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.
2.2.2 Determine Design Traffic. The next step is to determine the design traffic. The design
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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