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What is the Proctor Test?

By C. Martin
Updated May 21, 2024
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The Proctor test is a test that is used in geotechnical engineering to find out the maximum density that can be practically achieved for a soil or similar substance. The Proctor soil compaction test is performed by measuring the density, or dry unit weight, of the soil being tested at different moisture content points. The aim of the soil test is usually to determine the optimum moisture content for the soil. In addition to soils, other substances, such as aggregate, gravel, or sand, may be measured.

Soil testing equipment used for the Proctor test usually consists of a mold of a standard shape and size, and a device, such as a hammer, for compacting the soil into the mold. When soil testing machines are used, they must be able to measure how much force is applied to the soil in the mold. The hammer or other compacting tool is used to compact the soil or aggregate in the mold. In this scenario, compacting the soil means increasing its density by forcing air out of the soil.

By compacting the soil or aggregate at different moisture contents, an engineer can determine what is the optimum moisture content and compaction level of the soil or aggregate for a specific use in a particular engineering or construction project. As such, the Proctor test is an important tool in the field of geotechnical engineering, as it is a crucial test that can help in determining what the risks are in engineering or construction projects. It can also help determine how these risks may be mitigated by making optimum use of aggregates and other physical materials. An example of where the Proctor test may be used in a construction project might be in the selection of which aggregate to use in the foundation of a building.

The Proctor test was invented in 1933 by Ralph R. Proctor. It specifies the use of a mold, four inches in diameter, holding 1/30th of a cubic foot (0.0009 cubic meter) of soil. The Proctor test also specifies the weight and other specifics of the hammer used in compaction. Other variants on this soil compaction test have been defined, including the "Modified Proctor Test," which specifies the same mold but different requirements for the compaction hammer. Both the original and the modified Proctor tests also allow the use of a larger mold for measuring substances that contain larger particles, such as gravel.

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Discussion Comments
By afterall — On Feb 25, 2011

I wish everything in life could be determined for its usefulness based on something as exact as the Proctor test. Unfortunately, not everything is as explicable as the density of soils.

By BambooForest — On Feb 24, 2011

This is kind of amusing to me as a play on words. I thought that this would be about the act of "proctoring" a test, which is a term for giving a test to a group of people. Now I wonder if that term came after this one, or he just happened to be someone named Proctor who did lots of tests, and it is totally unrelated.

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