6.3.3 Factor affecting soil permeability and hydraulic conductivity

1. Grain-size

2. Void ratio

3. Composition

4. Fabric or structural arrangement of particles

5. Degree of saturation

6. Presence of entrapped air and other foreign matter.

 

Permeability varies with the square of particle diameter. It is logical that the smaller the grain-size, the smaller the voids and the lower the permeability. A relationship between permeability and grain-size is more appropriate in case of sands and silts than that in case of other soils since the grains are more equidimensional and its fabric changes are not significant.

 

This suggests a simple method for assessment of permeability of a soil at any void ratio when values of permeability are known at two or more void ratios. Once the line is drawn, permeability at any void ratio may be read directly.

 

Influence of soil composition on permeability is generally of little significance in case of gravels, sands, and silts, unless mica and organic matters are present. However, this is of major importance in the case of clays. Montmorillonite has the least permeability with sodium as the exchangeable ion (less than 107 cm/s, even at a very high void ratio of 15). Therefore, sodium montmorillonite is used by the engineer as an additive to other soils to make them impermeable. Kaolinite is hundred times more permeable than montmorillonite.

 

Fabric or structural arrangement of particles is an important soil characteristic influencing  permeability, especially of fine-grained soils. At the same void ratio, it is logical to expect that a soil in the most flocculated state will have the highest permeability and the one in the most dispersed state will have the lowest permeability. Remoulding of a natural soil invariably reduces its permeability. Stratification or macrostructure also has great influence on the permeability. it is more parallel to stratification than that perpendicular to stratification.

 

Higher the degree of saturation, higher the permeability. In case of certain sands permeability may increase three-fold when degree of saturation increases from 80% to 100%.

 

Entrapped air has pronounced effect on permeability. It reduces the permeability of soil. Organic foreign matter also has the tendency to move towards flow channels and choke them, thus decreasing the permeability. Natural soil deposits in the field may have some entrapped air or gas for several reasons. In the laboratory, air-free distilled water may be used as vacuum applied to achieve a high degree of saturation. However, this may not lead to a realistic estimate of the permeability of a natural soil deposit.