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Classification of Soil

Soil classification is the arrangement of soils into different groups such that the soils in a particular group have similar behavior under given set of physical conditions.Any soil classification system must provide us with information about the probable engineering behaviour of a soil.The most commonly used properties for soil classification are

i) Grain size distribution
ii) Plasticity of soil

Engineering classification System of Soil
i) Unified Soil Classification System(USCS)
ii) American Association of State Highway and Transport Officials(AASHTO)
iii) Indian Standard Soil Classification System(ISSCS)

Unified Soil Classification System(USCS)
This system was originally developed by A. Casagrande. According to the USCS, the coarse-grained soils are classified on the basis of their grain-size distribution and fine grained soils on the basis of their plasticity characteristics.

Soil type Prefix Subgroup Sufffix
Gravel G Well graded W
Sand S Poorly graded P
Silt M Silty M
Clay C Clayey C
Organic O <50% L
Peat Pt <50% H

Table 1: Prefix and Suffix of USCS

Coarse grained soils are those having 50% or more retained on the 0.075 mm sieve.They are further subdivided into gravels and sands. The coarse grained soils are designated as gravel (G) if 50% or more of these are retained on 4.75 mm sieve, otherwise they are designated as sand(S).

Fine grained soils having more than 50% material passing the 0.075 mm sieve.These are subdivided into silt(M) and clay(C) based on their liquid limit and plasticity index.

Highly organic soils,Fibrous in nature,usually peat and swampy soils having high compressibility are not subdivided.

AASHTO Soil Classification System

According to AASHTO system,the soils are classified into eight groups: A-1 through A-7 with an additional group A-8 for peat.This system includes several sub-groups.Soil within each group are evaluated according to the group index.

GI=0.2a + 0.005ac + 0.01bd

Where, a=that part of the percentage passing the 75 µ sieve greater than 35 and not exceeding 75

b=that part of the percentage passing the 75µ  sieve greater than 15 and not exceeding 55

c=that part of liquid limit greater than 40 and not greater than 60

d=that part of plasticity index greater than 10 and not exceeding 30

In general, the greater the group index value, the less desirable a soil is for highway construction within that sub-group. A group index 0 indicates a good subgrade material where group index 20 or more indicates a very poor subgrade material.

Indian Standard Soil Classification System(ISSCS)
The Indian soil classification system is basically the same as that of USCS with the slight modification that the fine grained soil have been subdivided into three sub-groups of low, medium and high compressibility.

In this system,coarse grain soils are classified on the basis of grain size distribution and fine soils on the basis of plasticity.

Coarse grained soil Fine Grained soil
Gravel Sand Silt
>300 300-80 20-80 4.75-20 2-4.75 0.425-2.0 0.075-0.425 0.002-0.075 <0.002

Table2.0: I.S. Classification (grain size distribution)

The fine grained soils are classified on the basis of their plasticity characteristics using the I.S. plasticity chart.

Plasticity chart as per Indian Soil Classification System
Fig: Plasticity chart as per Indian Soil Classification System

Coarse grained soils which contain more than 12% fines(<75 µ) are classified as GM or SM if the fines are silty in character;they are classified as GC or SC if the fines are clayey in character.

Coarse-grained soils having 5% to 12% fines are borderline cases and given a dual symbol.Fine grained soil also can have dual symbols if IP and WL values fall close to the A-line.

The above soil classification is based on a line called A-line,which is a boundary representing relationship between plasticity index IPand liquid limit WL

If IPof soil> IP of A-line

The soil will be above A-line and it will be clay(C)
If IP of soil< IP of A-line

The soil will be below A-line and it may be either silt(M) or organic clay(OI)

The IP of A-line is given by IP=0.73(WL-20)

Where WL is the liquid limit.

ISSCS Recommendation for field identification procedure of soil
The tests are-
(i) Visual Examination: Visual examination of soil is done by taking representative sample of soil and spreading it on a flat surface or the palm of hand.Visual examination is carried out with respect to size, angularity, touch and grading.

(ii) Dilatancy test: About 5cc of soil sample is taken and enough water is added to nearly saturate it. The pat of soil is placed in the open palm of the hand and shaken horizontally,striking vigorously against the other hand several times. The pat is then squeezed between the fingers.The appearance and disappearance of water with shaking and squeezing is referred to as a positive reaction. type of reaction is observed and recorded. Fine sand and silt exhibit a quick reaction whereas clays, none to show.

(iii) Toughness: It means consistency near plastic limit.In this test firstly,soil sample used in dilatancy test is dried by working and moulding until it reaches the consistency of putty.The time required to dry the sample is indicative of its plasticity.Further the moisture content is reduced by rolling and re-rolling into 3mm diameter thread till it reaches the plastic limit. The resistance to moulding at the plastic limit is called toughness.

Medium toughness is indicated by a medium thread and a lump formed of a thread slightly below the plastic limit crumbles;while low toughness is indicated by weak thread that breaks easily.Non plastic soil cannot be rolled into threads of 3 mmdiameter at any moisture content.

(iv) Dry Strength: It is also called crushing resistance. The prepared soil sample is dried completely in the sun or air drying. Its strength is tested by breaking a lump between fingers. Resistance to breaking, termed as dry strength is measure of the plasticity and is considerably influenced by the colloidal fraction content of the soil. If dry sample can be powdered easily, it is said to have a low dry strength, whereas if considerable finger pressure is required to break the lump it is said to have medium dry strength and if it cannot be powdered at all, it is said to have high dry strength.

(v) Organic content and colour: Organic soils have usually dark colour.It has pungent odour of organic matter and high total available water capacity.

(vi) Other identification Tests:These may be developed by an individual on the basis of experience,e.g. feel of soil in hands or between fingers.

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Kanwarjot Singh

Kanwarjot Singh is the founder of Civil Engineering Portal, a leading civil engineering website which has been awarded as the best online publication by CIDC. He did his BE civil from Thapar University, Patiala and has been working on this website with his team of Civil Engineers.

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