It takes an “interesting” article (“Using structured geomembranes in final solid-waste landfill closure designs,”) to generate a dozen pained e-mail comments to me even before I received my February/March copy of Geosynthetics! This topic had its beginnings in a paper I had rejected for the Geosynthetics-2007 conference in D.C. this past January. As those who know my work, it covers a product that my own specifications preclude.
So what happened?
Before addressing this particular article, let me comment on what I feel is a major problem in final cover design that is seemingly a secret known only by a few. The design of a final cover system that incorporates a geomembrane for the now common 3H:1V side slopes is very difficult and the final product has a very high probability of failure. Bob Koerner has indicated to me that final-cover side slopes are veneer failing at a rate more than 30 times that of liner systems. Based on my personal litigation experience and discussions with others, I feel that this is a realistic perspective.
More importantly, many failures are being hidden from the profession and many states and engineers are simply delaying doing closures on such slopes. These final cover side-slope veneer failures relate to landfill gas (LFG) pressures building up beneath the geomembrane or surface water seepage/infiltration exceeding the capacity of a drainage composite placed over the geomembrane. So essentially, I feel we all need to admit that the materials we have for such closures are marginal, and either [we need to] improve the materials or modify our closure designs to reflect what we now know can happen to such geosynthetic systems.
Now back to the article in question. To reduce the size of this response, let me first note that I am in full agreement with Rick Thiel’s comments (above). This paper was beefed up with some data and the “anti-Velcro®” comments reduced. The revised draft without pictures came across my desk as I headed out to South Dakota to kill God’s creatures. Haste does make waste.
However, buried within this paper is an indication that it is possible to integrate a drainage system into the structure of the geomembrane with adequate transmissivity to eliminate the failure above the geomembrane. Additionally, texturing as shown on Figure 6 may provide a significant increase in interface shear strength such that larger LFG pressures can be tolerated during that period that active systems are shut down for servicing.
Unfortunately, none of the products shown in the paper have the needed balance of properties. The high transmissivities in Table 3 were for 145-mil “Drain Stud” sheeting that is not discussed in Table 2 where interface shear strength is highlighted.
The attached photo shows a geomembrane/drainage net composite being developed that has similar transmissivities to those referenced in the article. Note the structure is significantly different to those shown in the article and provides “adhesion” due to geotextile intrusion.
Additionally, Table 2 seemingly implies that the micro-spike structure shown on Figure 2 can develop significant adhesion under the low normal loads associated with closures in marked contrast to the discussion giving the merits of a “Velcro®”-free surface. My experience is that “Velcro®” is critical to the performance of final cover system side slopes and that embossed textured sheets do not provide this adhesion. My hope was that a product such as the more aggressive “Drain Stud” sheet not pictured in the article would.
As a simple rule, if you can take a piece of nonwoven fabric and essentially “polish” the surface of the textured geomembrane by hand, then it lacks the level of adhesion required for slope applications—pretty common sense among experienced designers!
My thanks to Rick for his clarity, as usual, and I hope that this discussion will continue and lead either to a new generation of geosynthetics or an alternative approach to landfill side-slope closure design.
Letter submitted by: Greg Richardson