TM 5-822-5/AFM 88-7, Chap. 1
uniform by the required techniques of subgrade
field data and experience show this to be the case,
preparation. This will prevent or minimize
then a reduction in combined thickness of pavement
significant or objectionable differential heaving and
and base for frost conditions of up to 10 percent
resultant cracking of pavements. When the reduced
may be permitted for substantial fills.
subgrade strength method is used for F4 subgrade
(2) Flexible pavement criteria for nonfrost
soils, unusually rigorous control of subgrade
design should also be used to determine the thick-
preparation must be required. When a thickness
ness of individual layers in the pavement system,
determined by the reduced subgrade strength
and to ascertain whether it will be advantageous to
procedure exceeds that determined for limited
include one or more layers of bound base in the
subgrade frost penetration, the latter smaller value
system. The base course composition requirements
shall be used, provided it is at least equal to the
set forth must be followed rigorously.
b. Thickness of rigid pavements. Where frost is
thickness required for nonfrost conditions. In
situations where use of the reduced subgrade
expected to penetrate into a frost-susceptible sub-
strength procedure might result in objectionable
grade beneath a rigid pavement, it is good practice
frost heave, but use of the greater thickness of base
to use a nonfrost-susceptible base course at least
course indicated by the limited subgrade frost
equal in thickness to the slab. Experience has
penetration design procedure is not considered
shown, however, that rigid pavements with only a
necessary, intermediate design thickness may be
4-inch base have performed well in cold environ-
used. However, these must be justified on the basis
ments with relatively uniform subgrade conditions.
of frost heaving experience developed from existing
Accordingly, where subgrade soils can be made
pavements where climatic and soil conditions are
reasonably uniform by the required procedures of
comparable.
subgrade preparation, the minimum thickness of
a. Thickness of flexible pavements. In the re-
granular unbound base may be reduced to a
duced subgrade strength procedure for design, the
minimum of 4 inches. The material shall meet the
design curves herein (fig 8-1) should be used for
requirements set forth below for free-draining
road, street, and parking area design. The curves
material as well as the criteria for filter under
should not be entered with subgrade CBR values
pavement slab. If it does not also meet the criteria
determined by tests or estimates, but instead with
for filter over subgrade, a second 4-inch layer
the applicable frost-area soil support index from
meeting that criteria shall be provided.
table 18-3. Frost-area soil support indexes are used
(1) Additional granular unbound base
as if they were CBR values; the term CBR is not
course, giving a thickness greater than the minimum
applied to them, however, because being weighted
specified above, will improve pavement
average values for an annual cycle, their value
performance, giving a higher frost-area index of
cannot be determined by CBR tests. The soil sup-
reaction on the surface of the unbound base (fig.
port index for Si or S2 material meeting current
18-5) and permitting a pavement slab of less
specifications for base or subbase will be deter-
thickness. Bound base also has significant structural
mined by conventional CBR tests in the unfrozen
value, and may be used to effect a further reduction
state.
in the required thickness of rigid pavement slab.
Criteria for determining the required thickness of
rigid pavement slabs in combination with a bound
base course are contained in chapter 12. The
requirements for granular unbound base as drainage
and filter layers will still be applicable.
(2) The thickness of concrete pavement will
be determined in accordance with chapter 12, using
the frost-area index of reaction determined from
figure 18-5. This figure shows the equivalent
weighted average index of reaction values for an
(1) General field data and experience
annual cycle that includes a period of thaw-weak-
indicate that on the relatively narrow embankiments
ening in relation to the thickness base. Frost-area
of roads and streets, reduction in strength of sub-
indexes of reaction are used as if they were moduli
grades during frost melting may be less in sub-
of reaction, k, and have the same units. The term
stantial fills than in cuts because of better drainage
modulus of reaction is not applied to them because
conditions and less intense ice segregation. If local
being weighted average values for an annual cycle,
18-14
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