downward force on a mass =
= (mass) * (gravity) * (cosine ((90 degrees) - (slope angle in degrees)))
The angle of repose is the steepest angle at which material forming a slope will remain stable. Adding more material to the slope will result in slope failure.
The angle of repose may be affected by:
(1) type of material
(2) level of moisture
(3) how compacted the material is
(4) presence of stabilizers such as trees
(5) presence of ground shaking as in an earthquake or vibrations from the movement of heavy trucks
The slope angle for many materials seen in nature (sand dunes, rubble piles, etc) reflect the angle of repose. Common slope angles may range from 20 to 45 degrees.
safety factor for a slope =
= (shear strength) / (shear stress)
When the safety factor is less than 1 (stress exceeds strength) then the slope can be expected to fail.
