Classification of soil is an important aspect to be able to understand and deal with characteristics of soil. For general engineering purposes, soils may be classified by the following systems.

Systems for Soil Classification

  • Public Road Administration (PRA) system
  • Particle Size Classification System
  • Textural Classification System

Public Road Administration (PRA) System

PRA system is also called the highway research board (HRB) classification and is based on both the particle size composition as well as the plasticity characteristics. The system is mostly used for the pavement construction.

Soils are divided into 7 primary groups designed as A-1, A-2……A-7.

Group A-1 is divided into two sub groups and Group A-2 into four sub groups. A characteristic group index is used to describe the performance of the soils when used for pavement constructions.

Groups index is not used to place a soil in particular group. It is actually a mean of rating the value of a soil is a sub-grade material work in its own group. The higher the value of this index, the poor is the quality of the material.

The system is based on the following three soil proportion:

  • Particle Size Distribution
  • Liquid Limit
  • Plastic Limit

The group’s index of a soil depends upon:

  • The amount of material the ASTM sieves No 200.
  • The liquid limit.
  • The plastic limit and is given by the following equation is groups index may be determined from the equation.

Group Index (G-I) = 0.2a + 0.005ac + 0.01db


“a” That portion of percentage of soil particles passing number 200 ASTM sieve, greater than 35 and not exceeding 75, expressed as positive whole number (0 to 40)

“b” That portion of percentage of soil particles passing number 200 sieve, greater than 15 and not exceeding 55 expressed as a positive whole number (0 to 4).

“c” That portion of the liquid limit greater than 40 and not exceeding 60 expressed as a positive whole number (0 to 20)

“d” That portion of plasticity index greater than 10 and not exceeding 30, expressed as a positive while number (0 to 20).

Particle Size Soil Classification System

In this system soils are arranged according to the grain size. Term such as gravel, sand, silt and clay are used to indicate grain sizes. These terms are used only as designation of particles size and do not signify the naturally occurring soil sypes, which are mixtures of particles of different sizes and exhibit definite characteristic. It is preferable to use the world “silt size” and “clay size” in place of misplay ‘silt’ or ‘clay’ in this system.

There are various grain size classifications in use, but the most commonly used systems are:

  • US Bureau of soil and public road administration (PRA) system of United States.
  • International soil classification, proposed at the international soil congress at Washington DC in 1927.
  • The MIT classification proposed by Prof. Gilboy at Massachusetts institute of Technology as a simplification of the Bureau of soils classification.

Textural Soil Classification System

Soils occurring in nature are composed of different percentage of sand, silt and clay size particles.

Soil classification of composite soils exclusively based on the particle size distribution is known as “Textural classification”.

Probably the best known of these textural classifications is the triangular classification of US public roads administration. This classification is based on the percentages of sand, silt and clay sizes making upon the soil.

Such a classification is more suitable for describing course grained soils rather than clay soils whose properties are less dependent on the particle size distribution.

CES Editorial Team

An engineer by chance and blogger by passion. Trying to make a difference using words!

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