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Classification of Soil

The Classification of soil is very important in engineering design. The classifications are done with engineering basics. Therefore, those can be used to identify the behavior of the soil.

Characteristics of the soil are very important to determine the settlement criteria of a foundation. Doing design without technical backing would end up with an excessive settlement of the foundations.

There are three main areas that we consider classifying the soil.

Grain size distribution

Liquid Limit

Plastic Limit

In this classification, Atterberg limits play greater role.

The classification of soil is done mainly on the two types of classifying systems

  1. American Association of State Highway and Transportation Officials (AASHTO)
  2. Unified Soil Classification System – ASTM

American Association of State Highway and Transportation Officials (AASHTO)

According to this classification, soils are classified under eight main categories. They are called as A1, A2, A3, A4, A5, A6, A7 and A8.

Out of those types

A1-A3 – Coarse grained materials

A4-A7 – Fine grained materials

A8       – Peat, Muck and other highly organic soils

Group index (GI) is used to specify soil types. It is calculated based on the F200; percentage passing no. 200 sieve, liquid limit and plastic index.

Unified System

This classification system is widely used today in geotechnical works.

The method initially proposed was revised by United States Bureau of Reclamation and the US Army Corps of Engineer.

In this classification soil types such as gravel, sand, silt, clay, organic silts and clay, peat and highly organic soils, etc. are used.

This classification covers wide spread of the soils and there are many types of soils based on their Propeties. The same soil type is further sub divided into the different categories. For example, gravel can be categorized as well graded gravels, well graded gravel with sand, poorly graded gravel, poorly graded gravel with sand, and many more types.

You may work out soil type based on the soil properties such as Cu, Cc, percentage of fines, etc. Then this will be clear.

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