Friction angle

From the GMA Techline

RE: Friction angle
What sort of phi (interface friction angle) would you expect between #57 crushed limestone gravel (0.5in.–1.5in.) and a textured HDPE membrane? Surely more than 10 degrees, right? What would a conservative estimate be, I wonder?
(Brian | Kentucky)

Reply: It would be more on the order of 25–30 degrees depending on the normal stress.

Bob Koerner | GMA Techline