Question

If the proportion of soil passing 75 micron sieve is 50% and the liquid limit and plastic limit are 40% and 20% respectively, then the group index of the soil is

• A. 3.8
• B. 6.5
• C. 38
• D. 65

Answer 1

The correct option is B 6.5

Group index, GI = 0.2a + 0.005ac + 0.01 bd

a = % passing 75 mm sieve greater than 35 but not exceeding 75 (between 0 to 40)
b = % passing 75 mm sieve greater than 15 but not exceeding 55 (between 0 to 40)
c = Liquid limit greater than 40 not exceeding 60 (between 0 to 20)
d = Plasticity index greater than 10 and not exceeding 30 (between 0 to 20)
a = 50 - 35 = 15 < 40
b = 50 - 15 = 35 < 40
C = 40 - 40 = 0
d = 20 - 10 = 10 < 20
$\therefore$ G.I. = 0.2 $\times$ 15 + 0.005 $\times$ 40 $\times$ 0
+0.01 $\times$ 35 $\times$ 10
= 3 + 0 + 3.5
= 6.5