Sound velocity measurements in dense glass bead packs reveal significant softening effect at large amplitudes, due to the frictional nonlinearity at the grain contacts. Beyond a certain amplitude, the sound-matter interaction becomes irreversible, leaving the medium in a weakened and slightly compacted state. A slow recovery of the initial elastic modulus is observed after acoustic perturbation, revealing the plastic creep growth of microcontacts. The cross-correlation function of configuration-specific acoustic speckles highlights the relationship between the macroscopic elastic weakening and the local change of the contact networks, induced by strong sound vibration, in the absence of appreciable grain motion.

A significant dependence on the relative humidity H for the apparent mass (Mapp) measured at the bottom of a granular packing inside a vertical tube in relative motion is demonstrated experimentally. While the predictions of Janssen's model are verified for all values of H investigated (25% ≤ H ≤ 80%), Mapp increases with time towards a limiting value at high relative humidities (H ≥ 60%) but remains constant at lower ones (H = 25%). The corresponding Janssen length λ is nearly independent of the tube velocity for H ≥ 60% but decreases markedly for H = 25%. Other differences are observed on the motion of individual beads in the packing: for H = 25%, they are almost motionless while the mean particle fraction of the packing remains constant; for H ≥ 60% the bead motion is much more significant and the mean particle fraction decreases. The dependence of these results on the bead diameter and their interpretation in terms of the influence of capillary forces are discussed. PACS numbers: 45.70.-n, 68.08.Bc,81.05.RmDense granular flows in vertical channels are encountered in many industrial processes and often display intermittency or blockage effects representing important practical problems [1,2,3]. These effects depend on the force distribution in the moving grain packing and on its interaction with the walls: these depend in turn on the relative humidity H of the atmosphere. In static packings, capillary forces strongly influence, for instance, the stability of sand piles as a function of H [4,5,6,7,8,9]. In the present work, the influence of humidity on force transmission in a granular packing inside a vertical tube in relative motion is analyzed from variations of its apparent weight measured at the bottom of the packing.Stress transmission in static or quasistatic granular packings has been frequently investigated theoretically and experimentally [10, 11, 12, 13, 14, 16, 17, 18, 19? ] following the pioneering work and Janssen [20]. This latter study and subsequent ones predict that the vertical stress at the bottom of a grain packing inside a vertical cylindrical tube reaches exponentially a limiting value as the height of the packing increases. This is due to the redirection of vertical stresses towards the side walls where friction between grains and the walls occurs: this shields the lower grain layers from the weight of the upper ones so that the apparent mass M app measured at the bottom is lower than the total mass M of the grains.In a recent paper [17] we demonstrated that, for a constant relative humidity H = 50%, Janssen's predictions remain valid for a grain packing inside a vertical tube moving upwards relative to the grains at velocities between 10 −2 and 35 mm s −1 . The present study uses these results as a tool to analyse the dependence of stress transmission on the relative humidity H in the range 25% ≤ H ≤ 80%: H will be shown to influence strongly the variations and value of the apparent mass of the grains during the tube motion. Analyzing the motion of the grains during the tube displacement...