TM 5-822-5/AFM 88-7, Chap. 1
c. Design Index. 4 (from TM 5-822-2/AFM 88-
protection.
m. Flexible pavement design by reduced
7, Chap 5)
d. Design Freezing Index. 700 degree-days
subgrade strength method. From the REDUCED
e. Subgrade Material.
SUB-GRADE STRENGTH section, the frost-area
Uniform sandy clay, CL
soil support index is 3.5, which, from the design
Plasticity index, 18
curve (fig 8-1), yields a required combined
Frost group, F3
thickness of pavement and base of 20 inches. Since
Water content, 20 percent (average)
this is less than the 26-inch thickness (2.0 + 24)
Normal-period CBR, 10
required by the limited subgrade frost penetration
Normal-period modulus of subgrade reaction
method, the 20-inch thickness would be used. The
k=200 psi/inch on subgrade and 325 psi/
pavement structure could be composed of 2 inches
inch on 22 inches of base course.
of AC, 12 inches of crushed gravel (since the
f. Base course material.
crushed gravel contains only 1 percent passing the
Crushed gravel (GW), normal-period CBR =
no.200 sieve, it also serves as the free-draining layer
80, 30 percent passing no.10 sieve, 1 percent
directly beneath the pavement), and 6 inches of silty
passing no.200 sieve.
sand subbase material. Subgrade preparation would
g. Subbase course material.
be required to a depth of x 45 - 20 = 2 inches.
n. Rigid pavement design by limited subgrade
Coarse to fine silty sand (SP-SM), normal-
frost penetration method. From figure 12-1, the re-
period CBR=20,
11 percent passing no. 200 sieve, 6 percent
quired pavement thickness p, based on the normal-
finer than 0.02 millimeters, frost classification
period k = 325 psi per inch, the concrete flexural
52, meets filter criteria for material in contact
strength of 650 psi and the design index of 4, is 5.8
with subgrade.
inches (use 6 inch minimum). From figure 18-3, the
h. Average dry unit weight (good quality base
combined thickness of pavement and base for zero
and subbase). 135 pounds per cubic feet
frost penetration is 45 inches, equivalent to that for
i. Average water content after drainage (good
the flexible pavement. By use of r = 3 and a
quality base and subbase). 5 percent
thickness of base for zero frost penetration of 39
inches (45-6) in figure 18-3, the required thickness
face of subgrade
of base b is 22 inches, which would allow about 5.5
k. Concrete flexural strength. 650 psi
inches of frost penetration into the subgrade in the
1. Flexible pavement design by limited subgrade
design year. No subgrade preparation would be
frost penetration method. From figure 18-3, the
required.
o. Rigid pavement design by the reduced sub-
combined thickness a of pavement and base to pre-
grade strength method. Since frost heave has not
vent freezing of the subgrade in the design freezing
index year is 45 inches. According to criteria in
been a major problem, a minimum of 4 inches of the
chapter 8, the minimum pavement thickness is 2.5
free-draining base course material could be used,
inches over a CBR = 80 base course that must be at
plus 4 inches of the subbase that will serve as a filter
least 4 inches thick. The base thickness for zero
material on the subgrade. For this case (8 inches of
frost penetration is 42.5 inches (45-2.5). The ratio
base and subbase) the frost-area index of reaction
of subgrade to base water content is r = 20/5=4.
would be 50 psi per inch (fig. 18-5), requiring a
Since this is a highway pavement, the maximum
pavement slab 7 inches thick. Sub- grade
allowable r of 3 is used in figure 18-4 to obtain the
preparation to a depth of x 45-15=7.0 inches
required thickness of base b of 24 inches, which
would be required.
p. Alternative designs. Other designs using stabi-
would allow about 6 inches of frost penetration into
the subgrade in the design year. Subgrade
lized layers, including ABC pavements, should be
preparation would not be required since the
investigated to determine whether they are more
combined thickness of pavement and base is more
economical than the designs presented above.
than one-half the thickness required for complete
18-20
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