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# A geotextile filter for an edge drain

Q&A: GMA Techline | August 1, 2021 | By:

Q: I am doing a geotextile filter design for a highway edge drain. All I have from the site is a grain size analysis of the sandy subgrade soil. Can you point me in the right direction?

A: First you are going to need to calculate an estimate of hydraulic conductivity for the sandy soil by the Hazen method based on the grain size distribution of the soil as illustrated in the following:

k=Cd_10^2
k = hydraulic conductivity (cm/sec)
C = factor depending on grain size and sorting (well-graded vs. uniform)
i.e., very fine soil poorly sorted (C=40–80), coarse sand, well-sorted uniform clean (C=120–150)
d_10 = the grain diameter for which 10% of distribution is finer (usually “effective grain size” between 0.05 and 0.5 cm for sands)

After that, it is rather straightforward by adhering to the five steps for filter design that follow:

1. Permeability criteria
2. Retention criteria
3. Clogging check
4. Survivability assurance
5. Long-term flow check

If you want to pursue this further, please consult the following references. They are good ones.

Holtz, R. D., Christopher, B. R., and Berg, R. R. (1997). Geosynthetic Engineering, BiTech Publishers Ltd., Richmond, B.C., Canada.

Luettich, S. M., Giroud, J. P., and Bachus, R. C. (1992). “Geotextile filter design guide.” Jour. of Geotextiles and Geomembranes, 11(4–6), 19–34.