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Capillary water

The minute pores of soil serve as capillary tubes through which the moisture rise above the ground water table.

Capillary water


The minute pores of soil serve as capillary tubes through which the moisture rise above the ground water table.


Capillary water is the soil moisture located within the interstices and voids of capillary size of the soil.


Capillary water is held in the interstices of soil due to capillary forces. Capillary action or capillarity is the phenomenon of movement of water in the interstices of a soil due to capillary forces.


The capillary forces depend upon various factors such as surface tension of water, pressure in water in relation to atmospheric pressure, and the size and conformation of soil pores.


1 Contact moisture.


Water can also be held by surface tension round the point of contact of two particles (spheres) capillary water in this form is known as contact moisture (or) contact capillary water.




The pores of soil mass may be looked upon as a series of capillary tubes, extending vertically above water table.


The rise of water in the capillary tubes, or the fine pores of the soil, is due to the existence of surface tension which pulls the water up against the gravitational force.


The height of capillary rise, above the ground water (or free water) surface depends upon the diameter of the capillary tube (or fineness of the pores) and the value of the surface tension.

When a capillary tube is inserted in water, the rise of water will take place up to reach the

equilibrium. At this stage the rise of water in the tube is stopped. At this equilibrium position,  when the height of rise is hc, the weight of column of water is equal to The  weight  of  water  in  the  tube   s      orted   by   the   surface   tension(?w)of meniscus

e=Ts cosa / rwd




Soil undergoes a volume change when the water content cause shrinkage while increase of water content swelling.


Large volume changed in clayey soils lead to structural damage. For clayey soils, the degree of change in volume depends upon factors such as


i)  Type and amount of clay minerals present in the soil.


ii) Specific surface area of clay.


iii)  Structure of soil.


iv) Pore water salt concentration.


The two fundamental building blocks for clay minerals are


i) Silica tetrahedral unit




i) Silica tetrahedral unit


Four oxygen or hydroxyls having a configuration of tetrahedral enclose silicon. It is resembled in the symbol repressing the oxygen based layer and hydroxyl apex layer.


ii) Gibbsite


Aluminum, iron or magnesium atom is enclosed in six hydroxyls.

Surface tension                                                   

Surface tension of water is the which exists in the surface film of water tending to contract the contained volume in to a form having minimum superficial area possible


The surface tension of water is double the surface tension of other liquids.

Capillary tension (or) capillary potential

Tensile stress caused in water is the capillary tension or potential. It is also called as pressure deficiency or pressure reduction negtive pressure.





The tensile stress in the meniscus circumferences caused in water is called the capillary tension or the capillary potential. The capillary tension or capillary potential is the pressure deficiency, pressure reduction or negative pressure in the pore water (or the pressure below atmospheric) by which water is retained in a soil mass. It decreases linearly from a maximum value of hc?w at the level of the meniscus to zero value at the free water surface.


The pressure deficiency in the held water is also termed as soil suction or suction pressure.


Soil suction is measured by the height hc in centimeters to which a water column could be drawn by suction in a soil mass free from external stress.


The common logarithm of this height (cm) or pressure (g/cm2) is known as the pF value (Schofield, 1935): pF = log10 (hc)


Thus, a pF value of 2 represents a soil suction of 100 cm of water or suction pressure and capillarity of 100 g /cm2.


Factors affecting soil suction:


1.     Particle size of soil


2.     Water content


3.     Plasticity index of soil mass


4.     Soil structure


5.     History of wetting and drying


6.     Soil density


7.     Temperature


8.     Angle of contact

9.     Dissolved salts in water


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