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Sphericity and Roundness of Sand in Coastal Plain Soils and Relationships with Soil Physical Properties¹

ABSTRACT

Sand grain roundness and sphericity were evaluated for 50 soil samples from Atlantic Coastal Plain soils to determine relationships of sand grain roundness and sphericity to the soils' cone index (mechanical impedance), bulk density, and the dense soil angle of repose. A previous investigation collected undisturbed cores from 50 tillage-induced pans in the Atlantic Coastal Plain, and determined cone index, bulk density, and the dense soil angle of repose. One hundred sand grains were isolated from each of the 50 soil materials. Roundness was determined for each grain by comparing the grain's projected image to standard images in published roundness charts. Sphericity was also determined for the 100 grains per soil by measuring selected grain axes. Mean values for roundness and sphericity were determined for each soil sample. Mean roundness values ranged from 0.44 to 0.57 across sites, but 74% of the sites had mean roundness values between 0.45 and 0.49 inclusive. Mean sphericity values ranged from 0.74 to 0.81, but 72% of the sites had mean sphericities between 0.78 and 0.81 inclusive. Correlation coefficients for relations of cone index, bulk density, and dense soil angle of repose to roundness and sphericity were generally 0.26 and not significant (0.10 level). Sphericity and roundness were not significant variables in regression models describing variations in cone index and the dense soil angle of repose. Roundness was not a significant variable in regression models describing variations in bulk density among sites, but sphericity was a significant variable in models that were based on particle size characteristics alone.

¹ Contribution from the Dep. of Soil Science, North Carolina State Univ. Paper no. 10785 of the Journal Series of the North Carolina Agric. Res. Service, Raleigh, NC 27695-7601.

² Associate Professor and Professor of Soil Science, respectively, North Carolina State Univ., Raleigh, NC 27695-7619.

Received for publication November 19, 1986.