Concept:
Higher the value of group index, poorer is the soil as subgrade material.
Type of subgrade soil |
Group index range of subgrade |
Good |
0 – 1 |
Fair |
2 – 4 |
Poor |
5 – 9 |
Very poor |
10 – 20 |
Additional Information
Group index of a soil is given by
G.I = 0.2 a + 0.005 ac + 0.01 bd
a = It is the portion of % passing through 75 μ sieve greater than 35 but not exceeding 75 expressed as whole number in between [0-40].
b = It is the portion of % passing through 75 μ sieve greater than 15 but not exceeding 55 expressed as whole number in between [0-40].
c = It is the portion of the numerical liquid limit greater than 40 but not exceeding 60 expressed as whole number in between [0-20].
d = It is the portion of the numerical plasticity index greater than to 10 but not exceeding 30 expressed as whole number in between [0-20].