To the editor:
Regarding your article “Springtime Failure of a Cold-Climate Geotextile Riprap Slope” in the June/July issue of Geosynthetics magazine, I believe woven slit film (WSF) fabrics are inappropriate as “filter fabric” for subsurface drains or beneath hard armor in erosion control applications. WSF have insufficient porosity, percent open area and permeability to relieve buildup of pore pressure within the soil particles adjacent to the fabric. It does not take much porosity to relieve pore pressure in a fine-grained soil, and a WSF (2%–4% open area) may have adequate porosity for a uniform clay or silt. But sands need two to three times more fabric porosity.
Glacial soils are gap graded. They are an unpredictable mix of fine-grained soil and coarse stone that is prone to internal soil piping when water seeps through the saturated soil structure. No matter what filter fabric is used, if it will retain the fine particle, a filter cake will form, and that filter cake will restrict flow. That restricted flow causes the buildup in pore pressure.
A woven monofilament (WM) is most appropriate for filtration in glacial soils, because they have more porosity than a WSF. WM fabrics were the first ever evaluated as a filter fabric under riprap on sandy soils (Calhoun 1972). They have adequate percent open area (i.e., 8%–12%) to relieve pore pressures and fine enough pore size (EOS [equivalent opening size], aka AOS [apparent opening size]) to retain sand particles.
Less than 4% porosity is not enough once a filter cake forms on the fabric’s surface. And stay away from nonwoven filter fabrics. They’re great for uniform soils, but if soil piping occurs, beware of the buildup of pore pressure.
There is no foolproof solution, but an engineer has to consider the options and select the most capable. Do you agree that a WM like the one tested by Calhoun 1972 is most appropriate for glacial soils?
R. G. (Bob) Carroll Jr., P.E., CEO, Geosyn LLC, Fayetteville, Ga.
Response from the author:
Yes, we agree with Bob Carroll’s comments. The best way to avoid clogging would have been to allow the fine particles to pass through the geotextile and not have the silt particles trapped by using a woven slit film fabric, which tends to be tightly woven fabric with much less porosity. As pointed out in Holtz, Christopher and Berg (1998), tightly woven geotextiles are not recommended for subsurface drainage applications. The issue with the slope erosion application discussed in the article was not only with gap-graded soils but with water flow reversal between seasons. As we suggested, the geotextile might have performed adequately with water flow in one direction, but once it reversed, the geotextile clogged, generating sufficient water pressure to cause failure. Although this application was not a critical situation, it could be considered severe having gap-graded soils and water flow reversal. In this case, filtration tests would have been advised but would undoubtedly not have been cost-effective. Thus, using a high-porosity woven monofilament geotextile, as Bob Carroll recommends (Calhoun 1972), would have been advised.
Amaneh E. Kenarsari, Ph.D., Geotechnical Staff Project Manager, ECS Midwest, Chicago, Ill.
Stanley J. Vitton, Ph.D., P.E., Professor, Department of Civil and Environmental Engineering, Michigan Technological University, Houghton
Calhoun, C.C. (1972). “Development of design criteria and acceptance specifications for plastic filter cloth.” Technical Report S-72-7. U.S. Army Waterways Experiment Station, Vicksburg, Miss., 6–55.
Holtz, R.D., Christopher, B.R., and Berg, R.R. (1998). “Geosynthethic design and construction guidelines,” rev. ed. FHWA-HI-95-038. Federal Highway Administration, Washington, D.C.