Submitted to: Vadose Zone Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/2/2002
Publication Date: 2/20/2003
Citation: Lebron, I., Robinson, D.A. 2003. Particle size segregation during hand packing of coarse granular materials and impacts on local pore-scale structure. Vadose Zone Journal. v.2. p. 330-337. Interpretive Summary: In the present study we analyze binary mixtures demonstrating some particle size and particle shape effects on the stratification of granular mixtures, and the effect that pouring rates have on the homogeneity of packing as well as on the variability of porosity within the same heap. Soils and sediments are granular materials, as such they are neither solid nor fluid but exhibit characteristics of both, for instance, they flow when poured and act like solids when stationary and confined. It is this somewhat intermediate character that makes granular materials so fascinating. An area that has also caught much attention is the flow and avalanching behavior of granular materials, it also has the more obvious application in the earth sciences for understanding landslides and sand dune formation. It is many of the recent findings on granular flow that have particular application in soil science and the study of sediments. The simplification of considering soils and sediments as homogeneous materials is often an assumption made when measuring and modeling the transport properties of porous media.
Technical Abstract: Soils and sediments consist of granular particles with an intricate network of pores in between. The structure and orientation of these pores will determine how the material transports fluids and contaminants. A common practice in soil science to simplify experiments and to achieve a homogeneous medium against which to test transport equations is to repack a 2D (Hele-Shaw cell) or a 3-D column . Soil is broken up and sieved to remove large particles that could cause anomalous measurements and then it is repacked into the column. However, this procedure destroys the natural structure and imparts a new structural arrangement. The material may appear to have similar bulk properties such as bulk density and porosity but as we aim to demonstrate the structural properties will be a function of the method used to repack and are unlikely to achieve a uniform distribution at the micro-scale. Results in these experiments demonstrate how mixtures of particles of different sizes segregate when poured, forming banded structures. The rate at which a material is poured will determine the uniformity of the grain size distribution.