  Static granular packs have been studied in the last three decades in the
frame of a modified equilibrium statistical mechanics that assumes ergodicity
as a basic postulate. The canonical example on which this framework is tested
consists in the series of static configurations visited by a granular column
subjected to taps. By analyzing the response of a realistic model of grains, we
demonstrate that volume and stress variables visit different regions of the
phase space at low tap intensities in different realizations of the experiment.
We show that the tap intensity beyond which sampling by tapping becomes ergodic
coincides with the forcing necessary to break all particle-particle contacts
during each tap. These results imply that the well-known "reversible" branch of
tapped granular columns is only valid at relatively high tap intensities.
