numerically in loose granular materials through three‐dimensional DEM simulations. Stress controlled directional analysis are performed in Rendulic's plane, and a particular attention is paid to transient evolutions at the microscale. Thanks to a micromechanical analysis, the onset and development of transient mechanical instabilities is explored. It is shown that these instabilities result from the unjamming and bending of a few force chains associated with a local burst of kinetic energy. This burst of kinetic energy propagates to the whole sample and provokes a generalized unjamming of force chains. As force chains buckle, a phase transition from a quasi‐static to an inertial regime is observed. At the macroscopic scale, this results in a transient softening and a loss of controllability. After the collapse of existing force chains, the development of plastic strain is eventually stopped as new stable force chains are built.

Scaling laws for the mechanics of loose and cohesive granular materials based on Baxter's sticky hard spheres.

We have conducted discrete element simulations (pfc3d) of very loose, cohesive, granular assemblies with initial configurations which are drawn from Baxter's sticky hard sphere (SHS) ensemble. The SHS model is employed as a promising auxiliary means to independently control the coordination number z_{c} of cohesive contacts and particle volume fraction ϕ of the initial states. We focus on discerning the role of z_{c} and ϕ for the elastic modulus, failure strength, and the plastic consolidation line under quasistatic, uniaxial compression. We find scaling behavior of the modulus and the strength, which both scale with the cohesive contact density ν_{c}=z_{c}ϕ of the initial state according to a power law. In contrast, the behavior of the plastic consolidation curve is shown to be independent of the initial conditions. Our results show the primary control of the initial contact density on the mechanics of cohesive granular materials for small deformations, which can be conveniently, but not exclusively explored within the SHS-based assembling procedure.

Force Transmission Modes of Non-Cohesive and Cohesive Materials at the Critical State.

This paper investigates the force transmission modes, mainly described by probability density distributions, in non-cohesive dry and cohesive wet granular materials by discrete element modeling. The critical state force transmission patterns are focused on with the contact model effect being analyzed. By shearing relatively dense and loose dry specimens to the critical state in the conventional triaxial loading path, it is observed that there is a unique critical state force transmission mode. There is a universe critical state force distribution pattern for both the normal contact forces and tangential contact forces. Furthermore, it is found that using either the linear Hooke or the non-linear Hertz model does not affect the universe force transmission mode, and it is only related to the grain size distribution. Wet granular materials are also simulated by incorporating a water bridge model. Dense and loose wet granular materials are tested, and the critical state behavior for the wet material is also observed. The critical state strength and void ratio of wet granular materials are higher than those of a non-cohesive material. The critical state inter-particle distribution is altered from that of a non-cohesive material with higher probability in relatively weak forces. Grains in non-cohesive materials are under compressive stresses, and their principal directions are mainly in the axial loading direction. However, for cohesive wet granular materials, some particles are in tension, and the tensile stresses are in the horizontal direction on which the confinement is applied. The additional confinement by the tensile stress explains the macro strength and dilatancy increase in wet samples.

Phase Transformation States of Loose and Dense Granular Materials under Proportional Strain Loading

Depending on the rate of external loading, the permeability, and the boundary conditions, different drainage conditions are expected during the loading of sands. These drainage conditions are thus neither fully drained nor fully undrained as usually assumed for saturated soils, which implies simultaneous changes in pore volume and in pore water pressure. For a given material, the transition between contractive behavior (increase of the pore water pressure) and dilative behavior (decrease of the pore water pressure) defines the phase transformation state. It is found that the position of this phase transformation state, which is known to be dependent on the relative density, depends on the rate of dilatancy (e˙v/e˙1) imposed on the sample. Constraining a loose sample to dilate leads to an unstable state earlier than observed during undrained tests. Even dense samples, which do not exhibit a nonlocalized unstable state during undrained tests, are prone to collapse if the rate to dilatancy is high enough.

Inverse estimation of the non-acoustical parameters of loose granular absorbers by Simulated Annealing

Absorbing materials are fundamental for the acoustic design of buildings. They are used for increasing the insulation from either outdoor or neighbour noise and for raising the acoustic comfort of dwellings. If environmental and health issues are not of concern, conventional sound absorbers, namely synthetic foams and fibrous materials, offer an excellent acoustical performance at a moderate cost. However, the current trend towards eco-efficient products encourages the enhanced use of natural fibre and loose granular recycled materials. Whatever the material type, a model is required to reliably predict its acoustical performance. The microstructural Champoux–Stinson model has demonstrated to provide consistent acoustical predictions once five non-acoustical parameters are given. Instead of measuring these non-acoustical parameters, which is a rather sophisticated and time consuming procedure, an inverse method, based on Simulated Annealing, is proposed in this paper to estimate them from a single and simple measurement of the absorption coefficient. Experimental results on two samples of loose aquarium gravel validate the proposed inverse method.

