Lesson 5/Learning Event 3
Establish the OMC of the soil at 55 blows/layer.
Establish a moisture range for CBR investigation. The OMC +4 percent range of investigation may
not apply. It may be necessary to prepare samples over a wide range of moistures with the majority of
the work being done on samples wet at OMC. In the example, samples were prepared over a range of
OMC ƒ8 percent.
Compact samples within the moisture content range of investigation at different levels of compactive
effort. As swelling soils usually being tested will be cohesive and have a CBR less than 20, compaction
must be at no less than 90 percent of maximum CE 55 dry density. An upper limit can be established by
reason of practicality as expansive soils are very difficult to compact at levels greater than 100 percent
of maximum CE 55 dry density. Therefore, laboratory compactive efforts of 12, 26, and 55 blows per
layer are adequate for nearly all cases.
Soak samples and measure swell. This step is monitored more closely than in the non-swelling case.
For each sample, the expansion is measured, the percent swell is computed, and the percent swell is
plotted against the molding water content. As an example, the triangular data point at 10 percent
moisture on the attached "swell data curve" (Figure 38) was obtained as follows:
Initial sample height = 5.00 inches
Height after soaking = 5.25 inches
Molding water content = 10 percent
Level of compactive effort = 55 blows/layer
After obtaining all data points, a curve is fit from which it can be ascertained that only soil placement at
molding moisture contents of 14 percent or greater is acceptable.
NOTE: That to limit swell below 3%, values of moisture contents must be above 15%.
Perform CBR penetration tests and determine corrected CBR for each sample.
Plot the data on graphs of dry density versus molding moisture content and corrected CBR versus
molding moisture content.
Reformat data on CBR family of curves data sheet.
Plot the CBR family of curves.
Establish a density range at which soil will be placed in the field. For this example, it will be
assumed that prior experience or benefit of a test strip is not available. Hence, only the guidelines from
Table 1 will be used. The minimum level of compaction is 90 percent of maximum CE 55 dry density
and, assuming a reasonable specification range, the upper limit will