We talked about angle of internal friction of soil, in previous posts of this series. There is also the angle of repose, which is often confused with angle of internal friction. Angle of repose is the angle a soil forms with the horizontal, when poured over a surface in it’s loosest state. Although angle of repose and angle of internal friction is in direct proportion to each other, in other words, a higher value of one means a higher value of the other as well, they are not the same thing. Angle of internal friction can change with the density of sand. Angle of repose however, is the angle of internal friction in the soil’s loosest state (such as when poured over a surface by free fall).
Angle of repose is for sands only. Clays can not be just spilled on a surface and take this kind of shape. We cannot speak about angle of repose for clays, unless we submerge it into water, so that all cohesive bonds are broken (remember that clay is cohesive), to measure it.
In the next post of this series, we will discuss “Cohesion of Clay”