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Commentary RE #1: article on structured geomembranes

April 1st, 2007 / By: / From Our Readers

To the editor:

The February/March 2007 issue of Geosynthetics contains an article on the use of structured geomembranes in landfill final closure designs. The article contains many positive statements regarding polyethylene geomembranes with structured surfaces that are manufactured by the flat-die method. The article discusses many fine products, as well as attributes of those products that are useful for many projects.

However, the article presented negative overtones relative to coextruded geomembranes in a manner that is unbalanced in perspective, in my judgment.

I have designed, specified, tested, tendered bids, and performed CQA on millions of square feet of each the various materials described in the article on numerous projects for the past 20 years. The designs and testing have required the gamut of properties to be evaluated including thickness, asperity height, interface shear strength, seam strengths, transmissivity (where applicable), and numerous other properties and tests. In addition, the field constructability of the various materials is a significant part of that experience.

Relative to the subject of this article, a summary statement of this experience would read something such as: “There is a place for all materials, the owner deserves the benefit of a good bid, and the designer needs to understand the limitations of all materials and design/specify accordingly.” This general statement may end up precluding some materials and favor others in a bid situation.

The article begins with discussions of slope failures on final covers—most notably at the geomembrane/geotextile interface. Two of the more-recent failures of this type in the past few years of which I am aware involved flat-die molded textured surfaces. One of these failures occurred, to my understanding, because the geomembrane did not, in fact, have enough of the “Velcro® effect” during construction, and after several months of arguing, the contractor finally agreed to remove all of the molded-textured geomembrane and replace it with a coextruded geomembrane.

This was one of those rare situations that allowed the industry to discover GCL shrinkage panel-after-panel, but that is another discussion! The other failure was along the same interface (textured geomembrane over GCL on a final cover), but occurred many months after the final cover had been finalized and had a nice stand of grass beginning to grow. Obviously that was a much less desirable timing for a failure.

Are these examples meant to imply that coextruded textures are superior, in direct opposition to the article? By no means. Failures have occurred with all types of materials. These examples have simply been tugged out to balance the article.

In my opinion, one of the more blatant innuendos presented in the article occurs under the section titled “Potential for mechanical properties reduction.” The second paragraph mentions that the environmental stress cracking resistance of coextruded sheet is reduced because of the non-uniform surface. This is true.

The next sentence, however, implies that this is not true for flat-die structured surfaces. Has the author any such data? Looking at the potential stress concentrations caused by spikes the size of those on golf shoes, or 18+”-high perpendicular nubs, I would be highly suspicious of statements that imply the stress crack resistance of these surfaces are “close to that of smooth sheet,” as implied in the article.

The last paragraph in the section titled “Interaction at the shear surface” states that “embossed surface textures exhibit higher interface shear strength and lower post-peak strength loss at lower normal stresses commonly found in landfill closure designs.” As a general statement this is absolutely not true. In a specific case it may very well be true.

The embossed surfaces portrayed in the article’s Figures 2 and 6 will have very different relationships to interface shear strength relative to coextruded textured surfaces such as shown in Figure 5. One of them may be relatively lower than a typical coextruded textured surface at low normal loads, and one will be higher. Why was this not pointed out in the article?

In the same paragraphs the point is made twice that coextruded textured geomembranes exhibit larger post-peak strength loss against geotextiles than embossed geomembranes, and references some articles to support this. This statement must be taken in the context of the actual tests being performed.

Figure 1R shows the shear strength data presented by Richardson and Thiel (2001)—which was referenced in the article—for peak and large-displacement conditions for an embossed geomembrane/geotextile interface. New data has been added to show results from a coextruded geomembrane/geotextile interface. What do we notice about this particular data? (1) The results for this particular coextruded geomembrane interface (with asperity heights > 15 mils) are higher for both peak and large-displacement conditions compared to this particular embossed geomembrane over the normal load range of concern. (2) The coextruded geomembrane interface indicates the potential for a “Velcro® effect,” that is, shear strength at zero normal load, which is discussed next.

The article takes a negative light on the “Velcro® effect” requiring the use of slip sheets during construction. At the same time, it selectively avoided using statements from the Richardson and Thiel (2001) reference that was previously quoted that states: “… this adhesion is essential if the geomembrane is being placed on a slope or to minimize the potential for slippage of dozer tracks during placement of cover soils…” Perhaps this statement is not true of the “nubbed” molded surface shown in Figure 3, but it certainly may be true for the material shown in Figure 2.

In the end, it is the design engineer’s responsibility to know the material that is specified and in this regard take responsibility for constructability using that material.

Table 1 in the article is a very formatory manner of presenting ideas that in fact have a lot of relativity. Reducing engineering attributes to simple “Yes/No” columns is a posturing technique that leaves out the substantial experience and relatively reliable properties that can be achieved using coextruded materials. It is true that there is a level of variability in coextruded products that is not present in embossed products.

And yet very good, reliable designs and installations on hundreds of projects over hundreds of millions of square feet of material have been installed with these products because we have the tools to reliably perform QC and QA on these projects. This applies to thickness, texturing, asperity heights, and shear testing. Yes, it does require vigilance when it is critical. And contrary to what the table says, asperity heights in excess of 15 mils are definitely and commonly available on coextruded geomembranes if they are required.

Regarding the difficulties in deploying textured geomembrane, why did not the author provide a balanced discussion regarding the difficulties of welding the butt seams for the spiked and nubbed products shown in Figures 3 and 6? Severe grinding is necessary that requires great care, skill, and extra inspection. Of course it can be done with confidence, but it is certainly not to be taken for granted and it deserves special attention (just as some attention is warranted when welding across heavily textured coextruded geomembranes).

Finally, I heartily agree with the article’s point that the standards GRI GM-13 and GM-17 should seriously consider increasing the asperity height requirement for textured sheet from 10 to 15 mils, and that such a low number has also been to the detriment of the long-term good of the industry.

It is the design engineer’s responsibility to make sure that when referencing GM-13 or 17, it is pointed out that a higher asperity height is required (i.e. > 15 or 20 mils) if it is needed. Designers expecting textured geomembranes should not blindly call out GM-13 or 17 without a qualifier requiring higher asperities than are mentioned in those standards.

Letter submitted by: Richard Thiel
Thiel Engineering
Oregon House, Calif.

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