Activity of Soil

Activity of Soil :- ($A_t$)


Activity of soil os the ratio of plasticity index and percentage of clay friction (minus 2 cu size)



$I_p=$Plastic index

$f=$clay friction


where, $C%=$% of soil passing 2 cu size =% of soil finear than 2 mu $\lt 2u \rightarrow clay$


$A_t \propto (W_l-W_p)-plasticity$

$\propto Wl$ (Its compressibility)


If activity of soil is more then this soil will be more compressible or highly compressible.

Represented $\rightarrow$ shrinkage & swelling property of soil

  • The clay friction F is % finer than 2 u size
  • The amount of water is a soil mass depend upon the type of clay mineral present.
  • Activity is a measure of the water holding capacity of clay soil. The changes in the volume of a clay in soil during swelling or shrinkage depends upon activity.

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The soil containing the clay mineral:

  • Kaolinite
  • Ilite
  • Montomorillonite

  • The soil containing the clay mineral montomorillionite have been main activity $(A\gt4)$

  • The soil contains the mineral Kaolinite are least active $(A\lt 1)$

  • Whareas soil containing the mineral Illite are moderately active $(A=1 to 2)$

  • Depending on the activity, the soils are classified into three types.

Sr. No. Activity Soil Type
1 A$\lt$0.75 Inactive
2 A=0.75 to 1.25 Normal
3 A$\gt$1.25 Active
  • Activity gives information about the type of effect of class mineral in a soil. The following two points are worth nothing.
  • For a soil of specific origin. the activity is constant. The plastic index (P.I.) increases as the amount of class friction increases.
  • Montomorillonite can produce a large increase in the plasticity even when present in small quantity.
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