PARTICLE SIZE DISTRIBUTION
Particle Size Distribution (PSD) also known as a Grading, shows us the percentages of each size fraction contained in a sample. Size fractions are generally separated by each size sieve, and can be classified further into particle descriptions based on size fraction. In the highlighted example below, a fine gravel is the part of a sample that passes through a 6.7mm sieve, but is retained on a 2.36mm sieve.
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By using a ‘nest’ of sieves, the sample is screened, and particles that don’t fall through each sieve size are collected and weighed. These masses are then calculated against the original sample mass, giving you a percentage of the total sample.
PSDs are an important test because it can give an idea of the properties of a material. A gravel with no fine fraction (silts and clays) is excellent at allowing water to drain through. Clays are good when you need to stop water from getting through, like a dam wall.
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Well graded soils that contain a wide range of particles are preferred in construction applications because they can be easily compacted - eliminating voids, interlocking the particles, and resisting moisture absorption, allowing the soil to support heavier loads.
PSD results are usually reported with a cumulative ‘percentage passing’ each sieve in a table or with a line graph, an example of a well graded soil is shown below with three different visualisations
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