  By means of numerical simulations, we study the influence of confinement on
three-dimensional random close packed (RCP) granular materials subject to
gravity. The effects of grain shape (spherical or polyhedral) and
polydispersity on this dependence are investigated. In agreement with a simple
geometrical model, the solid fraction is found to decrease linearly for
increasing confinement no matter the grain shape. This decrease remains valid
for bidisperse sphere packings although the gradient seems to reduce
significantly when the proportion of small particles reaches 40% by volume. The
aforementioned model is extended to capture the effect of the confinement on
the coordination number.
