Compaction is the application of mechanical energy to a soil so as to rearrange its particles and reduce the void ratio.
It is applied to improve the properties of an existing soil or in the process of placing fill such as in the construction of embankments, road bases, runways, earth dams, and reinforced earth walls. Compaction is also used to prepare a level surface during construction of buildings. There is usually no change in the water content and in the size of the individual soil particles.

The objectives of compaction are:

• To increase soil shear strength and therefore its bearing capacity.
• To reduce subsequent settlement under working loads.
• To reduce soil permeability making it more difficult for water to flow through.

Laboratory Compaction
The variation in compaction with water content and compactive effort is first determined in the laboratory. There are several tests with standard procedures such as:

Indian Standard Light Compaction Test
Soil is compacted into a 1000 cm³ mould in 3 equal layers, each layer receiving 25 blows of a 2.6 kg rammer dropped from a height of 310 mm above the soil. The compaction is repeated at various moisture contents.

Indian Standard Heavy Compaction Test
It was found that the Light Compaction Test (Standard Test) could not reproduce the densities measured in the field under heavier loading conditions, and this led to the development of the Heavy Compaction Test (Modified Test). The equipment and procedure are essentially the same as that used for the Standard Test except that the soil is compacted in 5 layers, each layer also receiving 25 blows. The same mould is also used. To provide the increased compactive effort, a heavier rammer of 4.9 kg and a greater drop height of 450 mm are used.