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.
Summary:
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: tunreinforced = 8.0 in.
treinforced = 4.0 in.
A-7
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