Factors Affecting Soil Compaction

Factors Affecting Soil Compaction

Soil compaction refers to the process by which soil particles are compressed, reducing the pore spaces and increasing soil density. Following are the factors affecting Soil Compaction:

• The 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:

Soil that is too wet or too dry can be difficult to compact. Soil that is too wet can become sticky and difficult to work with, while soil that is too dry can become hard and resistant to compaction.

As water is added to the 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

Different compaction methods can affect soil compaction differently. For example, rolling or tamping can cause more compaction than using a vibrating plate compactor.

Increased compaction 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 the compaction of soil layers in the field.

Effects of Soil type on Compaction of Soil

Some soil types are more susceptible to compaction than others. For example, clay soils are more prone to compaction than sandy soils due to their smaller pore spaces. The 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 arrangement of soil particles into aggregates can also affect the compaction of soil. Soil with a good structure is easier to compact than soil that lacks structure.

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

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.