Angle of Repose
The maximum angle that a soil, sediment or other loose material can be placed or accumulate and be stable. The angle of repose varies for different types of materials and different moisture conditions. Example: once sand begins to pile up, ripples and dunes can form. Wind continues to move sand up to the top of the pile until the pile is so steep that it collapses under its own weight. The collapsing sand comes to rest when it reaches just the right steepness to keep the dune stable. This angle, usually about 30-34°, is called the angle of repose. Every pile of loose particles has a unique angle of repose, depending upon the properties of the material its made of.