What characteristic of soil is most important when determining the angle of repose?

The angle of repose refers to the steepest angle at which a sloped surface formed of loose material remains stable. Grain size plays a crucial role in determining this angle because it influences the frictional forces between particles. Coarser grains typically have a higher angle of repose due to their ability to interlock and create more resistance to movement, while finer grains tend to have lower angles of repose because they can flow more easily.

Other factors contribute to the angle of repose as well, such as moisture content, which can either increase or decrease stability depending on whether it acts to bind particles together or causes slippage through a film of water. Plasticity can also affect stability, especially in cohesive soils. However, the primary characteristic affecting the angle of repose is indeed the size of the grains, as it directly impacts the physical interrelations and stability of the granular structure. Color does not have a significant effect on the angle of repose; it is primarily an aesthetic feature without direct influence on the physical behavior of soil under gravitational forces.