Effectiveness of Laser Diffraction Method for Particle Size Evaluation of Residual Soil

The laser diffraction method (LDM) is a relatively new technique used to evaluate the particle size distribution (PSD) of soil. However, the lack of standard for setting the optical parameters such as refractive index (RI), absorption coefficient (AC), and small sample size results in difficulty of achieving a representative sample. To evaluate the effectiveness of LDM in carrying out the PSD of residual soil, this study was carried out to verify the reproducibility of the LDM at different obscuration levels and compare the results with that obtained by sieve-hydrometer method (SHM). Furthermore, a reasonable value of RI and AC was determined for the residual soil collected from a slope inside Universiti Teknologi Malaysia. Two soil fractions, the coarse fraction retained on 75 µm sieve and finer fraction passing through sieve 75 µm were analyzed. The LDM for fine fraction satisfied the established reproducibility goals of ISO 13320 and provided precise PSD curve at different obscuration levels. For coarse fractions, the wet method showed higher variability and failed to satisfy the ISO 13320 recommended reproducibility guidelines due to the difficulty in obtaining a proper representative sample and disintegration of the particle in the dispersion system. Conversely, the dry method of LDM, due to more sample for analysis and proper representation of the sample, provided a precise PSD curve satisfying the reproducibility criteria of ISO 13320. Based on the findings, a value of 1.55 and 0.1 for the RI and AC is recommended to achieve precise PSD for residual soil using LDM.