Permeability of soils and Factors affecting permeability

Ease with which the water can flow through soil is known as permeability. Larger the pores more the permeability and vice versa.

Gravel would have more permeability compared to the sand, silt and clay. Flocculated structure would have more permeability than the dispersed structure.

It helps us to understand engineering problems like seepage, settlement calculations, etc. 


Pore water pressure existed will develop the stress and it will dissipate with time based on the permeability and finally the settlement takes place.


Darcy's Law

Flow is assumed as laminar, valid for all the soils except for coarse gravel. Superficial velocity V is calculated considering the flow through the entire area.


Methods for the determination of permeability


  • Laboratory tests- Constant head permeability test and variable head permeability test.

  • Field test- Pumping out tests and Pumping in test.

  • Indirect methods- If the particle size and specific surface area are known then the permeability determination is done from the consolidation data.


Factors affecting permeability


  1. Particle size: It varies as the square of effective grain grain size. If the soil is coarse grained, permeability is more and if it is fine grained, permeability is low. Particle size can be gauged by performing sieve analysis

  2. Effect of void ratio: In general, Permeability increases with void ratio. But it is not applicable to all types of soils. For example, Clay has a higher void ratio than any other types of soil but permeability for clays is very low. This is due to the flow path through voids in case of clays is extremely small such that water can not permit through this path easily.

  3. Effect of permanent fluid flowing through the soil water.

  4. Shape of particles

  5. Degree of saturation: Unsaturated soils would have lesser permeability compared to the saturated soils.

  6. Effect of soil fabric: Dispersed soil will have lesser void ratio hence lesser permeability vice versa in case of flocculent structure.

  7. Impurities in water: Impurities would clog the pores and leads to lesser permeability.

  8. Effects of adsorbed cation on clay mineral surface

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