Granular Compaction Research Articles

Rate-and-state friction of epidote gouge under hydrothermal conditions and implications for the stability of subducting faults under greenschist metamorphic conditions

Epidote is a common hydrous mineral present in subduction zones subject to greenschist metamorphic conditions – and potentially an important control on the fault stability-instability transition observed under greenschist facies. We explore controls on this transition through shear experiments on simulated epidote gouge at temperatures of 100–500 °C, effective normal stresses of 100–300 MPa and pore fluid pressures of 30–75 MPa. We use rate-and-state friction to define these controls of temperature, effective stress and pore fluid pressure on gouge stability. Experimental results indicate that the epidote gouge is frictionally strong (μ ∼ 0.73) and the frictional strength is insensitive to variations in temperature or pressure. With increasing temperature, the epidote gouge exhibits a first transition from velocity-strengthening to velocity-weakening at sub-greenschist conditions (T < 100 °C) before transitioning to velocity-strengthening under greenschist metamorphic conditions (T > 300 °C). Elevating the pore fluid pressure or decreasing the effective stress promotes unstable sliding. The transition in gouge rheology at varied temperatures and pressures is explained by the competition between granular flow-induced gouge dilation and pressure solution-induced gouge compaction. Our results demonstrate that the rate-and-state frictional stability of epidote gouges support the potential for a fault stability-instability-stability transition for subduction under greenschist metamorphic conditions.

Structure evolution analysis of asphalt mixtures during compaction based on particle tracking and granular properties

Pavement compaction is a process of structure evolution of materials accompanied by energy dissipation, which is closely related to the inherent granular properties of asphalt mixture. This research achieved dynamic characterization of particle migration energy and contact force development through particle tracking technique and discrete element simulation. Analytical theories for multi-stage structure evolution and compaction mechanisms were proposed based on granular properties. Results indicate that kinetic, dissipative, and potential energies dynamically represent stages of structure evolution, determined by particle collisions, self-organization into arches, and interlocking reinforcement. The initial density state is established after interparticle collisions, while self-organization occurs as particles adaptively rearrange spatially by rotation in response to unbalanced forces, accompanied by a cyclical strengthening trend of arch formation and collapse. Arches facilitate stress redistribution toward isotropy, with strong contact forces exhibiting a top-down and side-center trend, progressively supported by particles larger than 4.75 mm. Theoretical measures for compaction control were proposed, including pre-calculation of IC, selection of compaction temperature corresponding to the optimum energy efficiency, and control of principal stress deflection within the friction cone. This research aims to provide insight into granular properties and compaction dynamics, thereby contributing to intelligent compaction.

On rapid compaction of granular materials: Combining experiments with in-situ imaging and mesoscale modeling

Grain and pore kinematics are important features of the response of granular materials to impact loading and rapid compaction. These kinematics and the associated material-phase stresses control solidification processes in shock-driven manufacturing and ignition in energetic materials. Diagnostics used in traditional gas-gun experiments cannot resolve spatially-heterogeneous grain and pore kinematics during granular compaction. Similarly, continuum models of the granular compaction process do not account for this spatial heterogeneity, making predictions of solidification or ignition challenging. Here, we propose a method of accessing spatially-heterogeneous grain and pore behaviors during rapid compaction which involves x-ray tomography, in-situ x-ray phase contrast imaging, and mesoscale numerical modeling. We use this method to study heterogeneous grain and pore kinematics and local stresses in a ductile aluminum powder impacted at velocities up to 800 m/s. We first validate the mesoscale model by comparing its predictions with x-ray measurements from an impact experiment on a sample that was used to generate a numerical microstructure. We then quantify the evolution of variables such as 3D pore sizes and local stresses. We comment on the role of microstructure on the granular material’s response and the sensitivity of the material response to changes in structure, impact velocity, and sample size.

