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AASHTO Soil Classification System - AASHTO Chart

By: Haseeb Jamal / On: Oct 01, 2019 / Notes, Types of
AASHTO Soil Classification Chart

Besides Soil Classification on other criteria, the AASHTO Soil Classification System classifies soils into seven primary groups, named A-1 through A-7, based on their relative expected quality for road embankments, sub-grades, sub-bases, and bases. Some of the groups are in turn divided into subgroups, such as A-1-a and A-1-b. Furthermore, a Group Index may be calculated to quantify a soil’s expected performance within a group. To determine a soil’s classification in the AASHTO system, one first determines the relative proportions of gravel, coarse sand, fine sand, and silt-clay.

AASHTO Soil Classification System Chart

General Classification Granular Materials (35% or less passing the 0.075 mm sieve) Silt-Clay Materials (>35% passing the 0.075 mm sieve)
Group Classification A-1 A-3 A-2 A-4 A-5 A-6 A-7
A-1-a A-1-b A-2-4 A-2-5 A-2-6 A-2-7 A-7-5 A-7-6
Sieve Analysis, % passing  
2.00 mm (No. 10) 50 max
0.425 (No. 40) 30 max 50 max 51 min
0.075 (No. 200) 15 max 25 max 10 max 35 max 35 max 35 max 35 max 36 min 36 min 36 min 36 min
Characteristics of fraction passing 0.425 mm (No. 40)  
Liquid Limit 40 max 41 min 40 max 41 min 40 max 41 min 40 max 41 min
Plasticity Index 6 max N.P. 10 max 10 max 11 min 11 min 10 max 10 max 11 min 11 min
Usual types of significant constituent materials stone fragments, gravel and sand fine sand silty or clayey gravel and sand silty soils clayey soils
General rating as a subgrade excellent to good fair to poor

Note: Plasticity index of A-7-5 subgroup is equal to or less than the LL - 30. Plasticity index of A-7-6 subgroup is greater than LL - 30

AASHTO Liquid Limit Vs Plasticity Index
AASHTO Liquid Limit Vs Plasticity Index

In the AASHTO Soil Classification System:

  • gravel is material smaller than 75 mm (3 in.) but retained on a No. 10 sieve;
  • coarse sand is material passing a No 10 sieve but retained on a No. 40 sieve; and fine sand is material passing a No. 40 sieve but retained on a No. 200 sieve.
  • Material passing the No. 200 sieve is silt-clay and is classified based on Atterberg limits.
  • It should be noted that the division between gravel and sand is made at a smaller size (No. 10 sieve) in the AASHTO Soil Classification System than in the unified system (No. 4 sieve).

Secondly, if any fines are present, Atterberg limits are determined and the plasticity index is calculated. A soil is a granular material if less than 35% of the soil by weight passes the No. 200 sieve (#200). Granular materials are classified into groups A-1 through A-3. Soils having more than 35% passing the No. 200 sieve are silt-clay and fall in groups A-4 through A-7. Having the proportions of the components and the plasticity data, one enters one of the two alternatives AASHTO classification tables and checks from left to right until a classification is found for which the soil meets the criteria. It should be noted that, in this scheme, group A-3 is checked before A-2. Soils classified as A-1 are typically well-graded mixtures of gravel, coarse sand, and fine sand. Soils in subgroup A-1-a contain more gravel whereas those in A-1-b contain more sand.

Soils in group A-3 are typically fine sands that may contain small amounts of non-plastic silt. Group A-2 contains a wide variety of “borderline” granular materials that do not meet the criteria for groups A-1 or A-3. Soils in group A-4 are silty soils, whereas those in group A-5 are high-plasticity elastic silt. Soils in group A-6 are typically lean clays, and those in group A-7 are typically highly plastic clays. Within groups containing fines, one may calculate a group index to further evaluate relative quality and supporting value of a material as sub-grade. The group index is calculated according to the following empirical formula:

Group index F 35 – ( )0.2 0.005 LL 40 – ( ) + [ ] + 0.01 F 15 – ( )PI 10 – ( )

AASHTO Classification of Soil Video Tutorial

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