ETL 1110-1-189
14 Feb 03
thickness design should be used for subgrade strengths of 0.5 CBR or less. The nonwoven
geotextile is placed directly on the subgrade followed by the geogrid and then the aggregate fill.
The designed construction platform serves as a bridge over very soft material, a compaction aid
for obtaining target densities, and a construction expedient.
2.1.2 Subgrade 0.5 < CBR < 2.0. For design subgrade CBR strengths of 2.0 or less, both the
mechanical subgrade stabilization and base reinforcement applications are mobilized. A
nonwoven geotextile is recommended for separation at subgrade strengths of 2.0 CBR or less.
The use of a biaxial geogrid for reinforcement is also generally cost-effective at subgrade CBR
values of 2.0 or less, in terms of aggregate savings. Thus, for this subgrade strength level both a
geotextile and geogrid are generally recommended, and the aggregate thickness can be reduced
using the appropriate reinforced bearing capacity factor presented in the following design
procedure section.
2.1.3 Subgrade 2.0 < CBR < 4.0. The use of a nonwoven geotextile for separation is generally
recommended for fine-grained subgrades with design subgrade CBR values less than or equal to
4. A nonwoven geotextile should also be used for separation when the designer has experienced
separation problems with the construction materials during previous construction projects. For
design subgrade CBR strengths between 2.0 and 4.0, the primary geogrid application is base
reinforcement. However, the cost effectiveness of using a geogrid at these subgrade strengths
should be determined by performing a life-cycle cost analysis. Research has indicated
substantial extensions in pavement service life and significant potential for base thickness
reductions for AC-surfaced pavements at these subgrade strengths. However, insufficient data is
available for aggregate-surfaced reinforced roads at these subgrade strengths to accurately define
an appropriate bearing capacity factor. Thus, it is recommended that the designer use the bearing
capacity factor for the inclusion of both a geotextile and a geogrid provided in the following
design procedure section. This recommendation is based on the assumption that the geotextile
serves to separate the different pavement materials and provides little reinforcement benefit.
2.1.4 Subgrade CBR > 4.0. Research has indicated significant reinforcement potential at these
subgrade strength values, however ERDC has yet to quantify the benefits conclusively. The
primary geogrid application at these subgrade strength values is base reinforcement. Until
additional research is accomplished, a life-cycle cost analysis should be conducted to determine
the economical feasibility of geogrid reinforcement for projects in which the design subgrade
CBR strength is greater than 4.0 using the design procedures described in this document. Future
research programs may develop conclusive results for designing geogrid-reinforced aggregate-
surfaced pavements at these subgrade strength values. Geogrids can be used as a construction
expedient to solve site-specific construction problems, such as site mobility and localized soft
soil deposits.
2.2 Reinforced Aggregate-Surfaced Pavement Design Procedure
2.2.1 Determine the Subgrade Shear Strength (C). The first step in designing a reinforced
aggregate-surfaced pavement is to determine the design subgrade conditions. The design
subgrade conditions can be determined using field CBR tests, vane shear tests, DCP tests, or
from laboratory soaked CBR tests as noted under Section 2.0. Once the design subgrade
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