ETL 1110-1-189
14 Feb 03
according to Table 7. The total required flexible pavement thickness for this example is 11 in.
using Figure 9. It is assumed that a subbase will not be used and the base course is capable of
producing an 80 CBR design strength. 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 9.0 in. of an 80 CBR
base course over the 8 CBR subgrade. To determine the total thickness requirements for a
geogrid-reinforced flexible pavement, Figure 8 is used. For this example, the equivalent
reinforced pavement thickness is 6.5 in. using Figure 8. Using the minimum layer thickness
values in Table 8, the reinforced flexible pavement cross section would consist of 2.0 in. of AC
and 4.5 in. of an 80 CBR base course over a 8 CBR subgrade. The net reduction in aggregate
thickness requirements based upon the inclusion of the geogrid reinforcement is 4.5 in. of
aggregate, a 50 percent reduction in required aggregate 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 = 8
(Based on measured values.)
Applicability: 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 = 11 in.
(Figure 8)
Reinforced Total Pavement Thickness = 6.5 in.
(Figure 9)
Minimum AC Thickness = 2.0 inches
(Table 8)
Minimum Base Course Thickness = 4.0 inches
(Table 8)
Design Base Course Thickness: tunreinforced = 9.0 in.
treinforced = 4.5 in.
Example A.2.3:
Description: Determine the reinforced design of a flexible pavement for an area located on a
sandy subgrade in Saudi Arabia. Estimates of the potential traffic for a 5-year design include
approximately 75,000 passes of heavily loaded tandem-axle trucks, approximately 4 percent of
the total expected traffic. A site investigation revealed that the design subgrade strength is
4 CBR.
Solution: The design subgrade CBR is 4. 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 not warranted since the subgrade is a sand material unless prior
experience has indicated separation problems. The design vehicle is identified as 75,000 passes
of a tandem-axle truck, which composes approximately 4 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 estimated as less than 100 based upon the total
vehicle volume of 1,875,000 vehicles over a 5-year design life. Thus, the design road is a
Class E road according to TM 5-822-2. Since the traffic includes 4 percent trucks with at least
3 axles, the traffic category is Category IV according to TM 5-822-5, Chapter 3. For a Class E
road with a Traffic Category of IV, the required design index (DI) is 4 according to Table 7. The
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