Civil and Environmental Applications of Geosynthetics - Reinforced Soil Engineering: Advances in Research and Practice - page 3

impregnated, they are used as containment. However, some newly developed
products perform better than geotextiles in certain functions. For example,
geogrids are developed specifically to tensile reinforce soil, while geonets are
used to convey large-capacity flow. Although geotextile may also be made
impermeable and used as containment by spraying bitumen or other polymers on
it, geomembranes should be considered for a watertight containment system. The
functions of geotextiles, geogrids, and geonets are described collectively in this
section, where one material can be referred to the other.
Geotextile sheets are manufactured from fibers or yarns. Polymers are
melted and forced through a spinneret to form fibers and yarns. They are
subsequently hardened and stretched. The manufacturing process produces
woven or nonwoven geotextiles. In producing woven fabrics, conventional
textile-weaving methodologies are used. For the nonwoven fabrics, the filaments
are bonded together by thermal, chemical, or mechanical means (i.e., heating,
using resin, or needle-punching).
Geogrids are mainly used as tensile reinforcement. Although biaxial
geogrids are available, most geogrids are manufactured to function uniaxially. In
manufacturing uniaxial geogrids, circular holes are punched on the polymer
sheet, which is subsequently drawn to improve the mechanical properties. For
biaxial geogrids, square holes are made on the polymer sheet, which is then
drawn longitudinally and transversely. For some geogrids, the junctions between
the longitudinal and transverse ribs are bonded by heating or knit-stitching.
Geogrid manufactured from yarns are typically coated with a polymer, latex, or
bitumen. Geogrids have higher stiffness and strength than most geotextiles.
The chapter now describes some major applications of geotextiles and
related products.
3.1 Reinforcement of Steep Slopes, Retaining Structures,
and Embankments
Geotextiles and geogrids are used to tensile reinforce steep slopes, retaining
structures, and embankments constructed over soft foundation (Fig. 1) . Sheets of
geotextile/geogrid are embedded horizontally in these soil structures. The shear
stress developed in the soil mass is transferred to the geotextile sheets as tensile
force through friction. The tensile strength of geotextile/geogrid and its frictional
resistance with the soil are the primary items required for design.
The tensile strength of geosynthetic is obtained from the wide-width test.
The ASTM standard specifies an aspect ratio (width-to-length) of 2 (i.e., 20 cm to
10 cm). Soil confinement may increase the stiffness and strength of nonwoven
spun-bonded needle-punched geotextile because of the interactions among the
fibers, but it has negligible effect on the heat-bonded nonwoven geotextiles and
woven geotextiles. Reduction factors (also known as partial factors of safety) are