Granular Matter最新文献

The operating frequency of the compaction equipment plays a pivotal role in the efficiency of the compaction process. However, it is not clear whether it is only the result of the frequency-dependent load amplitude or the frequency itself (affecting the dynamics of soil structure). This problem is difficult to solve experimentally since the operating frequency is strictly related to the load amplitude for specific equipment. In this paper, a numerical DEM simulation is conducted that allows compaction to be simulated with a centrifugal force independent of the compaction frequency. First, the material model is calibrated by taking into account particle size distribution, shapes, and mechanical behaviour. Next, the model is utilised in the simulation of compaction with a plate compactor at the operating frequency range of 50–90 Hz. The results obtained correspond well to the physical experiment and allow for additional conclusions to be drawn. The only advantage of compaction with higher frequency is the increased force amplitude. If the force amplitude is maintained, the same void ratio can be obtained for lower frequencies.

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The direct simple shear (DSS) test serves as a vital method in geotechnics, allowing the measurement of peak and post-liquefaction shear strengths, along with the critical state friction angle of soils. Additionally, the simple shearing mode applied in a DSS test is the predominant failure mode in many geotechnical engineering problems. Although the DSS test is widely used to determine soil strength, a significant challenge with the DSS device is the non-uniformity of stress and strain distributions at the specimen boundaries. This non-uniformity depends on not only the specimen size but also the size of soil particles. The influence of specimen size on boundary effects is typically evaluated using the ratio of specimen diameter (D) to height (H). The median particle diameter (D₅₀), as an indicator of a soil’s particle size, could be another influential factor affecting the non-uniformities of stress and strain on specimen boundaries in a DSS test. Through three-dimensional discrete element method (DEM) simulations, this research explores these factors. Specimens were generated with a particle size distribution (PSD) scaled from a coarse sand sample. Laboratory monotonic DSS testing results on the coarse sand were employed to calibrate the DEM model and ascertain the modeling parameters. Boundary displacements were regulated to maintain a constant-volume condition which represents undrained shearing behavior. Various specimen diameters were simulated with identical void ratios to investigate the influence of D/H on stress path, peak and post-peak shear strengths, and critical state behavior. DEM simulations allowed the generation of several particle size distributions through different scaling factors applied to the sand gradation to determine the combined effect D₅₀ and D/H. Limiting D/H and D₅₀/D ratios are subsequently proposed to mitigate specimen boundary effects.

Graphical Abstract

The study of acoustic emissions (AE) at soil-metal interfaces has gained increasing attention in geotechnical engineering due to its potential for developing acoustic-based early warning systems for structural stability and safety monitoring. Existing studies have paid limited attention to the fundamental mechanisms underlying soil-metal interface shearing across micro to macro scales and their associated acoustic emissions (AE). This study investigated the soil-metal interface shear and their AE responses through systematic tests using macromechanical and micromechanical interface shear testing apparatus, critically analyzing the shear response, geotribological aspects, and AE responses in the time and frequency domains to gain deeper insights and understand their interrelationships. The results revealed that soil-metal interface shear response and AE intensity (amplitude and frequency content) increased as normal stress and particle angularity increased. Unlike the shear response, the increase in displacement rate leads to a considerable increase in AE. Furthermore, the analysis of the test results reveal that the AE of soil-metal interfaces are strongly affected by the hardness of the continuum material, which, in turn, governs particle breakage and shear-induced surface changes during shearing. The novel micromechanical shear tests revealed that there is no AE during plowing, strain softening, or hardening; emissions are only observed when asperity breakage occurs, followed by micro-tapping during shearing. The findings of this study significantly advance the understanding of soil-structure interaction from an AE perspective and contribute to the design of efficient AE-based early warning devices.

This study investigates the impact of fabric anisotropy on the directional filtration mechanisms in granular filters, which arise from inherent particle shape variations and different preparation methods. Using the discrete element method, diverse filter samples underwent extensive numerical filtration tests in different directions. Subsequently, the pore space of these samples was analysed using an extraction algorithm. The results highlight the significant influence of particle shapes and preparation methods on intensifying anisotropy, which in turn remarkably affects directional filtration properties. Analysis of the pore space reveals variations in pore connectivity across different directions, explaining the observed differences in retention coefficients. This study emphasises the need for a comprehensive approach that accounts for constriction size, number, and connectivity to yield precise results. It contributes valuable insights into the role of anisotropy in granular materials, sheds light on complex directional filtration mechanisms, and advances related applications.

A new hypoplastic constitutive model with a modular structure is presented for granular soils. The modular structure allows the application of the constitutive model with very little material information under restriction of the soil effects to be reproduced. The more material information available, the better the stress–strain behaviour of the material can be represented. The basic model and six modules are presented that allow to model soil phenomena like barotropy, pyknotropy, load history, and small strain stiffness. Laboratory tests are simulated to show the performance of the constitutive model.

