FACTORS AFFECTING PERMEABILITY OF SOIL

Permeability Of Soil

The property of the soil by which soil permits the flow of water through it is called permeability of soil.There are voids in the soil which are interconnected to each other, provides the path for the flow of water through it.A soil with high porosity has high permeability.A soil with smaller value of permeability is categorized as impervious. 

Coefficient of permeability is derived from the comparison of poiseuille's law with Darcy's law:

                                        

                      k = C. D²  e³ /(1+e). Yw 

                                                                  µ  

Where,

            k  = Coefficient of permeability

            C = Shape constant

            D = Effective grain size

            e  = Void ratio

        Yw  = Unit weight of water

          µ  = Dynamic viscosity of water                                    

Factors that Affect Soil Permeability

1. Size of soil particle
2. Shape of soil particle
3. Specific surface area of soil particle
4. Void ratio
5. Soil structure
6. Degree saturation
7. Water properties
8. Temperature
9. Adsorbed water
10. Organic matter

1. Size of Soil Particle

Permeability varies according to size of soil particle.Fine particles of soil is smaller than 0.002 mm in diameter where as coarse grained soil particles size are 0.05 to 2.0 mm in diameter.Permeability is high in coarse grained soil and permeability is low in fine grained soil.The relation between coefficient of permeability (k) and particle size (D) can be shown in an equation as;

                             k ∝ D²

2. Shape of Soil Particle

Permeability varies according to shape of soil particle.Rounded particles will have more permeability than angular shaped.It is due to specific area of angular particle is more compared to rounded particles.

3. Specific Surface Area of Particle

Specific surface area of soil particle also effects the permeability.Higher the specific surface area lower will be the permeability.

                       k  ∝                 1                           

                               Specific Surface Area

4. Void Ratio

Void ratio is the ratio of the volume of voids in a soil to volume of solids.The void ratio is thus a ratio which can be greater than 1. It can be expressed as a fraction.The coefficient of permeability varies with void ratio.Permeability increases with void ratio and it is not applicable for all types of soils.Based on other concept it has been established that the permeability of a soil varies as;

                                   e² or e²/(1+e)                                        

5. Soil Structure

Soil structure affects permeability. Permeability in the direction of stratification is high as compared to the permeability perpendicular to stratification.If the soil contains dispersed structure, the particles are in face to face orientation hence, permeability is very low.The permeability of stratified soil deposits also varies according to the flow direction.If the flow is parallel permeability is more.If it is perpendicular permeability is less.

6. 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 a blockage to the passage of water.Consequently, the permeability of a partially saturated soil is considerably smaller than of fully saturated soil.Fully saturated soil is more permeable than partially saturated soil. 

7. Water Properties

Various properties of water or fluid such as unit weight and viscosity also effects the permeability because permeability is directly proportional to unit weight of water and inversely proportional to viscosity of water.

                           k  ∝    Yw  

                                       µ  

8. Temperature

Temperature also affects permeability of soil. Permeability is inversely proportional to the viscosity of the fluid.It is known that viscosity varies inversely to the temperature.Hence, permeability is directly related to temperature.Greater the temperature, higher will be the permeability.

                             k  ∝   1    ∝ temperature

                                      µ  

9. Adsorbed Water

Adsorbed water is the water layer formed around the soil particle especially in the case of fine-grained soils.This reduces the size of the void space by about 10%.

10. Organic Matter

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.