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Compaction of Soil and Factors Affecting Compaction

Roller for Soil Compaction

Compaction is the application of mechanical energy to a soil to rearrange the particles and reduce the void ratio.

Purpose of Compaction of Soil

The principal reason for compacting soil is to reduce subsequent settlement under working loads.

  • Compaction increases the shear strength of the soil.
  • Compaction reduces the voids ratio making it more difficult for water to flow through soil. This is important if the soil is being used to retain water such as would be required for an earth dam.
  • Compaction can prevent the build up of large water pressures that cause soil to liquefy during earthquakes.

Factors affecting Soil Compaction

  • Water content of the soil
  • The type of soil being compacted
  • The amount of compactive energy used

Effects of different factors on compaction of soil

Water content:

As water is added to a soil ( at low moisture content) it becomes easier for the particles to move past one another during the application of the compacting forces. As the soil compacts the voids are reduced and this causes the dry unit weight ( or dry density) to increase. Initially then, as the moisture content increases so does the dry unit weight. However, the increase cannot occur indefinitely because the soil state approaches the zero air voids line which gives the maximum dry unit weight for a given moisture content. Thus as the state approaches the no air voidsline further moisture content increases must result in a reduction in dry unit weight. As the state approaches the no air voids line a maximum dry unit weight is reached and the moisture content at this maximum is called the optimum moisture content.

Increased compactive effort

Increased compactive effort enables greater dry unit weights to be achieved which because of the shape of the no air voids line must occur at lower optimum moisture contents. It should be noted that for moisture contents greater than the optimum the use of heavier compaction machinery will have only a small effect on increasing dry unit weights. For this reason it is important to have good control over moisture content during compaction of soil layers in the field.

Effects of soil type

The table below contains typical values for the different soil types obtained from the Standard Compaction Test.

Typical Compaction Values

Type of Soil

(gdry )max (kN/ m3)

mopt (%)

Well graded sand SW



Sandy clay SC



Poorly graded sand SP



Low plasticity clay CL



Non plastic silt ML



High plasticity clay CH



Note that these are typical values. Because of the variability of soils it is not appropriate to use typical values in design, tests are always required.


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