ISO 11058 test specifies two test methods for the water permeability characteristics of a single layer of geotextile or geotextile-related product normal to the plane: the constant head method; and the falling head method.

ISO 11058:2019
Geotextiles and geotextile-related products — Determination of water permeability characteristics normal to the plane, without load. 

Water Flow through a Geotextile: Measuring Perpendicular Water Flow and Permittivity

This test standard specification discusses the methods for measuring water flow through geotextiles in the laboratory. This is an important characteristic, as it may be a crucial factor in determining the suitability of geotextiles for filtration, such as water flow into drainage layers. The main testing methods are 'vertical water flow' and 'permeability', which are similar but not identical. Broader information regarding the behavior of filter geotextiles is included in the technical specification titled 'Geotextile Filtration'. The vertical water flow measurement method, which measures vertical water flow (i.e., water flow perpendicular to the plane), is defined by EN ISO 11058, which is almost identical to ASTM D4491. 

The difference is that vertical water flow is defined as the flow rate (in liters per second) divided by the area of the geotextile being tested, but not divided by the head of water. EN ISO 11058 also uses a water head of 50mm for testing, and the results are expressed in velocity.

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