Thermal expansion effects in a silo

We consider a vertical container filled with granular material and exposed to temperature cycles which induce changes in the pressure distribution. Our starting point is an elastic model for the grains leading to classical Janssen behaviour in isothermal conditions. We assume complete mobilisation of the wall frictions. The dynamics superposes static friction plus a viscous friction. After a T jump favouring an expansion, we predict that the resulting overpressures are relaxed only in a region (of size comparable to the Janssen screening length lambda) near the top. When T returns to the initial value, we expect decohesion in a certain layer near the top (for rapid cooling). For slow cooling, we expect a smooth return to the original Janssen state. We also discuss a different, but related, effect: instead of changing the temperature, we switch off the gravity effects (on a small column) by suddenly rotating the column from vertical to horizontal. Here, a rarefaction wave should invade the whole column. (C) Academie des sciences/Elsevier, Paris.