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From the GMA Techline
Subject: Seams on a sideslope — I am currently reviewing a design for a piggybacked cell that is an extension of a cell that was built 4 years ago. The issue that I am most troubled by is that the 2 cells meet halfway up the slope of an existing landfill. Unfortunately, this will lead to a horizontal seam of the 60-mil HDPE liner of about 600ft long running across a slope.
Have you seen this before, and how was it addressed? (Amanda, Maine)
Reply: Good question, and in my mind I am considering our general approach toward horizontal seams on sideslopes. As you know, they are common, but the strategy is to stagger these seams from one roll end (about 30ft long) to the next so that they do not align with one another. That said, 600ft is indeed long.
Let’s now look into the strength of an overlapped seam in shear and the typical strength required (including Maine, I believe). It is about 90% of the unseamed sheet strength. If HDPE, you are then ending up with 90% of the yield strength, which is considerably higher than other types of geomembranes at 50- 100% elongation. I think that the situation is reasonable.
I am glad that you did not ask about differential settlement between the old landfill and the new cell(s). If that is a concern, it might call for a geogrid or high-strength geotextile support beneath the geomembrane.