Short for apparent opening size, also written O95. Glass beads of known sizes are dry-sieved through the geotextile, and AOS is the bead size of which 95 percent or more is retained. It is reported as a US sieve number, where a larger number means a smaller opening, or in millimeters, so No. 70 equals 0.212 mm. Filter design compares it with the grain size of the soil to be retained.
AOS is No. 70, so it holds this silty sand, but check permittivity before you approve it.
A bigger sieve number means smaller openings, which trips up nearly every newcomer reading a geotextile data sheet.
Also heard as
- apparent opening size
- O95
- EOS
Do not mix it up with
- Permittivity: AOS describes how big the openings are; permittivity describes how fast water actually flows across the fabric.
Related terms
Filtration
Letting water pass across a geotextile while holding back the soil particles it carries.
Permittivity
Rate of water flow across a geotextile per unit head, the basic measure of its cross-plane flow capacity.
Clogging
Gradual loss of flow through a geotextile as soil particles or deposits lodge inside its pores.
Monofilament woven
Woven geotextile of round single-strand yarns that leaves regular open pores for high flow and good filtration.