Soil Classification

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 from a sieve analysis, and its plasticity from the liquid limit and plasticity index, 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 or resilient modulus tell you how well it supports a pavement (Christopher et al., 2006¹).

Basic soil terms (gravel, sand, silt, clay) are defined in AASHTO Soil Terminology and 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, 2006²). 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, 2006²)

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).

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 refines the rating within a group. It is an empirical number that rises as the fines content and plasticity rise:

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, 2006²).

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.

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, 2006²).

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):

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, 2006²).

Calculator

Soil classification (AASHTO and USCS)

AASHTO M 145 · ASTM D2487

Classifies the material finer than 75 mm (3 in.) from its sieve analysis and Atterberg limits.

Percent passing

Passing 75 mm (3 in.), %

Passing 4.75 mm (No. 4), %

Passing 2.00 mm (No. 10), %

Passing 0.425 mm (No. 40), %

Passing 0.075 mm (No. 200), %

Atterberg limits (minus No. 40 fraction)

PlasticityPlastic (enter LL and PI)Nonplastic (NP)

Liquid limit, LL (optional)

Optional for a nonplastic soil that is mostly coarse.

Plasticity index, PI

Organic fines?NoYes: oven-dried LL under 0.75 × LL

Grain sizes (for coarse soils with little fines)

D₁₀, mm (optional)

D₃₀, mm (optional)

D₆₀, mm (optional)

Needed only when 12 % or less passes the No. 200 sieve.

CalculateExample values: a sandy lean clay. Rules follow FHWA NHI-06-088, chapter 4.

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

@misc{pavint-soil-classification,
  author = {{Pavement Interactive}},
  title = {Soil Classification},
  year = {2026},
  publisher = {HeadLight},
  howpublished = {\url{https://pavementinteractive.org/reference-desk/testing/soil-tests/soil-classification/}},
  note = {Accessed [date]}
}