Group index of soil

• Group index of soil is a number used to refer the quality of soil used as a subgrade material in highway construction. 
• Group index of soil is also used in AASHTO classification of soil. In AASHTO soil classification, group index is also included along with soil group and subgroup.
• Group index of soil as a subgrade material is calculated using liquid limit,  plastic limit, plasticity index and % passing from US sieve #200 (i.e, 0.075 mm opening sieve). 

Note:  Plasticity index, PI = (liquid limit - plastic limit) 

• So,  group index of soil can be defined as the combination of :
1.  Partial group index obtained from liquid limit. 
2. Partial group index obtained from plasticity index.

Group index of soil Formula

GI = (F₂₀₀-35) [0.2+0.005(LL-40)] + 0.01(F₂₀₀-15)(PI-10)

Here, (F₂₀₀ -35) [0.2+0.005(LL-40)] = partial group index obtained from liquid limit 

0.01(F₂₀₀ -15) (PI-10) = partial group index obtained from plasticity index

Note : In short form, the GI formula can also be written as

GI = 0.2a + 0.005ac + 0.01bd

here, a = (% passing # 200 sieve)-35

b = (%passing #200 sieve)-15

c =(LL-40)

d = (PI-10)

• Group index of soil is always rounded off to nearest whole number. If group index of soil comes to be negative , then group index of soil in this case is considered zero.
• The value of group index of soil ranges from 0 to 20. A soil having lower group index value is considered better subgrade material in comparison to that of soil having higher group index value.

• In AASHTO soil classification system, the group index value for
• Coarse grained soil
•  A-1-a, A-1-b,  A-2-4,  A-2-5,  A-3, GI= 0
• A-2-6 and A-2-7 , GI = 0.01(F₂₀₀-15)(PI-10) ,i.e, partial group index obtained from plasticity index.
• Fine grained soil
• GI = (F₂₀₀-35)[0.2+0.005(LL-40)] + 0.01(F₂₀₀-15)(PI-10)

Note : For coarse grained soil, the partial group index obtained from liquid limit is neglected.