Novel non-nuclear methodology for coarse granular soil compaction control: the Sherbrooke Method

The nuclear density gauge (NDG) is the most established device for compaction control. Several studies have proposed alternatives to it, but none have been widely adopted for a variety of reasons. This paper presents the Sherbrooke Method (SM), which involves the use of innocuous, user-friendly, and low-cost devices. It uses frequency domain reflectometry (FDR) sensors to obtain bulk density and gravimetric water content. In this research, 442 field test comparisons between the SM and the NDG device, and 117 between the SM and physical tests were obtained at sites where a type of well-graded gravel (MG 20 in Quebec) was used. The FDR technology showed promising results, with good ability to predict bulk density and gravimetric water content. Moreover, the method presented low complexity of execution and the data emitted by the probe can be obtained in 1 minute. Nowadays, the total lapse of time from setting up the device to the final response (of 3 tests) is approximately 20 minutes. The SM also presents a bulk density precision comparable to that of other devices reported in the technical literature.

Effect of particle size and concentration on low-frequency terahertz scattering in granular compacts [Invited

Fundamental knowledge of scattering in granular compacts is essential to ensure accuracy of spectroscopic measurements and determine material characteristics such as size and shape of scattering objects. Terahertz time-domain spectroscopy (THz-TDS) was employed to investigate the effect of particle size and concentration on scattering in specially fabricated compacts consisting of borosilicate microspheres in a polytetrafluoroethylene (PTFE) matrix. As expected, increasing particle size leads to an increase in overall scattering contribution. Scattering increases linearly at low concentrations, saturates at higher concentrations with a maximum level depending on particle size, and that the onset of saturation is independent of particle size. The effective refractive index becomes sublinear at high particle concentrations and exceeds the linear model at maximum density, which can cause errors in calculations based on it, such as porosity. The observed phenomena are attributed to the change in the fraction of photons propagating ballistically versus being scattered. At low concentrations, photons travel predominately ballistically through the PTFE matrix. At high concentrations, the photons again propagate ballistically through adjacent glass microspheres. In the intermediate regime, photons are predominately scattered.

Crossover from viscous fingering to fracturing in cohesive wet granular media: a photoporomechanics study.

We study fluid-induced deformation and fracture of cohesive granular media, and apply photoporomechanics to uncover the underpinning grain-scale mechanics. We fabricate photoelastic spherical particles of diameter d = 2 mm, and make a monolayer granular pack with tunable intergranular cohesion in a circular Hele-Shaw cell that is initially filled with viscous silicone oil. We inject water into the oil-filled photoelastic granular pack, varying the injection flow rate, defending-fluid viscosity, and intergranular cohesion. We find two different modes of fluid invasion: viscous fingering, and fracturing with leak-off of the injection fluid. We directly visualize the evolving effective stress field through the particles' photoelastic response, and discover a hoop effective stress region behind the water invasion front, where we observe tensile force chains in the circumferential direction. Outside the invasion front, we observe compressive force chains aligning in the radial direction. We conceptualize the system's behavior by means of a two-phase poroelastic continuum model. The model captures granular pack dilation and compaction with the boundary delineated by the invasion front, which explains the observed distinct alignments of the force chains. Finally, we rationalize the crossover from viscous fingering to fracturing by comparing the competing forces behind the process: viscous force from fluid injection that drives fractures, and intergranular cohesion and friction that resist fractures.

Pixelated sintering of α-Al2O3

The sintering of α-alumina by a brand new and innovative technique, called pixelated sintering (PS), is here studied. Densification and grain growth by PS of perfectly controlled granular compacts are analysed and compared to results obtained using Spark Plasma Sintering (SPS) and Pressure-Less SPS (PL-SPS). Materials are exposed to the same temperature profiles whatever the sintering technique used in order to assess the potential of PS in terms of microstructure control. It is shown that PS can be used as an alternative technique to SPS for fast sintering with the advantages of a much simpler and cost-effective set-up, as well as a better control of the localised heat input. PS also appears to be a very modular technology in the way it controls the temperature gradients allowing its implementation for multi-step sintering approaches, as well as for the fabrication of large and complex parts.

