There are numerous amounts of soils present on the surface of earth each containing soil particles of different sizes, shapes and composition. If we classify and categorize soils, it will be convenient for us to study them.
We arrange the soils into different groups such that soils in a particular group have similar behaviour. This arranging of soils is called classification system of soil.
Aashto Soil Classification System
The classification system based on the study conducted by the association of state highways and transportation gives guide lines for soil and aggregate classification. This is to find the ideal subgrade mixture in the highway construction. AASHTO classifies soils into seven groups, A-1, A-2, A-3, A-4, A-5, A-6 and A-7, which is based on their relative quality suited for the subbase, embankment material and subgrade etc.
This system was developed by Terzaghi in the year of 1929. It requires three major systems as below.
1. Mechanical analysis.
2. Liquid limit.
3. Plasticity index.
Soil classification:
• Soils categorized under the group of A-1, A-2 and A-3 will be considered as granular material. These soils which have less than or equal to 35% of particles passing through the sieve size of number 200.
• Soils which have more than or equal to 35% particles passing through the sieve number 200 are categorized under the category of A-4, A-5, A-6, and A-7. These type of soils are mostly clay and silt type materials.
Aashto Soil Classification System Purpose of AASHTO system:
• Sieve number 10 is used for the separation of gravel sizes from the sand particles size. The number 10 sieve size is mostly accepted as the upper limit of the sand.
• It is mostly used by the state and federal highway department.
Uscs Classification
It was first developed by Casagrande and later it was modified by Bureau of Reclamation and the Corps of Engineers of the USA. This system has also been adopted by American Society of Testing Materials, ASTM. It is the most popular system for use in all types of engineering problems involving soil. It is abbreviated as Unified Soil Classification System. It has been widely used throughout the country to perform the classification of soils. This system categorizes the soil into two broad classification as follows, coarse grained and fine-grained soil.
The gravel and sands come under the coarse grains soils, and the sample of soil passes through Number 200 sieve size with less than 50 percent retained on the sieve. The G or S as letters are prefixed to the name. Here, the G represents the gravel and S refers to the sand.
The soil which passes through Number 200 sieve, with less than 50 percent retained is said to be fine grained soil. The group symbol starts with prefix of C, M or O. Here the C stands for clays, M stands for inorganic silt and O stands to represent organic clays, silts. One more symbol exists, which is known as Pt and will stand for other highly organic soils like peak and muck.
Coarse-grained soil is classified as gravel or sand by means of retained amount of soil in sieve number 4. If less than 50 percent of soil is passed through sieve number 4 sieve, it is the gravel. Otherwise, the soil is said to be sand.
The fine-grained soil is classified based on liquid limit value and corresponding plasticity index value, with use of plasticity chart. Basically, it is classified into high plasticity and low plasticity soil and further it is classified as inorganic or organic. With the use of plasticity chart and the A line, the soil is classified into clay or silt.
