PSD measurement using the single particle optical sizing (SPOS) method

A new technique for measuring the particle size distribution (PSD) of a soil, known as single particle optical sizing (SPOS), has been evaluated. A series of tests was conducted to compare the results obtained from sieve analysis and the SPOS method, using both uniform and well-graded sands of varying particle size and angularity. It was found that the SPOS method oversizes particles compared with sieving by 20-30%, as predicted by theoretical analysis. Measurements of particle size gathered from sieving and SPOS methods are not equivalent. However, as neither of the definitions of particle size implicit in each method can be considered as 'correct', this exercise does not represent a 'validation' or otherwise of the SPOS method. The repeatability of the SPOS method was demonstrated by sizing multiple samples obtained by riffling from the same batch of sand. The accuracy of the SPOS method was demonstrated by the high correlation between the calculated and measured PSD of mixed samples. Finally, a series of tests showed that the system can easily resolve a small (<1%) introduction of fine material into a coarser sample. As the technique requires only a small sample volume, it is particularly suited to the assessment of changes in PSD during geotechnical model or element testing.