When a mass of granular or frictional material is pushed from one side, it builds up into a wedge shape. The angle of this wedge eventually reaches a state of equilibrium where the material slides forward as a solid block over the base friction rather than piling up further. Engineers use this concept to analyze material handling systems, bulk storage flow, and soil mechanics under lateral loads.
Heard on the job
The sand keeps piling up instead of sliding along the belt because the chute angle hasn't reached the critical taper yet.
Insider tip
In bulk material handling, designing hoppers and chutes below this angle guarantees that the material will bridge or rat-hole instead of flowing smoothly.