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. Ou r starting point is an elastic model for the grains leading to classical Ja nssen behaviour in isothermal conditions. We assume complete mobilisation o f the wall frictions. The dynamics superposes static friction plus a viscou s friction. After a T jump favouring an expansion, we predict that the resu lting overpressures are relaxed only in a region (of size comparable to the Janssen screening length lambda) near the top. When T returns to the initi al value, we expect decohesion in a certain layer near the top (for rapid c ooling). For slow cooling, we expect a smooth return to the original Jansse n state. We also discuss a different, but related, effect: instead of chang ing the temperature, we switch off the gravity effects (on a small column) by suddenly rotating the column from vertical to horizontal. Here, a rarefa ction wave should invade the whole column. (C) Academie des sciences/Elsevi er, Paris.