A coupled hydro-mechanical model for subsurface erosion with analyses of soil piping and void formation

A coupled hydro-mechanical erosion model is presented that is used for studying soil piping and erosion void formation under practical, in-situ conditions. The continuum model treats the soil as a two-phase porous medium composed of a solid phase and a liquid phase, and accounts for its elasto-plastic deformation behaviour caused by frictional sliding and granular compaction. The kinetic law characterizing the erosion process is assumed to have a similar form as the type of threshold law typically used in interfacial erosion models. The numerical implementation of the coupled hydro-mechanical model is based on an incremental-iterative, staggered update scheme. A one-dimensional poro-elastic benchmark problem is used to study the basic features of the hydro-mechanical erosion model and validate its numerical implementation. This problem is further used to reveal the interplay between soil erosion and soil consolidation processes that occur under transient hydro-mechanical conditions, thereby identifying characteristic time scales of these processes for a sandy material. Subsequently, two practical case studies are considered that relate to a sewer system embedded in a sandy soil structure. The first case study treats soil piping caused by suffusion near a sewer system subjected to natural ground water flow, and the second case study considers the formation of a suffosion erosion void under strong ground water flow near a defect sewer pipe. The effects on the erosion profile and the soil deformation behaviour by plasticity phenomena are elucidated by comparing the computational results to those obtained by modelling the constitutive behaviour of the granular material as elastic. The results of this comparison study point out the importance of including an advanced elasto-plastic soil model in the numerical simulation of erosion-driven ground surface deformations and the consequent failure behaviour. The numerical analyses further illustrate that the model realistically predicts the size, location, and characteristic time scale of the generated soil piping and void erosion profiles. Hence, the modelling results may support the early detection of in-situ subsurface erosion phenomena from recorded ground surface deformations. Additionally, the computed erosion profiles may serve as input for a detailed analysis of the local, residual bearing capacity and stress redistribution of buried concrete pipe systems.

Characterization of Sludge Morphology and Bacterial Community Evolution in the Rapid Activation of Freeze-stored PN/A Granular Sludge

Inoculating granular sludge is an alternative method for the quick start-up of a high-performance autotrophic nitrogen removal reactor. In order to establish the response relationship between sludge activation and reactor performance, the freeze-stored granular sludge was inoculated into a continuous-flow reactor, and a control strategy of the high loading rate and high hydraulic selective pressure was carried out in this study. As a result, a one-stage partial nitritation/ANAMMOX process was started up in 34 days, and the removal efficiency of total nitrogen was over 83%, with a removal loading rate of total nitrogen of 1.67 kg·(m3·d)-1. During this period, the Image pro-plus software was employed to analyze the evolution of the characteristic dimensions of particles. A good linear positive correlation (R2=0.988) between the projected area of the erythrine zone in the inner layer and the specific nitrogen removal rate of granules was found, which provide a simple method to estimate the activity of the PN/A granules. The results of MiSeq high-throughput sequencing showed that the enrichment of aerobic ammonia-oxidizing bacteria (Nitrosomonas) and the wash-out of heterotrophic bacteria (such as Denitratisoma and Haliangium, etc.) were achieved in the start-up of the reactor. Meanwhile, the improvement in granular compactness was in favor of activating anaerobic ammonia oxidizing bacteria (Candidatus_Kuenenia, abundance>30%) that colonized the inner layer of the granules.

Effect of hydraulic selection pressure on the characteristics of partial nitritation/anammox granular sludge in a continuous-flow reactor

