Worked example on how to calculate particle size analysis of soil

To evaluate: the quality of a soil as a highway subgrade material, one must also inmrporate a number called the grop index (Gi) with the groups and subgroup the soil. This index is written in parentheses after the group or subgroup designation.

The group index is given by the: equation

Equation 

Example

The results of the particle-size analysis of a soil are as follows:

- Percent passing through the No 10 sieve 100
- Percent passing through the No 40 sieve - 80
- Percent passing through the No 200 sieve

The liquid limit and plasticity index of the minus No. 40 fraction of the soil are 30 and 10, respectively. Classify the soil by the AASHTO system.

Solution
Using Table 3.1, since 58% of the soil is passing through the No. 200 sieve. it falls under silt-clay classifications - that is, it falls under group A-4, A-5, A-6, or A-7.

Proceeding from left to right, it falls under group A-4.
From Eq. (3.1)

Gi = (F200-35) + 0.005(LL - 40)) + 0.01(F200 - 15)(Pi-10)
= (58 - 35)(o.2 + o.o05(30 - 40)) + (o.o1)(58 - 15)(1o - 1o)
=3.45~3

So. the soil will be classified as A-4(3).

Classification of highway sub grade soil