  We present experimental results of the jamming of non-cohesive particles
discharged from a flat bottomed silo subjected to vertical vibration. When the
exit orifice is only a few grain diameter wide, the flow can be arrested due to
the formation of blocking arches. Hence, an external excitation is needed to
resume the flow. The use of a continuous gentle vibration is a usual technique
to ease the flow in such situations. Even though jamming is less frequent, it
is still an issue in vibrated silos. There are, in principle, two possible
mechanisms through which vibrations may facilitate the flow: (i) a decrease in
the probability of the formation of blocking arches, and (ii) the breakage of
blocking arches once they have been formed. By measuring the time intervals
inside an avalanche during which no particles flow through the outlet, we are
able to estimate the probability of breaking a blocking arch by vibrations. The
result agrees with the prediction of a bivariate probabilistic model in which
the formation of blocking arches is equally probable in vibrated and
non-vibrated silos. This indicates that the second aforementioned mechanism is
the main responsible for improving the flowability in gently vibrated silos.
