Geotechnical News • June 2015
37
GEOSYNTHETICS
extent of penetration that is achieved
(BS EN ISO 13433).
Soil-geotextile compatibility
The use of a geotextile in filtration
applications is predicated on it having
adequate strength to ensure no adverse
damage throughout the process of
installation (termed ‘construction sur-
vivability’) and that it can also endure,
thereafter, the working environment
of the application (termed ‘durabil-
ity’). The greatest physical demand
on the geotextile is typically encoun-
tered during the installation process.
The design approach, for construc-
tion survivability, requires that the
geotextile meet or exceed a required
strength category. A qualitative
category (for example a low, moder-
ate or high strength requirement) is
typically based on the type of field
application and anticipated severity
of loading imposed during placement
and subsequent construction. The
qualitative category is then expressed
in quantitative form, with reference to
a designated range of strength values
determined from laboratory index
testing .
Thereafter, the geotextile must also
be sufficiently durable to ensure it can
sustain its intended function over the
intended service life of the structure.
Durability is evaluated from a con-
sideration of any likely change in the
integrity of the geotextile over time, as
a consequence of in-service condi-
tions that would cause an unacceptable
degradation of its material properties.
The general intent is to avoid any
degradation that might compromise
the ability of the geotextile to act as a
filter. Durability varies with the type
of polymer and any additives during
the manufacturing process, and is
governed by physical, chemical and
biological influences (see for example,
Calhoun, 1972; Koerner et al., 1988;
Elias et al., 1999; Elias, 2001; Kay
et al., 2004). Accordingly, durability
must be evaluated on both a product-
specific and a site-specific basis.
Upon selecting a suitably strong and
durable geotextile, the requirement
for soil-geotextile filtration compat-
ibility is contingent on there being no
unacceptable erosion as a consequence
of soil loss through the geotextile
while, at the same time, providing for
unimpeded flow of water from the soil
through the geotextile. Therefore, the
principal requirements for compat-
ibility are those of (i) soil retention
and (ii) cross-plane permeability.
They represent competing interests,
insomuch as soil retention is assured
by smaller pore size openings in the
geotextile while, in contrast, a greater
cross-plane permeability is associated
with relatively larger pore size open-
ings.
Looking ahead to the next
article…
In summary, it is widely-accepted
practice to select a candidate geotex-
tile for routine construction works
with reference to (i) criteria for
strength and durability, given the
anticipated method of construction
service environment, (ii) an empirical
rule governing base soil retention, and
(iii) an empirical rule governing base
soil permeability. The approach has
been found conservative, and yields a
geotextile filter for which the margin
of safety is believed acceptable. How-
ever, the exact nature of that margin of
safety is not quantified. Accordingly,
there is need for a more comprehen-
sive means of evaluating soil-geo-
textile compatibility in applications
that are critical or severe, where filter
incompatibility is deemed problematic
within the context of either the ulti-
mate limit state (collapse) or the ser-
viceability limit state (deformation).
In principle, this would include filter
applications where the consequence
of failure is believed to be relatively
high, else the cost of remedial works
is anticipated to be significant. In such
projects, the state-of-practice is first
to identify a candidate geotextile on
the basis of the reported values for
its strength, opening size and permit-
tivity from index testing, and then to
evaluate its suitability for the proposed
construction application from labora-
tory compatibility testing of a sample
of the base soil in combination with
that candidate geotextile filter. My
next article in this series will address
compatibility testing.
References
ASTM D4491. Standard Test Meth-
ods for Water permeability of
geotextiles by permittivity.
ASTM
International
, USA.
ASTM D4595.Standard Test Method
for Tensile properties of geo-
textiles by the wide-width strip
method.
ASTM International
, USA
ASTM 4632. Standard Test Method
for Grab breaking load and elonga-
tion of geotextiles,
ASTM Interna-
tional
, USA
ASTM D4751. Standard Test Method
for Determining Apparent Open-
ing Size of a geotextile.
ASTM
International
, USA.
ASTM D5101. Standard Test Method
for Measuring the soil-geotextile
clogging potential by Gradient
Ratio.
ASTM International
, USA.
ASTM D5493. Standard Test Method
for Permittivity of geotextiles
under load.
ASTM International
,
USA.
ASTM D5567. Standard Test Method
for Hydraulic Conductivity Ratio
(HCR) Testing of Soil/Geotextile
Systems.
ASTM International
,
USA.
ASTM D6241. Standard Test Method
for the Static puncture strength of
geotextiles and geotextile-related
products using a 50-mm Probe.
ASTM International
, USA.
Bhatia, S.K. and Smith, J.L. (1996),
Geotextile characterization and
pore size distribution: Part I. A
review of manufacturing pro-
cesses.
Geosynthetics Interna-
tional,
3:85-105.
