Soil Permeability Testing Methods

Soil permeability measurements determine how well water flows through soil. Large pores in sand or granular soil allow water to move rapidly, while small pores in silt or clay cause water to seep through slowly. The main tests to measure soil permeability are the constant head, the falling head and the percolation test. Homeowners may need a permeability test for building, landscape or major gardening projects. You can easily do a percolation test yourself, but first check whether local laws require you to hire a professional.

Soil permeability measurements help determine the rate of soil settling, which you need to know before constructing buildings or determining how much water will flow toward an excavation. Good drainage from high soil permeability is needed for installation of a septic system. Low permeability, found in clay soils, works well for placement of ponds, such as a fish pond. Soil permeability measurements also help determine the stability of slopes and earth dams. Growing vegetables requires good drainage; a permeability test can indicate whether your soil is suitable or needs to be amended before planting.

The constant head test is a laboratory test done on sandy or granular soil samples. Under constant pressure, a piston forces water through a column of water-saturated soil to determine the flow rate of water. The water in the test is de-aired and kept at constant temperature. The test apparatus has a water reservoir on top and an outlet reservoir on the bottom. The permeability of the soil sample is calculated from the height of the soil sample, the sample's cross section, pressure measurements, the volume of passed water and the time interval.

The falling head permeability test is for low permeability soils, such as silts and clays. A relatively small soil sample is used, because water flow will be slow. After tamping down the sample and saturating it with water, a standpipe is connected to the container holding the soil. The standpipe is filled with water, and the initial water level is measured. The decline in water level in the standpipe is measured again after the water flows through the sample in a specified time. The permeability of the soil sample is calculated from the size of the soil sample, the cross section of the standpipe, the drop in water level and the time taken.

For the percolation test, a field test done in the area of interest, a tester digs a series of holes in the ground and fills them with water for a few hours or overnight to saturate the soil. Sandy or gravelly soils take shorter than silty or clay soils to become saturated. After water has saturated the soil surrounding the test holes, the tester adds new water and records the time it takes for the water level in the holes to drop. The permeability, or more accurately the percolation rate, is calculated from the drop in water level in inches or centimeters per specified time.