Geotextiles
Retention and permeability pull in opposite directions. A geotextile that satisfies only one of them either clogs or lets the soil through.
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Common criterion for stable soils is at or below 1
Decision-support only. Retention criteria vary with soil gradation, plasticity and whether flow is steady or cyclic, and a dispersive or gap-graded soil needs a stricter rule than the general one. Confirm against the governing design standard for the works.
Geotextile Soil Retention (AOS) & Permittivity Predictor — free, with the formula and a worked example, at Textile School.