The angle of repose, or critical angle of repose, of a granular material is the steepest angle of descent or dip relative to the horizontal plane to which a material can be piled without slumping.
Or it is the maximum horizontal angle at which granular particles can stack without using any other force or support.
Now, this might come to your mind that what is the maximum and what is the minimum angle of repose possible – So Theoretically speaking the angle will be between 0 and 90 degrees. Now let’s check which material has the largest and the least angle of repose –
Materials like Asphalt, Chalk, Coconut (Shredded), Coffee beans (Fresh), Earth, Gravel(Crushed stone) can have a maximum angle of repose up to 45 degrees. Whereas Looking to the least side we will have Urea, Wheat, Sand they can have as low as 27-30 degrees.