## 7 Factors Affecting The Permeability of Soil

Following are the 7 Factors Affecting The Permeability of Soil:

**1) Size of particles**

The permeability varies approximately as the square of the grain size. The relationship is [latex] K = CD^{2} \frac{e^{3}}{1+e}\frac{\gamma_{w} }{\mu } [/latex], where [latex] D [/latex] is the effective diameter of the soil particles in cm. From the above equation, we can say, the permeability of soil is directly proportional to the square root of the particle diameter. If the soil particle is large then permeability will be high, and if the soil particle is small then permeability will be less.

This happens because the large particles contain large volumes of voids, and other hand small particles contain small volumes of voids. Hence, the flow of water through soil mass will be more in the case of large particles.

**2. Specific Surface and Shape**

The permeability of coarse-grained soil is inversely proportional to the specific surface of particles at a given **porosity**. Relationship is [latex] K = \frac{1}{K_k\eta (S_s)^2} \times \frac{n^3}{1-n^2} [/latex] , where [latex] S_s [/latex] is the specific surface of particles.

**3) Properties of the pore fluid**

The permeability of soil is directly proportional to the unit weight of water and inversely proportional to its viscosity. Again the viscosity changes with the change in temperature. Normally, with the increase in temperature viscosity decreases and hence permeability increases.

**4) Void ratio of soil**

The permeability (k) is directly proportional to c.e3/ (1+e), where ‘c’ is a factor and ‘e’ is the **void ratio**. Hence permeability decreases with a decrease in void ratio & vice-versa.

**5) The structural arrangement of soil particles**

The permeability may vary with different geometric arrangements and shapes of voids.

If the particles are arranged in a flocculated structure, then the permeability will be low. On the other hand, if the particles are arranged in a dispersed structure, then permeability will be high.

The permeability of soil also depends on structural defects like – cracks.

**6) Entrapped air and organic impurities**

The permeability of soil is greatly reduced if the air is entrapped in the voids. The presence of organic foreign matter in soil mass also decreases the permeability.

**7) Adsorbed water**

The adsorbed water surrounding the fine soil particles reduces the effective voids available for the passage of water and thereby reduces the permeability.

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