CLASSIFICATION OF SOIL : Geotechnical Engineering

 

                CLASSIFICATION OF SOIL

There are number of classification system are  developed according to  different purpose of soil.

Soil are classified on the basis of soil particle,size,texture,colour,plasticity index etc.

Soil may broadly classified are as fallow :

1. Classification based on grain size

1. Textural classification

1. AASHTO classification system

1. Unified soil classification syste

1. Indian soil classification system

 

 

  CLASSIFICATION OF SOIL ON BASIS OF PARTICLE SIZE

 

SOIL TYPES	SIZE OF PARTICLE
Boulder	> 300 mm
Cobble	300 mm to 80 mm
Gravel	80 mm to 4.75 mm.
Coarse gravel	80 mm to 20 mm
Fine gravel	20 mm to 4.75mm
Sand	4.75 mm to 0.075 mm
Coarse sand	(4.75 mm to 2.0 mm
Mwdium sand	2.0 mm to 0.425 mm)
Fine sand	0.425 mm to 0.075 mm
Fine grained	less than 0.075 mm
Coarse grained	80 mm to 0.075 mm
Clay	less than 0.002 mm
Silt	0.075mm to 0.002 mm

 

 

 NOTE

 INDIAN SOIL CLASSIFICATION SYSTEM IS PREFERED MORE BECAUSE MAXIMUM OR NEARLY ALL QUESTIONS COMES IN GATE FROM ISCS.

 

INDIAN  SOIL  CLASSIFICATION SYSTEM

ISC System (ISC) was adopted by Bureau of Indian Standards.

The soil broadly classified into two categories:

1-COARSE GRAINED SOIL
2- FINE GRAINED SOIL
 

COARSE GRAINED SOIL

If 50% or more amount of soil are retained on 75 micron sieve the it termed as the COARSE GRAINED SOIL

 FINE GRAINED SOIL

If 50% or more amount of soil are passed through the 75 micron sieve the it termed as the FINE GRAINED SOIL

 

Coarse grained soil are further classified into 2 main categories on the basis % fineness :-

1-     IF % FINENESS LESS THAN 5%

     

 

 

2-     IF % FINENESS IS GREATER THAN 12%

 

 

3-     IF FINENESS % IS BETWEEN 5% TO 12%

 

Fine grain soil also subdivided into two categories based on particle size :

1- If the particle size is grater than 4.75 it is called gravel and

2- If particle size is less than 4.75mm it is called sand.

 

                A-LINE EQUATION

 

            EQUATION OF A-LINE :-

                                 IP = 0.73( WL-20)

 Where :--

IP = PLASTICITY INDEX
WL = LIQUID LIMIT
WP = PLASTIC LIMIT
 

Let the A-line equation is IP2 AND

                                          IP1 = WL – WP

IF  IP1 > IP2 soil lies above the A-line AND

 

IF  IP1 < IP2 soil lies below the A-line

 NOTE

Above the A-line the soil is clay.

Below the A-line the soil is Silt.

 

SOIL TYPE AND PREFIX AND SUFFIX

This is used for Nomenclature of any type of soil.

There are two groups of soil the main group and the subgroups.

 The subgroup are used before the main group but in symbol form main group is written before the subgroup.

 

MAIN GROUP	PREFIX
Gravel	G
Sand	S
Silt	M
Clay	C
Organic	O

 

 

SUBGROUP	SUFFIX
Well Graded	W
Poorly Graded	P
Sandey	S
Clayey	C
Siltey	M

 

 

EXAMPLE:-

The notation ‘’SC’’ as per INDIAN SOIL CLASSIFICATION SYSTEM refers to :-     (GATE 2019)

SOLUTION:-

In this S is the main group and C is the subgroup

So, according to rule we write subgroup first and then main group

S stands for silt

C stands for clayey

  The answer is CLAYEY SAND