Dear ggrirore,

This is not my area of expertise, but since nobody else has posted an answer, I will make some comments.

I have faced a similar problem with a different material. Through experimentation, we discovered that 26 degrees was the critical angle. But that was a coarser material an in a partially filled pipe.

Some factors that I imagine will be critical include the degree or aeration, the pipe diameter, and the standpipe effect.

Perhaps, by bumping this thread, some response from the experts might be forthcoming.

Regards ■