TM 5-822-5/AFM 88-7, Chap. 1
except where it is known that a higher density can
struction is available, CBR tests may be made in
be obtained practically, in which case the higher
place on material when it has attained its maximum
density should be required. Compaction of select
expected water content or on undisturbed soaked
materials will be as shown in table 4-1 except that
samples. The procedures for selecting CBR design
in no case will cohesionless fill be placed at less
values described for subgrades apply to select
than 95 percent or cohesive fill at less than 90
materials and subbases. CBR tests on gravelly
percent.
materials in the laboratory tend to give CBR values
higher than those obtained in the field. The
difference is attributed to the processing necessary
5-4. Drainage.
to test the sample in the 6-inch mold, and to the
Subbase drainage is an important aspect of design
confining effect of the mold. Therefore, the CBR
and should be accomplished in accordance with TM
test is supplemented by gradation and Atterberg
5-820-2/AFM 88-5, Chap. 2.
limits requirements for subbases, as shown in table
5-1. Suggested limits for select materials are also
5-5. Selection of Design CBR Values.
indicated. In addition to these requirements, the
The select material or subbase will generally be
material must also show in the laboratory tests a
uniform, and the problem of selecting a limiting
CBR equal to or higher than the CBR assigned to
condition, as described for the subgrade, does not
the material for design purposes.
ordinarily exist. Tests are usually made on remolded
samples; however, where existing similar con-
5-2
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