ETL 1110-1-189
14 Feb 03
3.0 Reinforced Flexible Pavement Design
Geogrids can be used to accomplish both mechanical subgrade stabilization and aggregate
base reinforcement in flexible pavements. Like the aggregate-surfaced pavement design, the
application is typically predetermined by the subgrade soil strength. Different combinations of
geosynthetics are recommended for use in flexible pavements based upon the subgrade soil
conditions. Geosynthetics used to construct roads and airfields over very soft subgrade
conditions typically serve to mechanically stabilize the subgrade. As the design subgrade
strength increases, the primary application of the geosynthetics transitions from mechanical
subgrade stabilization to base reinforcement. The reinforced road design procedures presented
herein can be used for airfields provided the appropriate design procedures for airfields are used
to generate the unreinforced design.
3.1 Geosynthetic Applicability Assessment For Flexible Pavement Design
3.1.1 Subgrade CBR < 0.5. For design subgrade CBR strengths of 0.5 or less, the primary
application is mechanical subgrade stabilization. At these soil strengths, it is recommended that
a construction platform be designed to facilitate the construction of the flexible pavement. The
construction platform should be designed using the procedures described in Section 2.0. The
construction platform will serve as the subbase for the flexible pavement system.
3.1.2 Subgrade 0.5 < CBR < 4.0. For design subgrade CBR strengths of 4.0 or less, both the
mechanical subgrade stabilization and base reinforcement applications are mobilized. A
nonwoven geotextile is recommended for separation for fine-grained subgrades at strengths of
4.0 CBR or less, and the use of a geogrid for reinforcement should be considered. Thus, for this
subgrade strength level both a geotextile and geogrid may be warranted, and the aggregate
thickness can be reduced using Webster's empirical reinforced pavement thickness equivalency
chart (Figure 8) presented in the following design procedure section.
3.1.3 Subgrade 4.0 < CBR < 8.0. For subgrade CBR strengths greater than 4.0, a geotextile
separator is not recommended unless the designer has experienced separation problems with the
construction materials during previous construction projects. For design subgrade CBR strengths
between 4.0 and 8.0, the primary geogrid application is base reinforcement. Research has
indicated substantial extensions in pavement service life and significant potential for base
thickness reductions. Thus, the designer should use Webster's reinforced pavement thickness
equivalency chart (Figure 8) to determine the required reinforced pavement thickness. A life
cycle cost analysis should then be made to determine the cost effectiveness of including geogrid
reinforcement.
3.1.4 Subgrade CBR > 8.0. A geotextile separator is not recommended unless prior separation
problems have been noted for the specific construction materials. The primary application of
geogrid reinforcement at high subgrade soil strengths is base reinforcement. These subgrade soil
strengths are outside the database used to develop Webster's reinforced pavement thickness
equivalency chart, and it should not be used for high subgrade soil strengths. An alternative
procedure is to base the reinforced design on test section results. This can be accomplished by
defining a Base Course Reduction (BCR) factor as the reinforced base thickness divided by the
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