Determination of coeficient of Permeability by constant head permeability test
INTRODUCTION
Darcy's Law:
Darcy experimentally demonstrated that the rate of flow of water through a soil medium of unit cross-sectional area is directly proportional to the applied hydraulic gradient.
The rate of flow of water per unit cross-sectional area is directly proportional to the hydraulic gradient.
Where,
q = Rate of flow of water,
A = Cross sectional area of soil,
i = Hydraulic gradient
To eliminate the proportionality in the equation, a constant is introduced, known as Darcy's coefficient of permeability (k), also referred to as the coefficient of permeability or permeability.
Therefore, Permeability may be defined as the property of soil that indicates the ease with which water flows or percolates through the continuously interconnected pore spaces within the soil.
Permeability may also be defined as the ratio of the rate of flow of water to the product of the cross-sectional area and the hydraulic gradient.
The permeability of soil depends on several major factors, including the grain size of soil particles, the properties of the pore fluid, the void ratio of the soil, the shape and arrangement of the pore spaces, and the degree of saturation.
In the laboratory, the permeability of soil is determined using either of the following two experiments:
- Constant head permeability:
In this experiment, a constant head of water is maintained in the overhead tank so that the water pressure remains constant throughout the test. This method is generally used for coarse-grained soils, where the permeability is relatively high.
- Variable head permeability:
In this experiment, a standpipe is connected to the permeameter to supply water. This method is generally used for fine-grained or cohesive soils, where the permeability is very low.
Constant Head Permeability:
The constant head permeability test is a commonly used laboratory method for determining the permeability of granular soils such as sand and gravel, which contain little or no silt.
In this method, the hydraulic gradient is maintained constant by keeping the water head in the overhead tank constant throughout the experiment.
The permeability in the constant head permeability test is calculated using the following formula:
Where,
k = Coefficient of permeability,
Q = Total quantity of water collected in time t,
L = Length of the soil sample,
t = Duration of water collected,
h = Constant head of water
| Type of soil | Permeability (cm/s) |
|---|---|
| Gravel | 1 |
| Coarse sand | 1 to 0.1 |
| Medium sand | 10-1 to 10-2 |
| Fine sand | 10-2 to 10-3 |
| Silty sand | 10-3 to 10-4 |
| Silt | 10-5 |
| Clay | 10-7 to 10-9 |