Q: I have a question about the Filter Criteria in the Federal Highway Administration (FHWA) procedure “Filter Performance and Design for Highway Drains” procedure, specifically the geotextile filtration clogging resistance equation 2-10. This seems to require a particularly large pore size; for example, if the D15 of the soil was for 70 microns, the minimum O95 of the fabric would be more than 200 microns, something we rarely see in a nonwoven geotextile. Wouldn’t the soil filter that forms upstream of the fabric in steady state flow resist clogging even with a smaller pore size?
A: Thank you for your GMA Techline question. As you know, filter design is a balancing act between piping and clogging. The FHWA is now incorporating geotextile filtration clogging resistance in designs and dealing with any fines that may flush through the system downstream. I have referenced a PowerPoint presentation from Petrasic (2018) that ushered in this design mentality. There is much discussion about active and passive fines management systems in the United States now. The formation of an upstream soil filter cake in advance of the geotextile is critical for success of such designs. Many are suggesting that the geotextile merely acts as a catalyst to initiate filter cake formation.
Petrasic, K. (2018). “Geotextile separation to prevent migration of subgrade soil into unbound aggregate subbase.” Proc., 30th ASCE Central PA Technical Conf., Hershey, Pa.