# Soil Classification

How the AASHTO and Unified systems sort soils into groups from gradation and Atterberg limits, and what the group says about a soil as subgrade.

- Canonical URL: https://pavementinteractive.org/reference-desk/testing/soil-tests/soil-classification/
- Last updated: 2026-11-01
- Standards: AASHTO M 145, ASTM D2487, AASHTO T 88, AASHTO T 89, AASHTO T 90, ASTM D2488

Soil classification sorts soils into groups that behave alike, so that a designer or inspector can predict how a soil will act from a few simple tests. Two systems are used on U.S. highway work:

- the **AASHTO system** (AASHTO M 145), written for highway subgrades and embankments, and
- the **Unified Soil Classification System** (USCS, ASTM D2487), used in geotechnical and foundation work.

Both start from the same two pieces of information: the soil's [gradation](https://pavementinteractive.org/glossary/gradation/) from a sieve analysis, and its plasticity from the [liquid limit](https://pavementinteractive.org/glossary/liquid-limit/) and [plasticity index](https://pavementinteractive.org/glossary/plasticity-index-pi/), together called the Atterberg limits. Neither system measures strength or stiffness directly. A classification tells you what kind of soil you have; tests such as the [California Bearing Ratio](https://pavementinteractive.org/reference-desk/design/design-parameters/california-bearing-ratio/) or [resilient modulus](https://pavementinteractive.org/reference-desk/design/design-parameters/resilient-modulus/) tell you how well it supports a pavement (Christopher et al., 2006[^1]).

Basic soil terms (gravel, sand, silt, clay) are defined in [AASHTO Soil Terminology](https://pavementinteractive.org/aashto-soil-terminology/) and [ASTM Aggregate and Soil Terminology](https://pavementinteractive.org/reference-desk/materials/aggregate/astm-aggregate-and-soil-terminology/). The two systems draw the gravel–sand boundary at different sieves, so a soil's description can change slightly between them.

## The AASHTO System

The AASHTO system places a soil in one of seven main groups, A-1 through A-7, several with subgroups. The first split is on the amount of material passing the 0.075 mm (No. 200) sieve:

- **Granular materials** have 35 percent or less passing the 0.075 mm sieve. They are groups A-1, A-3 and A-2.
- **Silt–clay materials** have more than 35 percent passing. They are groups A-4 through A-7.

Within each half, the liquid limit and plasticity index decide the group. The limits below follow the FHWA soils reference manual (Samtani and Nowatzki, 2006[^2]). To classify a soil, work from left to right through the groups and take the first one whose limits the soil meets.

Table 1. AASHTO soil classification limits (after Samtani and Nowatzki, 2006[^2])

| Group | Passing 2.00 mm (No. 10), % | Passing 0.425 mm (No. 40), % | Passing 0.075 mm (No. 200), % | Liquid limit | Plasticity index | Typical material |
|---|---|---|---|---|---|---|
| A-1-a | 50 max | 30 max | 15 max | — | 6 max | Stone fragments, gravel and sand |
| A-1-b | — | 50 max | 25 max | — | 6 max | Gravel and sand |
| A-3 | — | 51 min | 10 max | — | Nonplastic | Fine sand |
| A-2-4, A-2-5 | — | — | 35 max | 40 max / 41 min | 10 max | Silty gravel and sand |
| A-2-6, A-2-7 | — | — | 35 max | 40 max / 41 min | 11 min | Clayey gravel and sand |
| A-4 | — | — | 36 min | 40 max | 10 max | Silty soils |
| A-5 | — | — | 36 min | 41 min | 10 max | Silty soils |
| A-6 | — | — | 36 min | 40 max | 11 min | Clayey soils |
| A-7-5, A-7-6 | — | — | 36 min | 41 min | 11 min | Clayey soils |

A-7 soils split on how the plasticity index compares with the liquid limit: A-7-5 has a plasticity index no greater than the liquid limit minus 30, and A-7-6 has a plasticity index greater than that. A-7-5 soils tend to be elastic silty clays that rebound under load; A-7-6 soils are the highly plastic clays that shrink and swell most (see [Shrinking and Swelling Soils](https://pavementinteractive.org/reference-desk/design/design-parameters/shrinking-and-swelling-soils/)).

In general, the further a soil sits to the right of the chart, the poorer it is as a subgrade. A-1 and A-3 soils are usually rated excellent to good; A-4 through A-7 soils are usually rated fair to poor.

