PARTICLE SIZE DISTRIBUTION

What the sheet shows

The sheet plots three particle size distribution (PSD) curves: percentage passing on the vertical axis against particle size on a logarithmic horizontal axis. One curve is uniformly graded, one well graded and one gap graded.

How it works

The coarse part of the curve comes from sieving: a dried sample is shaken through a stack of sieves, and the mass retained on each is weighed ^[1]. The fine part, silt and clay, comes from a sedimentation test. Each point on the curve is the percentage of the sample finer than that size.

The shape tells you how the grains pack. A uniformly graded soil has most grains about the same size, so the curve is steep and nearly vertical. A well graded soil has a wide spread of sizes, so the curve is a long smooth S; small grains fill the gaps between large ones, which gives high density and good strength. A gap graded soil is missing a range of sizes, and the curve has a flat step.

Key equation

Cu = D60 / D10, Cc = (D30)² / (D10 · D60)

Worked example

From a curve, D10 = 0.1 mm, D30 = 0.4 mm and D60 = 1.2 mm. Then Cu = 1.2 / 0.1 = 12 and Cc = 0.4² / (0.1 × 1.2) = 0.16 / 0.12 = 1.3. Under the Unified Soil Classification System a sand is well graded when Cu is at least 6 and Cc is between 1 and 3 ^[2], so this is a well graded sand if its fines content is low.

In practice

Grading drives decisions well beyond classification. It controls whether a fill compacts well, whether a filter will stop the soil next to it washing through, how permeable the ground is, and whether a sand is prone to liquefaction. Uniform fine sands and gap graded soils are the ones to watch: the first can liquefy, the second can lose its fines by internal erosion.