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ANGLE OF REPOSE: http://en.wikipedia.org/wiki/Angle_of_repose ...the maximum angle of a stable slope determined by friction, cohesion and the shapes of the particles. When bulk granular materials are poured onto a horizontal surface, a conical pile will form. The internal angle between the surface of the pile and the horizontal surface is known as the angle of repose and is related to the density, surface area, and coefficient of friction of the material.
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When it comes to loose, unconsolidated, sediment, the steepest angle of repose is about 40°. This is for relatively large and angular fragments. The smaller and more rounded the fragments are the gentler the slope will be.
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Slopes in conical piles of cohesionless materials, typically dry sands and gravels, will stand at an angle around 30 degrees from the horizontal (plus or minus a couple of degrees). Slopes are unstable when at their angle of repose, and vibration or change of moisture content (especially drying) can cause sudden sliding. A pile of great size thought to be near its angle of repose should be understood to be unsafe to walk on or climb. Road salt will stand more steeply than sand because (a) its sharper and (b) it starts to cohere in storage. I'd love to measure the angle of repose of glass beads like the ones used in reflectorized paint. They really skitter around and their angle of repose could be as low as 18 degrees. (about 3:1 H/V) (?) The standard highway sideslope in soil cuts is 2:1 (26.5 degrees) and needs vegetative stabilization. This is too steep to mow. Soils are normally somewhat cohesive so other non-geometric rules apply.
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