Lesson 5/Learning Event 3
Plot CBR family of curves. This graph can be condensed into a single line. The three data points are
obtained by plotting the corrected CBR against the associated dry density at the limiting moisture
Establish a density range at which the soil will be placed in the field. Using the criteria from Table
5 and noting that the CBR for this soil is always greater than 20, the minimum level of compaction
allowed is 100 percent of maximum CE 55 dry density. As there is a lack of additional information,
specify 100 to 105 percent of maximum CE 55 dry density. This means compaction between:
1.00 x 120 = 120 pcf
1.05 x 120 = 126 pcf
Determine the design CBR and placement moisture content. From the family of curves, it can be
seen that the minimum CBR is 70 at 120 pcf. The placement moisture content necessary to insure that
this strength is obtained is 8 percent or greater.
It can be seen that considerably greater CBR values can be achieved if more field compaction is applied
to the soil. If this is not too costly, it may be advantageous to specify greater densities. Lastly, it must
be emphasized that the short cuts used in this procedure are dependent upon the fact that the soil is
actually free draining. This can be determined from the shapes of the compaction curves.
FIELD CBR TESTS (IN-PLACE TESTS)
As a part of flexible pavement design, it will be necessary to determine the strength of the subgrade soil
at its natural density. To accomplish this, the CBR penetration apparatus can be mounted on a vehicle.