Actually, angle of repose has little to do with establishing highwall angle. It’s more about rock strength and orientation of structure (faults, joints, etc.), and changing slope angle just a degree one way or the other can make a difference of millions of tons of stripping.
A slope engineer once told me that there are only two rules for determining slope angle. If it fails, it was too steep, and if it never fails, it wasn’t steep enough.
You are correct that the general term angle-of-repose is more suited to earthen slope issues and the analysis is much more complex analysis of the entire mountainside.
I am not in the design field, but I do get to build what those on that side of the fence draw and plan and I wanted to make clear that this was not some sort of sluff of tailings — a cosmetic failure almost — instead the whole side of pit has failed.