2021 Volume 77 Issue 1 Pages 59-69
This paper has tried to improve the understanding of a main cause for the difference of the particle-size distribution between the Sieve-Hydrometer Method (SHM) and the Laser Diffraction Method (LDM). The basis for this study is a series of particle-size analyses using 8 fine-textured soil samples applying the two methods. Photographs of fine-grained soils were taken by a Scanning Electron Microscope (SEM) at a magnification of 2,000 examining in detail the particulate shapes. A model calculation was executed using Heywood’s approximate method for obtaining terminal falling velocities of non-spherical particles. The above experimental values were compared with these calculation values, and the influence of the irregularity of particle shapes on settling velocity reduction was examined in detail based on the result.
As a result of this study, the following observations and considerations were made: 1) the particle-size analysis demonstrated that the SHM overestimated the percentages of fine-grained fraction (<10μm) with respect to the LDM; 2) the SEM photographs indicated that most of fine-grained soils were irregular in shape, or were platy, tubular, flat disc-like, and rod-like in shape; 3) in fine-grained fraction, SHM diameters tended to be considerably smaller than LDM diameters at the same cumulative percentage of the particle-size distribution; and 4) non-spherical particles in hydrometer measurements had longer settling time than their equivalent spheres, which resulted in an overestimation of the fine-grained fraction. There is a high probability that a critical factor differentiating the two methods is that the particle shapes in fine-grained fraction differ significantly from spherical forms.
