Soil is rarely uniform on a construction site. Beneath a single road alignment, the material can change from clean gravel at one end to soft clay at the other, and the design of the pavement depends on which material supports it. Classification is the first step engineers take to organize this variability. The AASHTO system, named for the American Association of State Highway and Transportation Officials, sorts soils into groups that share similar behavior under traffic loads. The same logic applies to the earth materials below the soil layer, where the geomechanics classification system of rocks for engineering purposes groups rock masses by origin, strength, and weathering before slopes and foundations are designed.
The AASHTO method is not the only classification in use, but it is the one most state and county highway departments rely on for subgrade evaluation. It combines particle-size distribution from sieve analysis with plasticity from Atterberg limit tests, and with those two inputs an engineer can assign a soil to a group, compute a group index, and estimate its performance under a pavement.
Why Engineers Classify Soils for Road Construction
Early classification systems were textural. They sorted soil entirely by particle size, using names like sand, silt, and gravel, with no regard for how the material behaved when wet. Textural classification is simple, but it misses the property that matters most in pavement design: plasticity. The amount and type of clay minerals present in fine-grained soils dictate to a great extent their physical properties, so a soil engineer must consider plasticity to interpret soil characteristics properly.
What the two modern systems share
Because textural systems ignore plasticity, they are inadequate for most engineering purposes, and two more elaborate systems are now common among soil engineers. Both take into consideration the particle-size distribution and Atterberg limits, but they organize the results differently. The Unified Soil Classification System groups soils by grain size and plasticity into categories such as GW, SM, and CH, and it is the system geotechnical engineers generally prefer for foundation and earthwork design. The AASHTO system, by contrast, was built for the highway engineer who needs a quick judgment about subgrade quality.
The two systems are complementary rather than competing. A geotechnical report may classify the same boring with both methods, because one describes the soil as a material while the other rates it as a pavement support layer.
The Seven AASHTO Soil Groups at a Glance
The AASHTO classification of soil in present use appears in a standard table, published as AASHTO method M 145 and ASTM designation D-3282. The system was developed in 1929 as the Public Road Administration classification and has undergone several revisions, with the present version proposed in 1945 by the Committee on Classification of Materials for Subgrades and Granular Type Roads of the Highway Research Board. Under this system, soils fall into seven major groups, A-1 through A-7.
Groups A-1, A-2, and A-3 are granular materials, of which 35% or less of the particles pass the No. 200 sieve. Groups A-4 through A-7 are silt-clay materials, with more than 35% passing the No. 200 sieve. The table below summarizes the groups and their typical field descriptions.
| Group | Material description | Rating as subgrade |
|---|---|---|
| A-1-a | Well-graded gravels and gravel-sand mixtures with little or no fines | Excellent |
| A-1-b | Coarse sands and sand-gravel mixtures with little or no fines | Excellent |
| A-2 | Silty or clayey gravel and sand, 35% or less passing No. 200 | Good to excellent |
| A-3 | Fine sands with little or no fines | Good |
| A-4 | Silty soils | Fair to poor |
| A-5 | Highly elastic silty soils | Fair to poor |
| A-6 | Clayey soils, plastic | Poor |
| A-7 | Highly plastic clayey soils | Poor |
How the group symbol is read
The letter after the dash refines the group. A-1-a covers well-graded gravel mixtures, while A-1-b covers coarse sands. A-2 carries a suffix borrowed from the silt-clay groups, so A-2-4 is a granular soil whose fines behave like the silty material in A-4, and A-2-6 behaves like the clayey material in A-6. A-7-5 and A-7-6 split the clay group by plasticity: A-7-5 has moderate plasticity with a high liquid limit, and A-7-6 high plasticity.
Where to find the full criteria
Full group definitions, including the exact sieve and Atterberg boundary conditions for every subgroup, appear in agency manuals and in online references. The same criteria appear in the AASHTO soil classification table at AboutCivil, which reproduces the M 145 limits with the subgroup boundaries spelled out.
Granular Materials: Groups A-1, A-2, and A-3
Granular soils make up the best subgrade material in the system. In groups A-1 through A-3, 35% or less of the particles pass the No. 200 sieve. These soils drain quickly, resist frost heave, and compact to high density.
Why the fines matter in granular groups
The dividing line between A-1 and A-2 is not grain size alone but the character of the fines. A-1 soils contain little or no fines, so they behave like clean gravel or sand. A-2 soils contain up to 35% fines, and the group splits into four subgroups by whether those fines are silty or clayey and by their plasticity. A-3 is a special case: fine sands with little or no fines, typically windblown or water-deposited sands that drain freely but do not hold together under load.
