A sample of inorganic soil has the following grain size characteristics
| Size (mm) | Percent passing |
| 2.0 (No. 10) | 95 |
| 0.075 (No. 200) | 75 |
The liquid limit is 56 percent, and the plasticity index 25 percent. Classify the soil according to the AASHTO classification system.
Percent of fine grained soil = 75
Computation of Group Index [Eq. (3.56a)]:
\text { Group Index }(G I)=0.2 a+0.005 a c+0.01 b d (3.56a)
a = 75 – 35 = 40
b = 75 – 15 = 60
c = 56 – 40 = 16, d = 25 -10 = 15
Group Index, G I=0.2 \times 40+0.005 \times 40 \times 16+0.01 \times 60 \times 15=20.2
On the basis of percent of fine-grained soils, liquid limit and plasticity index values, the soil is either A-7-5 or A-7-6. Since \left(w_{l}-30\right)=56-30=26>I_{p}(25), the soil classification isA-7-5(20).