What Are the Differences in the Permeability Tests to Determine Soil Properties?

Soil Types

• Gravel is the most permeable soil type, and clay is the least permeable. Water takes two minutes to travel through 3.37 feet (1 meter) of gravel. It requires 200 years for water to permeate 3.37 feet of clay soil. Most soils are a mixture of clay, silt, sand and gravel particle sizes. Knowledge of the correct soil type and its permeability are essential to the successful installation of home or commercial sewage systems, dams, ponds and buildings.

Percolation Test

• Two or three core soil borings are taken in the test area for a percolation test. This test is often used to determine the suitability of soil for sewage system installation. Core borings are made to 3 feet below the expected depth of the sewage system. A removable mesh bag of gravel is placed at the bottom of each core boring to protect the hole bottom from scouring when filled with water. The bag of gravel is removed after the test and used in subsequent test holes. Water is placed in the bored area and left overnight. The change in water level is determined by a percometer and used to calculate the soil’s permeability.

Falling Head Test

• The falling head permeability soil test is used in fine-grained soils such as silt and clay. It is used to determine the rate of seepage for underground construction projects and to analyze the stability of earth-holding constructions. The test involves flow of water through a soil sample connected to a standpipe. The standpipe provides the water head and allows measuring the volume of water passing through the sample, according to Geotechdata, a database of geotechnical information.

Constant Head Test

• A constant head permeameter test device is used to test soil seepage in coarsely grained soil such as gravel or sand. Water and the soil sample are placed in the chamber of the permeameter. Water is allowed to flow out naturally, and its rate and length of time are measured. A mathematical calculation using darcy units is obtained and used to determine the soil sample’s permeability. A darcy unit measures the permeability of porous substances such as soil.