Reverse Janssen effect in narrow granular columns

Janssen showed that the pressure of silos filled with grains matches the hydrostatic one for small filling heights but then saturates as the weight of the grains is supported by the lateral container wall through wall/grain frictional forces. Via a combined experimental and numerical investigation, we demonstrate a reverse Janssen effect whereby the pressure at the bottom exceeds the hydrostatic one. We show that this effect results from the emergence of compressive frictional forces, discuss the dependence on the various control parameters involved, and introduce a model accounting for our observations.

Our results suggest that a continuum description of the elastic properties of granular systems must incorporate constitutive relations more complex than previously envisaged: they are history-dependent and change as the external stress varies.