GEOSYNTHETICS: Geogrids for Foundations Engineering.

Figure 17.1 shows a photograph of a geogrid, which contains relatively high-strength polymer grids consisting of longitudinal and transverse ribs connected at their intersections. Geogrids have a large and open structure and the openings (i.e., apertures) are usually 0.5 to 4 in. (1.3 to 10 cm) in length and/or width. Geogrids can be either biaxial or uniaxial depending on the size of the apertures and shape of the interconnecting ribs. Geogrids are principally used as follows:

1. Soil reinforcement. Used for subgrade stabilization, slope reinforcement, erosion control (reinforcement), and mechanically stabilized earth-retaining walls. Also used to strengthen the junction between the top of soft clays and overlying embankments.

2. Asphalt overlays. Used in asphalt overlays to reduce reflective cracking.
The most common usage of geogrids is as soil reinforcement. Compacted soil tends to be strong in compression but weak in tension. The geogrid is just the opposite, strong in tension but weak in compression. Thus, layers of compacted soil and geogrid tend to compliment each other and produce a soil mass having both high compressive and tensile strength. The open structure of the geogrid (see Fig. 17.1) allows the compacted soil to bond in the open geogrid spaces. Geogrids provide soil reinforcement by transferring local tensile stresses in the soil to the geogrid. Because geogrids are continuous, they also tend to transfer and redistribute stresses away from areas of high stress concentrations (such as beneath a wheel load). "Figure 11.10 Two views of the installation of the geogrid for a mechanically stabilized earthretaining wall. The large arrows indicate the width of the mechanically stabilized zone and thesmall arrow points to a geogrid splice."  shows geogrids being used as soil reinforcement for a mechanically stabilized earth retaining wall.

Photograph of a geogrid.
FIGURE 17.1 Photograph of a geogrid. (From Rollings and Rollings
1996; reprinted with permission of McGraw-Hill, Inc.)
Similar to other geosynthetics, geogrids are transported to the site in 3 ft (0.9 m) to 12 ft (3.7 m) wide rolls. It is generally not feasible to connect the ends of the geogrid, and it is typically over- lapped at joints. Typical design methods for using geogrids are summarized by Koerner (1998).

Some of the limitations of geogrids are as follows:

1. Ultraviolet light. Even geogrids produced of carbon black (i.e., ultraviolet stabilized geogrids) can degrade when exposed to long-term ultraviolet light. It is important to protect the geogrid from sunlight and cover the geogrid with fill as soon as possible.

2. Nonuniform tensile strength. Geogrids often have different tensile strengths in different directions as a result of the manufacturing process. For example, a Tensar SS-2 (BX1200) biaxial geogrid has an ultimate tensile strength of 2100 lb/ft in the main direction and only 1170 lb/ft in the minor (perpendicular) direction. It is essential that the engineer always check the manufacturer’s specification and determine the tensile strength in the main and minor directions.

3. Creep. Polymer material can be susceptible to creep. Thus, it is important to use an allowable tensile strength that does allow for creep of the geosynthetic. Tensile strengths are often determined by using ASTM test procedures, such as ASTM D 6637-01 (“Standard Test Method for Determining Tensile Properties of Geogrids by the Single or Multi-Rib Tensile Method,” 2004) and ASTM D 5262-04 (“Standard Test Method for Evaluating the Unconfined Tension Creep Behavior of Geosynthetics,” 2004).

Many manufacturers will provide their recommended long-term design tensile strength for a specific type of geogrid. This recommended long-term design tensile strength from the manufacturer is usually much less than the ultimate strength of the geogrid. For example, for a Tensar SS-2 (BX1200) biaxial geogrid, the manufacturer’s recommended long-term design tensile strength is about 300 lb/ft, which is only one-seventh the ultimate tensile strength (2100 lb/ft). The engineer should never apply an arbitrary factor of safety to the ultimate tensile strength, but rather obtain the recommended long-term design tensile strength from the manufacturer.

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