Online AASHTO Classification and Group Index Calculator
The AASHTO M 145 classification groups soils according to their behaviour as pavement subgrade, from A-1-a (optimal) to A-7-6 (very poor). This tool applies the official decision table based on percentages passing the No. 10, No. 40 and No. 200 sieves, plus the Atterberg limits (LL, PI), and calculates the group index (GI) using the standard formula. It is the direct input for AASHTO 93 pavement design and for evaluating borrow materials in road projects under the BSI standard.
What is AASHTO and when to use it?
AASHTO M 145 was developed by the American Association of State Highway and Transportation Officials and is the standard reference in road works. Each soil receives a code (e.g. A-2-4, A-6) and a group index that weights the fine fraction, LL and PI to rank within the same group. It is used in subgrade studies, stabilisation design, classification of borrow pits for bases and subbases, and as input for CBR and MR. It is usually reported together with BS 5930 to give a complete view of the soil.
Applied formulas
Group index (GI):
GI = (F − 35) × (0.2 + 0.005 × (LL − 40)) + 0.01 × (F − 15) × (PI − 10)
Where F = percentage passing the No. 200 sieve, LL = liquid limit, PI = plasticity index. GI is rounded to the nearest integer. If the result is negative, GI = 0 is reported.
Decision rules: The AASHTO table is evaluated from left to right until the first group whose limits are met is found. Groups A-1, A-3 are granular; A-2 is granular with fines; A-4 to A-7 are fine-grained.
Calculation example
| Parameter | Value |
|---|---|
| Passing No. 10 sieve | 78 % |
| Passing No. 40 sieve | 55 % |
| Passing No. 200 sieve | 42 % |
| Liquid limit (LL) | 38 |
| Plasticity index (PI) | 14 |
With 42 % passing the No. 200 sieve, it is no longer A-1 or A-3 (which require ≤ 25 % and ≤ 10 % respectively). It also does not fall into A-2, which limits to 35 % passing the No. 200 sieve. With more than 35 % passing, it corresponds to a fine-grained group. LL = 38 (< 40) and PI = 14 (> 10): the table assigns A-6. Then GI = (42 − 35) × (0.2 + 0.005 × (38 − 40)) + 0.01 × (42 − 15) × (14 − 10) = 7 × 0.19 + 0.01 × 27 × 4 = 1.33 + 1.08 = 2.41.
Result: A-6 (2) — subgrade of fair to poor quality.
Interpretation of results
A-6 indicates a clayey soil with medium plasticity. The GI = 2 indicates that within A-6 it is not at the worst extreme but is still not acceptable without treatment. For permanent pavement, improvement with lime or granular stabilisation is generally recommended. GI values above 8-10 require subgrade replacement or reinforcement. For bases and subbases, groups A-1 or A-2-4 with GI = 0 are required.
Reference standards
- AASHTO M 145 — Classification of Soils and Soil-Aggregate Mixtures for Highway Construction Purposes
- AASHTO T 88 / T 89 / T 90 — Tests for particle size analysis and Atterberg limits
- BSI standard — Volume No. 3 Instructions and Design Criteria
- AASHTO Guide for Design of Pavement Structures 1993
Frequently asked questions
What GI is acceptable for pavement subgrade?
As a general reference, GI ≤ 4 allows standard design; GI between 4 and 8 requires CBR verification and possibly stabilisation; GI ≥ 8 usually requires subgrade replacement or reinforcement. The BSI standard sets specific criteria by type of work.
Can AASHTO be used without Atterberg limits?
Only for groups A-1 and A-3 which are granular with no plasticity. As soon as the soil has a significant fine fraction or measurable LL, the limits are mandatory to distinguish A-2-4 from A-2-6, or A-4 from A-6 and A-7.
Is the difference between A-7-5 and A-7-6 relevant?
Yes. A-7-5 has PI ≤ LL − 30 (silty clays, moderate expansion) while A-7-6 has PI > LL − 30 (plastic clays, high expansion potential). For projects in areas with marked moisture cycles, the difference changes the need for compaction control and treatment.
What value is reported if the GI is negative?
GI = 0 is reported. The formula can give a negative value when the soil has few fines with low plasticity; AASHTO M 145 indicates truncation to zero and negative values are not used.