Engineers - Compaction and optimum moisture content from Electronic Blueprint

As rollers, vibrating plate compactors, wackers or the like apply compactive effort to a soil, the particles move over one another to readjust their position within the matrix. The presence of moisture will "lubricate" this movement, although too much moisture will lead to loss of bearing capacity. The Optimum Moisture Content (OMC) is the moisture content that leads to the maximum soil density under the particular test conditions. It is a function of the soil type, and may be determined by the Modified or Standard Proctor density tests. It can be seen, from the diagram below, that Modified Compaction is appropriate in those circumstance, such as roads, where a need for higher finished density warrants increased compactive effort.

• Dry Density - The calculated density of soil, if there were no water in the voids.
• Moisture Content - The mass of the water in the voids divided by the mass of the dry soil, expressed as a percentage.
• Saturation Line (0 % Voids) - The line representing the dry density at various moisture contents, if no air is present.
• Optimum Moisture Content (OMC) - The moisture content at which the soil can achieve its maximum dry density during the test.
• Standard Compaction - The dry density a soil would adopt if compacted in three layers in a steel mould of volume 94,440 mm3 (102 mm diameter x 117 mm high cylinder) at various moisture contents by 25 blows of a 2.5 kg hammer dropped 305 mm.
• Modified Compaction - The dry density a soil would adopt if compacted in three layers in a steel mould of volume 94,440 mm3 (102 mm diameter x 117 mm high cylinder) at various moisture contents by 25 blows of a 4.5 kg hammer dropped 457 mm.