  We propose a simple model for arch formation in silos. We show that small
pertubations (such as the thermal expansion of the beads) may lead to giant
stress fluctuations on the bottom plate of the silo. The relative amplitude
$\Delta$ of these fluctuations are found to be power-law distributed, as
$\Delta^{-\tau}$, $\tau \simeq 1.0$. These fluctuations are related to large
scale `static avalanches', which correspond to long-range redistributions of
stress paths within the silo.
