ETL 1110-1-189
14 Feb 03
course thicknesses are determined from Table 8 as 2.5 and 4 in., respectively. Thus, the design
unreinforced pavement cross section is 2.5 in. of asphalt concrete (AC) and 13.5 in. of an
80 CBR base course over the 6 CBR subgrade. To determine the total thickness requirements for
a geogrid reinforced flexible pavement, Figure 8 is used. Figure 8 is entered along the y-axis
with the unreinforced total pavement thickness requirement and a horizontal line is drawn to the
intersection of the equivalency curve. A vertical line is then projected downward from the
intersection with the equivalency curve to the equivalent reinforced pavement thickness on the
x-axis. For this example, the equivalent reinforced pavement thickness is 14.0 in. Using the
minimum layer thickness values in Table 8, the reinforced flexible pavement cross section would
consist of 2.5 in. of AC and 11.5 in. of an 80 CBR base course over a 6 CBR subgrade. The net
reduction in aggregate thickness requirements based upon the inclusion of the geogrid
reinforcement is 2.0 in. of aggregate, a 12.5 percent reduction in required aggregate thickness. A
life cycle cost analysis should be performed to ensure a cost-effective design. Sample speci-
fications for the geogrid are provided in Table 3.
Summary:
Design Subgrade CBR = 6
(Based on measured values.)
Applicability: Geogrid
(Section 3.1 and Table 6)
Road Class: E
(TM 5-822-2)
Traffic Category: IVA
(TM 5-822-5)
Design Index (DI) = 5
(Table 7)
Unreinforced Total Pavement Thickness = 16 in.
(Figure 9)
Reinforced Total Pavement Thickness = 14 in.
(Figure 8)
Minimum AC Thickness = 2.5 inches
(Table 8)
Minimum Base Course Thickness = 4 inches(Table 8)
Design Base Thickness: tunreinforced = 13.5 in.
treinforced = 11.5 in.
Example A.2.2:
Description: Determine the reinforced design of a flexible pavement for an area located on a silt
plateau in Afghanistan. Estimates of the potential traffic for a 1-year design include
approximately 10,000 passes of heavily loaded tandem-axle trucks, approximately 8 percent of
the total expected traffic. A site investigation revealed that the design subgrade strength is
8 CBR.
Solution: The design subgrade CBR is 8. A geosynthetic applicability assessment based upon
the design subgrade CBR indicates that a geogrid may be a cost-effective alternative. A
geotextile for separation is generally not recommended at this design subgrade strength unless
prior experience indicated significant separation problems. The design vehicle is identified as
10,000 passes of a tandem-axle truck, which composes approximately 8 percent of the estimated
traffic. The location of the road indicates that the road will be in flat terrain. It is further
assumed that the road will lie in an open area rather than in a base camp and that two-lane traffic
will be required. A design hourly volume (DHV) of traffic can be assumed to range from 0 to
100 since the total traffic is only 10,000 trucks and 125,000 total vehicles for 1 year. Thus, the
design road is a Class E road according to TM 5-822-2. Since the traffic includes 8 percent
trucks with at least 3 axles, the traffic category is Category IV according to TM 5-822-5, Chap-
ter 3. For a Class E road with a Traffic Category of IV, the required design index (DI) is 4
A-5
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