Response Of Granular Media Research Articles

Mechanical response of dense pedestrian crowds to the crossing of intruders

The increasing number of mass events involving large crowds calls for a better understanding of the dynamics of dense crowds. Inquiring into the possibility of a mechanical description of these dynamics, we experimentally study the crossing of dense static crowds by a cylindrical intruder, a mechanical test which is classical for granular matter. The analysis of our experiments reveals robust features in the crowds’ response, comprising both similarities and discrepancies with the response of granular media. Common features include the presence of a depleted region behind the intruder and the short-range character of the perturbation. On the other hand, unlike grains, pedestrians anticipate the intruder’s passage by moving much before contact and their displacements are mostly lateral, hence not aligned with the forces exerted by the intruder. Similar conclusions are reached when the intruder is not a cylinder, but a single crossing pedestrian. Thus, our work shows that pedestrian interactions even at high densities (3 to 6 ped/m2) do not reduce to mechanical ones. More generally, the avoidance strategies evidenced by our findings question the incautious use of force models for dense crowds.

Reply to “Discussion of ‘Experimental investigation of shear strength of sands with inherent fabric anisotropy’ by Tong et al. (DOI 10.1007/s11440-014-0303-6)” by Gao (DOI 10.1007/s11440-015-0383-y)

The authors read with interest the discussion of their paper by Dr. Zhiwei Gao. The discusser correctly points out that a general failure criterion without a priori knowledge of the failure plane can be used in general analysis, while a criterion addressing failure at a specific plane is more restricted. However, the latter criterion is more practical for problems where the failure plane is known, and its parameters can be directly determined by the direct shear test which is relatively easy to perform. It must be pointed out that our paper does not propose such a criterion explicitly, but it lays down the experimental evidence and foundation for such a possible proposition. Having said that, we pleasantly notice that the simulations of the model of the discusser agree with, in every aspect examined, our experimental data, both the ones obtained earlier by DEM [1] and the ones on real sand samples of the paper under discussion. It should be mentioned that the model used by the discusser is based on the recently developed Anisotropic Critical State Theory (ACST) by Li and Dafalias [2] that brings into the picture the role of fabric on the response of granular media, and enhances the classical Critical State Theory (CST) by one more condition on the critical state value of the fabric (in fact Y. F. Dafalias is a co-author of that paper that develops the model). There are a few additions to the brief presentation that may interest the soil modeling community. We hope the discusser may consider these in the future.

Electrical‐hydraulic relationships observed for unconsolidated sediments in the presence of a cobble framework

AbstractMechanistic models now exist to predict hydraulic conductivity (K) from the spectral‐induced polarization (SIP) response of granular media. We examined the predictions of such a model on unconsolidated coarse fluvial sediments and compared them to those obtained with a modified Kozeny‐Carman (KC) model. Samples were retrieved from the Boise Hydrogeophysical Research Site (BHRS), located on a gravel bar adjacent to the Boise River, Idaho. A sample holder (0.102 m diameter and 0.12 m in length) was designed to include the cobble framework in reconstituted samples representing the primary stratigraphic units defined based on porosity variation at this site. SIP (0.001–1000 Hz) and K (from Darcy tests) measurements were recorded for 12 samples, with SIP measurements made as a function of pore fluid conductivity (3–300 mS/m), grain size distribution (GSD), and total porosity. K prediction with the KC model was improved after discounting of the cobble framework and multiplying by the tortuosity resulting from matrix “capillaries” around the cobbles, resulting in estimates within a factor of 5 of the measurements. K prediction with a mechanistic SIP model based on Stern layer polarization (SLP model) that requires an estimate of the GSD also required discounting for the cobble framework to obtain estimates within 0.5 orders of magnitude of the measurements. Similarly, the SLP model overpredicts the measured imaginary conductivity ( ) unless the cobble framework is discounted, which then results in estimates of within a factor of 2 of the measurements. This can be explained by the fact that the cobbles polarize at frequencies well below the minimum measurement frequency (0.001 Hz). The SLP model for K prediction parameterized in terms of the formation factor and imaginary conductivity performed well for the 10 samples with a cobble framework without modification as the imaginary conductivity directly senses the matrix grain size characteristics, whereas the formation factor captures the porosity reduction and tortuosity resulting from the presence of the cobble framework (capillary tortuosity). Our findings suggest that the estimation of contrasts in K in coarse sediments may be achievable through measurements of electrical properties after appropriate consideration of the cobble fraction.

Shock-induced deformation in wetted particle beds

The high-strain-rate response of granular media has received considerable attention due to increasing interest in granular penetration. In the present study, we investigate the response of wetted packed particle beds under varying flyer plate-induced shock loadings. We investigate the critical conditions for the onset of particle deformation in systems of spherical macroscopic glass beads. Resulting particle deformations from the shock compression are characterized using microscopy as well as particle size analysis, and the effects of shock strength are compared. A fracturing response with a bimodal particle distribution is observed, with an increasing shift to the lower particle size range as shock loading is initially increased. As the transmitted shock pressure exceeds 1 GPa, a significant decrease in the mean particle size is observed.

Discrete particle model for morphologic and mechanical study

The behaviour of three-dimensional granular media was studied using spheres, examining the response of coarse particles in a 1 m edge cubic cell. The media were adjusted to a predetermined material grain size distribution and subjected to the same boundary conditions as in previous laboratory tests. The study observed how forces are distributed and how the particles are deformed when an external load is introduced to the system. The equations considered are the Hertzian elastic contact law and the Coulomb slip law. In this way, the force of the contacts is calculated with Hertz's law and the force developed is limited by that established using Coulomb's law, allowing relative slip between particles when such a force is reached. Both morphologic and mechanical analyses were carried out. The morphologic analysis studied the way particles adjust in the cell and the mechanical analysis studied the forces and settlements developed. In addition, both analyses were related to determine if there is a relation between the morphology of a coarse granular medium and its mechanical response. Using these two analyses, the paper attempts to explain and model the behaviour and the response of granular media, comparing this response with one obtained from previous laboratory tests, to deduce if the proposed model is adequate for representing laboratory tests.

真三軸応力プローブに対する粒状体の増分非線形応答について

真三軸応力プローブに対する粒状体の増分非線形応答について

Configurational entropy, non-associativity and uniqueness in granular media

It has long been recognized that fabric, i.e., the grain configuration in a domain, plays a significant role in the constitutive response of granular media. Here we establish that this effect has bearing on the suitability of non-associative plasticity laws that describe the observed deviation of the plastic strain rate, from the normal to the yield surface. It is known that when non-associated plasticity laws are used, uniqueness of the solution of the initial value problem cannot be proved.

Derived failure criteria for granular media

AbstractThe failure criterion for a special rate‐type material is derived using a method similar to that of Tokuoka.7 For particular choices of constitutive parameters it is shown that this failure criterion contains, as special cases, failure criteria suggested in four recent works dealing with elastic‐plastic response of granular media.