Measured by passing water through a specimen at constant or falling head, it is reported in inverse seconds and equals the fabric's permeability divided by its thickness. Because thickness varies so much between products, permittivity compares them more fairly than permeability. Typical needlepunched nonwovens fall around 1 to 2 per second, while slit-film wovens are far lower, often near 0.05.
Permittivity on the slit film is 0.05, that's a separator, not a filter for the edge drain.
Data sheet permittivity is measured with clean water; field soils, fines and water chemistry always pull it down.
Also heard as
- cross-plane flow
Related terms
AOS
Apparent opening size: an index of the largest openings in a geotextile, given as a US sieve number or in millimeters.
Filtration
Letting water pass across a geotextile while holding back the soil particles it carries.
Transmissivity
In-plane flow capacity of a geosynthetic drainage layer per unit width, under a given gradient and normal load.
Clogging
Gradual loss of flow through a geotextile as soil particles or deposits lodge inside its pores.