From the GMA Techline
Subject: Rub sheets— What kind of developments are going on these days with rub sheets in leachate sumps with sideslope risers? I’ve seen multiple sheets of geomembrane, but I am more familiar with thick (100- and 120-mil) stock sheet material.
Also, what do you think of sandwiching a nonwoven cushion layer between a 60-mil HDPE and thick-stock HDPE rub sheet for a riser pipe? (Rob, Texas)
Reply: Good questions. Here is what I’d say, currently:
I like a rub sheet between the pipe risers and the uppermost geomembrane, since any downdrag deformation caused by the subsiding waste will cause pipe deformation and not be transferred to the underlying geomembrane. The rub sheet will probably cause the pipe(s) to deform down into the sump area, which ideally can be accommodated accordingly.
Now, to the type of rub sheet. What I often see is a 40- or 60-mill smooth HDPE sheet. Even if placed on a textured geomembrane, its friction value is so low that downdrag of the pipe can be envisioned readily. Your values of 100- and 120-mils are large, and if the rub sheet deforms along with the pipe, you don’t want any restriction and the 120-mil HDPE approaches “boilerplate.”
If you put a nonwoven geotextile between the geomembrane and the smooth rub sheet, you will definitely not affect the underlying geomembrane and force the rub sheet to deform with the overlying pipe system. Thus, it is a conservative alternative.