Specifying Permittivity in Geotextiles

There is much confusion surrounding permittivity and permeability relating to geotextiles. As a specifier, the most important point is to understand is that permeability as a geotextile property is not supported by the geosynthetic industry. The following will hopefully clarify the differences and underscore why specifying permeability is very problematic.

Permittivity [ASTM D 4491]
first-image-sm_360.jpgPermittivity is the mechanism by which water moves through the fabric. ASTM defines it as “the volumetric flow rate of water per unit cross sectional area per unit head under laminar flow conditions, in the normal direction through a geotextile” (Illustration B) The permittivity test measures the quantity of water which can pass through a geotextile perpendicular to the surface of the geotextile. The permittivity may be measured either in a constant head or falling head test, although constant head testing is more common due to the high flow rates through geotextiles which makes it difficult to obtain readings of head change versus time in the falling head test.

In the constant head test, a head of 50 mm water is maintained on the geotextile throughout the test. The quantity of flow is measured versus time.

In the falling head test, a column of water is allowed to flow through the geotextile and a reading of head change versus time is taken. The flow rate of water through the geotextile needs to be slow enough to obtain accurate readings.

Permeability [ASTM D 4491]

ASTM defines permeability as “the rate of flow of a liquid under differential pressure through a material.” (Illustration B) Geotextile permeability is derived from pemittivity using the nominal thickness of the geotextile. (Illustration A) ASTM notes that “nominal thickness is used as it is difficult to evaluate the pressure on the geotextile during the test, thereby making it difficult to determine the thickness of the fabric under these test conditions.” Keep in mind that nonwoven thickness will decrease under load. It is also important to note that nominal thickness is just that: “existing in name only.” In addition, the geotextile thickness value is only relevant at the time of manufacture. Packaging of the product and method of shipment can negatively impact the geotextile’s thickness. All of these factors make permeability an unreliable property for geotextiles.

History

Permeability soil coefficients are well established and used in various calculations such as structural coefficient for subgrades. The thought was defining a permeability for geotextiles would allow one to compare the geotextile’s permeability to the soil’s permeability. But adding a geotextile’s thickness to the equation does not make the geotextile a “soil” or create a test value to compare to soil just because both now have cm/sec as their units.

At least, it would seem to offer an index test to compare one geotextile to another. However, because geotextiles vary in thickness using permeability nullifies a designer’s ability to compare them, since the permeability value is related to geotextile thickness rather than geotextile cross-plane flow.

Summary

Permittivity is the volumetric flow through a cross section of material. Permeability is the advancement of that water in conjunction with thickness. Geotextile thicknesses vary and are easily impacted by packaging, shipping and load. Furthermore, permeability relies on the nearly meaningless “nominal” thickness value. As such, permeability is an unreliable index test that offers little understanding in how a geotextile will function in situ or how one geotextile will perform compared to another.

Permittivity [ASTM D 4491] is the test method preferred by the geosynthetic industry.

ILLUSTRATION A

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ILLUSTRATION B

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