ETL 1110-1-162
30 Sep 01
fabric's opening sizes. Soil particle size is often represented by the percent passing a specific sieve size.
For example, the term D85 refers to the sieve size that will allow 85% of a soil sample to pass. The
opening size distribution of a geotextile is often represented by the percentage of a given particle size
that will be retained by a geotextile. For example, O95 indicates that a geotextile will retain 95% of a
given particle size. Geotextile design methods also differ depending on other criteria, such as whether
the soils are cohesive or noncohesive, if the fabric is woven or nonwoven, soil gradation, etc. The
designer should select a method that is well documented, has proven to be reliable, and is appropriate
for site-specific conditions. Designers should refer to AASHTO-ABC-ARBTA Task Force Number
25 Specification for Geotextiles, which provides complete detailed design procedures regarding the
following simplified guidance: For soils with 50% or fewer particles by weight passing U.S. No. 200
sieve, apparent opening size (AOS) should be less than 0.6 mm. For soils with more than 50%
particles by weight passing U.S. No. 200 sieve, AOS should be less than 0.297 mm. AOS and O95 are
equivalent terms.
(b) Cross-plane Permeability. The design of the geotextile must allow an adequate flow of
water from the select fill through the filter to the drainage layer. Consequently, the design must ensure
that the cross-plane permeability of the geotextile is greater than the vertical permeability of the overlying
select fill by some factor of safety. Depending on the method selected, the factor of safety can range
from 1 to over 10. Because of the compressible nature of nonwoven geotextiles, the manufacturer often
specifies a permittivity value for the geotextile. Permittivity is defined as the volumetric flow rate of
water per unit area, per unit head, under laminar flow conditions.
(c) Clogging (Long-term Compatibility). In addition to meeting the soil retention and
permeability requirements, the geotextile should continue to allow sufficient cross-plane flow even if a
percentage of the pores become clogged during the life of the system. For filters used in covers,
clogging is usually a result of soil particles embedding in the open spaces of the fabric. Although
generally not a problem in landfill covers, chemical or biological clogging, or both, could also occur if the
filter is exposed to leachate or high pH liquids. There have been a number of empirical methods
developed to evaluate the clogging potential of a geotextile. If necessary, the long-term compatibility of
the geotextile can be directly determined by evaluating the select fill and geotextile filter in the laboratory
using the gradient ratio test as described in ASTM D 5101. Clogging potential can also be evaluated
using ASTM D 1987.
(d) Survivability. Survivability refers to the ability of the fabric to withstand the construction
and installation process. Items that should be considered include the type of construction equipment,
construction technique, and subgrade material. The index tests commonly referred to as the survivability
criteria for geotextiles are puncture (ASTM D 4833), grab tensile strength (ASTM D 4632), and
trapezoidal tear strength (ASTM D 4533). The American Association of State Highway and
Transportation Officials AASHTO-ABC-ARBTA Task Force No. 25 and other organizations have
published recommended minimum values for these and other tests that are intended to minimize the
potential for construction damage. It is also essential that proper construction techniques be specified
and used to ensure that the filter will not be damaged during construction.
B-10
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