Compaction and Moisture Testing of Soil

by John E. Vavroch, Civil Engineering Technician
Natural Resources Conservation Service (NRCS)
Hays, Kansas

The American Society for Testing and Materials (ASTM) D-698, sometimes referred to as a Standard Proctor test, is a test of compaction characteristics of the soil. It is a laboratory compaction method to determine the relationship between water content and dry unit weight of soil. If you are sealing the fish pond you constructed for the grandkids or meeting the Kansas Department of Health and Environment’s permeability requirements for a livestock waste storage pond, this test is a valuable tool to ensure your pond holds water.

The test uses a 5.5 pound hammer dropped (freefall) from a height of 12 inches. The test produces a compactive effort of 12,400 ft/lb per cubic foot of soil. The test starts by filling a cylinder or mold in three nearly equal increments with soil. Each third of the mold is compacted by 25 blows of the 5.5 pound hammer. Seventy-five freefall blows of the hammer result in 12,400 ft/lb of energy on the soil sample. After the initial test, three or four more identical tests are performed with a 2 to 3 percent increase in moisture content each time. The weight and volume of the mold are constant, so by simply multiplying the weight of the soil times the volume of the mold, we compute the wet density of the soil in pounds per cubic foot of soil.

After computing the moist density of the soil, a sample is taken from the newly formed soil cylinder to check the actual moisture content. Usually, a 60- to 70-gram sample is placed in a microwave oven and dried. Normally, 4 or 5 one-minute sessions in the microwave remove the moisture in the sample. By dividing the weight of the moisture into the weight of the dry soil, the percent moisture is computed. Dry density is computed by taking (wet density x 100) divided by (100 + % moisture).

After the 4 or 5 tests are completed with increasing moisture contents, the densities and moisture curve are plotted in graph form. By plotting this curve, one can easily determine maximum dry density and the optimum moisture content for that particular soil sample.

For more information and assistance, please contact your local Natural Resources Conservation Service office or conservation district office located at your local county USDA Service Center.

For more information about NRCS programs, visit the Kansas NRCS Web site at www.ks.nrcs.usda.gov.

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Last Modified: 12/09/2008