Strong hydraulic selection pressure is a control co-factor for maintaining the stability of granular sludge in a continuous-flow bioreactor. In this study, efficient nitrogen removal was achieved in a continuous partial nitritation/anammox (PN/A) granular reactor in a short hydraulic retention time (HRT), decreasing from 100 to 40 min, where more than 95% of ammonium and 80% of total inorganic nitrogen (TIN) were removed at a constant nitrogen loading rate of 2.0 kg (m 3 d) −1 . Results showed that the decreased HRT substantially influenced the observed biomass growth curve, with a trade-off between the growth and washout of microbial populations. Further, sludge characterization indicated that shorter HRT improved granular compactness with the accumulation of extracellular polymeric substances and increased the specific nitrogen removal rate, reaching q (TIN) = 39.9 mg N g −1 VSS ⋅ h −1 . These observations were consistent with shifts in the bacterial community structure of the PN/A sludge with different particle sizes. High-throughput pyrosequencing revealed that strong hydraulic selection pressure promoted the niche segregation of aerobic and anaerobic ammonium-oxidizing bacteria (AOB and AnAOB) and washout of nitrite-oxidizing species in the granules. The effective retention of k -strategists AnAOB (dominated by genus Candidatus Kuenenia) occupying the interior layer of the granules exhibited a solid retention time (SRT) 10-fold longer than that of AOB (identified as Nitrosomonas ) enriched in the aerobic zone. A strategy involving HRT and SRT coordination in operation can enhance the efficiency and stability of PN/A systems, considering the potential development of small aggregates into mature granules. • Strong hydraulic selection pressure was applied in a CSTR with PN/A granules. • NH 4 + -N removal ≥ 95% and TIN removal ≥ 80% were achieved at an HRT of 40 min. • HRT influenced the observed biomass growth curve and granular functional activity. • Shorter HRT promoted niche segregation of AOB and AnAOB in PN/A sludge.

Behavior of granular column-improved clay under cyclic shear loading

To investigate the influence of granular column on cyclic shear characteristics of treated clay, a total number of 42 large-scale cyclic direct shear test was performed. Clay undrained shear strength, granular column diameter, relative density of aggregates, shear displacement half-amplitude and numbers of loading cycles were all varied so that the behavior of the treated clay could be evaluated. The results of the experiments showed that the use of granular column improved up to 30% the maximum cyclic shear resistance compared to the untreated clay, particularly in the initial loading cycles. This improvement was much more considerable as column diameter and relative density of the aggregates increased. As well, increasing the number of loading cycles was found to decrease the magnitude of damping ratio. However, increasing both the column diameter and aggregates relative density has shown to reduce the corresponding value. In addition, the magnitude of secant shear stiffness was observed to reduce by progressing the number of loading cycles. Whereas, this downward trend became flatter as aggregates relative density increased; hence, bearing witness to the positive influence of granular column compaction on mitigation of the shear stiffness degradation with the progress of cycles.

Ejecta from Granular-Medium Cratering by a Supersonic Jet Entering a Continuum Atmosphere

High-fidelity three-dimensional numerical simulations are analyzed to investigate cratering on a granular bed of particles due to a supersonic multi-species fluid jet. In the simulations, the volume-averaged two-phase equations are solved with a multi-species large-eddy simulation formulation for the fluid and a kinetic-theory-based model for the particle bed. The simulations vary according to the ratio of the jet-to-ambient-fluid density (and pressure) and the ambient density. The characteristics of the jets and crater are different according to the cross-sectional shape, which is determined by the jet/ambient density ratio: a conical shape is observed when the ratio is near unity, and a parabolic shape occurs when this ratio is large. Craters are characterized by their diameter, an upper depth, and a lower depth, with the difference between the two depths accounting for the granular medium compaction zone. The variation of the crater diameter timewise evolution is evaluated, and the evolution of the ratio between the upper or lower crater depths and the crater diameter is examined. Joint probability density functions of the Mach number based either on the fluid velocity or the relative velocity between the fluid and particles show that a large portion of the granular medium is dominated by subsonic conditions. The azimuthally averaged ejecta’s radial variation is inspected, and joint probability density functions elucidate the regions where gravity effects are important.

Effects of humidity and glidants on the flowability of pharmaceutical excipients. An experimental energetical approach during granular compaction

