FACTORS AFFECTING SOIL COMPACTION - As water is added to 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. 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. Normally, heavy clay, clay & silt offer higher resistance to compaction whereas sandy soils and coarse grained or gravelly soils can be easily compacted.

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Factors Affecting Soil Compaction

By: Haseeb Jamal 

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.

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 whereas 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	22	7
Sandy clay SC	19	12
Poorly graded sand SP	18	15
Low plasticity clay CL	18	15
Non plastic silt ML	17	17
High plasticity clay CH	15	25

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.

Haseeb Jamal. I am a Civil Engineer, graduated from University of Engineering and Technology, Peshawar, Pakistan in 2010. I also have a PG-Diploma in Disaster Management and MS in Urban Infrastructure Engineering (In Progress). My expertise include civil related softwares like AutoCAD, SAP2000, MS Project, Primavera, MS Office and GIS. My technical skills include project management, monitoring and evaluation, structural assessment, disaster risk management, Quantity survey, land survey, material testing, site management and technical writing. I am trained in writing project progress reports as well as proposals and concept papers. I have also received advanced training on surveying, proposal writing, Monitoring and Evaluation of projects as well as organizations.

I have worked as Project Engineer at National Research and Development Foundation, Peshawar and CENCON Associates. I also worked with Spectra Engineering Solutions as Senior Civil Engineer in monitoring of World Bank and UNDP funded projects all over Khyber Pakhtunkhwa and FATA. Currently, I am working as Deputy Manager Development at NayaTel, Peshawar.

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