Granular Matter最新文献

On June 24, 2017, a catastrophic landslide destroyed the village of Xinmo (Maoxian County, Sichuan, China). A 2.87 × 10⁶ m³ rock mass in source area collapsed and entrained the surface soil layer along the run-out path. This disaster took eighty-three people’s lives and destroyed more than 103 houses. It is worth noting that rock fragmentation and grinding could expand the spreading area of danger zone in a landslide event. The Xinmo landslide provided a rare opportunity to infer the dynamic fragmentation and grinding of rock masses from the particle size and shape distribution in the entrainment and deposition area. A field investigation combined with an automatic image analysis system was conducted to study the characteristics of particle size and shape distribution along the debris channel. The image analysis of these field data showed that the median size (D₅₀) of particles ranged from 0.41 to 27.71 m in the landslide area. Particle fractal dimension (D) obtained from the Number-size distribution ranged from 1.77 to 2.97 over the entire study area. Moreover, the evolution of D₅₀ and D along the run-out path confirmed that the degree of cumulative rock fragmentation increased as the travel distance increased. Additionally, the particle roundness (R) ranged from 0.51 to 0.88 along the run-out path, which peaked twice during the motion of granular flow, once was in the entrainment area, and another was in the end of the deposition area. Rock scraping occurred in the entrainment area could increase the degree of rock grinding, and reshape coarse stones into smooth particles of large R values (larger roundness of particles could lead to longer spreading distance in a landslide event, due to the lower internal friction among particles). Based on analysis above, the rock scraping phenomena occurred between the source materials and entrainment materials were confirmed to influence the translation and spread of granular flows in landslides.

Graphical Abstract

Granular flow is a scientific problem with applications in many engineering problems such as sinkholes, ground surface collapse, and soil loss into a defective pipe. In most previous studies, however, only single size grain has been considered in mass flow rate calculations for simplicity. In nature, most granular materials have different sizes of grains. It is believed that the grain size distribution plays an essential role in the flow mechanics of granular material, which has not been well studied. This study investigates the effects of the grain size distribution on the granular flow through a slot experimentally. It also examines the development of a free-fall arch during the flow process. The results show that the grain size, grain size distribution, and slot width are the controlling parameters that determine the mass flow rate. The mass flow rate is found to be independent of the material height above the slot for any given grain size distribution. This observation suggests the existence of a free-fall arch that controls the flow rate above the slot. Besides, the mass flow rate is governed by the finer portion of the material rather than the coarser materials. In calculating the mass flow rate using the existing formula, there are some discrepancies between the calculated and observed flow rates which are attributed to the simplifications in the formula, which does not consider the effects of grain shape and flow density. An attempt is made to identify the characteristic grain size based on experimental measurements for non-uniform grain size distributions to calculate the mass flow rate.

Pore fluid plays a crucial role in many granular flows, especially those in geophysical settings. However, the transition in behaviour between dry flows and fully saturated flows and the underlying physics that relate to this are poorly understood. In this paper, we report the results of small-scale flume experiments using monodisperse granular particles with varying water content and volume in which the basal pore pressure, total pressure, flow height and velocity profile were measured at a section. We compare the results with theoretical profiles for granular flow and with flow regimes based on dimensional analysis. The runout and the centre of mass were also calculated from the deposit surface profiles. As the initial water content by mass was increased from zero to around 10%, we first observed a drop in mobility by approximately 50%, as surface tension caused cohesive behaviour due to matric suction. As the water content was further increased up to 45%, the mobility also increased dramatically, with increased flow velocity up to 50%, increased runout distance up to 240% and reduced travel angle by up to 10° compared to the dry case. These effects can be directly related to the basal pore pressure, with both negative pressures and positive pore pressures being measured relative to atmospheric during the unsteady flow. We find that the initial flow volume plays a role in the development of relative pore pressure, such that, at a fixed relative water content, larger flows exhibit greater positive pore pressures, greater velocities and greater relative runout distances. This aligns with many other granular experiments and field observations. Our findings suggest that the fundamental role of the pore fluid is to reduce frictional contact forces between grains thus increasing flow velocity and bulk mobility. While this can occur by the development of excess pore pressure, it can also occur where the positive pore pressure is not in excess of hydrostatic, as shown here, since buoyancy and lubrication alone will reduce frictional forces.

Graphical abstract

Particle shape affects the mechanical behavior of crushable granular media, especially in the context of phenomena such as impact and penetration. However, shape descriptors are rarely incorporated into fracture criteria for single grains, which focus instead on size effects and assume idealized spherical geometries. This study aims to extend multiple frameworks used to predict the crushing resistance of individual sand grains by incorporating the effect of particle shape. We conducted an experimental study for varying grain geometries, as revealed by x-ray tomography, and propose a series of analytical models incorporating the grains’ aspect ratio, computed by ellipsoidal approximation fitting. Specifically, non-spherical shape descriptors are incorporated into different contact laws, fracture criteria, and statistical failure models providing closed-form expressions of the strength of single particles as a function of their size and shape. We compare the performance of these models and assess their accuracy against a series of compression experiments on Ottawa sand grains. Experimentally, we find that elongated grains tend to break at lower compression stress than spherical particles of equal size and that their strength depends more on their shape than on their size. By comparing the performance of the proposed models, it was found that the modified Weibull model for non-spherical grains provides the best overall performance. However, the proposed centre crack model for ellipsoidal grains was found to have a similarly satisfactory ability to capture the experimental evidence, while requiring a simpler parameter calibration procedure. By providing criteria to rationalize and predict the effect of the shape on the crushing resistance of single particles, these results offer an analytical foundation to model shape-dependent particle strength in discrete and continuum models for particle crushing which require this quantity as an input for their analyses.

