Soil Moisture Tension, Thixotrophy, Activity of clay

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Soil moisture tension(S.M.T) is the force per unit area required by plant to suck the water from the soil. It means that S.M.T is inversely proportional to water content in soil(more the water content less S.M.T required and vice versa) i.e. at field capacity it is easy for plant to extract water from soil but at wilting point maximum energy is liberate by plant to extract the water from soil which is equals to the tension with which soil holds the water.

Liquefaction: When there is an intense earthquake occurs the deposit of loose coarse grained soil  structure is activated towards more compact packing due to shaking, because pore water does not have enough time during the earthquake to drain off. The pore water pressure then shoots up as the incompressible water takes up the applied stress and the effective stress approaches zero. Hence zero effective stress and no shear strength results in liquefaction.


Thixotrophy is generally the property of soil due to which the loss in shear strength(depend on material properties) caused by remoulding can be regained with time if left alone. Hence, higher the sensitivity(i.e. more than 16) higher the thixotrophy.If sinsitivity is equals to 1 means there will be no loss in shear strength of soil on remoulding.

The loss in shear strength is due to

    • Permanent distortion of bond between molecules and it is not recoverable.
    • Disturbance in orientation of molecules.
Activity of clay is generally defined as the ratio of the plasticity index and the percentage of clay fraction.
Hence,                           A = Ip / F


where , Ip = plasticity index
F = clay fraction
Activity Types of Soil
<0.75 Inactive
0.75-1.25 Normal Active
>1.25 Active
The variations in the volume(i.e. shrinkage or swelling) of clayey soil generally depends upon the activity.
• Soil containing mineral kaolinite are very low active ( A<1)


• Soil containing the mineral illite are moderately active ( A=1 or 2)


• Soil containing the mineral montmorillonite are highly active (A>4).

Pressure Bulb in Soils:

A pressure bulb is a type of stress contour that joins all points of equal vertical stress below the ground surface. For a particular load system many y isobars can be drawn for different values of stresses.

Since the vertical stress on a given horizontal plane is same in all the directions, an isobar is a curved surface of shape of an onion bulb. Within a pressure bulb, a soil mass will be stressed to stresses higher than the designated stress of the pressure bulb, while the soil mass beyond the pressure bulb will be stressed to lower stresses.

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Arbaj Demrot is the founder of VideRime Online Learning, a leading engineering website. He did his BE Civil and M.Tech Structure from RGPV University, Bhopal and has been working as an Assistant Professor in a reputed college.

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