ETL 1110-1-189
14 Feb 03
Summary:
Design Subgrade CBR = 3
(Based on measured values.)
Applicability: Geotextile Recommended
(Equation 1)
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 16-kip tandem-axle gear
(Use Figure 7)
Unreinforced Nc = 2.8 GT-GG Reinforced Nc = 5.8
(Table 5)
Initial: tunreinforced = 8 in.
treinforced = 4 in. (Figure 7)
Minimum Thickness Adjustment:
(Section 2.2.4)
Design: tunreinforced = 8 in.
treinforced = 6 in.
Geotextile Only Design Alternative: Nc = 5.0
(Table 5)
Initial: tunreinforced = 8 in.
treinforced = 4.5 in. (Figure 7)
Minimum Thickness Adjustment:
(Section 2.2.4)
Design: tunreinforced = 8 in.
treinforced = 6 in.
Select Geotextile Only Alternative Based Upon Costs
Example A.1.3:
Description: Determine the reinforced design of an unpaved road for an area located in the
Saudi Arabian desert. Estimates of the potential traffic include approximately 1,000 passes of an
M1000 HET. A site investigation revealed that 75 percent of the soil strengths in the upper
18 in. of the sand subgrade were greater than a 3 CBR.
Solution: The design subgrade CBR is 3. A geosynthetic applicability assessment based upon
the design subgrade CBR indicates that a geotextile is not required for separation with a sand
subgrade unless prior experience has indicated separation problems. A geogrid may be cost-
effective and should be considered for use. Entering Figure 4 with a 3 CBR and drawing a
horizontal line to the intersection of the shear strength scale produces a design subgrade shear
strength, C, of 10.1 psi. The design vehicle is identified as 1,000 passes of an M1000 HET.
Table 4 indicates that the typical tandem-axle gear weight for the design vehicle is 37 kips. 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 a geogrid is 5.8 according to Table 5. The effective
subgrade bearing capacity, CNc, of the unreinforced subgrade is 28.3 psi, and the reinforced
subgrade is 58.6 psi. Using Figure 7, the required aggregate thicknesses for the unreinforced and
reinforced designs are 14 and 6 in., respectively. As noted in Section 2.2.2, the design aggregate
thickness should be increased by 10 percent to account for the abrasive action of the HET's
multiple heavy wheel loads. This adjustment results in adjusted design thicknesses for the
unreinforced and reinforced pavements of 15 and 7 in. Thus, the final design unreinforced and
reinforced aggregate thicknesses are 15 and 7 in., respectively. The net reduction in aggregate
thickness requirements based upon the inclusion of the geogrid reinforcement is 8 in. of
aggregate, a 53 percent reduction in required aggregate thickness. A life-cycle cost analysis
should be performed 1to ensure a cost-effective design. Sample specifications for the geotextile
and geogrid are provided in Tables 2 and 3, respectively.
Summary:
Design Subgrade CBR = 3
(Based on measured
values.)
A-3
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