Compaction of soil is a process of densification of soil by displacing air from the pores by applying external stress on soil at different moisture content.

The increase in dry density of soil by compaction depends upon the following factors:

Water Content:

At low water content, the soil is stiff and offers more resistance to compaction. As the water content is increased, the soil particles get lubricated. The soil mass becomes more workable and the particles have closer packing. The dry density of the soil increases with an increase in the water content till the optimum water content in reached. At that stage, the air voids attain approximately a constant volume. With further increase in water content, the air voids do not decrease, but the total voids (air plus water) increase and the dry density decreases. Thus the higher dry density is achieved upto the optimum water content due to forcing air voids out from the soil voids. After the optimum water content is reached, it becomes more difficult to force air out and to further reduce the air voids.

The effect of water content on the compaction of soil can also be explained with the help of electrical double layer theory. At low water content, the forces of attraction in the adsorbed water layer are large, and there is more resistance to movement of the particles. As the water content is increased, the electrical double layer expands and the inter-particle repulsive forces increase. The particles easily slide over one another and are closely packed. This results in higher dry density.

Amount of Compaction:

The compaction of soil increases with the increase in amount of compactiv effort. With increase in comactive effort, the optimum water content required for compaction also decreases. At a water content less than the optimum, the effect of increased compaction is more predominant. At a water content more than the optimum, the volume of air voids become almost constant and the effect of increased compaction on soil is not significant.

It may be mentioned that the maximum dry density does not go on increasing with an increase in the compactive effort. For a certain increase in the compactive effort, the increase in the dry density becomes smaller and smaller. Finally a stage is reached beyond which there is no further increase in the dry density with an increase in the compactive effort.

The line of optimums which join the peaks of the compaction curves of different compactive efforts follows the general trend of the zero-air void. This line corresponds to air voids of about 5%.

Type of Soil:

The compaction of soil depends upon the type of soil. The maximum dry density and the optimum water content for different soils are shown in figure. In general, coarse grained soils can be compacted to higher dry density than fine-grained soils. With the addition of even a small quantity of fines to a coarse-grained soil, the soils attain a much higher dry density for the same compactive effort. However, if the quantity of the fines in increased to a value more than that required to fill the voids of the coarse-grained soils, the maximum dry density decreases. A well graded sand attains a much higher dry density than a poorly graded soil.

Cohesive soils have high air voids. These soils attain a relatively lower maximum dry density as compared with the cohesionless soils. Such soils require more water than cohesionless soils and therefore the optimum water content is high. Heavy clays of very high plasticity have very low dry density and a very high optimum water content.

Method of Soil Compaction:

The dry density achieved depends not only upon the amount of compactive effort but also on the method of compaction. For the same amount of compactive effort, the dry density will depend upon whether the method of compaction utilizes kneading action, dynamic action or static action. For example, in Harvard Miniature compaction test, the soil is compacted by the kneading action, and therefore, the compaction curve obtained is different from that obtained from the other conventional tests in which an equal compactive effort is applied.

Different methods of compaction curve give their own compaction curves. Consequently, the lines of optimums are also different.

Compaction curves for different soils

Fig: Compaction curves for different soils


  1. Sir, I left two / three feet wide strip along the perimeter of plot to plant decorative shrubs / flowering plants by replacing original black cotton soil with gardening soil from local river bank. Now I find that soil has become so dence that it holds water like a pitcher and has become so hard that a crowbar penetrates with difficulty. I wonder if it is due to over watering such that all air voids are removed and the soil has been compacted during dry and hot weather of NAGPUR summer, Could you kindly suggest how to get over the problem and how to prevent it? Regards.

    I realise it is not an engineering problem but may be you have experienced similar problem and have found an answer to it. Thanks

  2. In the pipe line trench sand bedding is provided at the bottom of the pipe upto 150mm deep and sand is filled again after laying the above upto a height of 300mm above the pipe. The height of the trench to be back filled above the sand is 700mm. The excavated matereial is black cotton soil. Can this soil be used for back fill. If not why?. The diameter of the pipe (steel) is DN 300, PN 25 and the pipe is for transmission of water. Please send your comments to [email protected]

  3. 1- I’m working on a back-filling of a causeway in the sea, can I use salt water in watering for compaction or is it necessary to use fresh one, form a scientific point of view?
    2- The consultant always asks me to remove stones bigger than 1/3rd the thickness, what is its effect scientifically on compaction? and is this proportion is a rule of thumb ?

Leave a Reply