Bulk granular materials when poured on a flat surface will create a conical pile. The internal angle between the surface of the pile and the horizontal surface is called as ‘angle of repose’. Angle of repose is depended on the density, surface area, and the shapes of the particle also the coefficient of friction.
Antlion larvae’s sand pit works on critical angle of repose to catch its prey, see below image
IMSBC Code defines angle of repose as the maximum slope angle of non-cohesive (i.e., free-flowing) granular material. It is measured as the angle between a horizontal plane and the cone slope of such material.
Angle of repose of UN 1942 AMMONIUM NITRATE is 27 deg to 42 deg
IMDG Code Compliance Centre 