The odd weight of grains: Anomalous Janssen effect in narrow granular columns

Granular matter exhibits unusual mechanical properties. For instance, when filling a cylindrical column with grains the weight measured at the bottom of the column does not scales linearly with the added mass, but asymptotically saturates towards a constant "apparent" weight. This observation is the well-known Janssen effect, and the "lost weight" is supported by frictional forces between the grains and the column wall. Here, we present an experimental study addressing the mechanical behavior of static granular packings confined within a narrow vertical column: by reproducing the classical experiment designed by Janssen in 1895 but using narrower columns, we observe a reverse Janssen effect whereby the fraction of the weight supported by the base overcomes one.