protection.

7, Chap 5)

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

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

be required to a depth of * *x 45 - 20 = 2* *inches.

Coarse to fine silty sand (SP-SM), normal-

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

combined thickness of pavement and base for zero

frost penetration is 45 inches, equivalent to that for

the flexible pavement. By use of r = 3 and a

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

inches of frost penetration into the subgrade in the

design year. No subgrade preparation would be

required.

combined thickness a of pavement and base to pre-

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.

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

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