Understanding the migration trajectory characteristics of top coal in longwall top coal caving (LTCC) is crucial for studying the flow properties of granular top coal, drawing laws, and optimizing the coal drawing process. To monitor the migration trajectory of top coal during the drawing process, an experimental platform was developed for monitoring the top coal migration trajectory in LTCC. Using this platform, physical simulation experiments of LTCC were conducted. A multi-step experimental procedure was designed, including “model construction, marker point installation, simulated coal drawing, data collection, and trajectory inversion.” The migration trajectories of top coal at different layers during the coal drawing process were obtained, and the drawing body of top coal was inferred. Additionally, a bi-directional top coal drawing body equation was theoretically derived, establishing a quantitative relationship between the gangue content (cumulative and instantaneous) and top coal recovery. Based on this, field process optimization was carried out, adjusting the “four-level” method to a double-opening group coal drawing method. The instantaneous gangue content threshold at the coal drawing openings was set to 35%. The measured top coal recovery at the working face reached 90.12%, an increase of approximately 14.87% points compared to the previous recovery. The cumulative gangue content was controlled at around 9.25%, and the coordination efficiency of coal caving reached 68.2%, which is close to the theoretically derived results. This indicates that the theory can provide certain theoretical guidance for determining relevant process parameters in coal drawing operations.

Graphical Abstract

Top - Coal Migration Trajectory and Optimization of Coal Caving Technology

Liquefaction behaviors of sand deposits with impervious stratum are quite different from that of homogeneous geological conditions. However, the micro- liquefaction behaviors of the interlayered deposits have been infrequently documented. This study introduces a novel experimental methodology aimed at examining the influence of silt interlayer on the liquefaction mechanisms of sand deposits from both macro and micro perspectives. In the experiments, the Excess Pore Water Pressure (EPWP) was analyzed in conjunction with recorded micro liquefaction images. The migration mechanism of fine sand particles beneath the silt interlayer was revealed. The existence of low permeability interlayer leads to prolonged retention of EPWP beneath the silt interlayer. Substantially, the water film on the base of the interlayer is demonstrated to be the mixture of pore water and silt particles flowing with high velocity under seismic motions, thereby resulting in significant strain localization. An agminated zone of loose fine sand particles is usually generated beneath the silt interlayer after the dissipation of EPWP.

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The motion of wireless capsule endoscopes (WCE) in the gastrointestinal tract is complex and variable. Measuring its motion patterns accurately is crucial for optimizing diagnostic, therapeutic procedures and improving diagnostic accuracy. To gain a deeper understanding of the motion patterns of WCE in the gastrointestinal tract, particularly its behavior in different regions. A simulation measurement system based on magnetic localization technology is proposed in a laboratory environment. We designed a cylindrical-conical-cylindrical structure simulation device. The free fall motion of soft hydrogel granules is designed to mimic fluid motion in the gastrointestinal tract. A hard-targeted pellet with a permanent magnet simulated the WCE. It measured parameters such as trajectory, vertical velocity, vertical acceleration, and attitude angle of the targeted pellet during its drop at different initial positions in a silo during unloading in a soft granules environment were measured. The experimental results reveal the motion characteristics of a hard pellet in a silo during unloading in a soft granules environment, in the specific wide channel region, as well as in the transition region from the wide channel to the narrow channel. These findings are valuable for understanding the complexity of flow behaviours in different regions of the soft granules environment. And, these findings provide data references for understanding the dynamic behavior of WCE in the gastrointestinal tract, thereby aiding in optimizing WCE design and enhancing its clinical efficacy.

Analyzing geotechnical problems associated with granular material like cohesionless soil typically necessitate constitution of a physical model exhibiting a homogenous soil structure. Such well-conditioned model aids in reliable and reasonable interpretation of soil’s in-situ behavior under controlled conditions. However, such well-conditioned model needs to be reconstituted multiple times with a high degree of consistency. To this motive, the present study aims at the development and performance assessment of a novel mechatronic assisted air pluviation system (MAPS). The modular design of MAPS and the user commanded mechatronics integrated within its operational ecosystem were smartly used to facilitate a uniform and controlled reconstitution of specimen from a cohesionless soil. The reconstitution performance of MAPS was assessed by conducting several air pluviation trails with a poorly graded fine sand (D₅₀ = 0.22 mm), and further examining the effect of pluviation control parameter such as height of fall, sieve porosity, number of diffuser sieve, and diffuser ratio on characteristics of reconstituted sand. A wide range of relative density, ranging from 12 to 90% was achieved for reconstituted specimen upon utilizing the developed MAPS. Further, the mean coefficient of variation in relative density in horizontal and vertical direction of specimen was found to be well within acceptable limit of 5% and 7% respectively.

In this study, we investigate the stability and solid fraction of columns comprised of highly non-convex particles. These particles are constructed by extruding arms onto the faces of Platonic solids, a configuration we term Platonic polypods. We explore the emergence and disappearance of solid-like behavior in the absence of adhesive forces between the particles, referred to as geometric cohesion. This investigation is conducted by varying the number of arms of the particles and the thickness of these arms. To accomplish this, columns are assembled by depositing particles within a cylindrical container, followed by the removal of the container to evaluate the stability of the resulting structures. Experiments were carried out using three distinct materials to assess the influence of the friction coefficient between the grains. Our findings reveal that certain granular systems exhibit geometric cohesion, depending on their geometrical and contact properties. Furthermore, we analyze the initial solid fraction of the columns, demonstrating that these arrangements can achieve stability even at highly loose states, which contrasts with traditional granular materials.

Graphical Abstract

The particles were Platonic polypods with varying arm thickness and different numbers ofarms. Depending on their shape and friction characteristics, these systems can exhibit either frictional or cohesivebehavior.