Relation between water content and dry density of soil

What is water content?

The amount of water contained in the soil is known as water content. The presence of water in the soil makes it weak. Water content in soil decreases the shear resistance of the soil. The settlement may occur below the structure if the required amount of water did not take out from the soil.

What is the dry density of soil?

The dry density of soil is nothing but the density of soil mass in the absence of water content in it.

Now, let us see the relationship between water content and the dry density of soil.

In general, compaction is the densification of soil by the removal of air, which requires mechanical energy. The degree of compaction of soil is measured in terms of its dry unit weight.

In 1993, R.R. Proctor show that for any soil there exists a definite relationship between the soil water content and the dry density to which the soil can be compacted and that for a specific amount of compaction energy or effort applied to the soil there is a particular water content at which the soil attains its maximum dry density.

Figure 1 shows the relation between water content and dry density of soil at particular compaction energy or effort.

Compaction Curve - Relation between water content and dry density of soil
Fig:- 1

The addition of water to dry soil, it helps in bringing the soil particles together coating them with a thin film of water. i.e., it acts as a softening agent on the soil particles.

As the water content is increased, water starts acting as a lubricant, the soil particles start coming closer due to increased workability, and under a given amount of compactive effort, the soil water-air mixture starts occupying less volume, thus causing a gradual increase in dry density.

As more water has added a stage is reached when the air content of the soil attains minimum volume, thus making the dry density maximum. The water content corresponding to maximum dry density is called optimum water content or optimum moisture content ( O.M.C. ).

At water content lower than O.M.C., the soil is rather stiff and has a lot of void spaces and, therefore dry density is low. however, the water content beyond the O.M.C. reduces the dry density because the extra water starts occupying the space which otherwise could have been occupied by soil grains under compaction.

The curve between water content and dry density of soil is shown in figure 1 and is known as the water content – dry density curve or the moisture content – dry density curve or compaction curve. The state at the peak of the curve is said to be that of 100 % compaction at a particular compactive effort. The curve is usually of a hyperbolic form when the points obtained from the tests are smoothly joined.

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