  We present a numerical model for a two dimensional (2D) granular assembly,
falling in a rectangular container when the bottom is removed. We observe the
occurrence of cracks splitting the initial pile into pieces, like in
experiments. We study in detail various mechanisms connected to the
`discontinuous decompaction' of this granular material. In particular, we focus
on the history of one single long range crack, from its origin at one side
wall, until it breaks the assembly into two pieces. This event is correlated to
an increase in the number of collisions, i.e. strong pressure, and to a
momentum wave originated by one particle. Eventually, strong friction reduces
the falling velocity such that the crack may open below the slow, high pressure
`dynamic arch'. Furthermore, we report the presence of large, organized
structures of the particles' angular velocities in the dense parts of the
granulate when the number of collisions is large.
