Soil Compaction

Soil compaction is defined as the method of mechanically increasing the density of soil. In construction, this is a significant part of the building process. If performed improperly, settlement of the soil could occur and result in unnecessary maintenance costs or structure failure. Almost all types of building sites and construction projects utilize mechanical compaction techniques.


Soil compaction occurs when the weight of heavy machinery compresses the soil, causing it to lose pore space. Soil compaction may also occur due to a lack of water in the soil.

Affected soils become less able to absorb rainfall, thus increasing runoff and erosion. Plants have difficulty in compacted soil because the mineral grains are pressed together, leaving little space for air and water, which are essential for root growth. Burrowing animals also find a hostile environment, because the denser soil is more difficult to penetrate.

In construction, the soil will increase the density along with its side effects of increased strength and decreased permeability, is usually desirable in earthwork construction and below building foundation.

Compaction is accomplished by use of heavy equipment. In sands and gravels, the equipment usually vibrates, to cause re-orientation of the soil particles into a denser configuration.

In silts and clays, a sheepsfoot roller is frequently used, to create small zones of intense shearing, which drives air out of the soil.

The result of soil compaction is measured by determining the bulk density and optimum moisture of the compacted soil and comparing it to a maximum dry density, for example, obtained from a Proctor compaction test, to determine the relative compaction.

The Proctor compaction test and the related modified Proctor compaction test, are tests to determine the maximum practically-achievable density of soils and aggregates.

The test consists of compacting the soil or aggregate to be tested into a standard mould using a standardized 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.