Angle of repose

The steepest slope at which loose granular material remains stable without sliding or collapsing. Its value depends on grain properties, packing, and measurement conditions.

Internal friction: Resistance to sliding between parts of a material under applied stress. Friction between grains helps hold a pile's slope against gravity.
Coefficient of friction: A dimensionless measure of friction between contacting surfaces. Greater grain-contact friction generally permits steeper stable piles.
Angle of repose test: A laboratory test that estimates granular flow and handling behavior from a pile's slope. The test uses the measured angle to compare powders and bulk solids.
Friction angle: An angle representing a material's resistance to shear, often defined through a stress-strength relationship. It is a material parameter, whereas repose angle describes a particular free pile.
Granular material: A collection of discrete solid particles whose bulk behavior differs from that of a continuous solid or fluid. The angle emerges from interactions among particles rather than from a single solid surface.
Particle shape: The geometry of an individual particle, including its roundness, elongation, and angularity. Interlocking angular grains can resist motion more than smooth, rounded grains.
Bulk material handling: The storage, transport, and processing of large quantities of loose solid materials. Equipment and storage layouts must account for how granular piles build and flow.
Critical state soil mechanics: A framework describing soil behavior as it shears toward a steady state of density and stress. It characterizes sustained soil deformation rather than a pile's observed stable slope.
Coulomb friction: A friction model in which sliding resistance is proportional to the normal force between surfaces. It approximates the contact resistance that stabilizes grains on a slope.
Particle size distribution: The range and relative abundance of particle sizes in a granular sample. A mixture of sizes can alter packing and the stability of the pile.

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