What is group index of soil?

Before discussing about the Group Index (GI) we will first discuss about the need and significance of GI.


This group index is developed by US Bureau of Public Road. They wanted to develop the soil classification system called the Public Road Administration classification system which was meant for road construction. Actually the famous name for this classification system is AASHTO classification system.

Calculation of Group Index

According to AASHTO soil is classified into eight groups A-1 to A-7 and A-8 for peat or muck.

$$ GI=0.2a+0.005ac+0.01bd$$

Also read: permeability of soil, void ratio, specific gravity

Where :

a= that part of the percent passing the \(75\mu\) sieve (\(-75\mu\) greater than 35 and less than 75. Expressed as a positive whole number ( Suppose the percentage passing the \(75\mu\) is 40 then the value of “a” is 5)

b= the part of percentage passing the \(75\mu\) sieve greater than 15 and not exceeding 55, expressed as positive whole number (range 1- 40)( Suppose the percentage passing the (75\mu) is 40 then the value of “b” is 25)

c=the part of liquid limit greater than 40 and not greater than 60, expressed as positive whole number (range 1 to 20 )

d = the part of the plasticity index greater than 10 and not exceeding 30 expressed as positive whole number (range 1 to 20 )

You must take care that the calculation of GI from the above formula will always be positive. If you found the value of GI is coming out negative, than you have to take the value of GI as zero.

So, if you want to classify the soil according to GI formula then you have to find following values:

  1. Grain size distribution
  2. Liquid limit
  3. Plasticity index value

In practice if you want to take idea. about the GI values. Than if you find the higher GI value, lesser the soil is suitable for highway construction with in that subgroup. A group index of 0 indicates a good subgrade material. You also keep in in mind that a GI of 20 or more indicates a very poor subgrade material .

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