Rapid compaction of granular material: characterizing two- and three-dimensional mesoscale simulations

There have been a variety of numeric and experimental studies investigating the dynamic compaction behavior of heterogeneous materials, including loose dry granular materials. Mesoscale simulations have been used to determine averaged state variables such as particle velocity or stress, where multiple simulations are capable of mapping out a shock Hugoniot. Due to the computational expense of these simulations, most investigators have limited their approach to two-dimensional formulations. In this work we explore the differences between two- and three-dimensional simulations, as well as investigating the effect of stiction and sliding grain-on-grain contact laws on the dynamic compaction of loose dry granular materials. This work presents both averaged quantities as well as distributions of stress, velocity and temperature. The overarching results indicate that, with careful consideration, two- and three-dimensional simulations do result in similar averaged quantities, though differences in their distributions exist. These include differences in the extreme states achieved in the materials.

An investigation on loose cemented granular materials via DEM analyses

This paper presents the results of a numerical study carried out by 2D discrete element method analyses on the mechanical behavior and strain localization of loose cemented granular materials. Bonds between particles were modeled in order to replicate the mechanical behavior observed in a series of laboratory tests performed on pairs of glued aluminum rods which can fail either in tension or shear (Jiang et al. in Mech Mater 55:1–15, 2012). This bond model was implemented in a DEM code and a series of biaxial compression tests employing lateral flexible boundaries were performed. The influence of bond strength and confinement levels on the mechanical behavior and on the onset of shear bands and their propagation within the specimens were investigated. Comparisons were also drawn with other bond models from the literature. A new dimensionless parameter incorporating the effects of both bond strength and confining pressure, called BS, was defined. The simulations show that shear strength and also dilation increase with the level of bond strength. It was found out that for increasing bond strength, shear bands become thinner and oriented along directions with a higher angle over the horizontal. It also emerged that the onset of localization coincided with the occurrence of bond breakages concentrated in some zones of the specimens. The occurrence of strain localization was associated with a concentration of bonds failing in tension.

Diffuse instabilities with transition to localization in loose granular materials

SUMMARYThis paper is concerned with diffuse and other ensuing failure modes in geomaterials when tested under homogeneous states of shearing in various loading programs and drainage conditions. Material instability is indeed the basic property that accounts for the instability of an initially homogeneous deformation field leading to diffuse failure and strain localization in geomaterials. The former is normally characterized by a runaway type of failure accompanied with a sudden and violent collapse of the material in the absence of any localization phenomena. Against this backdrop, we present a brief overview of material instability in elastoplastic solids where one finds a rich source of theoretical concepts including bifurcation, strain localization, diffuse failure and second‐order work, as well as a considerable body of experiments. Some compelling laboratory experimental studies of material instability with focus to diffuse failure are then presented and interpreted based on the second‐order work. Finally, various material instability analyses using an elastoplastic constitutive and a general finite element analysis of the above‐mentioned laboratory experimental tests are presented as a boundary value problem. It is shown that instability can be captured from otherwise uniform stress, density and hydraulic states, whereas uniform deviatoric loads are being applied on the external boundaries of a specimen. Although the numerical simulations reproduce well the laboratory experimental results, they also highlight the hierarchy of failure modes where localization phenomena emerge in the post‐bifurcation regime as a result of a degradation of homogeneity starting from a diffuse mode signalled by a zero second‐order work. Copyright © 2012 John Wiley & Sons, Ltd.

