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.