Granular subgrades also need less compaction energy, and in cold regions their drainage advantage is decisive because frost heave forms where water collects in fine-grained soil.
Classification results feed into design decisions beyond the pavement. The same sieve and plasticity data used to rate a subgrade also tell the engineer how fast water moves through the soil, which matters for on-site wastewater systems because a drain field must sit in soil that drains at the right rate. The factors that determine how long a septic system lasts start with the soil classification of the drain field area, and sandy granular soils give the longest service life.
Silt-Clay Materials: Groups A-4 Through A-7
When more than 35% of the soil passes the No. 200 sieve, the soil falls into groups A-4 through A-7. These are silt-clay materials, and their engineering behavior is governed by plasticity rather than grain size.
| Group | Typical material | Liquid limit | Plasticity index |
|---|---|---|---|
| A-4 | Silty soils | 40 max | 10 max |
| A-5 | Highly elastic silty soils | 41 min | 10 max |
| A-6 | Clayey soils | 40 max | 11 min |
| A-7 | Highly plastic clayey soils | 41 min | 11 min |
Reading the plasticity criteria
A-4 soils are silty, with liquid limits of 40 or less and plasticity indexes of 10 or less. A-5 soils share the low plasticity index but have liquid limits above 40, which makes them elastic silts that respond slowly to changes in moisture. A-6 soils are clayey, with plasticity indexes of 11 or more. A-7 splits into A-7-5 and A-7-6 at the line where the plasticity index equals the liquid limit minus 30; A-7-5 falls at or below that line, and A-7-6 above it.
Silt-clay subgrades are the ones that move. They swell when wet, shrink when dry, and lose strength under repeated traffic. A pavement on A-6 or A-7 soil needs a thicker base layer or a stabilized subgrade to spread wheel loads before the surface is placed.
Mapping these materials across a site requires a soil survey with reliable ground control. Soil boundaries are located with auger borings and test pits tied to a coordinate network, and the types and applications of triangulation systems in surveying determine how accurately the soil map lines up with the site.
Computing the Group Index
The group symbol alone does not tell the whole story. Soils in the same group can still differ in quality, so the AASHTO system adds a numeric rating called the group index (GI), which combines the percentage passing the No. 200 sieve (F), the liquid limit (LL), and the plasticity index (PI) in one formula:
GI = (F – 35)[0.2 + 0.005(LL – 40)] + 0.01(F – 15)(PI – 10)
When any term inside a bracket is negative, that term is taken as zero. A group index of 0 indicates a good subgrade, and higher values indicate poorer material. The computed GI is written after the group symbol, for example A-6(12).
Worked example
Consider a soil with 62% passing the No. 200 sieve, a liquid limit of 38, and a plasticity index of 14. The group index calculation runs in four steps:
- Record the inputs: F = 62, LL = 38, PI = 14.
- Evaluate the first term: (62 – 35) x [0.2 + 0.005(38 – 40)] = 27 x 0.19 = 5.13.
- Evaluate the second term: 0.01 x (62 – 15) x (14 – 10) = 1.88.
- Add and round: GI = 5.13 + 1.88 = 7.01, which rounds to 7.
The soil classifies as A-6(7), a clayey material of fair-to-poor subgrade quality.
Particle-size terminology differs between systems, and the MIT soil classification system sorts particles by size into gravel, sand, silt, and clay fractions using its own boundary sizes, with its categories appearing in some laboratory reports alongside the AASHTO groups.
Applying AASHTO Classification in the Field
The classification is only useful if the sample represents the material in the ground. Field procedure starts with sampling: test pits or auger borings along the alignment, one sample from each layer that will carry the pavement. Sieve analysis and Atterberg limits follow in the laboratory, and the results are plotted against the AASHTO criteria.
Reporting and specification use
Highway specifications commonly require the contractor to place fill that classifies within a stated range, for example A-1 or A-2 material with a group index of 4 or less. The classification gives the designer and the inspector a common language for accepting or rejecting material. A field checklist keeps the results consistent:
- Take disturbed samples from each distinct layer; a blended sample hides the worst layer.
- Run a sieve analysis on the full sample, including the fraction retained on the No. 200 sieve.
- Run Atterberg limits on the fraction passing the No. 40 sieve.
- Compute the group index and record it with the group symbol.
- Reclassify the material whenever the borrow source changes.
Stable subgrade material matters for structures as well as roads. Foundations and retaining structures built on well-graded granular fill perform predictably, while structures on soft clay settle over years. Even simple building systems feel the difference: a dry-stacked interlocking masonry system relies on a firm, well-drained base to keep its block courses aligned, and the soil classification tells the builder whether that base exists on the site.