Abstract

The particle size distributions of sand-size material can be determined either directly by microscopic measurement, or indirectly by sieving or by sedimentation techniques. Routine size analyses by microscopic measurements are impractical because they are too time consuming, although other properties such as grain shape or roundness can be determined when making size analyses by this method. Sieve analyses, because of their convenience and resulting fractionation, have been most extensively used to determine the size frequency distribution of sands. however, since most sediments are deposited in fluid media, sedimentation size analyses are often desirable because they most nearly approach the hydrodynamic conditions of deposition. This paper discusses a sedimentation technique employed for the analysis of sand-size material, and demonstrates, by means of experiments with glass spheres, that single grain terminal settling velocities can be used as suitable approximations for fall rates of grain groups. The settling velocities for groups of grains are 10 to 15 percent faster than those for single grains of the same diameter. Comparative results of size analyses of the same sample indicate that sedimentation diameters generally were coarser than sieve diameters but finer than microscopically measured diameters.

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