The phenomenon of allowing water to pass through soil particles is called permeability. We know velocity is proportional to the hydraulic gradient.
V ∞ I
⇒V=KI
This K means the coefficient of permeability. There are various factors affecting the permeability of soil. These factors will be discussed in the following
- Grain size distribution
- Pore size distribution
- Fluid viscosity
- Void ratio
- Roughness of mineral particles
- Degree of soil saturation
- Soil structure
- Temperature
- Grain size distribution
- Pore size distribution
- Fluid viscosity
- Void ratio
- Roughness of mineral particles
- Degree of soil saturation
- Soil structure
- Temperature
Grain size distribution:
The size of the particle controls permeability. In coarse grained soil, permeability is high. On the contrary, in fine grained soil permeability is low. Permeability is proportional to the square of the diameter of soil particles.
Pore size distribution:
There are voids in soil particles. these voids sizes are indicated as pore size. The pore size distribution affects the permeability. If the pore size is greater permeability is high. If the pore is less the permeability is low. it is a proportional relationship.
Fluid viscosity:
Viscosity is the internal friction of property of a moving fluid. If viscosity is higher, the permeability will be less and vice versa. Viscosity is also affected by temperature. If temperature increases, viscosity decreases which eventually affects the permeability.
K ∞ Ɣw/μ
Void ratio:
The ratio of the volume of void in a soil sample to the volume of solid of that soil sample is called the void ratio. The high value of that ratio means high void in sample resulting in high permeability condition.
Void ratio = Vv/Vs
Roughness of mineral particles:
Soil particles have different shapes like round, angular, etc. Round particles are more favorable to permeability than angular shaped particles.
The reason behind this is the presence of more surface area in the case of angular particles than round particles. More surface area resists water to flow freely resulting in fewer permeability conditions.
Degree of saturation:
The degree of saturation means how many soil voids are filled with water. When all the voids of a soil sample are filled with water, it will be called 100% saturation or fully saturated soil.
Partially saturated soil has entrapped air in its voids. This resists the water to flow through the soil sample. Fully saturated soil is more permeable than partially saturated soil.
Soil structure:
Temperature:
Temperature indirectly influences the Coefficient of Permeability (k) through unit weight and viscosity of water. Unit weight and viscosity of water decrease with an increase in temperature. Coefficient of permeability of soil increases with increase in temperature.
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