Environmental Engineering Reference
In-Depth Information
controlled heat on one side, they become stiffer on that side while remaining compliant on
the other side.
Multilayer geotextiles
are produced by bonding together several layers of fabrics. Two tech-
niques for manufacturing multiplayer geotextiles are stitch bonding and needle-punching.
Stitch bonding consists of stitching together several layers of fabric. This is often used to pro-
duce high strength geotextile from slit film woven fabrics. The needle-punching process
(described above for non-woven geotextile manufacturing) is used, for example, to associate
woven and needle-punched non-woven fabrics.
Yarns, filaments, fibers, and tapes
are usually made from polypropylene or polyester.
Other polymers used include polyamide (commercially known as Nylon), polyethylene and
polyaramide (commercially known as Kevlar). Some natural fibers such as jute and
coconut fibers are also used.
There are also other geotextile related products such as geomats, geonets, geogrids and
geocells. These are described in Giroud and Bonaparte (1993).
When considering the use of geotextiles as filters in dam construction, the important
properties are the “particle size” that affects its ability to act as a filter and its “permeabil-
ity”, or ability to allow water to pass through. The “particle size” of the geotextile is usu-
ally measured by sieving a standard soil or single sized glass spheres on the geotextile for
a fixed period and observing the quantity and particle size of the soil which passes
through. As described in Bertacchi and Cazzuffi (1985), ICOLD (1986), Heerten (1993),
Fischer et al. (1992) and Christopher et al. (1993) there is no one accepted standard and dif-
ferent methods use dry or wet sieving and consider the geotextile size as that allowing
zero, 2% or 5% of the soil passing.
Koerner (1986) and most authors suggest the use of the apparent opening size AOS
(also known as equivalent opening size EOS). The test uses known size glass beads and
determines by sieving using successively finer beads, that size of bead for which 5% or less
pass the fabric. The AOS or EOS is the US standard sieve number of this size bead. It is
also, more commonly, quoted as the equivalent sieve opening in millimetres or 95% open-
ing size O
95
. This is the form used for most filter design criteria as discussed below. Many
manufacturers provide data on the 95% opening size in their product literature and it
appears to have become the industry standard, at least for the time being. However for
important designs, given the dependence of the D
95
(or O
90,
O
50
) on the test method, cau-
tion should be exercised in using the manufacturers' values, unless the test method is com-
patible with the design method.
Some typical values are given in Table 9.12.
In most cases, the ability of the geotextile to transmit water across the fabric is also
important.
This is assessed as the permittivity
kn
t
(9.4)
Table 9.12.
Some typical values for geotextiles.
Product name
Structure woven (W)
EOS (mm)
Permeability cm/sec
Terrafirma s/2100
W
0.06-0.07
0.03
Polyfelt TS500
NW Needle P
0.21
0.5
Polyfelt TS800
NW Needle P
0.13
0.4
Bidim U14
NW Needle P
0.1-0.12
N/A
Bidim U64
NW Needle P
0.06-0.075
Polytrac
W
0.07
