A comparative study of particle size analyses by sieve-hydrometer and laser diffraction methods

Combined sieve-hydrometer method (SHM) is one of the two conventional methods commonly used in research and practice in all branches of science and engineering dealing with soils. SHM, however, has some inborn defects, which lead to systematical errors. The paper discusses the major causes of such errors in the analysis with examples from saprolitic soils derived from granites and volcanics. It is concluded that the deviations may be explained by three factors: differences in definitions and measurements of particle sizes; variations in densities of constituent grains; and turbulence in flow pattern of suspension. Particle size distribution (PSD) curves produced by the laser diffraction method (LDM) generally agree with those of SHM. However, some discrepancies between the results of these methods become evident upon closer inspection, and vary in extent according to the soil type and specific size fraction. For the same soil sample and a given fraction, such discrepancies can be mainly attributed to variations in density, shape, and mineralogy of particles. LDM with its many advantages over SHM should be adopted as the standard in geoengineering and geoenviromental research, particularly involving problematic soils.