30 June 2001
a. Laboratory Tests. Tests results should include a full family of curves (Figure 6-1) as described
in CRD-C 654. These curves show the three-way relationship of water content at the time of
compaction, compacted density, and CBR after soaking. These curves should be studied in view of the
actual water contents and densities that can be expected considering the natural scatter when specific
control values are specified. The scatter that can be expected with normal control procedures will vary
with the soil type. A spread of plus or minus 2 percent can be anticipated for soils with low optimum
moisture contents (in the range of 10 percent), whereas a spread of plus or minus 4 percent can be
anticipated for soils with high optimum moisture contents (in the range of 25 percent). Poor construction
control may result in even greater scatter. A comparable scatter in the density can also be expected.
After the range of moisture contents and densities that can be expected during actual construction is
estimated, the range of CBR values that will result from these variations in moisture and density should
be determined. The design CBR value for the specific soil tested should be selected near the lower part
of the range. The following steps along with Figure 6-1 illustrate the selection of a design CBR value.
(1) Step A. Determine moisture/density relationship (CRD-C 653) at 12, 26, and
55 blows/layer. Plot density to which soil can be compacted in the field. For the clay of this example,
use 95 percent of maximum density. Plot the desired moisture content range. For the clay of this
example, use 1-1/2 percent of optimum moisture content for approximately 13 and 16 percent. Shaded
area represents compactive effort greater than 95 percent and within 1-1/2 percent of optimum
(2) Step B. Plot laboratory CBR (CRD-C 654) for 12, 26, and 55 blows/layer.
(3) Step C. Plot CBR versus dry density at constant moisture content. Plot attainable
compaction limits of 1,770 and 1,840 kg/m3 (110.6 and 115 lb/ft3) for this example. The hatched area
represents attainable CBR limits for desired compaction 1,770 and 1,840 kg/m3 (110.6 to 115 lb/ft3) and
moisture content (13 to 16 percent). CBR varies from 11 (95 percent compaction and 13 percent
moisture content) to 26 (15 percent moisture content and maximum compaction). For design purposes,
a CBR at the low end of range is used. In the example, a CBR of 12 with a moisture content specified
between 13 and 16 percent is selected.
b. In-place Tests and Tests on Undisturbed Samples. Where an existing pavement at the site has
a subgrade constructed to the same standards as the job being designed, in-place tests or tests on
undisturbed samples may be used in selecting the design CBR value. Also, where no compaction is
anticipated, as in the layers below the zone of compaction, tests should be conducted on the natural
material. The in-place CBR may be used where little increase in moisture is anticipated, such as coarse
grained cohesionless soils, soils which are at least 80 percent saturated in the natural state, and soils
under existing similar pavements which have reached the maximum water content expected, and thus
no soaking is required. When in-place tests or tests on undisturbed soils are used, a statistical approach
is recommended for selecting the design CBR. An illustration of selecting the design CBR is as follows:
Given 20 CBR test values from a runway site.
(1) CBR = 4, 4, 4, 4, 5, 5, 5, 5, 5, 6, 6, 6, 6, 6, 7, 7, 8, 8, 10, and 11. This is a total of
20 separate tests.
(2) Percent of CBR values equal to or greater than each different value: