Soil Permeability Testing
Permeability of Soil, continued
Materials with a high permeability coefficient will allow fluids to move rapidly through them while those with a low permeability will not.
While testing permeability, it is important to make sure all specified conditions are met. The discharged water must be under laminar flow conditions and move under a unit hydraulic gradient at standard temperature conditions (20°C). Additionally, consistency of pressure is important to maintain. Certain control products are useful in regulating the testing environment.
A permeability coefficient is most commonly determined through the use of two main laboratory tests: the constant head permeability test and the falling head permeability test. For highly granular soils such as sands and gravels, the constant head method is best and can yield accurate results even if the sample has been disturbed or reconstituted. The falling head method is typically used for fine-grained soils and is more accurate when testing undisturbed samples. These two methods work in different ways to determine a sample’s permeability coefficient.
The constant head permeability test:
The constant head permeability test utilizes a test apparatus with a reservoir on the top to hold de-aired water and one at the bottom to hold water that has permeated through the soil sample. Because the samples are large-grained, their hydraulic conductivity is rather fast. The coefficient of permeability can be calculated using the pressure measurements and volume of the permeated water during the set time interval, as well as the height and cross-sectional area of the soil sample. To ensure accurate results, repeat the test three or more times and find an average coefficient.
The falling head permeability test:
The falling head permeability test is performed using a standpipe and a relatively small sample size. A small sample size is due to the fine-grained nature of the test samples, which results in slow hydraulic conductivity. The soil sample is first placed in a container and saturated with water. From there, the sample is attached to standpipes filled with a specific level of de-aired water. The permeability coefficient can be calculated when the water in the standpipe has reached a previously determined level. The calculation takes into account the size of the sample, the cross-sectional area of the standpipe, as well as the time it took to change the water level.
A good knowledge of soil permeability is important for both civil engineering and agricultural purposes. Although determining a sample’s permeability can require many steps, the process can often be simplified with the proper tools. Some useful products include:
- Soil Permeameters: are used to determine the coefficient of permeability in both constant head and falling head test methods
- Permeability Test Cells: work to hold the soil samples during testing when not using permeameters
- Pressure Controllers: provide an accurate permeability setup by controlling saturation, consolidation and permeation functions
- FlexPanels: deliver a highly-efficient distribution system for providing air, water, and de-aired water for use in permeability testing applications