SSC JE Civil Engineering 23-Mar-2021 Shift 1 Paper

Concepts:
The proportion of clay mineral flakes (< 0.002 mm size) in a fine soil increases its tendency to swell and shrink with changes in water content. This is called the activity of the clayey soil, and it represents the degree of plasticity related to the clay content. Mathematically, it is the ratio of Plasticity Index(IP) of soil and percentage of clay size particles 9 less than 2 micron).
$\mathrm{A}=\frac{\mathrm{IP}}{\%\mathrm{of}\mathrm{Clay}\mathrm{size}\mathrm{fraction}}$
Plasticity Index, IP = Liquid Limit (LL) – Plastic Limit (PL)
Calculation:
Given: LL = 45 5; PL = 25 %; % clay size fraction = 20 %
The Activity of soil is given as:
$\mathrm{A}=\frac{45-25}{20}$
∴ A = 1
Important Point:
Based on Activity, soils can be classified as:

Activity	Classification
< 0.75	Inactive
0.75 - 1.25	Normal
> 1.25	Active