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To study the screening process and explore the segregation mechanism of wet sand and gravel particles, the screening of wet particles is carried out by discrete element method. The accurate parameter settings of wet particles are obtained by parameter calibration of the cylinder lifting. Then, the distribution of wet particles shows the triangular cone and ellipsoid particles have been screened in feeding and screening area. Moreover, there is obvious splash of particles in dry screening while there is no splash in wet screening, which explains the serious dust pollution in dry screening process. Meanwhile, the number of triangular cone and ellipsoid particles in feeding and screening area in wet screening process is more than that in dry screening process, which indicates wet screening process has higher screening efficiency. By comparing the average distances of different particles to the screen surface, the segregation mechanism of different particles is obtained. Furthermore, the square of velocity trends are in good agreement with the average distances of particles, which indicates that square of velocity can also be used as the evaluation index for particle segregation. The above researches provide a reference for optimization of the wet screening process parameters.

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Classical avalanche defending structures aim to catch and deflect the motion of avalanches, this paper proposes several types of step-pool-type and side braking structures to reduce the avalanche impact and investigates their energy dissipation efficiency. In our study, the adoption of µ(I) rheology into the framework of N–S(Navier–Stokes)-type governing equations enables the 3D (three-dimensional) description of the hard-to-predict dynamic properties of avalanche with low computational cost. In particular, our approach overcomes limits imposed with depth-averaged models currently used, and has the potential to capture the braking effect of these defending structures accurately. A numerical program was developed on the open-source platform OpenFOAM specifically for the full model to simulate the entire evolutionary process of the avalanche as well as the obstruction of braking structures. Laboratory experiments are also conducted to verify the simulation. Clearly, our analysis of different cases indicates that avalanches are effectively blocked by side and step-pool-type structures as well as baffle piles, whose energy dissipation effect are significantly affected by their configurations. Simulation results deliver supportive information for the design of avalanche defending structures.

Particle breakage plays a crucial role in determining the macroscopic mechanical behaviors of granular materials, such as compressibility and shear strength. This study aims to investigate the mechanical behavior and particle shape evolutions of three types of granular materials, namely Leighton Buzzard sand (LBS), glass bead (GB), and carbonate sands (CSs), through a series of 1D compression tests. The study employs micro-computed tomography (micro-CT), image processing, and analysis techniques to build a comprehensive fragmentation database and elucidate the statistical mechanical behavior of granular materials. A set of samples were prepared for each granular material type and compressed to a desired stress level. The compressed samples and natural sand particles were then scanned using micro-CT, and the irregular particle morphologies were reconstructed through a series of image processing techniques. By analyzing the particle size distributions and the evolutions of the particle shape, a detailed comparison between the LBS, GB, and CS particles was conducted. The study reveals that the mechanical behavior and fracture patterns of granular materials are influenced by the initial particle morphology and mineralogy. The CS particles, which exhibit abundant intra-particle pores and irregular morphology, have lower compressive strength and higher compressibility compared to LBS and GB particles. Furthermore, the study finds that the particle size of the newly generated fragments for LBS, GB, and CS particles is primarily concentrated around 0.3 mm, 0.65 mm, and 0.18 mm, respectively, indicating significant differences in the particle failure modes between them. The statistical analysis of the newly generated fragments provides quantitative results that help us better understand the development of particle breakage and gain deep insights into the role of grain shape in the mechanical behavior of granular materials.

Graphical abstract

Dynamic disturbance is an essential factor leading to rock discontinuity’s deterioration and instability, which induces geological disasters such as collapses and landslides. To study the mechanical response characteristics and deterioration mechanism of rock discontinuity under different dynamic disturbances, the discrete element PFC is used to apply different waveform stress loads on the rock discontinuity with different roughness. The deterioration process of the structural plane is monitored and observed in real time to analyze the evolution of the deterioration process and the disaster mechanism of rock discontinuity under different dynamic disturbances. The results show that the peak shear stress of the structural plane under triangular wave disturbance is smaller than that under sine wave disturbance, and the difference is less than 1 MPa. With the increase in disturbance cycles, the loose degree of the strong force chain under sine wave disturbance is more significant, and the disturbance deterioration is more serious. The micro-cracks gradually develop and penetrate from the edge to the interior under sine wave disturbance and the opposite under triangular wave disturbance. Compared with the triangular wave disturbance, the crack growth rate is faster, the number of micro-cracks is higher, and the range is more extensive under sine wave disturbance, indicating it is more prone to deterioration.

To explore the characteristics of the flow field and the movement of granular media in horizontal vibration, the motion behavior of granular media was studied based on the horizontal vibratory finishing blade process. The evolution of the fluidization process of granular media under different frequencies and amplitudes was analyzed. The relationship between granular media's long-term and short-term movements and their effects was clarified. And the results were verified by PIV technology. The results show that increasing the frequency and amplitude can improve the fluidization degree of the particle system. Moreover, increasing the amplitude is more effective than frequency. Due to the blade's obstruction, the flow field's characteristic is an asymmetric double rolls. Granular media's macro-movement is divided into long-term and short-term motion based on particles' movement characteristics. The short-term motion of granular media mainly realizes the polishing and finishing of the blade, while the long-term motion realizes the renewal and replacement of granular media. The long-term and short-term movements are positively correlated. This study provides a reference for selecting process parameters and regulating particle flow fields in horizontal vibration.

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