INFLUENCE OF PARTICLE DENSITY AND OTHER STOKE'S LAW ASSUMPTIONS ON THE PRECISION AND ACCURACY OF PARTICLE SIZE ANALYSES

Particle size analyses are undoubtedly the most widely performed tests carried out on sediments by geologists, soil scientists, and engineers. The methods used emphasize precision, rather than accuracy, and include, hydrometer and pipette procedures, X-ray measurement (Sedigraph®), electronic measurement (Coulter Counter®), settling tubes, and several others. All methods, however, involve the indirect measurement of grain size and are necessarily biased by particle shape or particle density (or both). Shape is an important consideration when samples are sieved and may preclude meaningful comparisons if analyses are carried out on materials that differ significantly in this property. A similar problem arises when methods are used to evaluate silt- and clay-sized particles by methods whose fundamental basis is Stoke’s Law. When this principle was formulated in 1851 it was clearly predicated on a number of basic assumptions, one of which is particle sphericity and another that assumes all particles have the same density. The latter is rarely realized because sediments often contain mixtures of quartz, clay minerals, carbonates, and other mineral phases. Because of this, comparison of size analyses of samples differing greatly in mineral composition may be fraught with error. As long as the sediments are composition similar, however, any particle size method may be used to obtain relative comparisons (with the understanding that some methods do provide somewhat higher precision than others). Some solace may be also gained by results that clearly showed that density differences from the generally assumed value of “2.65" do not significantly alter the statistical measures of central tendency and dispersion (mean, median, sorting, skewness, etc.). Analysis of 77 samples whose empirically-calculated densities varied from 1.7 to over 4.5 showed no statistical differences when a value of 2.65 was used to calculate the Stoke’s Law settling velocity, rather than the true particle density.