ETL 1110-1-189
14 Feb 03
separation geotextile for aggregate layers less than 14 in. For pavements with a design base
thickness greater than or equal to 14 in., the geogrid should be placed in the middle of the base
course layer (Webster 1993). However, Collin et al. (1996) and Haas et al. (1988) recommend
that the geogrid be placed in the middle of the base course layer for layers in excess of 10 in.,
otherwise placement should occur at the layer interface. Regardless of the placement location of
the geogrid, the separation geotextile is always placed at the subgrade-base interface.
1.2 Reinforcement Mechanisms
Three fundamental reinforcement mechanisms have been identified involving the use of
geogrids to reinforce pavement materials: (a) lateral restraint, (b) improved bearing capacity,
and (c) tensioned membrane effect (Perkins and Ishmeik 1997a). Lateral restraint refers to the
confinement of the aggregate material during loading, which restricts lateral flow of the material
from beneath the load. Since most aggregates used in pavement systems are stress-dependent
materials, improved lateral confinement results in an increase in the modulus of the base course
material. The effect of increasing the modulus of the base course is an improved vertical stress
distribution applied to the subgrade and a corresponding reduction in the vertical strain on the
top of the subgrade. Figure 1 illustrates the lateral restraint reinforcement mechanism. The
second mechanism, improved bearing capacity, is achieved by shifting the failure envelope of
the pavement system from the relatively weak subgrade to the relatively strong base course
material. Figure 2 shows the improved bearing capacity concept. The third fundamental
reinforcement mechanism has been termed the "tensioned membrane effect." The tensioned
membrane effect is based upon the concept of an improved vertical stress distribution resulting
from tensile stress in a deformed membrane. Figure 3 illustrates the tensioned membrane effect.
In the early stages of research regarding geogrid reinforcement of pavement systems, the
tensioned membrane effect was thought to be the primary reinforcement mechanism. However,
subsequent investigations have shown that reinforcement benefits are obtained without
significant deformation of the pavement section. Thus, lateral restraint has been identified as the
primary reinforcement mechanism, followed by the improved bearing capacity concept and the
tensioned membrane effect. The actual contribution of each of these mechanisms to the overall
reinforcement provided to the pavement system has yet to be quantified.
1.3 Material Properties
Many attempts have been made to link geogrid reinforcement of pavement systems to the
geometric and engineering properties of the geogrid. Unfortunately, researchers have
experienced little success in correlating geogrid reinforcement with individual material
properties (Perkins and Ishmeik 1997a). Table 1 lists many of the engineering properties
commonly reported and used in specifications for geogrid products. Tables 2 and 3 list typical
specifications for separation geotextiles and reinforcement geogrids used in road construction.
2.0 Aggregate-Surfaced Reinforced Pavement Design
Geogrids in aggregate-surfaced roads can be used to support two pavement applications:
mechanical subgrade stabilization and aggregate base reinforcement. The application is
predetermined by the subgrade soil strength, and the type of geosynthetics recommended for use
construct pavements over very soft subgrade conditions typically serve to mechanically stabilize
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