TM 5-822-5/AFM 88-7, Chap. 1
CHAPTER 9
RIGID PAVEMENT DESIGN
9-1. Soil Classification and Tests. All soils
density from ASTM D 1557, no rolling is necessary
should be classified according to the Unified Soil
other than that required to pro- vide a smooth
Classification System (USGS) as given in ASTM D
surface. Compaction requirements for cohesive soils
2487. There have been instances in construction
(LL > 25; PI > 5) will be 90 percent of maximum
specifications where the use of such terms as
density for the top 6 inches of cuts and the full
"loam," "gumbo," "mud," and "muck" have resulted
depth of fills. Compaction requirements for
in misunderstandings. These terms are not specific
cohesionless soils (LL < 25: PI <5) will be 95
and are subject to different interpretations
percent for the top 6 inches of cuts and the full
throughout the United States. Such terms should
depth of fills. Compaction of the top 6 inches of
not be used. Sufficient investigations should be
cuts may require the subgrade to be scarified and
performed at the proposed site to facilitate the de-
dried or moistened as necessary and recompacted to
scription of all soils that will be used or removed
the desired density.
c. Special soils. Although compaction increases
during construction in accordance with ASTM D
2487; any additional descriptive information con-
the stability and strength of most soils, some soil
sidered pertinent should also be included. If Atter-
types show a marked decrease in stability when
berg limits are a required part of the description, as
scarified, worked, and rolled. Also, expansive soils
indicated by the classification tests, the test pro-
shrink excessively during dry periods and expand
cedures and limits should be referenced in the con-
excessively when allowed to absorb moisture. When
struction specifications.
soils of these types are encountered, special
treatment will usually be required. For nominally
expansive soils, water content, compaction effort,
9-2. Compaction.
and overburden should be determined to control
a. General. Compaction improves the stability of
swell. For highly expansive soils, replacement to
the subgrade soils and provides a more uniform
depth of moisture equilibrium, raising grade, lime
foundation for the pavement. ASTM D 1557 soil
stabilization, prewetting, or other acceptable means
compaction test conducted at several moisture con-
of controlling swell should be considered (see TM
tents is used to determine the compaction charac-
5-818-7 for guidance).
teristics of the subgrade soils. The range of maxi-
mum densities normally obtained in the compaction
9-3. Treatment of Unsuitable Soils. Soils not
test on various soil types is listed in TM 5-825-2.
suitable for subgrade use (as specified in TM 5-825-
This test method should not be used if the soil
2/AFM 88-6, Chap 2, and MIL-STD-619) should
contains particles that are easily broken under the
be removed and replaced or covered with soils
blow of the tamper unless the field method of
which are suitable. The depth to which such adverse
compaction will produce a similar degradation.
soils should be removed or covered depends on the
Certain types of soil may require the use of a
soil type, drainage conditions, and depth of freezing
laboratory compaction control test other than the
temperature penetration and should be determined
above-mentioned compaction test. The unit weight
by the engineer on the basis of judgment and
of some types of sands and gravels obtained using
previous experience, with due consideration of the
the compaction method above may be lower than
traffic to be served and the costs involved. Where
the unit weight that can be obtained by field com-
freezing temperatures penetrate a frost-susceptible
paction; hence, the method may not be applicable.
subgrade, design procedures outlined in chapter 17
In those cases where a higher laboratory density is
herein, or TM 5-852-3 as applicable, should be
desired, compaction tests are usually made under
followed. In some instances, unsuitable or adverse
some variation of the ASTM D 1557 method, such
soils may be improved economically by stabilization
as vibration or tamping (alone or in combination)
with such materials as cement, flyash, lime, or
with a type hammer or compaction effort different
certain chemical additives, whereby the
from that used in the test.
characteristics of the composite material become
b. Requirements. For all subgrade soil types, the
suitable for subgrade purposes. Criteria for soil
subgrade under the pavement slab or base course
stabilization are in TM 5-822-4. However, subgrade
must be compacted to a minimum depth of 6 inches.
stabilization should not be attempted unless the
If the densities of the natural subgrade materials are
costs reflect corresponding savings in base-course,
equal to or greater than 90 percent of the maximum
9-1
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