ETL 1110-1-189

14 Feb 03

total required flexible pavement thickness for this example is 18.5 in. using Figure 9. It is

assumed that a 10-CBR subbase will be used with a base course capable of producing an 80 CBR

design strength. Since a subbase layer is to be used, Figure 9 must be used to determine the

aggregate thickness requirements above the 10-CBR subbase. Entering Figure 9 with 10 CBR,

the required pavement thickness above the subbase is 10.0 in. Using this information, the

minimum surface and base course thicknesses are determined from Table 8 as 2.0 and 4.0 in.,

respectively. Thus, the design unreinforced pavement cross section is 2.0 in. of asphalt concrete

(AC) and 8.0 in. of an 80 CBR base course over 8.5 in. of the 10 CBR subbase. To determine

the total thickness requirements for a geogrid-reinforced flexible pavement, Figure 8 is used. For

the 10-in. combined surface and base course thickness, the equivalent reinforced pavement

thickness is 6.0 in. using Figure 8. Thus, the geogrid reinforcement material is placed at the

subbase-base interface. Using the minimum layer thickness values in Table 8, the reinforced

flexible pavement cross section would consist of 2.0 in. of AC and 4.0 in. of an 80 CBR base

course over a 8.5 in. of a 10-CBR subgrade. The net reduction in aggregate base thickness

requirements based upon the inclusion of the geogrid reinforcement is 4.0 in. of aggregate, a

50 percent reduction in required aggregate base thickness. A life cycle cost analysis should be

performed to ensure a cost-effective design. Sample specifications for the geogrid are provided

in Table 3.

Design Subgrade CBR = 4

(Based on measured values.)

Applicability: Geotextile Not Warranted (Section 3.1 and Table 6)

Geogrid Should Be Considered

(Section 3.1 and Table 6)

Road Class: E

(TM 5-822-2)

Traffic Category: IV

(TM 5-822-5)

Design Index (DI) = 4

(Table 7)

Unreinforced Total Pavement Thickness = 18.5 in.

(Figure 9)

Unreinforced Subbase Thickness = 8.5 in. (Figure 9)

Reinforced Total Pavement Thickness = 14.5 in.

(Figure 8)

Minimum AC Thickness = 2.0 inches

(Table 8)

Minimum Base Course Thickness = 4.0 inches

(Table 8)

Design Base Course Thickness: **t**unreinforced = 8.0 in.

A-7

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