Geotextile filtration and drainage

Two requirements that pull against each other

A filter has to do two things at once, and they are in tension.

It has to retain the soil, which means its openings must be small relative to the particles it is holding back. And it has to keep passing water, which means its openings must not be so small that they blind over.

Get the first wrong and soil washes into the drain. Get the second wrong and the drain fills with the soil that was supposed to stay put. Both failures look the same from the surface, which is to say they do not look like anything at all.

Where the numbers come from

The equivalent opening size is the property that governs retention. Across our range, both fabrics are quoted at an O90 to O95 of 0.07 to 0.2 mm.

Vertical permeability governs how freely water crosses the fabric, and it is quoted as K x (10-1 to 10-3) cm/s, where K is between 1.0 and 9.9. Both fabrics carry the same figures.

Note that the opening size does not change with mass per unit area. The filter behavior is fixed across the range, so the mass can be chosen on strength rather than on filtration.

The part that is easy to miss

A geotextile does not only pass water through its thickness. It also conducts water in its own plane, which is what discharges excess liquid and gas out of the soil structure rather than letting it collect behind the fabric.

On a drainage trench or behind a retaining structure, that in-plane flow is often what the design depends on. A fabric specified only for its opening size, with no thought to how the water leaves once it has crossed, is half a specification.

What the fabric is not

A geotextile is permeable by definition, so it is not a barrier. Where the job needs to hold water rather than filter it, the layer is a geomembrane.

The two are frequently used together, and the arrangement is a good one: the membrane does the containing, and the fabric protects the membrane from the ground it sits on. Specify them as a pair and the roles stay clear.