HRB Classification of Soils

The Highway Research Board (HRB) classification of soils is a widely accepted system of soil classification used for highway construction and design. This system is based on both particle size composition and plasticity characteristics of the soil. Let's understand the classification system in detail.

Particle Size Composition

In the HRB classification system, soils are classified into seven groups based on their particle size composition. These groups are:

1. Gravel: Soil particles larger than 4.75 mm in diameter
2. Coarse sand: Soil particles ranging from 2.00 to 4.75 mm in diameter
3. Medium sand: Soil particles ranging from 0.425 to 2.00 mm in diameter
4. Fine sand: Soil particles ranging from 0.075 to 0.425 mm in diameter
5. Silt: Soil particles ranging from 0.002 to 0.075 mm in diameter
6. Clay: Soil particles smaller than 0.002 mm in diameter
7. Organic soil: Soil composed primarily of organic material

Plasticity Characteristics

In addition to particle size composition, the HRB classification system also takes into account the plasticity characteristics of the soil. The plasticity of a soil refers to its ability to be molded or shaped without breaking. Soils are classified based on their plasticity characteristics into three groups:

1. Non-plastic: Soils that do not exhibit plasticity and crumble easily when dry.
2. Low plasticity: Soils that exhibit some plasticity but do not form a cohesive mass when rolled into a thread 1/8 inch in diameter.
3. High plasticity: Soils that exhibit significant plasticity and form a cohesive mass when rolled into a thread 1/8 inch in diameter.

Final Classification

The final classification of a soil in the HRB system is determined by combining its particle size composition and plasticity characteristics. For example, a soil that is predominantly clay and exhibits high plasticity would be classified as CH (clay, high plasticity).

Conclusion

The HRB classification system is an important tool for engineers and geologists to understand the properties of soils for highway construction and design. By combining particle size composition and plasticity characteristics, this system provides a comprehensive classification of soils that helps in selecting appropriate construction materials and designing safe and efficient highways.