pavement and base of 6 inches (2 inches of

pavement and 4 inches of base). Therefore, the

section using materials 1 and 2 will consist of a 4.5-

inch subbase course of material 1, a 4-inch base

course of material 2, and a 2-inch pavement.

layers within a flexible pavement provides the op-

portunity to reduce the overall thickness of pave-

The total thickness and thicknesses of the various

ment structure required to support a given load. To

subbase and base layers are determined as follows:

design a pavement containing stabilized soil layers

requires the application of equivalency factors to a

base, base, and pavement will be governed by the

layer or layers of a conventionally designed

CBR of the compacted subgrade. From the flexible-

pavement. To qualify for application of equivalency

pavement design curves shown in figure 8-1, the

factors, the stabilized layer must meet appropriate

required total thickness above the compacted sub-

strength and durability requirements set forth in TM

grade (CBR of 10) is 11 inches. A check must be

5-822-4. An equivalency factor represents the

made of the adequacy of the strength of the un-

number of inches of a conventional base or subbase

compacted subgrade and of the weak layer within

which can be replaced by 1 inch of stabilized

the subgrade. From the curves in figure 8-1, the

material. Equivalency factors are determined as

required cover for these two layers is 14.5 and 21

shown in table 8-1 for bituminous stabilized

inches, respectively. If the design thickness is 11

materials, and from figure 8-2 for materials

inches and the subgrade is compacted to 9 inches

stabilized with cement, lime, or a combination of

below the subgrade surface, the natural subgrade

flyash mixed with cement or lime. Selection of an

will be covered by a total of 20 inches of higher

equivalency factor from the tabulation is dependent

strength material. Similarly, the soft layer occurring

upon the classification of the soil to be stabilized.

24 inches below the subgrade surface will be

Selection of an equivalency factor from figure 8-2

protected by 35 inches of total cover. Thus, the

requires that the unconfined compressive strength

cover is adequate in both cases.

as determined in accordance with ASTM D 1633 be

known. Equivalency factors are determined from

a design index of 5 the minimum base thickness is 4

figure 8-2 for subbase materials only. The

inches and the pavement thickness is 2 inches as

relationship established between a base and subbase

indicated in table 6-1. If, however, the CBR of the

is 2 to 1. Therefore, to determine an equivalency

base material had been 100 rather than 80, a

factor for a stabilized base course, divide the

minimum pavement thickness of 2 inches would

subbase factor from figure 8-2 by 2.

have been required.

design thickness of each layer of materials 1 and 2

will depend upon the CBR design value of each

material. The total thickness of subbase, base, and

pavement, as determined above, is 11 inches. The

thickness required above material 1 (CBR = 35), as

determined from figure 8-1, is 3 inches; there- fore,

the required thickness of material 1 is 8 inches (11 -

3 inches). The 3-inch layer required above material

1 will be composed of material 2 and pavement;

however, adjustments must be made in the

thicknesses of material 2 and the pavement to

conform with minimum base and pavement

thickness, which is a combined thick-ness of

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