Foundation, Concrete and Earthquake Engineering

Significance of Activity of Soil

In United States, large parts of Africa and India there have severe problems associated with expansive potential of clay soil. The three clay minerals we concern mainly in foundation engineering are:

a. Kaolinite

b. Illite

c. Montmorillonite

If we can identify the mineral structure of clay soils, we can predict their expansion and shrinkage potential. 

Many lighter building in the regions of expansive soil, subjected to severe settlement in the regions stated above of the world; even before they were left for operation.
Structural unit that form montmorillonite
Montmorillonite is the main mineral element of black cotton soil. The mineral structure of clay is indicative to amount of water and form of their presence in clay minerals. Water can combine separated mineral units to form a sheets stakes over another unit.

Activity of clay soil defines its water retaining capacity and we know that swelling and shrinkage of soil depends on the water within soil. Thus both phenomena of volume change depend on activity of soilSoils are classified as three types as follows:

a. Inactive soil
b. Normal soil
c. Active soil

Inactive soil

Soil having activity less than 0.75 is considered as inactive soil.

Normal soil

Soil having activity in between 0.75 and 1.25 are taken as normal soil.

Magnified view of Kaolinite

Active soil

Soil that has activity greater than 1.25 is considered as active. Identifying clay minerals depending on activity:


Soil having activity greater than 4 have significant amount of montmorillonite.


Soil having activity less than 1 has mainly kaolinite

Illite :

Soil having activity in between 1 and 2 have significant amount of illite.

Thus activity furnishes information about effect and type of mineral in clay soil. We conclude following points about activity:

a. Activity remains constant for a soil from specific origin. In such case, plasticity index reduces as quantity of clay fraction reduces and vice versa.

b. In case of highly active clay minerals like montmorillonite, shows great increase in plasticity index under small quantity of clay fraction.

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