Classification systems are used to group soils according to their order of performance under given set of physical conditions. Soils that are grouped in order of performance for one set of physical conditions will not necessarily have the same order of performance under some other physical conditions.
Therefore, number of classification systems have been developed depending on the intended purpose of the system. Soil classification has proved to be a very useful tool to the soil engineer.It gives general guidelines in an empirical manner for making use of the field experience of others.
Different Classification of Soils for Engineering Purpose
Soil may be broadly classified as follows:
- Classification based on grain size
- Textural classification
- AASHTO classification system
- Unified soil classification system
(i) Grain Size Classification System for Soils
Grain size classification systems were based on grain size. In this system the terms clay, silt, sand and gravel are used to indicate only particle size and not to signify nature of soil type. There are several classification systems fin use, but commonly used systems are shown here.
(ii) Textural Classification of Soil
The classification of soil exclusively based on particle size and their percentage distribution is known as textural classification system. This system specifically names the soil depending on the percentage of sand, silt and clay. The triangular charts are used to classify soil by this system.
Figure – 1 shows the typical textural classification system.
Fig-1: Textural Classification of U.S. Public Roads Administration
(iii) AASHTO classification system of Soil
AASHTO classification, (table-2) is otherwise known as PRA classification system. It was originally developed in 1920 by the U.S. Bureau of Public Roads for the classification of soil for highway subgrade use.
This system is developed based on particle size and plasticity characteristics of soil mass. After some revision, this system was adopted by the AASHTO in 1945.
In this system the soils are divided into seven major groups. Some of the major groups further divided into subgroups. A soil is classified by proceeding from left to right on the classification chart to find first the group into which the soil test data will fill.
Soil having fine fractions are further classified based on their group index. The group index is defined by the following equation.
Group index = (F – 35)[0.2 + 0.005 (LL – 40)] + 0.01(F – 15)(PI – 10)
F – Percentage passing 0.075mm size
LL – Liquid limit
PI – Plasticity index
When the group index value is higher, the quantity of the material is poorer.
(iv) Unified Soil Classification System
Unified soil classification system was originally developed by Casagrande (1948) and was known as airfield classification system. It was adopted with some modification by the U.S. Bureau of Reclamation and the U.S. Corps of Engineers.
This system is based on both grain size and plasticity characteristics of soil. The same system with minor modification was adopted by ISI for general engineering purpose (IS 1498 – 1970).
IS system divides soil into three major groups, coarse grained, fine grained and organic soils and other miscellaneous soil materials.
Coarse grained soils are those with more than 50% of the material larger than 0.075mm size. Coarse grained soils are further classified into gravels (G) and sands (S). The gravels and sands are further divided into four categories according to gradation, silt or clay content.
Fine grained soils are those for which more than 50% of soil finer than 0.075 mm sieve size. They are divided into three sub-divisions as silt (M), clay (c), and organic salts and clays (O). based on their plasticity nature they are added with L, M and H symbol to indicate low plastic, medium plastic and high plastic respectively.
GW – well graded gravel
GP – poorly graded gravel
GM – silty gravel
SW – well graded sand
SP – poorly graded sand
SM – silty sand
SC – clayey sand
CL – clay of low plastic
CI – clay of medium plastic
CH – clay of higher plastic
ML – silt of medium plastic
MI – silt of medium plastic
MH – silt of higher plastic
OL – organic silt and clays of low plastic
OI – organic silt and clays of medium plastic
OH – organic silt and clays of high plastic.
Fine grained soils have been sub-divided into three subdivisions of low, medium and high compressibility instead of two sub-divisions of the original Unified Soil Classification System.
Table-3 below shows the classification system. Table 2 lists group symbols for soils of table-3.
Table-2: Significance of letters for group symbol in table-3.
Table – 3
|Applicable to Coarse grained Soils||Well graded||W|
|Applicable to Fine grained soils||Low compressibility|
(WL 35 to 50)
The standard recommends that when a soil possesses characteristics of two groups either in particle size distribution or in plasticity, it is designed by combination of group symbols.
Field identification is recommended through the following tests:
For fine grained soils
a) Visual examination
b) Dilatancy test
c) Toughness test
d) Dry strength test
e) Organic content and colour
f) Other identification test
Indian Standard Classification System for Soil
Indian Standard Classification System (ISC) was adopted by Bureau of Indian Standards is in many respect similar to the Unified Soil Classification (USC) system.
Soils are divided into three broad divisions:
- Coarse grained soils, when 50% or more of the total material by weight is retained on 75 micro IS sieve.
- For fine grained soils, when more than 50% of the total material passes through 75 micron IS sieve.
- If the soil is highly organic and contains a large percentage of organic matter and particles of decomposed vegetation, it is kept in a separate category marked as peat (Pt).
In all there are 18 groups of soils: 8 groups of coarse grained, 9 groups of fine grained and one of peat.
Fig.2: Indian Standard Classification Plasticity Chart
Click to view following ISC Charts