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Geotechnical News •June 2015
GEOSYNTHETICS
Hydraulic conductivity of the
geotextile
Methods used to determine the per-
mittivity (cross-plane permeability)
of a geotextile involve variations on
laboratory permeameter testing, the
two most common of which are a
‘constant-head’ method and a ‘falling-
head’ method (see for example, ASTM
D4491 and ISO 11058). The geotextile
specimen is not subject to any normal
load in these index tests, and therefore
is tested in an uncompressed state.
Where appropriate, provision exists
to perform the test under a specified
compressive stress (see for example,
ASTM D5493 and ISO 10776).
Experience suggests the compressive
stress yields no significant change to
the permittivity of a woven geotextile
or a nonwoven heat-bonded geotextile,
but causes a reduction in permittivity
in nonwoven needle-punched geotex-
tiles. Using image analysis techniques,
Palmeira and Gardoni (2000) attri-
bute the reduction in flow capacity to
an increase in contacts between the
needle-punched fibres, and therefore a
greater constriction of pore channels
across the plane of the geotextile.
Multiplying the permittivity by the
nominal thickness of the geotextile
yields a nominal value of permeability
or hydraulic conductivity. However,
given the range in thickness of differ-
ent types of geotextile, it is generally
recognized that reporting a value of
permittivity avoids the potential for
any misleading comparison of perme-
ability between different products. As
for granular filters, the permittivity
values of geotextiles vary over several
orders of magnitude.
Geotextile tensile strength
Woven and nonwoven geotextiles
exhibit a very different characteristic
response to loading which, again, is a
direct consequence of the manufactur-
ing process. Woven geotextiles exhibit
a significantly greater stiffness upon
loading, a response that arises from
the preferential alignment of polymer
strands during the weaving process. In
contrast, nonwoven geotextiles have a
random layout of polymer strands that
must progressively deform in order
to align themselves in the direction
of imposed loading. Methods used to
determine the strength of a geotex-
tile involve loading it to failure as a
result of tensile rupture of the polymer
strands (see Fig. 3):
the two most com-
mon methods are
uniaxial-tension testing of a rectangu-
lar specimen (using either a full ‘wide-
width’ clamp, else a partial ‘grab’
clamp) and axisymmetric-tension
testing of a circular specimen (using a
rod to puncture).
In the ‘wide-width’ style of test, a rect-
angular specimen is clamped across
its entire width and the load-extension
response then measured over a speci-
fied gauge length, for loading imposed
at a constant rate of axial displace-
ment (see for example, ASTM D4595
and ISO10319). In a variation to this
method, only the central portion of a
rectangular specimen is clamped over
a specified gauge length in the ‘grab’
style of test (see for example, ASTM
4632). Static puncture resistance is
measured by advancing a probe of
specified diameter into a specimen that
is clamped between circular rings, at a
constant rate of displacement, in order
to determine the maximum resistance
(see for example, ASTM D6241 and
ISO 12236). In a variation to this
concept, a cone is dropped through a
specified distance onto a circular test
specimen, in order to measure the
Figure 2. Standard test methods for pore size opening.
Figure 3. Standard test methods for strength.
