14 Feb 03
1.1 Geogrids in Roads and Airfields
Geogrids used within a pavement system perform two of the primary functions of
geosynthetics: separation and reinforcement. Due to the large aperture size associated with most
commercial geogrid products, geogrids are typically not used for achieving separation of
dissimilar materials. The ability of a geogrid to separate two materials is a function of the
gradations of the two materials and is generally outside the specifications for typical pavement
materials. However, geogrids can theoretically provide some measure of separation, albeit
limited. For this reason, separation is a secondary function of geogrids used in pavements. The
primary function of geogrids used in pavements is reinforcement, in which the geogrid
mechanically improves the engineering properties of the pavement system. The reinforcement
mechanisms associated with geogrids will be discussed in Section 1.2.
The three primary uses of a geogrid in a pavement system are to (a) serve as a construction
aid over soft subgrades, (b) improve or extend the pavement's projected service life, and
(c) reduce the structural cross section for a given service life. Geogrids have been successfully
used to provide a construction platform over soft subgrades (Cancelli et al. 1996, Douglas 1997,
Haas et al. 1988, Halliday and Potter 1984, and Santoni et al. 2001). In this application, the
geogrid improves the ability to obtain compaction in overlying aggregates, while reducing the
amount of material required to be removed and replaced. Numerous research programs have
also reported results documenting extended service lives for pavement sections with geogrids
compared to similar sections without geogrids (Al-Qadi et al. 1997, Barksdale et al. 1989,
Cancelli et al. 1996, Collin et al. 1996, Haas et al. 1988, Miura et al. 1990, Perkins et al. 1997a/b,
and Webster 1993). Finally, research has shown that the required base course thickness for a
given design may be reduced when a geogrid is included in the design (same references).
Relative agreement exists that substantial benefits can be achieved from the inclusion of geogrids
within pavement systems; however, the quantity of the improvement is in relative disagreement.
For this reason, this document will be based primarily on the results of ERDC research
supplemented with industry results as appropriate.
Geogrids have traditionally been used in three different pavement applications:
(a) mechanical subgrade stabilization, (b) aggregate base reinforcement, and (c) asphalt concrete
(AC) overlay reinforcement. The contents of this document will focus upon the first two of these
applications. The latter application has received mixed results in historical research programs.
The ERDC has not produced definitive results concerning the inclusion of polymer geogrids as
asphalt concrete (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 have demonstrated success in
reinforcing AC overlays, primarily in regards to reflective crack retardation. Mechanical
subgrade stabilization and aggregate base reinforcement will be discussed in detail throughout
the remainder of this document.
For mechanical subgrade stabilization and base reinforcement applications the geogrid
should be placed at the bottom of the base for aggregate layers less than 14 in. If a geotextile is
to be used for separation of the subgrade and base materials, the geotextile should be placed
directly on top of the subgrade. The reinforcement geogrid is then placed directly on top of the