ETL 1110-1-189

14 Feb 03

conditions have been determined, an assessment of the applicability of geosynthetics should be

conducted using the guidance presented in Section 2.1 and the design subgrade soil strength. If

the use of a geotextile and/or geogrid is warranted based upon the applicability assessment, the

following procedure can be used to design the aggregate-surfaced road, otherwise the procedures

described in Technical Manual TM 5-822-12 should be used to design an unreinforced aggregate

road. If the use of a geotextile and/or geogrid is warranted, the subgrade soil strength must be

converted from CBR to shear strength (C) if not previously accomplished. The shear strength

(C) can be determined using Figure 4 based upon either cone index or CBR. The shear strength

(C) can also be directly measured using vane shear devices.

traffic gear should be based upon the gear configuration of the heaviest vehicle expected in the

traffic mix, defined as either a single-wheel load, a dual-wheel load, or tandem-wheel gear load.

The combined weight on the selected gear is used as the design vehicle weight. For example,

use one-half of the single- or dual-wheel axle load for single-axle vehicles. For multiple-axle

vehicles, use one-half of the total load on the heaviest two neighboring axles. Table 4 provides

typical traffic loading values for Army vehicles. The design aggregate thickness presented in

this procedure is based upon the development of a 2-in. rut after 1,000 passes of an 18-kip

equivalent axle load. The aggregate thickness should be increased by 10 percent for 2,000-pass

designs and 20 percent for 5,000-pass designs. An additional 10 percent increase in the design

aggregate thickness should be added for HET traffic to account for the abrasive action of mul-

tiple heavy wheel loads.

2.2.3 Determine the Reinforced Bearing Capacity Factor (Nc)*. *Both the unreinforced and

reinforced bearing capacity factors were determined using empirical data from full-scale ERDC

test sections. The unreinforced bearing capacity factor (Nc) is 2.8. The reinforced bearing

capacity factor for a geotextile alone is 5.0 based on TM 5-818-8. However, recent research has

shown that this factor should be reduced to approximately 3.6 for conservative designs. The

bearing capacity factor, Nc, for the use of a geotextile separator and geogrid reinforcement is 5.8.

Insufficient data exist to determine a value of Nc for geogrid reinforcement alone. In the absence

of sufficient data, an Nc of 5.8 is recommended based upon engineering judgment from

observations of geotextile and geogrid reinforced pavement sections. This assumes that the

geotextile serves as a separation fabric with little reinforcement benefit. Bearing capacity factor

recommendations are summarized in Table 5.

2.2.4 Determine the Required Aggregate Thickness. Finally, the required aggregate thickness is

determined using Figures 5 through 7 for single-wheel, dual-wheel, and tandem-wheel gear

loads, respectively. The subgrade bearing capacity (CNc) is determined by multiplying the

subgrade shear strength (C) in psi by the appropriate bearing capacity factor (Nc). The

appropriate design curve, Figures 5 through 7, is entered with the computed subgrade bearing

capacity (CNc) value on the x-axis. A vertical line is drawn from the subgrade bearing capacity

to the appropriate gear weight design curve. A horizontal line is projected from that point of

intersection to the required aggregate thickness in inches on the y-axis. The required aggregate

thickness for aggregate-surfaced pavements should be rounded up to the next higher inch. The

required aggregate thickness for the unreinforced condition should always be determined using

Nc of 2.8 as a basis for comparing the relative savings of the proposed reinforced design. The

reinforced design should then be computed using the appropriate Nc depending upon the type of

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