Effect of the pendular state on the collapse of granular columns

Granular materials exhibit special phase transitions between different mechanical states. They behave like solids or fluids when particular conditions occur. Generally, the presence of water in the granular medium greatly affects mechanical stability, as well as jamming and blockage in dynamic phenomena, even in very small amounts. In this work the triggering and the stopping of flow was studied with regards to granular column collapse experiments. Glass beads of different grain-size were mixed with liquids of different surface tension values. The amount of liquid was varied from 0 to 5% in order to maintain the pendular state. The material was first poured in a rectangular box and then allowed to flow by removing a lateral wall. The movement of the mass was captured by a high-speed CCD camera. Repose angles, run-out and slope height were measured in order to explore the effect of the different initial dry and wet conditions on the profile evolution. DEM numerical simulations were also carried out in a 1:1 scale reproducing the same experimental configuration and conditions. The classical Discrete Element model with spring dashpot at the normal contact was coupled with a capillary attraction force based on the minimum energy approach. Using DEM, the effect of liquid content and liquid surface tension were also evaluated. A good agreement between experiments and DEM simulations was found with respect to the kinematic and the final slope profile. In particular, both the techniques highlight the effect of the liquid that reduces the run-out distance and time even for small liquid contents. This work demonstrates the suitability of the DEM approach for the study of wet granular materials in static as well as in dynamic conditions.