Permeability is a property of soil by virtue of which the soil mass allows water (or any other fluid) to flow through it.
The expression for coefficient permeability which is derived from the comparison of poiseuille’s law with Darcy’s law.
`k=C**D^2**e^3/(1+e)**gamma_w/mu`
Where k = Coefficient of permeability
C = Shape constant
D = Effective grain size
e = Void ratio
`gamma_w=` Unit weight of water
`mu`=Dynamic Viscosity of water
Following are factors effecting permeability of soils.
Size of soil particle
Specific Surface Area of Soil Particle
Shape of soil particle
Void ratio
Soil structure
Degree of saturation
Water properties
Temperature
Adsorbed water
Organic Matter
1) Size of particles
The permeability varies approximately as the square of the grain size. Relationship is
`k=C**D^2**e^3/(1+e)**gamma_w/mu`
, where
`D` 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 particle contain large volume of voids, and other hands small particle contain small volume of voids. Hence, the flow of water through soil mass will be more in case large particle.
2. Specific Surface Area of Particles
Specific surface area of soil particles also effects the permeability. Higher the specific surface area lower will be the permeability.
`k prop 1/( s p r c i f i c \ S u r f a c e \ A r e a)`
3. Shape of Soil Particle
Rounded Particles will have more permeability than angular shaped. It is due to specific surface area of angular particles is more compared to rounded particles.
4. Void Ratio
In general, Permeability increases with void ratio. But it is not applicable to all types of soils. For example, Clay has high 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 cannot permit through this path easily.
The relation between coefficient of permeability and void ratio can be expressed from equation (1) as
For Clay
`k prop (C e^3)/(1+e)`
Where, C = Shape of the flow path,
e = Void ratio.
For coarse grained soil, “C” can be neglected. Hence
`k prop e^3/(1+e)`
5. Soil Structure
The permeability may vary with different geometric arrangement and shape 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.
Permeability of soil also depends on the structural defects like crack.
6. Degree of Saturation
Partially saturated soil contain air voids which are formed due to entrapped air or gas released from the percolating fluid or water. This air will block the flow path thereby reduces the permeability. 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. However, unit weight of water will not affect much since it does not change much with temperature.
But when temperature is increased viscosity decreases rapidly. From equation (1), permeability increase when viscosity decreases.
`k prop gamma_w/mu`
8. Temperature
Temperature also affects the permeability in soils. From equation (1), 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. That is the reason, seepage is more in summer seasons than in winter.
`k prop 1/mu prop` temperature.
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%. Hence, permeability reduces.
10. Organic matter
Presence of organic matter decreases the permeability. This is due to blockage of voids by the organic matter.
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