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Soil compaction can be a very 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 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 (but not necessarily desirable) to expel all the air from the voids, in which case the dry density would correspond to a zero-air voids state (ie. the sample would be saturated with water).  Since perfect compaction is not possible (except at high moisture contents and low dry density) the compaction curve will always fall below the ideal or zero-air voids curve.

It should be noted that there are a number of standards for compaction tests, each differing in the amount of energy input into compaction.  For a given soil the different tests will produce different maximum dry densities and optimum moisture contents (ie. these parameters are NOT soil properties).  The maximum dry density and optimum moisture content are only relevant for a specified compaction procedure which should be stated when presenting the results.

In earthworks it is common to specify a dry density within a certain percentage of the maximum determined from a specified compaction test.  For this to be a sensible procedure it is important that the compactive effort used in the laboratory is comparable to that supplied by the field equipment.

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