Compaction Test

Compaction is the process of increasing the density, or the amount of solids per unit volume, of soil by mechanical means to improve such soil properties as strength, permeability, and compressibility. Compaction is a standard procedure used in the construction of earth structures, such as embankments, subgrades, and bases for road and airfield pavement.

In the field, compaction is accomplished by rolling or tamping the soil with special construction equipment. In the laboratory, compaction can be accomplished by the impact of hammer blows, vibration, static loading, or any other method that does not alter the water content of the soil. Usually, however, laboratory compaction is accomplished by placing the soil into a cylinder of known volume and dropping a tamper of known weight onto the soil from a known height for a given number of blows. The amount of work done to the soil per unit volume of soil is called compactive effort.

For most soils and for a given compactive effort, the density of the soil will increase to a certain point as the moisture content is increased. That point is called the maximum density. After that point, the density will start to decrease with any further increase in moisture content. The moisture content at which maximum density occurs is called the optimum moisture content (OMC). Each compactive effort for a given soil has its own OMC. As the compactive effort increases, the maximum density generally increases and the OMC decreases.