  In this work we use a multi-scale framework to calculate the fluidization
threshold of three-dimensional cohesive granulates under shear forces exerted
by a creeping flow. A continuum model of flow through porous media provides an
analytical expression for the average drag force on a single grain. The balance
equation for the forces and a force propagation model are then used to
investigate the effects of porosity and packing structure on the stability of
the pile. We obtain a closed-form expression for the instability threshold of a
regular packing of mono-disperse frictionless cohesive spherical grains in a
planar fracture. Our result quantifies the compound effect of structural
(packing orientation and porosity) and dynamical properties of the system on
its stability.
