Building Design and Construction Handbook - Merritt - Ventech!

6.116 SECTION SIX
and redistribute stresses away from areas of high stress concentrations (such as
beneath a wheel load).
Some of the limitations of geogrid 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. Non-uniform 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 specifications
and determine the tensile strengths 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.
Oftentimes, this allowable tensile design strength is much less than the ultimate
strength of the geogrid. For example, for a Tensar SS-2 (BX1200) biaxial geogrid,
the manufacturer’s recommended 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 allowable geogrid tensile design strength from the manufacturer.
6.11.2 Geotextiles
Geotextiles are the most widely used type of geosynthetic. Geotextiles are often
referred to as fabric. For example, common construction terminology for geotextiles
includes geofabric, filter fabric, construction fabric, synthetic fabric, and
road-reinforcing fabric. As shown in Figs. 6.49 and 6.50, geotextiles are usually
categorized as either woven or nonwoven, depending on the type of manufacturing
process. Geotextiles are used for many different purposes, as follows:
1. Soil Reinforcement. Used for subgrade stabilization, slope reinforcement, and
mechanically stabilized earth retaining walls. Also used to strengthen the junction
between the top of soft clays and overlying embankments.
2. Sediment Control. Used as silt fences to trap sediment on-site.
3. Erosion Control. Installed along channels, under riprap, and used for shore and
beach protection.
4. Asphalt Overlay. Used in asphalt overlays to reduce reflective cracking.
5. Separation. Used between two dissimilar materials, such as an open graded
base and a clay subgrade, in order to prevent contamination.
6. Filtration and Drainage. Used in place of a graded filter where the flow of
water occurs across (perpendicular to) the plane of the geotextile. For drainage
applications, the water flows within the geotextile.
Probably the most common usage of geotextiles is for filtration (flow of water
through the geotextile). For filtration, the geotextile should be at least 10 times
more permeable than the soil. In addition, the geotextile must always be placed