### Group Index

The [group index](https://pavementinteractive.org/glossary/group-index/) refines the rating within a group. It is an empirical number that rises as the fines content and plasticity rise:

$$GI = (F - 35)\left[0.2 + 0.005(LL - 40)\right] + 0.01(F - 15)(PI - 10)$$

where:

- $$GI$$ = group index, rounded to the nearest whole number
- $$F$$ = percent passing the 0.075 mm (No. 200) sieve
- $$LL$$ = liquid limit
- $$PI$$ = plasticity index

Each bracketed term is set to zero when it would be negative, and a negative group index is reported as zero. For the A-2-6 and A-2-7 subgroups only the plasticity-index term is used. The group index is written in parentheses after the group, for example A-6(12). A group index of 0 marks a good subgrade within its group; values of 20 or more mark a very poor one (Samtani and Nowatzki, 2006[^2]).

## The Unified Soil Classification System

The USCS gives each soil a two-letter symbol. The first letter names the main soil type and the second describes it:

- **G** gravel, **S** sand, **M** silt, **C** clay, **O** organic, **Pt** peat
- **W** well graded, **P** poorly graded, **L** low plasticity, **H** high plasticity

A soil is **coarse grained** when more than half of it is retained on the 0.075 mm (No. 200) sieve. It is a gravel when more than half of that coarse fraction is retained on the 4.75 mm (No. 4) sieve, and a sand otherwise. What follows depends on the fines:

- **Less than 5 percent fines:** the second letter comes from the shape of the gradation curve (W or P), using the coefficients of uniformity and curvature below.
- **More than 12 percent fines:** the second letter comes from the plasticity of the fines (M or C), read from the plasticity chart.
- **5 to 12 percent fines:** the soil gets a dual symbol, such as SW-SM.

$$C_u = \frac{D_{60}}{D_{10}} \qquad C_c = \frac{(D_{30})^2}{D_{10}\,D_{60}}$$

where:

- $$C_u$$ = coefficient of uniformity
- $$C_c$$ = coefficient of curvature
- $$D_{10}, D_{30}, D_{60}$$ = particle sizes at which 10, 30 and 60 percent of the sample passes

A gravel is well graded (GW) when $$C_u$$ is at least 4 and $$C_c$$ is between 1 and 3; a sand (SW) needs $$C_u$$ of at least 6 with the same $$C_c$$ range (Samtani and Nowatzki, 2006[^2]).

A soil is **fine grained** when half or more passes the 0.075 mm sieve. Its symbol comes from the plasticity chart, a plot of plasticity index against liquid limit. The "A-line" on that chart separates clays (above) from silts (below):

$$PI = 0.73\,(LL - 20)$$

A liquid limit below 50 gives L (low plasticity) and 50 or more gives H (high plasticity), so the four common fine-grained groups are CL, ML, CH and MH. A small zone just above the A-line at low plasticity (a plasticity index of about 4 to 7) is given the dual symbol CL-ML.

## Visual Description in the Field

Laboratory classification takes a day or more. In the field, inspectors describe soils by eye and feel using the visual-manual procedure (ASTM D2488): estimating the percentages of gravel, sand and fines, and judging dry strength, dilatancy and toughness by hand. A visual description is useful for logging test pits and spotting a change in material, but it is not a substitute for laboratory classification when a specification or design depends on the group (Samtani and Nowatzki, 2006[^2]).

Calculator: Soil classification (AASHTO and USCS) — https://pavementinteractive.org/tools/soil-classification/

## Comparing the Two Systems

The two systems often agree in broad terms but not group for group. A soil with 40 percent fines is an A-4 to A-7 "silt–clay" under AASHTO, but still a coarse-grained SM or SC under USCS, because the USCS split is at 50 percent fines rather than 35. Specifications usually name the system they mean; when a project report gives only one, don't convert between them without the underlying gradation and limits.

## Standard Test Methods

- AASHTO M 145: Classification of soils and soil–aggregate mixtures for highway construction
- ASTM D2487: Classification of soils for engineering purposes (Unified Soil Classification System)
- ASTM D2488: Description and identification of soils by the visual-manual procedure
- AASHTO T 88, T 89 and T 90: particle-size analysis, liquid limit and plastic limit of soils

## Sources

[^1]: Christopher, B.R.; Schwartz, C. and Boudreau, R. (2006). Geotechnical Aspects of Pavements Reference Manual. FHWA-NHI-05-037. National Highway Institute, Federal Highway Administration. Washington, D.C. https://www.fhwa.dot.gov/engineering/geotech/pubs/05037/05037.pdf
[^2]: Samtani, N.C. and Nowatzki, E.A. (2006). Soils and Foundations Reference Manual, Volume I. FHWA-NHI-06-088. National Highway Institute, Federal Highway Administration. Washington, D.C. https://www.fhwa.dot.gov/engineering/geotech/pubs/nhi06088.pdf