From the GMA Techline
RE: Technical reference for LLDPE cover soil depth
We are working on a project and our installation procedures are being questioned—specifically, what basis do we have for claiming that 36in. of cover soil will protect a 40 mil liner from damage. Our firm has always used 12, 24, or 36 inches as the minimum depending on ground pressure of the equipment.
Do you know of any technical reference we can cite to support our minimum requirement?
(Steve | Minnesota)
Reply: The 36in.-thick cover soil is a somewhat historical value originally aimed at protecting a compacted clay liner from freeze-thaw degradation of its hydraulic conductivity. In my mind, it has nothing to do with damage protection of the geomembrane.
You are completely correct in your use of thinner layers for protection of the geomembrane. The value of thickness, however, is subjective and does depend on the construction equipment.
Regarding references, we will send a laboratory study on required thickness over a GCL to prevent damage in the form of lateral squeezing of the bentonite. Using ratios to real-life equipment we come out with 12in. to which the Corps of Engineers uses 18in. Thus, the values you suggest are certainly more in line than the frost depth values which have nothing to do with damage to the geomembrane.
Incidentally, freeze-thaw cycling of geomembranes is a complete nonissue insofar as geomembrane degradation. We have a white paper on our website to that issue: “Cold Temperature and Free-Thaw Cycling Behavior of Geomembranes and Their Seams”.