Permeability of soil


Soil permeability is the feature of the soil to transmit water and air. It is one of the most important qualities to give a thought for fish culture. A pond constructed in impermeable soil will lose water through the seepage. If the soil is more permeable then there will be greater porosity.

   A material that has continuous voids is called permeable. The soils with the more permeability also have the greatest seepage. Larger grains with smaller surface areas clear the best. Clay has a small grain but has a large surface area, due to which doesn’t drain easily. Permeability in the sand is huge.

Study of seepage of water through soil:

1. Resolution of the rate of settlement of a soaked compressible soil layer. 

2. Determination of seepage through the part of earth dams and stability of slopes for roadways.

3.. Determination of uplift pressure under the hydraulic structure and their safety against piping.

4. Groundwater flow as to near well and drainage of the soil.  

Darcy’s Law of Permeability:

This assumed for laminar flow conditions in soaked soil, the rate of the discharge per unit time is directly proportional to the hydraulic gradient.

q = Kia

v = q/A = Ki … (1)

Where q = discharge per unit time

A = total cross-sectional area of a soil mass.

i = hydraulic gradient

k = Darcy’s coefficient of permeability

v = velocity of flow or discharge velocity.

 Vs=Q/Av   ……..(2)

(Vs= seepage velocity

Q= discharge or rate of flow.

Av= net sectional area of voids)

Also if Vv is the volume of voids in a soil mass of total volume V and L, since (Av*L)=Vv

we have

Av/A =Vv/V=n (porosity)

from Eqn (1) and (2) we get,

Q=Av Vs 


 Vs directly proportional to i


# Factors affecting permeability are:

1. Grain size. 

     k=C D²

   where k is effective dia (cm) and C, constant approximately equal to 100

2. Properties of pore fluid. 

    k is directly proportional to the unit weight of water and inversely proportional to viscosity.

    Permeability at 27c, k=n/n27c

where n= viscosity 

           n 27c=viscosity at 27c 

3. Void ratio of the soil. 

     Effect of void ratio



  4. The structural arrangement of the soil particle. 

        1.Disturbed              2. Undisturbed

       The effect of structural disturbance on permeability is more pronounced in fine-grain soil .K should be measured in the Undisturbed soil sample. 

5. Entrapped air and foreign matter. 

     k will reduce if the air is entrapped in the voids thus decreasing its degree of saturation. Percolating water in the field may have some gas content, it may appear more realistic to use the actual field water for testing in the laboratory.  

6. Adsorbed water in clayey soil.  

The surface assimilation from the water surrounding the fine soil particle is not free to move and decreases the effective pore space available for the passage of water. 

7. Organic matter

The presence of organic matter reduces permeability. This is due to the choking of voids by the use of organic matter.

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