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The Proctor compaction test, and the related Modified Proctor compaction test, are tests to determine the maximum practically-achievable density of soils and aggregates, and are frequently used in geotechnical engineering.

The test consists of compacting the soil or aggregate to be tested into a standard mold using a standardised compactive energy at several different levels of moisture content. The maximum dry density and optimum moisture content is determined from the results of the test.

Soil in place is tested for in-place dry bulk density, and the result is divided by the maximum dry density to obtain a relative compaction for the soil in place.

Compaction is the process of increasing the bulk density of a soil or aggregate by driving out air. For any soil, for a given amount of compactive effort, the density obtained depends on the moisture content. At very high moisture contents, the maximum dry density is achieved when the soil is compacted to nearly saturation, where (almost) all the air is driven out. At low moisture contents, the soil particles interfere with each other; addition of some moisture will allow greater bulk densities, with a peak density where this effect begins to be counteracted by the saturation of the soil.

The original Proctor test, ASTM D698, uses a 4-inch diameter mold which holds 1/30th cubic foot of soil, and calls for compaction of three separate lifts of soil using 25 blows by a 5.5 lb hammer falling 12 inches, for a compactive effort of 12,400 ft-lbf/ft³. The "Modified Proctor" test, ASTM D1557, uses the same mold, but uses a 10 lb. hammer falling thorugh 18 inches, with 25 blows on each of five lifts, for a compactive effort of about 56,000 ft-lbf/ft³. Both tests allow the use of a larger mold, 6 inches in diameter and holding 1/13.333 ft³, if the soil or aggregate contains too large a proportion of gravel-sized particles to allow repeatability with the 4-inch mold. To insure the same compactive effort, the number of blows per lift is increased to 56.

CalTrans has developed a similar test, California Test 216, which measures the maximum wet density, and controls the compactive effort based on the weight, not the volume, of the test sample. The primary advantage of this test is that maximum density test results are available sooner, as evaporation of the compacted sample is not necessary.

There is also a test (ASTM D4253) which uses a vibrating table using standard vibrations for a standard time to densify the soil. This test method prevents particle breakage, but is only usable for granular soils. The test method also includes a method to determine the minimum density of the soil; density of soils in place are compared against the maximum and minimum to obtain a relative density.

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