Geosynthetics February/March 2024 - 41

material is exposed to extreme environmental
conditions in the field.
Skewing is a similar condition in which
filling yarns or cross-machine elements are
angularly displaced from a line perpendicular
to the edge or side of the material
due to uneven distribution of tension.
Figure 2a is a photograph of a biaxial
geonet where the " controlled " skew is
intentional, and Figure 2b is a schematic
diagram of the property being measured.
In the determination of skew, a full
width geosynthetic test specimen is placed
on a flat surface. One limb of a square is
aligned along the machine direction of
the center-most longitudinal rib or yarn.
The lower tip of the opposite limb of the
square is then brought into contact with a
cross-machine rib or yarn. The skew is the
measurement of length that the machine
direction rib or yarn is out of square from
the cross-machine direction rib or yarn
measured at the inside elbow of the square.
Distortion in the consistent construction
of the material may constitute a defect. It
is also interesting to note that the pattern
on one side of the material may be different
from or ahead of the pattern on the
opposite side.
Another geometric property that was
not mentioned in the new test method is
" trueness of edge. " This term is used in
relation to serpentine or snaked edges
that are usually present in some degree on
all geosynthetic rolls. Geosynthetic rolls
that " drift " can often be corrected if each
outward drift corresponds to an inward
drift on the opposite side of the material
width and if pulling one side of the geosynthetic
to a straight line straightens the
corresponding opposite side.
Trueness of edge is measured by pulling
a straight line lengthwise down the
roll from one corner to another or to a
corresponding " peak, " as shown in Figures
3a and 3b, where 3a is a photograph of a
woven and 3b is a schematic diagram of
the property being measured. The distance
of the " valley " from the straight
line should establish the amount of drift.
If the material suddenly moves inward
or outward on one side without having
a corresponding peak or valley on the
opposite side, it is unlikely that stretching
the affected area will correct the problem.
Bowing, skewing and trueness of edge
in geosynthetic rolls usually occur during
operations where there is a risk of uneven
tension across the width. Needless to
say, the lighter the material in mass per
unit area, the more likely these conditions
are present. As there are no industry
standards for such variations, individual
producers have developed their own
manufacturing tolerances of these three
conditions. Now, certified quality auditor
(CQA) personnel have a standard way of
measuring these conditions to hold manufacturers
to tolerances pre-installation.
It is freely admitted that this new standard
is a rudimentary attempt at measuring
a few geometric manufacturing
parameters. ASTM D3882, Standard Test
Method for Bow and Skew in Woven
and Knitted Fabrics, has been around
since the 1940s and we know manufacturers
have all sorts of in situ monitoring
technologies to control their processes.
However, by adhering to specifications
and using products that meet GRI GS-34,
field personnel can ensure that the geosynthetic
will be consistent. Following
such methods will help prevent common
installation errors and material issues due
to inconsistent manufacturing. G
>> For more, search GSI at
GeosyntheticsMagazine.com.
FIGURE 2A Photograph of biaxial geonet
FIGURE 2B Schematic diagram of the
measurement of " skew "
FIGURE 3A Photograph of a woven
FIGURE 3B Schematic diagram of the
measurement of " trueness of edge "
GeosyntheticsMagazine.com
41
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