Soil Impermeability Testing Methods

For the homeowner, the most common reason to test soil impermeability is to determine where and how to construct an on-site wastewater treatment system. Depending on the estimated use of such a system, the soil in which the system will be built will need to have a measurable capacity for drainage. Soils with poor drainage or with a high impermeable layer of bedrock may not be suitable for wastewater treatment, so building authorities require soil tests before issuing building permits. The most common is the perc---or percolation---test.

Hire an Expert

• Hire an expert. Percolation tests must normally be conducted by a licensed engineer trained in the methodology of testing. There are rules that must be followed if the test data are to be accepted. These have to do with where and how test holes are dug, how the soil removed is examined and how drainage tests are conducted in the testing holes.

Trenches and Borings

• Make way for excavation. Depending on the requirements of your locality, your tester may dig a series of holes with an auger or boring equipment or may dig larger holes with a backhoe. Borings must normally extend at least several feet below the bottom of the proposed soil treatment system.

Examining Soil

• Ask about your soil types. The tester will evaluate the soil taken from the boring to determine its makeup. This can be useful information for you later, when you make decisions about landscaping your property. Drainage rates are different in different types of soil, and soil types are generally layered within the boring, so it is important to know the depth of layers of such soil types as clay, clay loam and sandy loam.

Percolating Water in the Test Hole

• Watch how water behaves in the test holes. Your tester will measure the drainage---percolation---rate by filling the holes with water and then measuring the drop in water level over time. This determines the percolation rate, which is reported in minutes per inch, or MPI.

Nonscientific Applications

• Conduct your own nonscientific version of an official perc test before planting your garden or selecting trees for your property. Dig a hole deeper than the root system of the plants you intend to grow, and fill the hole with a garden hose to get a general idea of how well the soil drains. Depending on the results you observe, you may want to amend the soil with organic material to improve drainage or choose plants that are tolerant to wet areas.