Classification of Soils According to AASHTO
Introduction
AASHTO stands for American Association of State Highway and Transportation Officials. AASHTO proposed soil classification in 1929 and had undergone several revisions till now with the present version proposed by the Committee on Classification of Materials for Subgrades and Granular Type Roads of the Highway Research Board in 1945 (ASTM designation D-3282; AASHTO method M145). It is widely used to classify soil and soil-aggregate mixtures for construction of roads, highways, and airfield (runways, taxiways) especially for sub-grade material.
Objective(s) of the Experiment
This practical is carried out to classify soils according to AASHTO method of classification
Equipments and Materials Needed
Procedures
The AASHTO classification in present use is given in Table 1 and 2 below. According to this system, soil is classified into seven major groups: A-1 through A-7. Soils classified under groups A-1, A-2, and A-3 are granular materials of which 35% or less of the particles pass through the No. 200 sieve. Soils of which more than 35% pass through the No. 200 sieve are classified under groups A-4, A-5, A-6, and A-7. These soils are mostly silt and clay-type materials.
This classification system is based on the following criteria:
Please click on the tables below to view the full size
Notes:
To classify a soil according to Tables 1 and 2, one must apply the test data from left to right. By process of elimination, the first group from the left into which the test data fit is the correct classification. Figure 1 shows a plot of the range of the liquid limit and the plasticity index for soils that fall into groups A-2, A-4, A-5, A-6, and A-7.
Group Index (GI)
To evaluate the quality of a soil as a highway subgrade material, one must also incorporate a number called the group index (GI) with the groups and subgroups of the soil. This index is written in parentheses after the group or subgroup designation. The group index is given by the equation:
GI = (F200 - 35) [0.2 + 0.005 (LL - 40)] + 0.01 (F200 - 15) (PI - 10).......... Eq. (1)
Where
F200 = percentage passing through the No. 200 sieve
LL = Liquid Limit
PI = Plasticity Index
The first term of Eq. (1) — that is, (F200 - 35) [0.2 + 0.005 (LL - 40)] — is the partial group index determined from the liquid limit. The second term—that is, 0.01 (F200 - 15) (PI - 10) — is the partial group index determined from the plasticity index.
Rules for Determining the GI
Following are some rules for determining the group index:
Discussion and Conclusion
In general, the quality of performance of a soil as a subgrade material is inversely proportional to the group index.
References
Credits: Samuel Alalade. Obafemi Awolowo University, Ile-Ife, Osun State, Nigeria.