The dry density of the soil is increased by compaction. The increase in the dry density 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 dry density of the 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 effect of increasing the amount of compactive effort is to increase the maximum dry density and to decrease the optimum water content. 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 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 dry density achieved 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 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