14 Feb 03
The following examples for the design of reinforced aggregate-surfaced and flexible
pavements are included to clarify the design process and demonstrate the simplicity of the
procedures documented previously. A detailed example of each design is provided with
additional abbreviated examples.
A.1 Aggregate-Surfaced Road Design Examples
Description: Determine the reinforced design of an unpaved road for an area located in the
floodplain of the Sava River. Estimates of the potential traffic include approximately
2,000 passes of heavily loaded tandem-axle trucks, weighing approximately 55 kips each.
Approximately 37.5 kips of the gross vehicle weight is supported by the tandem axles. 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 1 CBR.
Solution: The design subgrade CBR is 1. A geosynthetic applicability assessment based upon
the design subgrade CBR indicates that both a geotextile (for separation) and a geogrid are
recommended for use. To determine the subgrade shear strength, C, the design subgrade CBR is
input in the nomograph provided in Figure 4. Entering Figure 4 with a 1 CBR and drawing a
horizontal line to the intersection of the shear strength scale produces a design subgrade shear
strength, C, of 4.8 psi. The next step is to determine the design traffic. The design vehicle is
identified as 2,000 passes of a tandem-axle truck with a tandem-axle gear weight of 37.5 kips.
Then, determine the appropriate bearing capacity factors. An unreinforced pavement design
should always be performed for comparison to the reinforced cross section. The unreinforced
bearing capacity factor, Nc, is 2.8 according to the text and Table 5. The reinforced bearing
capacity factor (Nc) for use with both a geotextile and a geogrid is 5.8 according to Table 5. The
effective subgrade bearing capacity, CNc, is calculated by multiplying the design subgrade shear
strength, C, by the appropriate bearing capacity factor, Nc. Thus, the unreinforced subgrade
bearing capacity is 13.4 psi, and the reinforced subgrade bearing capacity is 27.8 psi. Finally,
the required aggregate thickness should be determined using the appropriate aggregate-surfaced
road design curve. Figure 7 for tandem-axle gear loads should be used to determine the design
aggregate thickness. Using Figure 7, a vertical line is drawn from the from the subgrade bearing
capacity on the x-axis to the intersection of the appropriate design curve, in this case the
37,500-lb curve. A horizontal line is then projected from the point of intersection to the y-axis to
determine the required aggregate thickness. The required aggregate thicknesses for the
unreinforced and reinforced designs are 24 and 14 in., respectively. Since the design curves are
based upon 1,000 passes of the design vehicle and this design requires 2,000 passes, the required
aggregate thicknesses must be increased by 10 percent. Multiplying 24 in. and 14 in. by a factor
of 1.1 results in final unreinforced and reinforced design thicknesses of 26 and 15 in.,
respectively. The net reduction in aggregate thickness requirements based upon the inclusion of
the geotextile separator and geogrid reinforcement is 11 in. of aggregate, a 42 percent reduction
in required aggregate thickness. A life-cycle cost analysis should be performed to ensure a cost-
effective design. Sample specifications for the geotextile and geogrid are provided in Tables 2
and 3, respectively.