Use of a Laser Diffractometer to Obtain the Particle Size Distribution of Fine-Grained Soils

Obtaining the particle size distribution (PSD) of fine-grained soils in hydrometer testing has several shortcomings, particularly long test duration, whereas a laser diffractometer can provide a soil’s PSD relatively expediently. PSDs of the finer fraction of several Hawaiian soils using laser diffraction were compared with those from hydrometer testing. The objective was to investigate some key test variables affecting laser diffractometer PSDs including circulation duration, soil pre-treatment with a deflocculant, and the values of test parameters. The following conclusions are drawn. PSD becomes finer with increasing circulation duration. Circulation duration can be excessively large when attempting to match the laser diffractometer and hydrometer PSDs for the untreated soil. Soil should be pre-treated with sodium hexametaphosphate to decrease the circulation or test duration. The larger the real portion of the refractive index (RI), the coarser the soil. RI values must be calibrated for different laser diffractometer models and for different mineralogical compositions as the RI is a function of not only particle size and mineral composition but also the particle shape and light wavelength and different laser diffractometer models utilize different light wavelengths. The smaller the imaginary component of the RI, the more pronounced the finer fraction. The imaginary component also affects the shape of the PSD, especially in the finer spectrum. The optimum real and imaginary values of RI are 1.4 and −0.1i for the laser diffractometer used and for the fines tested in this study, respectively.