angle of repose
(of a granular material) steepest angle of descent or dip relative to the horizontal plane to which a material can be piled without slumping. At this angle, the material on the slope face is on the verge of sliding
en-GB/ˈæŋɡəl əv ɹɪˈpəʊz/
en-US/ˈæŋɡəl ʌv ɹiˈpoʊz/