Packing Fractions and Maximum Angles of Stability of Granular Materials
  In two-dimensional rotating drum experiments, we find two separate influences of the packing fraction of a granular heap on its stability. For a fixed grain shape, the stability increases with packing fraction. However, in determining the relative stability of different grain shapes, those with the lowest average packing fractions tend to form the most stable heaps. We also show that only the configuration close to the surface of the pile figures prominently. 
J. Olson, M. Priester, J. Luo, S. Chopra, R.J. Zieve