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
8. Angle of contact
9. Dissolved salts in water