To obtain the moisture content-dry density relationship for a soil and hence to determine
its maximum dry density (MDD) and optimum moisture content (OMC).
Soil compaction is an economical method of soil improvement, and it is often used to
make ground suitable for the foundations of roads and buildings. It is also used in the
placing of soil fills and in the construction of earth dams to ensure suitable soil
properties. The compaction is normally achieved through the input of energy into the soil
by impact, kneading, vibration or static means.
The extent of compaction depends on the moisture content of the soil and the compactive
effort used. In a compaction test the object is to determine the optimum moisture content
and maximum dry density achievable with a given compactive effort. A plot of dry
density versus moisture content (Figure 1) indicates that compaction becomes more
efficient up to a certain moisture content, after which the efficiency decreases. The
maximum dry density is obtained at this optimum moisture content. If the compaction
process were completely efficient, it would be possible to expel the air from the voids, in
which case the dry density would correspond to a zero-air voids state (i.e. the sample
would be saturated with water). Since perfect compaction is not possible (except at high
moisture contents) the compaction curve will always fall below the ideal or zero-air voids
curve (Figure 1).