To perform well, concrete structures must be built on firm soil. Soil consists of solids and voids that are filled with either air or water. It is the voids that compress when a soil is loaded, and the fewer the voids, the less chance there is that excessive settlement or sliding failures will occur. Poor compaction or no compaction may cause a concrete foundation, floor slab or pavement to fail. The failure may take place immediately or it may occur weeks, months, or even years after construction. Compaction is achieved by applying a pressure on the surface or by vibrating the soil mass. Different compaction methods are needed for different types of soils and the amount of compaction required for different soils must be established using standard testing procedures.


To find out how well a soil has been compacted we must measure the dry unit weight or dry density in pounds per cubic foot. American Society for Testing and Materials (ASTM) standards describe several methods for measuring the dry density of an in-place soil. In the rubber balloon method (ASTM D 2167), water is used to find the volume of the hole and in the sand-cone method (ASTM D 1556) sand is used to find the volume. There is also a method for determining the in-place density of soils by nuclear methods (ASTM D 2922).


For the results of a field density test to have meaning, a reference point is needed. There are two tests that provide a reference point for granular soils. One (ASTM D 4524) determines the minimum index density of a cohesionless (granular) soil and the other (ASTM D 4253) determines the maximum index density of the soil. These are laboratory tests that determine how loose or how dense a given granular soil can be made in the laboratory. These become the reference points.


In the standard Proctor test (ASTM D 698), samples of the soil are mixed at several different water contents in the laboratory, allowed to stand for at least 16 hours, and then compacted using a standardized procedure. After the wet weight is obtained, the soil is dried and the dry weight determined. For different moisture contents, the dry density is calculated and a graphical plot of dry density versus moisture content is made.


Clean granular materials (sands and gravels) are free-draining so that water can enter or leave the voids with relative ease. Vibration causes the particles to bounce around and roll or slide into a dense configuration.