USCS classification step by step: ASTM D2487

Updated: 2026-08-04 · ISKRIT

The Unified Soil Classification System reduces a soil to a two letter symbol through a fixed decision sequence. Every branch of it depends on numbers you already have from sieving, except the last one, which needs Atterberg limits.

The decision sequence

First split on the material passing the No. 200 sieve, 0.075 mm. If that fraction is 50 % or more, the soil is fine-grained and the symbol comes from the plasticity chart: ML, CL, MH or CH. If it is under 50 %, the soil is coarse-grained and the sequence continues.

For coarse-grained soil, compare the gravel fraction, 4.75 to 75 mm, with the sand fraction, 0.075 to 4.75 mm. The larger one sets the prefix: G for gravel, S for sand. Then the fines content sets the suffix.

Well graded or poorly graded

Clean coarse soils are graded on two shape numbers taken off the curve:

Cu = d60 / d10 and Cc = d30² / (d10 · d60)

Gravel is well graded when Cu ≥ 4 and Cc falls between 1 and 3. Sand needs the stricter Cu ≥ 6 with the same Cc window. Fail either condition and the soil is poorly graded, P.

A sand with d10 = 0.15 mm, d30 = 0.55 mm and d60 = 2.4 mm gives Cu = 16 and Cc = 0.84. The spread is wide, but Cc is below 1, which signals a gap in the middle sizes, so the symbol is SP and not SW.

Oversize material

Particles coarser than 75 mm are excluded from the classification and reported separately as a percentage of total mass. The gravel, sand and fines percentages are then recomputed on the sieved portion. Skipping this step inflates the gravel fraction and can flip the prefix from S to G.

Where the curve stops being enough

Three of the branches above cannot be closed by sieving alone. Fine-grained soils need liquid limit and plasticity index for the plasticity chart. Soils with more than 12 % fines need the plasticity of those fines to choose between M and C. Dual-symbol soils in the 5 to 12 % band need the same thing. A report that assigns CL or SC without Atterberg limits has guessed.

Questions

What if gravel and sand fractions are exactly equal? ASTM D2487 assigns the sand prefix on a tie only when sand exceeds gravel, so equality goes to gravel.

Are d10, d30 and d60 read or interpolated? Interpolated from the cumulative curve, normally on a logarithmic size axis.

Does USCS have a silt-clay size boundary? No. Unlike EN ISO 14688, USCS separates silt from clay by plasticity, not by particle size.

Enter the sieve curve and get the symbol, Cu, Cc and the characteristic diameters in the grain size distribution tool, which also carries the EN ISO 14688 and GOST branches.