Theory: USCS Classification
How the Unified Soil Classification System (USCS) groups soils by gradation and plasticity, and how to read Casagrande's plasticity chart.
A system for earthworks
Arthur Casagrande proposed the system in 1942 for military airfields, and the U.S. Army Corps of Engineers and the Bureau of Reclamation unified it in 1952. Standardized today as ASTM D2487, it's the most widely used classification in geotechnical engineering.
Each soil gets a two-letter symbol: the first names its main type — G (gravel), S (sand), M (silt), C (clay), O (organic) — and the second a property — W (well graded), P (poorly graded), L (low liquid limit) or H (high liquid limit). Borderline soils get a dual symbol, such as SW-SM.
Coarse-grained soils
A soil is coarse-grained when more than half of it is retained on the No. 200 (0.075 mm) sieve. It's a gravel when more than half of the coarse fraction is retained on the No. 4 (4.75 mm) sieve, and a sand otherwise. The second letter depends on the fines (F, percent passing No. 200):
| Fines | Criterion | Gravel | Sand |
|---|---|---|---|
| F < 5% | Cu ≥ 4 (gravel) or 6 (sand) and 1 ≤ Cc ≤ 3 | GW | SW |
| F < 5% | Doesn't meet the grading | GP | SP |
| 5 ≤ F ≤ 12% | Grading and fines | GW-GM, GP-GC… | SW-SM, SP-SC… |
| F > 12% | Fines ML or MH | GM | SM |
| F > 12% | Fines CL or CH | GC | SC |
| F > 12% | Fines CL-ML | GC-GM | SC-SM |
The fines are classified on the plasticity chart, as a fine-grained soil would be. With 5 to 12% fines, the dual symbol joins the grading and the kind of fines; CL-ML fines count as clay.
Fine-grained soils and the plasticity chart
With 50% or more passing the No. 200 sieve, the soil is fine-grained and is classified by its Atterberg limits on the plasticity chart. The A-line separates clays (above) from silts (below), and LL = 50 separates low (L) from high (H) liquid limits.
- CL: LL < 50, PI > 7 and on or above the A-line
- CL-ML: LL < 50, 4 ≤ PI ≤ 7 and on or above the A-line
- ML: LL < 50, PI < 4 or below the A-line; also non-plastic soils
- CH: LL ≥ 50, on or above the A-line
- MH: LL ≥ 50, below the A-line
- OL and OH: organic soils, whose oven-dried LL is under 75% of the undried LL
The U-line is the empirical upper bound of natural soils; a point above it suggests an error in the tests.
Group names
ASTM D2487 completes the symbol with a name that describes the other fractions. In a fine-grained soil, 15 to 29% coarse material adds "with sand" or "with gravel", whichever predominates; with 30% or more, the soil is "sandy" or "gravelly", and the other fraction is added as "with…" if it reaches 15%. In a coarse-grained soil, the other coarse fraction is named from 15%.
Limitations
- The limits are compared using the values as entered; rounding can change the group of a soil on a boundary.
- It was developed for temperate soils and doesn't capture well the behavior of tropical lateritic soils; for those, the MCT classification is used.
- Peat and highly organic soils (Pt) are identified visually and fall outside this page.
References
- ASTM. D2487: Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System). West Conshohocken, 2017.
- Casagrande, A. Classification and identification of soils. Transactions of the ASCE, v. 113, p. 901–930, 1948.
- ASTM. D4318: Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils. West Conshohocken, 2017.
- Das, B. M.; Sobhan, K. Principles of Geotechnical Engineering. 9th ed. Boston: Cengage, 2018.