Static and dynamic angles of repose in loose granular materials under reduced gravity

Granular materials avalanche when a static angle of repose is exceeded and freeze at a dynamic angle of repose. Such avalanches occur subaerially on steep hillslopes and wind dunes and subaqueously at the lee side of deltas. Until now it has been assumed that the angles of repose are independent of gravitational acceleration. The objective of this work is to experimentally determine whether the angles of repose depend on gravity. In 33 parabolic flights in a well-controlled research aircraft we recorded avalanching granular materials in rotating drums at effective gravitational accelerations of 0.1, 0.38 and 1.0 times the terrestrial value. The granular materials varied in particle size and rounding and had air or water as interstitial fluid. Materials with angular grains had time-averaged angles of about 40° and with rounded grains about 25° for all effective gravitational accelerations, except the finest glass beads in air, which was explained by static electricity. For all materials, the static angle of repose increases about 5° with reduced gravity, whereas the dynamic angle decreases with about 10°. Consequently, the avalanche size increases with reduced gravity. The experimental results suggest that relatively low slopes of granular material on Mars may have formed by dry flows without a lubricating fluid. On asteroids even lower slopes are expected. The dependence on gravity of angle of repose may require reanalysis of models for many phenomena involving sediment, also at much lower slope angles.

Intensification of cooling of phosphatic salts and fertilizers

Processes of cooling of various loose (granular) materials in industrial rotary drums, fluidized-bed apparatuses, conductive plate heat exchangers, etc., have been investigated. Methods have been recommended for cooling heat carriers (heat-transfer agents), as applies to specific production conditions. Designs of coolers-separators and coolers-conditioners for powdery and granular products have been developed.

Optimal design of soil dynamic compaction using genetic algorithm and fuzzy system

Computational intelligent techniques, such as fuzzy and genetic algorithm, have proven to be useful in modeling of complex nonlinear phenomena such as dynamic compaction. Dynamic compaction method is used to improve the mechanical behavior of underlying soil layers especially loose granular materials. The method involves the repeated impart of high-energy impacts to the soil surface using steel or concrete tampers with weights ranging 10–40 ton and with drop heights ranging 10–40 m. A relatively exact estimation of dynamic compaction level is of major concern to geotechnical engineers. This paper develops a fuzzy set base method for the analysis of dynamic compaction phenomenon. In this model, the input variables are tamper weight, height of tamper drop, print spacing, tamper radius, number of impact and soil layer geotechnical properties. The main shortcoming of this technique is uncertainty to locate the best sketch of dynamic compaction to gain maximum effect of this method of soil improvement. Therefore, this paper describes the incorporation of genetic algorithm methodology using fuzzy system for determining the optimum design of dynamic compaction. Subsequently, it will be shown that the genetic algorithm has some abilities in the optimization of dynamic compaction design. Also different manners of this algorithm are compared and then the optimized structure of genetic algorithm will be suggested for dynamic compaction.

Stochastic Modeling of Load-Induced Settlement in Loose Granular Materials

The response of loose cohesionless granular material to surface applied loads is investigated from the viewpoint of probabilistic mechanics of particulate media. A model is proposed that is based on the combined propagation of intergranular forces and an excess volume of voids. In this regard, it provides a bridge between earlier theories developed independently for the diffusion of stresses and for the propagation of settlements. In its general formulation, the theory can model three-dimensional, transient effects. However, the model is believed to be limited to normally consolidated or noncompacted, fully drained or dry, granular materials that do not exhibit dilatancy effects. The derived numerical modeling of steady state deflection patterns under a rigid footing is found to be in good agreement with x-ray images of laboratory model tests using noncompacted silt. The proposed theory recognizes the discrete and inherently random nature of natural granular materials such as cohesionless soils and builds upon these fundamental characteristics to predict responses of such materials to boundary applied load. This is achieved by modeling intergranular force and excess pore volume propagation as Markovian diffusion-advection processes. This approach, which departs from traditional continuum mechanics models, seems to have potential for addressing some of the challenging aspects of granular material mechanics in lunar or Martian environments.

Bifurcation and second‐order work in geomaterials

AbstractIn this paper, the ability of a material rate‐independent system to evolve toward another mechanical state from an equilibrium configuration, with no change in the control parameters, is investigated. From a mechanical point of view, this means that the system can spontaneously develop kinetic energy with no external disturbance from an equilibrium state, which corresponds to a particular case of bifurcation. The existence of both conjugate incremental strain and stress such that the second‐order work vanishes is established as a necessary and sufficient condition for the appearance of this bifurcation phenomenon. It is proved that this fundamental result is independent of the constitutive relation of the rate‐independent material considered. Then the case of homogeneous loading paths is investigated, and, as an illustration, the subsequent results are applied to interpret the well‐known liquefaction observed under isochoric triaxial loading conditions with loose granular materials. Copyright © 2006 John Wiley & Sons, Ltd.