Factors affecting permeability of soils

The following factors affect permeability of soils.

• Particle Size. It was studied by Allen Hazen that the coefficient of permeability (k) of a soil is directly proportional to the square of the particle size (D). Thus permeability of coarse grained soil is very large as compared to that of fine grained soil. The permeability of coarse sand may be more than one million times as much that of clay.
• Impurities in water. Any foreign matter in water has tendency to plug the flow passage and reduce the effective voids and hence permeability of soil.
• Void ratio (e). Coefficient of permeability varies with void ratio as e³/(1+e). For a given soil, the greater the void ratio, the higher is the value of coefficient of permeability. Here 'e' is void ratio.

Based on other concepts it has been established that permeability of a soil varies as e² or e²/(1+e). Whatever may be the exact relationship, all soils have e versus log k plot as a straight line.^[1]

• Degree of Saturation. If the soil is not fully saturated, it contains air pockets. The permeability is reduced due to the presence of air which causes blockage to passage. Consequently, the permeability of a partially saturated soil is considerably smaller than that of fully saturated soil. In fact, Darcy's Law is not strictly applicable to such soils.
• Adsorbed water. The fine grained soil have a layer of adsorbed water strongly attached to their surface. This adsorbed layer is not free to move under gravity. It causes an obstruction to the flow of water in the pores and hence reduces permeability of soils.

^[2]

• Entrapped air and organic matter.

The air entrapped in the soil and organic matter block the passage of water through soil, hence permeability considerably decreases. In permeability tests, the sample of soil used should be fully saturated to avoid errors.^[3]

See also

• Permeability
• Darcy's Law
• Soil
• Soil Mechanics

References