TM 5-822-5/AFM 88-7, Chap. 1
CHAPTER 8
FLEXIBLE PAVEMENT DESIGN
instances, be modified rather than stabilized. In
8-1. General.
addition, when unbound granular layers are
Flexible pavement designs will provide the follow-
employed between two bound layers (e.g., an un-
ing:
bound base course between an asphalt concrete
a. Sufficient compaction of the subgrade and of
(AC) surface course and a stabilized subbase
each layer during construction to prevent objec-
course), it is imperative that adequate drainage be
tionable settlement under traffic.
provided the unbound layer to prevent entrapment
of excessive moisture in the layer. Additional in-
c. Adequate thickness above the subgrade and
formation on soil stabilization may be - obtained
above each layer together with adequate quality of
from TM 5-818-1.
the select material, subbase, and base courses to
c. All-bituminous concrete. All-bituminous con-
prevent detrimental shear deformation under traffic
crete pavements are also designed using equivalency
and, when frost conditions are a factor, to control
factors (see para 8-6). The procedure is the same as
or reduce to acceptable limits effects of frost heave
for stabilized soil layers discussed above.
or permafrost degradation.
d. A stable, weather-resistant, wear-resistant wa-
8-3. Design Index.
terproof, nonslippery pavement.
The design of flexible pavements for roads, streets,
parking areas, open storage, and similar areas will
8-2. Design Procedure.
be based on a design index, which is an index rep-
a. Conventional flexible pavements. In designing
resenting all traffic expected to use a flexible
conventional flexible pavement structures, the
pavement during its life. It is based on typical
design values assigned to the various layers are
magnitudes and compositions of traffic reduced to
applied to the curves and criteria presented herein.
equivalents in terms of repetitions of an 18,000-
Generally, several designs are possible for a specific
pound, single-axle, dual-tire load. Selection of the
site, and the most practical and economical design
design index will be accomplished as stated in
is selected. Since the decision on the practicability
chapter 3. The designer is cautioned that in select-
of a particular design may be largely a matter of
ing the design index, consideration will be given to
judgment, full particulars regarding the selection of
traffic which may use the pavement structure during
the final design (including cost estimates) will be
various stages of construction and to other
included in the design analysis. For computer aided
foreseeable exceptional use.
design, see paragraph 1-6.
b. Stabilized Soil Layers. Flexible pavements
8-4. Thickness Criteria-Conventional Flexible
containing stabilized soil layers are designed
Pavements.
through the use of equivalency factors. A conven-
tional flexible pavement is first designed and the
Thickness design requirements are given in figure 8-
equivalency factors applied to the thickness of the
1 in terms of CBR and design index. Minimum
layer to be stabilized. When stabilized materials
thickness requirements are shown in table 6-1. For
meeting all gradation, durability, and strength re-
frost condition design, thickness requirements will
quirements indicated in TM 5-822-4, and in chap ter
be determined from chapter 18 of this manual. In
17 herein are utilized in pavement structures, an
regions where the annual precipitation is less than
appropriate equivalency factor may be applied. Soils
15 inches and the water table (including perched
which have been mixed with a stabilizing agent and
water table) will be at least 15 feet below the
which do not meet the requirements for a stabilized
finished pavement surface, the danger of high
soil are considered modified and are designed as
moisture content in the subgrade is reduced. Where
conventional pavement layers. When portland
in-place tests on similar construction in these
cement is used to stabilize base course materials in
regions indicate that the water content of the
Air Force Pavements, the treatment level must be
subgrade will not increase above the optimum, the
maintained below approximately 4 percent by
total pavement thickness, as determined by CBR
weight to minimize shrinkage cracking which will
tests on soaked samples, may be reduced by as
reflect through the bituminous concrete surface
much as 20 percent. The minimum thickness of
course. In this case, the base course will, in most
pavement and base course must still be met; there-
8-1
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