What is soil classification?

Soil classification is a method of categorizing soil based on its properties, types, and uses. The motive of soil classification is to arrange various types of soils and divide them into groups for various engineering and agricultural purposes. Soil classification is carried out by various agencies as per their purposes. From an engineering perspective, soil classification is done to find out the suitability of soil for various construction purposes like the construction of dams, foundations, highways, infrastructures, and so on. For engineering purposes, the soil is classified by the following systems:

• Particle size classification
• Textural classification
• Highway Research Board (HRB) classification
• Unified soil classification system

Particle size classification

In the particle size classification system, the particles of soils are arranged based on grain size. In this system, terms like gravel, sand, silt, and clay are used to denote the grain size. The most commonly used particle size classification systems are:

• U.S. Bureau of Soil and Public Road Administration (PRA) system of United States
• International soil classification system

U.S. Bureau of soil and Public Road Administration (PRA) systems

As per this system, the soil particles are classified as below-

1. Soil particles that are below 0.005 mm are termed clay.
2. The soil particles that are between 0.005 mm to 0.05 mm are termed silt.

• Sand is categorized into four types, very fine sand, fine sand, medium sand, and coarse sand.

1. The soil particles that are between 0.005 mm to 0.10 mm are termed very fine sand.
2. The soil particles that are between 0.10 mm to 0.25 mm are termed fine sand.
3. The soil particles that are between 0.25 mm to 0.50 mm are termed medium sand.
4. The soil particles that are between 0.50 mm to 1.0 mm are termed coarse sand.

• The soil particles that are between 1.0 mm to 2.0 mm are termed fine gravel.
• The soil particles that are above 2.0 mm are termed gravels.

International soil classification system

As per the international soil classification system, the soil particles are classified as below-

• The soil particles below 0.0002mm are termed ultra clay or colloids. Clay is categorized into two types: fine clay, and coarse clay.

1. The soil particles between 0.0002 mm and 0.0006 mm are termed fine clay.
2. The soil particles between 0.0006 mm and 0.002 mm are termed coarse clay.

• Silt is categorized into two types, fine silt, and coarse silt.

1. The soil particles between 0.002 mm and 0.006 mm are termed fine silt.
2. The soil particles between 0.006 mm and 0.02 mm are termed coarse silt.

• A new term Majla (Mo) is introduced in this system.

1. The soil particles between 0.02 mm and 0.05 mm are termed fine majla.
2. The soil particles between 0.05 mm and 0.1 mm are termed coarse majla.

• Sand is categorized into four types, Fine sand, medium sand, coarse sand, and very coarse sand.

1. The soil particles between 0.1 mm and 0.2 mm are termed fine sand.
2. The soil particles between 0.2 mm and 0.5 mm are termed medium sand.
3. The soil particles between 0.5 mm and 1.0 mm are termed coarse sand.
4. The soil particles between 1.0 mm and 2.0 mm are termed very coarse sand.

• The soil particles above 2.0 mm are termed gravels.

Textural classification

Soil occurring in nature is a composition of different percentages of sand, silt, clay, and gravel. Soil classification of such composite soils based on the particle size distribution is known as textural classification. The best known and most commonly used textural classification system is adapted from U.S Public Road Administrations. The classification is based on the different percentages of sand, silt, and clay making up the soil graphed in the shape of a triangle. The method is suitable for determining the properties of coarse-grained soils, rather than fine-grained soils.

Highway research board classification

The Highway Research Board (HRB) classification is also known as Public Road Administration (PRA) classification system. The classification in this system is based on both particle size and plasticity characteristics. In this classification, soils are divided into 7 primary groups, A-1, A-2, A-3, ...., A-7. Group A-1 is divided into two sub-groups and group A-2 is divided into four sub-groups. An additional property, group index is introduced into the classification which is used to describe the performance of the soils when they are used in pavements. the soil is classified based on the elimination method on the classification chart.

Unified Soil Classification System (USCS)

The UCSC was developed by Casagrande. According to this system, the coarse-grained soils are classified on the basis of their particle size, and the fine-grained soils are classified on the basis of their plasticity. The soils are classified into four major groups- coarse-grained, fine-grained, organic soils, and peat. There is a total of 15 groups, 8 for coarse-grained soils, 6 for fine-grained soils, and 1 for peat.

Coarse-grained soils

If more than 50% of the soil is retained on the No. 200 US sieve, which is 0.075 mm in size, then the soil is designated as coarse-grained soil. The coarse-grained soil is termed gravel if more than 50% or more of the fraction of soil is retained on the No. 4 US sieve, which is 4.75 mm in size, else it is termed as sand. If the soil contains less than 5% fines, then it is termed as well-graded gravels and well-graded sand if they are well graded, and poorly-graded gravel and poorly graded sand if they are poorly graded. If the soil contains more than 12% fines, then the soil is termed Silty gravel, clayey gravel, silty sand, and clayey sand.

Fine-grained soils

If less than 50% of the soil is retained on the No. 200 US sieve, which is 0.075 mm in size, then the soil is designated as fine-grained soil. The fine-grained soils are classified based upon Casagrande's Plasticity chart.

Characteristics of soil

The basic characteristics of soil are important for the initial classification of soil. Following are the various characteristics of soil-

• Composition
• Color
• Texture
• Chemistry

The composition of the soil is used to determine the basic type of soil, which are inorganic and organic. The color is used to determine the preliminary type and composition of the soil, if the soil is red, then the soil is rich in iron, if the soil is black, the soil is rich in organic matter, and so on. The texture is used to distinguish the soil between sand, silt, clay, and gravel. The chemistry is used to determine the alkaline or acidic nature of the soil. If the pH of the soil is below 7, it is classified as acidic and if the pH is above 7, it is classified as alkaline.

Context and Applications

The soil classification and characteristics are useful for the students undergoing the following courses-

• Bachelors in Technology (Civil Engineering)
• Masters in Technology (Civil Engineering)
• Bachelors in Technology (Geotechnical Engineering)
• Masters in Technology (Geotechnical Engineering)

Practice Problems

1. Which of the following is not a method of classification of soil?

1. Particle size classification
2. Textural classification
3. Ductility based classification system
4. Unified soil classification system

Answer: Option c

Explanation: Ductility based classification system is not a method of classification of soil.

2. As per the U.S. Bureau of Soil and Public Road Administration (PRA) system, the soil particles that are below 0.005 mm are termed what?

1. Clay
2. Sand
3. Silt
4. Gravel

Answer: Option a

Explanation: As per the U.S. Bureau of Soil and Public Road Administration (PRA) system, the soil particles that are below 0.005 mm are termed clay.

3. As per the International soil classification system, the soil particles between 0.1 mm and 0.2 mm are termed what?

1. Gravel
2. Fine sand
3. Fine silt
4. Coarse sand

Answer: Option b

Explanation: As per the international soil classification system, the soil particles between 0.1 mm and 0.2 mm are termed fine sand.

4. In HRB classification, the soils are divided into how many primary groups?

1. Five
2. Six
3. Seven
4. Eight

Answer: Option c

Explanation: In HRB classification, the soils are divided into seven primary groups.

5. As per USCS, if more than 50% of the soil is retained on the No. 200 US sieve, then the soil is designated as what?

1. Fine-grained soil
2. Coarse-grained soil
3. Plastic soil
4. Deformable soil

Answer: Option b

Explanation: As per USCS, if more than 50% of the soil is retained on the No. 200 US sieve, then the soil is designated as coarse-grained soil.