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Particle Size Segregation during Hand Packing of Coarse Granular Materials and Impacts on Local Pore-Scale Structure

George E. Brown Jr. Salinity Laboratory, USDA-ARS, 450 W. Big Springs Road, Riverside, CA 92507

View larger version (109K): [in a new window]   Fig. 1. Upper, mixture of 300- and 800-µm glass spheres. Lower, mixture of 400- and 200-µm quartz sand grains.

View larger version (34K): [in a new window]   Fig. 2. Schematic diagram of the Hele-Shaw cell used in the experiments; the maximum angle of stability and angle of repose are illustrated.

View larger version (20K): [in a new window]   Fig. 3. An overlay of the structures caused by packing that can be observed in a photograph of a repacked sandy soil presented in Fig. 1d of Wang et al. (2002).

View larger version (95K): [in a new window]   Fig. 4. Upper, photograph of a binary mixture of a 50:50 mixture of 300- and 800-µm glass spheres. Banding caused by segregation can be seen parallel to the slope angle. The lower diagram is a textured image used to highlight the dark areas and better show the banding.

View larger version (29K): [in a new window]   Fig. 5. Binary mixtures of quartz sand with (left to right) 60, 40, and 20% small grains. The small grains are colored black and tend to accumulate under the apex of the pile.

View larger version (70K): [in a new window]   Fig. 6. Poured binary mixtures of glass spheres, 47% small (300 µm) and 53% large (800 µm). The small grains appear black in the photos on the left side and light green in the negative images on the right side used to highlight the distribution of beads. The three photos represent piles poured at differing velocities (a) 50 g s-1, (b) 3.7 g s-1, and (c) 0.83 g s-1. The dashed lines show the approximate positions of where the piles were sectioned for the data presented in Fig. 7.

View larger version (29K): [in a new window]   Fig. 7. Upper, the mass fraction of small beads relative to large in Sections 1, 2, and 3, corresponding to Fig. 6a and 6c. The lower diagram shows the respective porosity values.

View larger version (19K): [in a new window]   Fig. 8. Cohesion data for a silty soil as a function of water content. Our suggestion is that the best uniform mixing is achieved below saturation but above air dry, where the cohesion aids the mixing.

View larger version (59K): [in a new window]   Fig. 9. Pictures of two sands of different colors. We used negative imagery to highlight the contrast between the color of the grains. Shown are the initial dry and segregated state, air dry mixed, and results of mixing with increasing water contents. Thorough mixing occurs at a gravimetric water content of about 0.03. At saturation the mixing is better than air dry, but percolation of the small particles begins to occur.