Granular materials are part of the design, production and final products of different industrial sectors. Powder flowability is a major topic in manufacturing and transport as it is closely related to process feasibility. Nonetheless, the flows of granular materials are not easy to describe or quantify, even in the simple case of dry monodisperse cohesionless particles. Flowability assessment is not a standard or normalized issue; still, no test is able predict powder flow behavior in all the different mechanical situations encountered during processing.This study aims (1) to evaluate flowability, as device-related, through the force or the energy supplied to the powder bed and (2) to study the effect of glidants and moisture content on flowability. To illustrate these aims, the flowability of two well-known pharmaceutical excipients, Avicel® PH-102 and Retalac® mixed with four different types of precipitated nano-silica (SIPERNAT® D10, D17, 50 S and 500 LS), was assessed using two granular compaction devices: Densitap® and FT4® compaction cell. Our results show that the hydrophilicity of colloidal silica affects surface coverage, ranging from 6% to over 80%. Binary mixtures with hydrophobic additives, D10 and D17, generated smaller silica aggregates with a wider spread on the surface of host particles. For Retalac® conditioned at 20% RH, HR values changed from 1.30 (acceptable flow) to 1.17 (good flow). For Avicel® PH-102, conditioned at 60% RH, HR values changed from 1.22 (fair flow) to less than 1.10 (excellent flow).

Explosively driven dynamic compaction of granular media

This paper reports experimental investigations into the dynamic compaction of particle rings subjected to moderate explosions confined in a radial Hele-Shaw cell. The findings reveal marked transitions in the flow regimes corresponding to the evolution of the transient pressure fields inside the granular medium induced by unsteady gas infiltration. As the pressure fields evolve from being localized to diffusive with a substantial reduction in intensity, three sequent flow regimes with distinct rheologies are identified. Specifically, these flow regimes are found to be governed by the localized strong pressure field, then the competition between the diffusive pressure field and wall friction, and finally, solid stresses in the presence of rarefaction waves. A Bingham-type rheology can adequately describe the granular compaction when the pressure gradients remain the dominant driving forces, whereas the frictional nature of the granular flows becomes increasingly significant as the solid stresses set in. As the pressure gradients phase out, rarefaction decompaction commences. However, this only manages to relax the innermost layers of the compacted particles due to a distinctive compressive deformation pattern, giving rise to a discontinuous flow field. These findings shed light on the rheology of dense granular flows subjected to unsteady pressure loadings involving diverse flow–particle and particle–particle interactions.

Simulation of granular crystallization of cubic particles in twisting container using discrete element method

Granular compaction is a process in which the volume fraction, or density, of the granular materials increases when an excitation is applied. A recent experiment reported that twisting a large number of cubic particles in a cylindrical container leads to an ordered and dense arrangement. This structure is similar to the crystal lattice formed in solidification process. In this article, this phenomenon is repeated by using discrete element method (DEM) simulation. Two different shaped containers are used and it is found that the rectangular angles between the sidewalls and the bottom,namely wall effect, plays a key role. In addition, gravitation is also a very important parameter in this process. The higher gravitation added, the faster crystallization process is achieved. On the contrary, shear force due to friction between particles may slow down this process.

WITHDRAWN: Use of geo-grid reinforcement and stone column for strengthening of mat foundation base

Geogrid reinforced stone column system was used in present research to check the effectiveness of the system in soft soils of Kathmandu Valley. The area consists of silty sand and clay of low plasticity and initial bearing capacity of 75 kPa. Geogrid layers are laid over granular backfill material and compaction was carried out. The results show that there is a significant increase in the bearing capacity and reduction in settlements. The results show that combination of stone column with geogrid reinforcement is an effective solution for ground improvement to reduce settlement, increase bearing capacity and strengthening mat foundation base.

Influence of Geological and Environmental Factors on the Reconsolidation Behavior of Fine Granular Salt

Understanding the consolidation behavior of granular salt is of great significance for salt cavern repository design and the corresponding long-term safety assessments. To evaluate the influence of environmental and geological factors on the reconsolidation behavior of fine granular salt, a series of orthogonal array design experiments were performed. The experimental results showed that temperature, compressive stress, moisture content and compression time had significant impacts on the compaction properties of granular salt. The compressive stress exerted the greatest impact on porosity of the granular salt, which decreased the porosity. An increase in moisture content (within 3%) in the granular salt sample substantially reduced the porosity of the sample, whereas this effect was limited when moisture exceeded 3%. In the process of granular salt compaction, all of the deformation mechanisms were temperature dependent. The correlation between porosity and permeability of the dried compacted salt samples was independent of the environmental and geological factors during the compaction of the granular salt. The use of granular salt with a smaller particle size in the compacted salt backfill can improve its sealing performance. In the transient loading stage of granular salt compaction, the porosity decreased rapidly under stress to form a denser layer, which was mainly accomplished by particle rearrangement and cataclastic flow. However, in the constant-load creep stage, the additional porosity reduction was dominated by grain boundary processes and crystal plasticity mechanisms, including dislocation creep, pressure-solution creep and recrystallization.

