By means of extensive contact dynamics simulations, we investigate the mechanical equilibrium and deformation of a granular material composed of irregular polyhedral particles confined between two horizontal frictional planes. We show that, as a consequence of mobilized wall-particle friction forces at the top and bottom boundaries, the transient deformation induced by a constant vertical load is controlled by the aspect ratio (thickness over width) of the packing as well as the stress ratio. The transient deformation declines considerably for increasingly smaller aspect ratios and grows with the stress ratio. From the simulation data for a large number of independent configurations, we find that sample-to-sample fluctuations of the deformation have a broad distribution and they scale with the average deformation. We also analyze the evolution of particle connectivity during settlement and with the applied force. The face-face and edge-face contacts between polyhedral particles concentrate strong force chains with a growing proportion as a function of the applied force.