Figure 13-2 shows the test results and calculations for a compaction test. As you can see, this test used a 10-pound tamper and Proctor mold. Five runs were made. After compaction, the weight of the compacted soil and mold was recorded for each run. From this, the weight of the mold was subtracted to get the weight of the soil for each run. Then the wet unit weight was computed using the formula shown.

Lines A, B, C, D, and E contain the data for the moisture-content test for each run. Note that for each run, there were two tests: one from the top of the mold and the other of soil from the bottom. The averages were set down beside average moisture content. Finally, the dry unit weight (density) in pounds per cubic foot (pcf) for each run was calculated by the formula shown. As you can see, for the same compactive effort, the density varied with the average moisture content.

The ultimate objective of the compaction test is to determine the OMC; that is, the moisture content that yields maximum density for a given compactive effort.

Figure 13-3.Determination of optimum moisture content.

You determine this by applying the test results to plot a curve like the one shown in figure 13-3. In this curve, the horizontal coordinates are the average moisture contents; the vertical coordinates are the dry densities. For the test results used in the example, the curve indicates that the maximum attainable density for the given compactive effort was 127.2 pcf for which the OMC was 10.9 percent.

The dotted line marked "98% maximum density" indicates that, in this case, the project specifications

Figure 13-4.Sand-displacement method apparatus.

required that 98 percent of the maximum density be obtained through compaction. The maximum attainable was 127.2 pcf; 98 percent of this is 124.7 pcf. The dotted line is drawn at the 124.7 pcf level. Any moisture content lying in the crosshatched area above this line would produce the specified density for a given compactive effort; therefore, the range of permissible moisture content is from 9 to 13 percent.