Technical Notes


Thrace-LINQ TECH NOTE #6
PERMEABILITY? PERMITTIVITY?
WHAT SHOULD BE SPECIFIED?


The state of geosynthetic specification has come a long way from gleaning test methods from ASTM's long standing textile section. There are specific geotextile test methods developed by ASTM Committee D-35, and most of these test methods are index test methods. Index test methods allow the designer and user to compare the properties of geotextiles, tested under the same, controlled, highly repeatable conditions, so that a comparison can be made between products. This paper will focus on test method ASTM D-4491, Test Method for Water Permeability of Geotextiles by Permittivity, and will discuss the difference between permeability and permittivity. This paper will show why permittivity should be specified as the correct index test value for cross plane flow rate.

A specification should do two things. It should define a project's needs by listing the physical properties required. This assures that the product approved and used will serve the intended function for proper performance both during construction and over the design life of the project. It should also define the product using standard test methods that will enable the specifier to fairly evaluate and compare the different products submitted for approval (apples to apples).

On any project requiring permeable separation, a geotextile's ability to pass water through its plane is an important consideration. The ideal first choice in determining a geotextile's ability to achieve this function with a designer's site specific soil is to use a performance test method such as a long term flow test. A standard test method has not been developed to date. The long term flow test measures the geotextile/soil's filtration mechanism's performance. A second approach is to use ASTM D-5101, Measuring the Soil-Geotextile System Clogging Potential by the Gradient Ratio. Both can provide the designer with valuable information that can be used to evaluate various geotextile products' performance with on-site soils. Unfortunately, these two test methods are not always practical. A long term flow test method takes a long time to get results, and both approaches are expensive since multiple products must be tested. Also, the results of these test methods do not provide data for specification or certification.

The index test method, ASTM D-4491, measures a product's ability to pass water normal to the geotextile. While this index test method provides relevant cross plane flow data for specification/certification purposes, it has been, on occasion, misused by modifying the resulting permittivity value, in units of sec-1, to obtain permeability in cm/sec. This is done so that one can compare the geotextile's permeability to a soil's permeability, but this destroys the credibility of the "index" value.

Refer to the following table:

THE EFFECT OF THICKNESS
  Product A Product B

FLUX (gpm/ft2)

100 100

PERMITTIVITY (sec-1)

1.3 1.3
THICKNESS (mils) 20 100 20 100

PERMEABILITY (cm/sec)

0.66 x 10-1 3.30 x 10-1

Geotextiles vary in thickness. As the table shows, introducing thickness into the equation nullifies a designer's ability to compare geotextiles because the resultant permeability value obtained is related to geotextile thickness, rather than the geotextile's cross-plane flow. Not only do geotextile thicknesses vary, but certain geotextile thicknesses (needlepunched nonwoven geotextiles) decrease under load. Permittivity is the correct index test method because it allows a designer a direct comparison of flux between materials. This "apples to apples" comparison directly relates to the ability of the geotextile to pass water. Dave Suits, Chairman of ASTM D-35 committee, wrote in ASTM Standardization News, 1986:
"The permittivity rather than Darcy's coefficient of permeability, was chosen for geotextiles for two reasons. First, in determining the coefficient of permeability, it is necessary to know the thickness of the specimen being tested. This would be difficult to determine under the conditions of the test. Secondly, permittivity is an indicator of the flow of the geotextile. While it is possible for two geotextiles to have the same coefficient of permeability, they may have different flow capacities."
Mr. Suits' statement, "permittivity is an indicator of the flow of the geotextile," defines exactly what a designer requires for his specification. Permittivity gives the designer a reliable index value. Adding a geotextile's thickness to the equation does not make the geotextile a "soil" or give one a design test value to compare to soil just because both now have cm/sec as their units. Multiplying thickness times a geotextile's permittivity value only makes a good index test method into a bad index method dependent on an irrelevant thickness factor.

Permeability values are appropriate for soil comparisons. Since a geotextile is not a soil, permeability should not be used to compare geotextiles. Permittivity is the correct index test method for geotextile comparison. Contact your Thrace-LINQ distributor for appropriate permittivity levels for all of your applications.