From the GMA Techline
RE: More silt fence
Also, I have another question regarding geo properties and their specifications.
Permittivity is an indicator of how well water can flow through the fabric. AOS is an indicator of the soil particle size held back by the fabric. The permittivity specification is a minimum value while AOS is a maximum value, so the fabric design should prevent soil particles from washing out while still allowing a minimum flow of water to pass through. Is my understanding correct?
(Barry | Georgia)
Reply: You have the concept perfectly. Now while these are seemingly conflicting criteria, it works out really neat. The coarse-grained soils with high flow rates need open fabrics, while the fine-grained soils with low flow rates need tighter fabrics.
P.S. Incidentally, in Designing With Geosynthetics (6th edition), I give numerous examples illustrating this exact issue.