TM 5-822-5/AFM 88-7, Chap. 1
compacted, and the material replaced, or the
should be 85 percent of maximum density to
thickness of select material or subbase must be so
conform to fill requirements.
increased that the densities in the uncompacted
subgrade will be adequate.
4-4. Selection of Design CBR Values.
b. Cohesive subgrade. Assume a lean clay, a
Flexible pavements may be designed using the lab-
design CBR of 7, and a natural in-place density of
oratory soaked CBR, the field in-place CBR, or the
83 percent of maximum density extending below the
CBR from undisturbed samples as described in
depth of exploration of 6 feet. Compaction of the
MIL-STD-621A, Method 101. For the design of
subgrade from the surface would be impracticable
flexible pavements in areas where no previous ex-
with ordinary equipment beyond the 6- to 8-inch
perience regarding pavement performance is avail-
depth that could be processed; therefore, the
able, the laboratory soaked CBR is normally used.
minimum depth of cut would be limited by the in-
Where an existing pavement is available at the site
place density. From table 4-1 for a design index of
that has a subgrade constructed to the same
5, it is found that the 83 percent in-place natural
standards as the job being designed, in-place tests
density would be satisfactory below depths of about
or tests on undisturbed samples may be used in se-
25 inches from the pavement surface. From CBR
lecting the design CBR value. In-place tests are
design curves (explained subsequently), the top of
used when the subgrade material is at the maximum
the subgrade will be 14.5 inches below the
water content expected in the prototype. Contrarily,
pavement surface; therefore, a zone 10.5 inches
tests on undisturbed samples are used where the
thick below the top of the subgrade requires treat-
material is not at the maximum water content and
ment. The bottom 6 to 8 inches of this can be proc-
thus soaking is required. Sampling involves
essed in place; so about 4 inches of material must be
considerably more work than in-place tests; also,
removed and replaced. Compaction to 95 percent of
"undisturbed" samples tend to be slightly disturbed;
maximum density is required for all cohesive
therefore, in-place tests should be used where
material that lies within 12 inches of the pavement
possible. Guides for determining when in-place tests
surface. Since the subgrade does not fall within this
can be used are given in details of the CBR test in
zone compaction requirements in the replaced
MIL-STD-621A, Test Method 101.
material should be 90 percent to conform to fill
requirements, and the layer processed in place
4-2
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