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Factors Affecting Soil Compaction

By: Haseeb Jamal / On: Oct 03, 2019 / Factors Affecting, Shallow
Factors Affecting Soil Compaction

Factors Affecting Soil Compaction

Following are the factors affecting Soil Compaction:

  • Water content of the soil at the time of compaction.
  • The amount of compactive energy used e.g. type of plant (weight, vibration, number of passes)
  • Nature and the type of soil being compacted like sand or clay, grading, plasticity
  • Site conditions, e.g. weather, type of site, layer thickness

Effects of different factors on compaction of soil

Effect of Water content on compaction of soil:

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. 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 zero air voids line further moisture content increase 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.

Also See: Foundation Engineering

Increased compactive effort

Increased compactive effort enables greater dry unit weights to be achieved. Because of the shape of the no air voids line, the dry unit weights 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 on Compaction of Soil

Type of soil has a great influence on its compaction characteristics. Normally, heavy clay, clay & silt offer higher resistance to compaction where as sandy soils and coarse grained or gravelly soils can be easily compacted. The coarse grained soils yield higher densities in comparison to clay. It should also be noted that a well graded soil can be compacted to higher density.

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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