The maximum bulking occurs at water content between 4 to 5 %. On further additions of water, the meniscus is destroyed and the soil particles are able to shift to a closer packing.

The amount of compaction affects maximum dry density and optimum water content of a given soil. The effect of increasing the compactive energy results in an increase in the maximum dry density and decrease in the optimum water content.

The attainment of maximum density at full saturation is due to the reduction of effective pressure between soil particles by hydrostatic pressure.

The initial decrease of dry density at lower water content is a characteristic feature of black cotton soils, high swelling clays and fat clays. The optimum water content for such soils ranges between 20 to 25%.

When only a relatively small amount of water is present in the soil, it is firmly held by the electrical forces at the surface of soil particles with a high concentration of the electrolyte.

The density reaches the maximum value when the cohesion less soil is fully saturated, on further addition of water, the dry density again increases.

It has been seen from a laboratory experiment that, as the water content is increased the compact density goes on increasing, until a maximum dry density is achieved after which further addition of water decreases the density.

Well graded coarse-grained soil attains a much higher density and lower water optimum water content then fine grained soil which requires more water.

In case of cohesion less soil the dry density decreases with an increase in water content under a low compactive effect, this is due to bulking of sands where in the capillary tension resists the tendency of soil particles to take a density state.

Water content, type of soil, addition of admixtures, amount and type of compaction are the various factors which affect the compacted density.