4 Oct 10 17:13
Continuing with f-d's explanation...
ASTM D6938 is a method to determine the in-place density and moisture content of the soil or a soil-aggregate mixture such as a pavement base material.
The in-place density is first determined as the wet density, then the moisture content at the same location is determined. Let say for example that your wet density of the in-place material was 114.2 pounds per cubic foot. Then the moisture content is determined and found to be 11.3 percent. From these two values, the dry density is determined by the equation
((wet density/(1+moisture content))=dry density
So your dry density is 102.6 pcf
Meanwhile, a sample of the material in the field has been taken back to the laboratory to perform the D1557 test (commonly called a Modified Proctor). This test is the moisture-density relationship of laboratory compacted soil.
In the laboratory, the technicians divide sample into 4 specimens and allow them to dry. They then add moisture at different amounts so as to develop a relationship curve between moisture and dry density. The moistened soil is then compacted in the lab using standardized procedures so that the same amount of compaction energy is put into each specimen.
After compaction the wet density and the moisture content are determined to find the dry density of each specimen.
The dry density is then plotted against the moist content of the specimen at the time of compaction and a curve is developed. The peak of this curve is called the maximum dry density and the corresponding moisture content is called the optimum moisture content. Let's say that such a test was done on your soil and it was found that the maximum dry density was 109.6 pcf and the optimum moisture content was 11.1 percent.
Your in-place density divided by the maximum dry density is the amount of compaction you have achieved in the field. Usually that is specified to be somewhere between 95 and 100 percent, depending on the application. For your case...
102.6/109.6 yields 94 percent compaction, or slightly failing the typical specification requirement. Looking at the results, we see that the in-place moisture content is almost at optimum, so this tells us that there just needs to be more compactive effort put forth in the field...roll it some more, then retest it.