What Does Permeability Test Mean?
The permeability test is conducted on soils to determine the rate at which the soil allows water to flow through it.
Soil permeability depends on the grain structure of the soil and the void spaces in the soil. It is important to know how easily water can travel through the layers of soil in the test area for geotechnical investigation purposes based on which structures will be designed.
The soil permeability test determines the rate at which water-saturated soil settles under load, its drainage characteristics, and the rate of consolidation.
Trenchlesspedia Explains Permeability Test
There are two methods to perform the soil permeability test - the constant head method and the falling head method.
In the constant head test, the same relative elevation of the top of the water column remains over the sample for the entire duration of the test. The test is performed on soils with a high flow rate like sands, gravels, and some clay soils.
The apparatus used for the test consists of two reservoirs, one at the top that holds de-aired water and one at the bottom that collects the water that has permeated the soil. After a fixed amount of time, the coefficient of permeability is calculated using factors such as the height and cross-section of the sample, the pressure measurement, and the volume of water that has flowed through the soil in that time interval.
In the falling head test, the head decreases as water infiltrates the soil sample causing a reduction in pressure as the test progresses. This test can only be performed on fine-grained soils.
The test requires only a small soil sample and a standpipe. The soil sample is placed in a mold, completely saturated, and attached to standpipes that are filled with de-aired water up to a certain level.
After determining a lower water level, the water in the standpipe is allowed to reach that level. Once the level is reached, the coefficient of permeability is determined using the volume of the soil sample, the cross-section of the standpipe, change in water level, and time interval it took to attain that level.