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Still further comment about the impact of water

February 1st, 2009 / By: / From Our Readers

I read with interest the exchange between Gordon Stevens with Maccaferri and Michael Adams with FHWA in your August/September “Letters to the editor” section about the use of woven geotextiles for soil reinforcement in MSE structures.

While the geosynthetics industry has a great deal of respect for Mr. Adams, what we have here is an interesting difference in perspective between Maccaferri, with thousands of geosynthetic soil reinforcement projects worldwide in their base of experiences, and a public agency that is just starting to use geosynthetics in critical structures.

In my opinion, Mr. Stevens could not be more correct in his warnings about the lack of filtration design regarding long-term permeability of tightly woven geotextiles used for soil reinforcement. The simple truth is that virtually none of the thousands of MSE structures designed with reinforcement geotextiles each year worldwide are designed with any consideration about the long-term permeability of the geotextile. And very few designs call for the use of completely free-draining backfill in the reinforced zone.

We know that, long-term, water is usually going to flow vertically through these woven geotextiles and potentially carry fine soil particles that could clog the openings. We also know from many hundreds of general failures of SRWs that the entrapment of water inside a reinforced-soil zone is a major contributor to the instability of the entire structure.

The analytical methodology exists for such a long-term permeability analysis using the pore size and permittivity of the fabric combined with the soil gradation, hydraulic gradient, and other factors. In fact, these designs are usually performed when geotextiles are used as filters in other critical civil engineering projects. Why is this analysis not part of every MSE design, especially in a really serious application such as reinforcing the soil under a bridge?

In an industry as conservative as ours, with so many intelligent, dedicated people involved in the writing of testing standards and codes, I feel that this is a glaring omission and should be addressed as an industry.

Finally, regarding the comment by Mr. Adams about the cost of geogrids being so much higher than geotextiles, I suspect that he has not recently checked geogrid prices. The gap between the prices of geogrids and high-strength geotextiles is now quite small, actually insignificant when compared to the total cost of the project, and long-term permeability will never be an engineering concern with geogrids.

Chip Fuller, President, Strata Systems Inc.

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