GCL and HDPE questions

From the GMA Techline

RE: Two questions

• Does a geosynthetic clay liner (GCL) form whales from gas or water pressure, the same way as with a flexible membrane liner (FML), or does gas or water under pressure pass upward through a GCL?
• When a high-density polyethylene (HDPE) pipe crushes—say, due to lack of bedding when backfill is compacted in the trench above it—does it just pinch off or does it shear?

(Brian | Kentucky)

Reply: Regarding your first question, GCLs do not whale up from pressures beneath them since they have sufficient porosity for the underlying gas to escape. Even if the voids are saturated, I believe that permselectivity will function whereby the gases will work their way through the thin layer of saturated bentonite. Remember also that the GCL weighs three times that of a geomembrane.

Regarding HDPE pipe when it is overstressed, it does, indeed, crush rather than fracture. The deformation eventually will pinch off the flow within the pipe itself. I have never seen HDPE pipe shear since it is quite ductile. The only caveat that I can think of is in extremely cold environments.

Bob Koerner | GMA Techline