Theory: AASHTO Classification
How the AASHTO classification groups soils for highway construction from their gradation and plasticity, and what the group index tells.
A classification for subgrades
The AASHTO classification grew out of the Highway Research Board (HRB) system and is the standard one in highway engineering. It ranks soils by their expected performance as highway subgrades, from the best (A-1) to the poorest (A-7).
It needs only the percent passing three sieves and the Atterberg limits of the fraction passing the No. 40 sieve: the liquid limit (LL) and the plasticity index (PI = LL − PL).
Groups
Soils are granular when 35% or less passes the No. 200 (0.075 mm) sieve, and silt-clay materials above that. The group is the first, left to right in M 145's table, whose limits the soil meets; A-3 comes before A-2 in this order, though it isn't a better soil.
| Group | No. 10 | No. 40 | No. 200 | LL | PI |
|---|---|---|---|---|---|
| A-1-a | ≤ 50 | ≤ 30 | ≤ 15 | — | ≤ 6 |
| A-1-b | — | ≤ 50 | ≤ 25 | — | ≤ 6 |
| A-3 | — | > 50 | ≤ 10 | — | NP |
| A-2-4 | — | — | ≤ 35 | ≤ 40 | ≤ 10 |
| A-2-5 | — | — | ≤ 35 | > 40 | ≤ 10 |
| A-2-6 | — | — | ≤ 35 | ≤ 40 | > 10 |
| A-2-7 | — | — | ≤ 35 | > 40 | > 10 |
| A-4 | — | — | > 35 | ≤ 40 | ≤ 10 |
| A-5 | — | — | > 35 | > 40 | ≤ 10 |
| A-6 | — | — | > 35 | ≤ 40 | > 10 |
| A-7-5 · A-7-6 | — | — | > 35 | > 40 | > 10 |
A-7 is split by the line PI = LL − 30: below it, A-7-5, with a moderate PI for its LL and subject to high volume change; above it, A-7-6, with a high PI.
- A-1: well-graded stone fragments, gravel and sand with little binder
- A-3: fine sand without plastic fines, such as beach and dune sands
- A-2: gravels and sands with silty (A-2-4, A-2-5) or clayey (A-2-6, A-2-7) fines
- A-4 and A-5: silty soils; A-5 is elastic, with a high LL
- A-6 and A-7: clayey soils, with high volume change between wet and dry states
Plasticity chart
On the LL × PI plane, the lines LL = 40 and PI = 10 separate the fine groups 4, 5, 6 and 7, and the line PI = LL − 30 splits A-7. The A-2 groups follow the same regions for soils with 35% fines or less. No soil can plot above the line PI = LL, since the plastic limit isn't negative.
Group index
The group index (GI) rates a soil within its group: the higher, the poorer the subgrade. It's written in parentheses after the group, as A-6 (12). By AASHTO M 145:
It's rounded to the nearest whole number and reported as zero when negative. Groups A-1, A-3, A-2-4 and A-2-5 always have GI = 0, and for A-2-6 and A-2-7 only the PI term counts. There's no upper limit: very plastic clays can exceed 20, the cap of older versions and of DNIT's, which bound each term to a range.
For non-plastic soils, PI is taken as zero and the LL term, which can't be determined, is left out.
Limitations
- The limits are compared using the values as entered; they're usually reported as whole numbers, which can change the group of a soil on a boundary.
- It was developed for temperate soils. It tends to underrate tropical lateritic soils: lateritic clays classed as A-7 can make excellent subgrades. For those, the MCT classification is used.
- It doesn't account for organic matter; peat and highly organic soils fall outside it (group A-8 in some versions).
References
- AASHTO. M 145: Classification of Soils and Soil-Aggregate Mixtures for Highway Construction Purposes. Washington, 2021.
- ASTM. D3282: Standard Practice for Classification of Soils and Soil-Aggregate Mixtures for Highway Construction Purposes. West Conshohocken, 2015.
- 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.