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Geotechnical News •June 2015
GEOSYNTHETICS
Jonathan Fannin, Editor
Professor of Civil Engineering, University of British Columbia
You may recall that, in the
GN:December 2014 issue, I sought to
compare the origins of current practice
for the specification of a geotextile
filter with those for the specification of
a granular filter. In contrast to a granu-
lar filter, the opening size distribution
of a geotextile is controlled directly
through the process of manufactur-
ing. Accordingly, the properties of
a geotextile filter are specified with
reference to a characteristic open-
ing size of its fabric, with additional
consideration given to the polymer
type and also to the strength of the
fabric. In the subsequent GN:March
2015 issue, written in collabora-
tion with my good friend and fellow
geosynthetics (and rugby!) enthusiast
Kelvin Legge, Chief Engineer with the
Department of Water and Sanitation in
South Africa, we then reviewed select
regulatory guidance for granular and
geotextile filters, placing emphasis
on applications in embankment dam
engineering. More specifically, we
addressed matters pertaining to base
soil-filter layer compatibility because
Kelvin, through his involvement with
the South African National Committee
on Large Dams (SANCOLD), is seek-
ing to update the now wholly-outdated
1985 ICOLD Bulletin 55 on “Geotex-
tile Filters in Dams”. In this current
article, I have chosen to remain with
the theme of filtration applications,
and a return to geotextile ‘basics’.
Geotextile material properties
Geotextiles are generally made from
one of three polymer types: polypro-
pylene (PP), polyester (PET), and
polyethylene (PE), with the first two
polymers accounting for the major-
ity of geotextile manufacture. The
manufacturing process yields three
principal styles of geotextile: woven,
nonwoven and knitted fabric, with
the first two styles accounting for
the majority of production and use in
filtration applications. The manufac-
ture and use of woven geotextiles in
filtration applications pre-dates that of
nonwoven geotextiles. The two styles
of geotextile are inherently different.
A woven geotextile is made from
individual polymer strands that are
aligned and interwoven on an indus-
trial loom, yielding a planar fabric.
The strand itself is usually a tape, a
monofilament, or a multifilament yarn.
A fibrillated strand is one that has been
intentionally split along portions of its
length, as a part of the manufacturing
process, to condition its properties.
In contrast, a nonwoven geotextile
comprises a layer of many randomly
oriented polymer strands that are
bonded to obtain a planar fabric. The
individual strands are usually a short
fibre or a continuous filament. The
common methods of bonding are
either physical entanglement of the
strands, yielding a needle-punched
nonwoven geotextile, or thermal
fusing of contact points between the
strands during a calendaring operation,
which produces a heat-bonded nonwo-
ven geotextile.
Inherent differences between each
of these manufacturing processes
impart differences to the opening
size distribution of the fabric and,
by association, differences to the
capacity for flow of water across the
plane of the fabric. Likewise, there
is a difference in tensile strength and
stiffness that results from the manu-
facturing process. Accordingly, in
Figure 1. Standard test methods (adapted from Fannin,2001).
Jonathan Fannin
