ETL 1110-1-189
14 Feb 03
Applicability: Geotextile Not Required
(Section 2.1 and Table
5)
Geogrid Should Be Considered
(Section 2.1 and Table
5)
3 CBR Subgrade = 10.1 psi Shear Strength, C
(Figure 4)
Traffic = 1,000 passes 37-kip tandem-axle gear
(Use Figure 7)
Unreinforced Nc = 2.8 Reinforced Nc = 5.8
(Table 5)
Initial: tunreinforced = 14 in.
treinforced = 6 in. (Figure 7)
HET Abrasion Thickness Adjustment:
(Section 2.2.2)
Adjusted Design: tunreinforced = 15 in. treinforced = 7 in.
Minimum Thickness Adjustment:
(Section 2.2.4)
Design: tunreinforced = 15 in.
treinforced = 7 in.
A.2 Flexible Pavement Design Examples
Example A.2.1:
Description: Determine the reinforced design of a flexible pavement for an area located in the
floodplain of the Sava River. Estimates of the potential traffic include approximately
20,000 passes of heavily loaded tandem-axle trucks, approximately 40 percent of the total
expected traffic. A site investigation revealed that 75 percent of the soil strengths in the upper
18 in. of the fine-grained subgrade were greater than a 6 CBR.
Solution: The design subgrade CBR is 6. 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 next step is to determine the
design traffic. The design vehicle is identified as 20,000 passes of a tandem-axle truck, which
composes approximately 40 percent of the estimated traffic. The design of a conventional
flexible pavement requires that several assumptions be made to determine the Design Index (DI).
First, 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 20,000 trucks and 50,000 total vehicles. Thus, the design road is a
Class E road according to TM 5-822-2. Since the traffic includes 40 percent trucks with at least
3 axles, the traffic category is Category IVA according to TM 5-822-5, Chapter 3. For a Class E
road with a Traffic Category of IVA, the required design index (DI) is 5 according to Table 7.
The required flexible pavement thickness is then determined using Figure 9, the design subgrade
CBR, and the design index. Enter Figure 9 on the x-axis with the design subgrade CBR (6) and
draw a vertical line to the intersection of the appropriate design index curve, 5 in this case. A
horizontal line is then projected from the intersection with the design index curve to the y-axis to
determine the total required pavement thickness. The total required pavement thickness for this
example is 16 in. 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
A-4
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