TM 5-818-8/AFJMAN 32-l030
CHAPTER 1
INTRODUCTION
1 - 1 . Purpose
as shown in fig 1-1). Woven construction produces
geotextiles with high strengths and moduli in the
This manual describes various geotextiles, test
warp and fill directions and low elongations at
methods for evaluating their properties, and rec-
rupture. The modulus varies depending on the rate
ommended design and installation procedures.
and the direction in which the geotextile is loaded.
1-2. Scope
When woven geotextiles are pulled on a bias, the
modulus decreases, although the ultimate break-
This manual covers physical properties, functions,
ing strength may increase. The construction can
design methods, design details and construction
be varied so that the finished geotextile has equal
procedures for geotextiles as used in pavements,
or different strengths in the warp and fill direc-
railroad beds, retaining wall earth embankment,
tions. Woven construction produces geotextiles
rip-rap, concrete revetment, and drain construc-
with a simple pore structure and narrow range of
tion. Geotextile functions described include pave-
pore sizes or openings between fibers. Woven
ments, filtration and drainage, reinforced embank-
geotextiles are commonly plain woven, but are
ments, railroads, erosion and sediment control,
sometimes made by twill weave or leno weave (a
and earth retaining walls. This manual does not
very open type of weave). Woven geotextiles can be
cover the use of other geosynthetics such as geo-
composed of monofilaments (fig l-2) or multifila-
grids, geonets, geomembranes, plastic strip drains,
ment yarns (fig 1-3). Multifilament woven con-
composite products and products made from natu-
struction produces the highest strength and modu-
ral cellulose fibers.
lus of all the constructions but are also the highest
cost. A monofilament variant is the slit-film or
1-3. References
ribbon filament woven geotextile (fig l-4). The
Appendix A contains a list of references used in
fibers are thin and flat and made by cutting sheets
this manual.
of plastic into narrow strips. This type of woven
1-4. Geotextile Types and Construction
geotextile is relatively inexpensive and is used for
separation, i.e., the prevention of intermixing of
two materials such as aggregate and fine-grained
propylene, polyester, polyethylene, polyamide
soil.
(nylon), polyvinylidene chloride, and fiberglass.
Polypropylene and polyester are the most used.
(2) Manufacturers literature and textbooks
S e w i n g thread for geotextiles is made from
should be consulted for greater description of
Kevlar L or any of the above polymers. The physi-
woven and knitted geotextile manufacturing pro-
cal properties of these materials can be varied by
cesses which continue to be expanded.
the use of additives in the composition and by
(3) Nonwoven geotextiles are formed by a
changing the processing methods used to form the
process other than weaving or knitting, and they
molten material into filaments. Yarns are formed
are generally thicker than woven products. These
from fibers which have been bundled and twisted
geotextiles may be made either from continuous
together, a process also referred to as spinning.
filaments or from staple fibers. The fibers are
(This reference is different from the term spinning
generally oriented randomly within the plane of
as used to denote the process of extruding fila-
the geotextile but can be given preferential orien-
ments from a molten material.) Yarns may be
tation. In the spunbonding process, filaments are
composed of very long fibers (filaments) or rela-
extruded, and laid directly on a moving belt to
tively short pieces cut from filaments (staple
form the mat, which is then bonded by one of the
fibers).
processes described below.
b. Geotextile Manufacture.
(1) In woven construction, the warp yarns,
punching involves pushing many barbed needles
which run parallel with the length of the geotex-
through one or several layers of a fiber mat
tile panel (machine direction), are interlaced with
normal to the plane of the geotextile. The process
yarns called fill or filling yarns, which run perpen-
causes the fibers to be mechanically entangled (fig
dicular to the length of the panel (cross direction
l-5). The resulting geotextile has the appearance
of a felt mat.
1
Kevlar is a registered trademark of Du Pont for their aramid
fiber.
1-1
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