  We study numerically the influence of confinement on the solid fraction and
on the structure of three-dimensional random close packed (RCP) granular
materials subject to gravity. The effects of grain shape (spherical or
polyhedral), material polydispersity and confining wall friction 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. Furthermore, 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 confinement effect on
the coordination number is also captured by an extension of the aforementioned
model.
