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We present experimental results on slow flow properties of a granularassembly confined in a vertical column and driven upwards at a constantvelocity V. For monodisperse assemblies this study evidences at low velocities($1<V<100 \mu m/s$) a stiffening behaviour i.e. the stress necessary to obtaina steady sate velocity increases roughly logarithmically with velocity. On theother hand, at very low driving velocity ($V<1 \mu m/s$), we evidence adiscontinuous and hysteretic transition to a stick-slip regime characterized bya strong divergence of the maximal blockage force when the velocity goes tozero. We show that all this phenomenology is strongly influenced by surroundinghumidity. We also present a tentative to establish a link between the granularrheology and the solid friction forces between the wall and the grains. We baseour discussions on a simple theoretical model and independent grain/walltribology measurements. We also use finite elements numerical simulations toconfront experimental results to isotropic elasticity. A second system made ofpolydisperse assemblies of glass beads is investigated. We emphasize the onsetof a new dynamical behavior, i.e. the large distribution of blockage forcesevidenced in the stick-slip regime

Slow dynamics and aging of a confined granular flow