  We investigate the relaxation dynamics of a dense monolayer of bidisperse
beads by analyzing the experimental data previously obtained in a fluidized
bed. We show that the dynamics is formed by elementary relaxation events called
cage jumps. These aggregate on a very short time into clusters. Increasing the
packing fraction makes the spatio-temporal organization of the clusters evolve
from a rather scattered and random distribution towards a collection of sparse
and large events, called avalanches. The avalanche process is a manifestation
of dynamical facilitation. The study of its evolution with density reveals that
dynamical facilitation becomes less conserved and play a lesser role for the
structural relaxation approaching the granular glass transition.
