A soil is said to be permeable when it allows water through it. There are various factors such as void ratio, size, and shape of the particle, of saturation os soil etc. which are affecting permeability property of soils and these factors are briefly discussed in this article.
Before going to know about these factors, take a look at the general expression for coefficient permeability which is derived from the comparison of poiseuille’s law with Darcy’s law.
Where k = Coefficient of permeability
C = Shape constant
D = Effective grain size
e = Void ratio
Factors Effecting Permeability of Soils
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
- Adsorbed water
- Organic Matter
1. Size of Soil Particle
Permeability varies according to size of soil particle. If the soil is coarse grained, permeability is more and if it is fine grained, permeability is low. The relation between coefficient of permeability (k) and particle size (D) can be shown from equation (1) as follows.
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.
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
Where, C = Shape of the flow path,
e = Void ratio.
For coarse grained soil, “C” can be neglected. Hence
5. Soil Structure
Structure of any two similar soil masses at same void ratio need not be same. It varies to the level of compaction applied. If a soil contains flocculated structure, the particles are in random orientation and permeability is more in this case.
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 is parallel, permeability is more. If it is perpendicular, permeability is less.
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.
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.
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.