Falling head permeability test can be used for less permeable soil

The falling head permeability test is a laboratory test used to determine the permeability of soils. It is particularly suitable for less permeable soils, where the flow of water is slow. The test measures the time it takes for water to flow through a soil sample under a constant hydraulic gradient.

Procedure:
1. A soil sample is taken and prepared in a permeameter, which is a cylindrical tube with a known cross-sectional area.
2. The bottom of the permeameter is covered with a filter paper to prevent the soil from escaping.
3. Water is poured into the permeameter until it is completely saturated. The water level is then adjusted to a desired height.
4. The stopwatch is started, and the time it takes for the water level to drop a certain distance is recorded.
5. The test is repeated with different water levels to obtain multiple data points.

Analysis:
The falling head permeability test is based on Darcy's Law, which states that the flow of water through a porous medium is directly proportional to the hydraulic gradient and the cross-sectional area, and inversely proportional to the length and coefficient of permeability of the soil.

By measuring the time it takes for the water level to drop, the coefficient of permeability can be calculated using the following formula:

k = (L * A * t) / (h * log(e))

Where:
k = coefficient of permeability
L = length of the soil sample
A = cross-sectional area of the soil sample
t = time for the water level to drop
h = initial water level
e = natural logarithm base

Advantages:
1. The falling head permeability test is simple and easy to perform.
2. It is suitable for less permeable soils where the flow of water is slow.
3. It provides a direct measure of the coefficient of permeability.

Limitations:
1. The test is time-consuming and may not be suitable for coarse-grained soils where the flow of water is rapid.
2. The permeability values obtained from the test may not be representative of the field conditions.

Therefore, the falling head permeability test is commonly used for less permeable soils, where the flow of water is slow and other tests may not be applicable.