Introduction to Soil Classification Systems

Because the soil deposits of the world are infinitely varied, it has not been found possible to create a universal system of soil classification for dividing soils into various groups and subgroups on the basis of their important index properties. However, useful systems based on one or two index properties have been devised. Some of these systems are in such common use by workers in various fields involving soils that the engineer must have at least a general knowledge of them. At the same time it is essential to keep in mind that no system can adequately describe soil of all engineering purposes.

Detailed testing of soils for determining their suitability as a load supporting material or as construction material is difficult, expensive and time consuming.  Engineers, on the basis of their experience, therefore, have divided soils into various groups.  Each group of soil has its own peculiar engineering properties.  This segregation of soils into various groups on the basis of their engineering properties is known as soil classification.

The purpose of soil classification is to provide a systematic method of categorizing soils into different groups in accordance with their engineering performance.

PRINCIPLES OF SOIL CLASSIFICATION

A good soil classification system must satisfy the following basic principles:

1)         The terms used in the system must be universal, brief; comprehensive and meaningful for the user.

2)          The system must utilize some simple field and/or laboratory identification and classification tests.

The groups and sub-groups must categorize soils of similar characteristics and engineering behavior.

REQUIREMENT OF SOIL CLASSIFICATION SYSTEM

It should be:

1)                  Based on scientific method

2)                  Simple

3)                  Permits classification by visual and manual tests

4)                  Acceptable to all engineers

5)                  Describe engineering properties

TYPES OF SOIL CLASSIFICATION SYSTEMS

Engineering classification system takes into account the engineering performance of the soils and accordingly the system is known as Engineering Classification System. All classification system relies on results of particle size distribution.  Some of the widely used classification systems rely on sieve analysis results for coarse-grained soils and results of the consistency tests for fine grained particles. Different engineering classification systems are:-

1)                  AASHTO (American Association for State Highway and Transportation Officials)

2)                  USCS (Unified Soil Classification System)

3)                  M.I.T. and British Standards (Massachusetts Institute of Technology)

4)                  US Dept. of Agriculture Classification System (USDA)