What is compaction and why is it important?

Compaction is the rearrangement of soil particles to a denser state using mechanical equipment.  Compaction increases the unit weight (sometimes referred to as density) of a soil by decreasing the volume of the voids in the soil structure.  Compaction is important because it strengthens soil which increases the bearing capacity of a soil and decreases the settlement potential of a soil.  For example, compaction of soil beneath footings for a house will decrease the overall settlement of the house.  Compaction recommendations are typically given as a percentage of a soil's maximum dry unit weight (or density) as determined by the Proctor Test.  The Proctor Test (ASTM D698 & D1557) is a test that is performed in the lab on a sample of soil taken in the field.  The results of this test determine the maximum unit weight (or density) and corresponding optimum moisture content for a specific soil.  Once the specific soil has been compacted in the field, it is tested to determine its unit weight (or density) as compared to the soil's maximum density.  It is important that the soil being tested for compaction in the field is the same type of soil that was used for the Proctor test in the lab.