Frictional slip weakening and shear-enhanced crystallinity in simulated coal fault gouges at slow slip rates

Abstract. Previous studies show that organic-rich fault patches may play an important role in promoting unstable fault slip. However, the frictional properties of rock materials with nearly 100 % organic content, e.g., coal, and the controlling microscale mechanisms remain unclear. Here, we report seven velocity stepping (VS) experiments and one slide–hold–slide (SHS) friction experiment performed on simulated fault gouges prepared from bituminous coal collected from the upper Silesian Basin of Poland. These experiments were performed at 25–45 MPa effective normal stress and 100 ∘C, employing sliding velocities of 0.1–100 µm s−1 and using a conventional triaxial apparatus plus direct shear assembly. All samples showed marked slip-weakening behavior at shear displacements beyond ∼ 1–2 mm, from a peak friction coefficient approaching ∼0.5 to (nearly) steady-state values of ∼0.3, regardless of effective normal stress or whether vacuum-dry or flooded with distilled (DI) water at 15 MPa pore fluid pressure. Analysis of both unsheared and sheared samples by means of microstructural observation, micro-area X-ray diffraction (XRD) and Raman spectroscopy suggests that the marked slip-weakening behavior can be attributed to the development of R-, B- and Y-shear bands, with internal shear-enhanced coal crystallinity development. The SHS experiment performed showed a transient peak healing (restrengthening) effect that increased with the logarithm of hold time at a linearized rate of ∼0.006. We also determined the rate dependence of steady-state friction for all VS samples using a full rate and state friction approach. This showed a transition from velocity strengthening to velocity weakening at slip velocities &gt;1 µm s−1 in the coal sample under vacuum-dry conditions but at &gt;10 µm s−1 in coal samples exposed to DI water at 15 MPa pore pressure. The observed behavior may be controlled by competition between dilatant granular flow and compaction enhanced by the presence of water. Together with our previous work on the frictional properties of coal–shale mixtures, our results imply that the presence of a weak, coal-dominated patch on faults that cut or smear out coal seams may promote unstable, seismogenic slip behavior, though the importance of this in enhancing either induced or natural seismicity depends on local conditions.

Substitutional effect of copper replacement by cadmium on structural, microstructural and electrical properties of Cu1–xCdxO oxides

In this communication, sol–gel method was employed to synthesize Cd substituted CuO oxides. The substitutional effect of Cd at Cu site in Cu1–xCdxO oxides on the structural, microstructural and electrical properties has been understood. Structural studies reveal single phase nature without any detectable impurity. Scherrer’s formula and Williamson–Hall (W–H) plot analysis suggest that size of the particles increases with increase in Cd content. Microstructural studies show the enhancement in an average grain size as well as improvement in the granular compactness in the CCO samples having higher Cd content. To realize substitutional effect of Cd at Cu site in CCO system on the electrical properties, frequency dependent dielectric, impedance and ac conductivity were recorded. Variations in these electrical properties with frequency and Cd content have been understood in detail. Different theoretical aspects like relaxation mechanism, universal dielectric behavior (UDR) and power law fits have been employed.

Compaction of dispersed granular material by a vibratory compactor with polyharmonic oscillation exciter

This paper addresses problems connected with machine engineering for compaction of powder materials by vibration. The authors present an engineering solution of disperse granular material compaction in pone operating cycle of a compacting equipment. The structural layout of a vibratory compactor involves a polyharmonic oscillation exciter. Some research findings on the compaction process of granular material with the proposed facility are given, and its efficiency is compared with a compacter with a harmonic oscillation exciter. The research shows the promising nature of this method, while the polyharmonic oscillation exciter expands the application range of vibratory equipment in many industries.