ETL 1110-1-189
14 Feb 03
Table 6
Recommendations for Geosynthetic Use in Flexible Pavements
Step 1: Determine Design Subgrade Soil Strength and Geosynthetic Applicability
CBR < 0.5
0.5 < CBR < 4.0
4.0 < CBR < 8.0
CBR > 8.0
The design subgrade
Use a geotextile and A geotextile is
A geotextile is
strength exceeds the
recommended for fine- generally NOT
a geogrid at the
existing database. Do
grained subgrades with recommended
subgrade-base
not use this design
a design subgrade
unless prior
interface. No
procedure. Consider
CBR < 4. Geogrid
experience has
aggregate thickness
using a Base Course
noted separation
reduction is
reinforcement should
problems. Design an Reduction (BCR)
recommended. Use be evaluated. Design
factor for reinforced
unreinforced
the aggregate-
an unreinforced
(determined from Figure 9) and the minimum AC thickness (determined from Table 8) is the
design aggregate thickness of the flexible pavement.
The reinforced aggregate thickness is determined by using the Webster's reinforced
pavement thickness equivalency chart shown in Figure 8. Entering the chart with the
unreinforced flexible pavement thickness, a line is drawn to the intersection of the equivalency
curve. A second line is drawn from the intersection with the equivalency curve to the required
geogrid-reinforced flexible pavement thickness. The reinforced aggregate thickness is deter-
mined by subtracting the minimum AC thickness in Table 8 from the equivalent reinforced
flexible pavement thickness. Finally, the reinforced aggregate layer thickness replaces the
unreinforced aggregate thickness in the pavement design. All other thicknesses remain the same.
Note that Figure 8 is only valid for AC layer thickness values of 3 in. or less. The geogrid is
placed at the subgrade-base interface for base thicknesses less than 14 in. and in the middle of
the base layer for aggregate thicknesses greater than 14 in. Reinforced flexible pavement design
examples are provided in Appendix A. Please note that standard frost design procedures should
be used for regions in which significant frost penetration occurs. Additional resources should
beconsulted for designing pavements to minimize damage when exposed to adverse climatic
conditions, such as frost heave.
4.0 Asphalt Concrete (AC) Overlay Reinforcement
The use of polymeric geogrids for reinforcing AC overlays has received mixed results in
historical research programs. The ERDC has not produced definitive results concerning
theinclusion of polymer geogrids as AC overlay reinforcement. Other referenced literature
includes Brown et al. (1984), Haas (1984), Kennepohl et al. (1984), and Chang et al. (1999).
Thus, the use of geogrids for reinforcing asphalt concrete overlays is not recommended at this
time. However, geotextiles (TM 5-818-8), geosynthetic composites, and fiberglass grids
havedemonstrated success in reinforcing AC overlays, primarily in regards to reflective crack
retardation.
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