From the GMA Techline
Subject: GCLs and slope stability — I’d like to find out what the current thinking is on designing for slope stability with GCLs … perhaps summarize the current stateof- the-practice regarding residual strength of reinforced GCLs used in bottom liners at landfills. (Myron, Washington)
Reply: Of course, there are ongoing tests on GCLs constantly, but the focus is on the upper and lower interface shear tests and somewhat less with respect to internal shear strengths. I think that most designers are satisfied with the reinforcing systems of needlepunched GCLs at this time. I have not heard of any failures in this regard.
To more specifically answer your question, however, the residual strength of the internal shear strength is the shear strength of hydrated bentonite clay. Depending on the normal pressure, it is between 7° and 11°. To get there, you must fail the fiber reinforcement that, as I just mentioned, has not happened, to my knowledge.