Introduction of AASHTO
Although the textural classification of soil is relatively simple, it is based entirely on the particle-size distribution.
The soil classification table and group index formula AASHTO are essential tools in this regard.
The amount and type of clay minerals present in fine-grained soils dictate to a great extent their physical properties.
Hence, the soil engineer must consider plasticity, which results from the presence of clay minerals, to interpret soil characteristics properly.
Because textural classification systems do not take plasticity into account and are not totally indicative of many important soil properties, they are inadequate for most engineering purposes.
Currently, two more elaborate classification systems are commonly used by soil engineers. Both systems take into consideration the particle-size distribution and Atterberg limits.
They are the American Association of State Highway and Transportation Officials (AASHTO full name American Association of State Highway and Transportation Officials) classification system and the Unified Soil Classification System.
The AASHTO classification system is used mostly by state and county highway departments. Geotechnical engineers generally prefer the Unified system.
Also, Read: Unified Soil Classification System Chart
What Is the AASHTO Classification System?
The AASHTO system of soil classification was developed in 1929 as the Public Road Administration classification system.
It has undergone several revisions, 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).
This practice is carried out to classify soils according to the AASHTO method of classification
- Set of U.S. Sieves
- Atterberg Limit Tests Apparatus
Atterberg Limits testing is a group of tests used to define the properties of cohesive soils at different moisture contents. Gilson carries all the equipment needed to perform liquid limit, plastic limit, and shrinkage limit tests in accordance with ASTM D4318 and ASTM D4943 standards, and other associated test methods.
- Weighing Balance
- Drying Oven
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AASHTO Classification System Procedures
The AASHTO classification of soil in present use is given in the AASHTO table below. According to this system, the soil is classified into seven major groups: A-1 through A-7.
Soils classified under groups A-1-a, A-1, A-2, and A-3 are granular materials, of which 35% or less of the particles pass through the No. 200 sieve. This is an example of AASHTO soil classification example.
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:
#1. Soil size classification and Grain size
a. Gravel: fraction passing the 75-mm (3-inch.) sieve and retained on the No. 10 (2-mm) U.S. sieve
b. Sand: fraction passing the No. 10 (2-mm) U.S. sieve and retained on the No. 200 (0.075-mm) U.S. sieve
c. Silt and clay: fraction passing the No. 200 U.S. sieve
#2. Plasticity index of soil and Plasticity
The term silty is applied when the fine fractions of the soil have a plasticity index of 10 or less. The term clayey is applied when the fine fractions have a plasticity index of 11 or more.
#3. If cobbles and boulders (size larger than 75 mm) are encountered, they are excluded from the portion of the soil sample from which classification is made. However, the percentage of such material is recorded.
Table-1. Classification of Highway Subgrade Materials
a For A-7-5, PI ≤ LL – 30
b For A-7-6, PI ≤ LL – 30
To classify soil according to the below figure, 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.
The below figure 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.
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Group Index (GI)
To evaluate the quality of a soil as a highway subgrade material, one must also incorporate a number called the AASHTO soil classification chart and 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 = (F 200 – 35).[0.2 +0.005 ( LL – 40)] + 0.01 ( F 200 – 15).(PI -10) ……….(Eq. 1)
Figure 1. Range of liquid limit and plasticity index for soils in groups A-2, A-4, A-5, A-6, and A-7.
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:
- If Eq. (1) yields a negative value for GI, it is taken as 0.
- The group index calculated from Eq. (1) is rounded off to the nearest whole number (for example, GI = 3.4 is rounded off to 3; GI = 3.5 is rounded off to 4).
- There is no upper limit for the group index.
- The group index of soils belonging to groups A-1-a, A-1-b, A-2-4, A-2-5, and A-3 is always 0.
- When calculating the group index for soils that belong to groups A-2-6 and A-2-7, use the partial group index for PI, or
GI = 0.01(F200 – 15) (PI – 10)……….(Eq. 2)
In general, the quality of performance of soil as a subgrade material is inversely proportional to the group index.
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