Permittivity is determined by either a constant or falling head test. Constant Head Test—A head of 50 mm 2 in. The quantity of flow is measured versus time. The constant head test is used when the flow rate of water through the geotextile is so large that it is difficult to obtain readings of head change versus time in the falling head test. Falling Head Test—A column of water is allowed to flow through the geotextile and readings of head changes versus time are taken.

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These test methods cover procedures for determining the hydraulic conductivity water permeability of geotextiles in terms of permittivity under standard testing conditions, in the uncompressed state.

Included are two procedures: the constant head method and the falling head method. The values stated in SI units are to be regarded as the standard. The inch-pound units stated in parentheses are provided for information only. This standard does not purport to address all of the safety concerns, if any, associated with its use.

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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. 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.


ASTM D4491


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