Particle size segregation during hand packing of coarse granular materials and impacts on local pore-scale structure

Soils and sediments consist of granular particles with an intricate network of pores between them. 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 quasi two-dimensional (2-D) Hele Shaw cell or a column. Soil is broken up and sieved to remove large particles that could cause anomalous measurements; 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 porosity and bulk density, but as we aim to demonstrate, the structural properties will be a function of the method used to repack, and it is unlikely that one can achieve a uniform distribution at the micro-scale. We present results of experiments using granular materials, demonstrating 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 packed sample.