HRB classification system, also known as public road administration (PRA) is based on both the particle-size composition and plasticity characteristics.

Triangular classification of U.S. public road administration is a commonly used system for textural classification.

Since the textural classification is based on the percentages of sand, silt, and clay size making up the soil. Such a classification is more suitable for describing coarse-grained soil.

In the particle soil classification system, soils are arranged to the grain size.

Terms such as gravel, sand, silt, and clay are used to indicate grain sizes. These terms are used only as a designation of particle size.

Based on HRB system, the soil is divided into 7 primary groups, designated as A-1, A-2…A-7.

Soil occurring in nature is composed of a different percentage of sand, silt, and clay size particles.

Particle size classification used only as a designation of particle sizes, and do not signify the naturally occurring types which are mixtures of a particle of different sizes.

A triangle, having equal length of sides (i.e. Equilateral triangle) is used in textural classification chart.

As the system is classified based on size, it is preferable to use the word ‘silt size’ and ‘clay size’ in place of simply ‘silt’ or ‘clay’.